What is Electric Current?
How
does it
resemble
the flow
of water
in a
pipe?
Can you get a flashlight bulb to
light, with a battery and a
single wire?
Electric Circuits and Electric
Current
A flashlight, an electric toaster, and a car’
car’s starting
motor all involve electric circuits and electric current.
For the flashlight bulb
to light, there must be
a closed or complete
path from the bulb to
both ends of the
battery.
Such a path is called
a circuit.
Electric Circuits and Electric
Current
A flashlight, an electric toaster, and a car’
car’s starting
motor all involve electric circuits and electric current.
In this circuit, the battery is the energy source,
using energy from chemical reactions to separate
positive and negative charges.
This leads to a voltage difference, with an excess
of positive charges at one end of the battery and
an excess of negative charges at the other.
These charges will tend to flow from one terminal
to the other if we provide an external conducting
path (the circuit).
1
A flow of electric charge is an electric current:
current:
q where I is electric current, q is charge,
I=
t and t is time.
The standard unit for electric current is the ampere:
ampere:
1A=1C/s
A flow of electric charge is an electric current:
current:
q where I is electric current, q is charge,
I=
t and t is time.
The standard unit for electric current is the ampere:
ampere:
1A=1C/s
For example, if 3 C of
charge flow through a wire in
2 s, then the electric current
The direction of current is
defined as the direction that
positive charges would flow.
I is 3 C / 2 s = 1.5 A.
In reality, the charge
carriers in a metal wire are
negatively charged
electrons.
Positive charges moving to
the right have the same
effect as negative charges
moving to the left.
In addition to an energy source and a conducting path, a
circuit also includes some resistance to the current.
In the flashlight bulb, a very thin
wire filament restricts the current
because of its very small crosssectional area.
The wire filament gets hot as
charges are forced through this
constriction.
Its high temperature makes it
glow, and we have light.
Two arrangements of a battery, bulb, and
wire are shown below. Which of the two
arrangements will light the bulb?
a)
b)
c)
d)
Arrangement (a)
Arrangement (b)
Both
Neither
The bulb will light in
arrangement A in which the
filament of the bulb is
connected to the two sides of
the battery for a closed circuit.
In B there is no voltage across
the filament and thus no
current in the filament.
2
Water flowing in a pipe is similar to electric current flowing
in a circuit.
The battery is like the pump.
The electric charge is like the water.
The connecting wires are like the thick pipe.
The filament is like the nozzle or narrow pipe.
The switch is like the valve.
In the circuit shown, the wires are
connected to either side of a wooden block
as well as to the light bulb. Will the light
bulb light in this arrangement?
a)
b)
c)
d)
Yes
No
Maybe
Impossible to tell
from the picture
The bulb will not light since
(dry) wood is a very poor
conductor. The resistance will
be so high that virtually no
current is in the lamp circuit.
In a waterwater-flow system, a high pressure difference will
produce a large rate of water flow or current.
High pressure can be produced by raising the storage tank: this
pressure is related to the gravitational potential energy.
Likewise, a large difference in potential energy between the charges
charges
at the two ends of a battery is associated with a high voltage and
and a
greater tendency for charge to flow.
In the circuit shown, could we increase the
brightness of the bulb by connecting a wire
between points A and B?
a)
b)
c)
d)
Yes
No
Maybe
Impossible to tell
from the picture
No. Connecting A and B will
provide a short circuit for the
battery that will damage it
while allowing virtually no
current in the bulb.
3
Which of the two circuits shown will cause
the light bulb to light?
Arrangement (a)
Arrangement (b)
Both
Neither
a)
b)
c)
d)
a)
b)
c)
d)
Diagram B will allow the
light bulb to light since
there is a closed circuit
providing current from
the battery through the
bulb. Whether the
switch is open or closed
is immaterial here since
it is in parallel with another conductor.
In diagram A no potential difference is in the closed circuit.
Ohm’s Law and Resistance
The electric current flowing through a given portion of a
circuit is directly proportional to the voltage difference
across that portion and inversely proportional to the
resistance:
Ohm' s Law :
Suppose we use an uncoated metal clamp to
hold the wires in place in the batterybattery-andandbulb circuit shown. Will this be effective in
keeping the bulb burning brightly?
I=
Yes
No
Maybe
Impossible to tell
from the picture
No. The metal clamp will provide a
conducting path across the battery
causing the battery to discharge. If
we want to use a clamp we can put
insulating tape between one of its
jaws and the electrical connection.
Consider the two signs shown, located in
different physics labs. Which of the two
would be reason for greater concern?
∆V
R
Resistance R is the ratio of the voltage difference to the current for a given
given
portion of a circuit, and is in units of ohms:
1 ohm = 1 Ω = 1 V / A.
A.
The resistance of a wire is proportional to the length of the wire, inversely
proportional to the crosscross-sectional area of the wire, and inversely
proportional to the conductivity of the material.
It also depends on the temperature of the material.
a)
b)
c)
d)
The one on the
left.
The one on the
right.
Both
Neither
We had better pay attention to the high voltage
warning. The other is a practical joke. The danger to
the body, and even to life, comes from electrical
current in the body, which could occur if you
accidentally make contact with a large potential
difference across parts of your body. The effect of a
high resistance is to limit current in a circuit if a
voltage source is present; it is not dangerous at all.
4
BODILY EFFECT
DIRECT CURRENT (DC) 60 Hz AC
10 kHz AC
-----------------------------------------------------------------------------------------------------------------------Slight sensation
Men =
1.0 mA
0.4 mA
7 mA
felt at hand(s)
Women =
0.6 mA
0.3 mA
5 mA
-----------------------------------------------------------------------------------------------------------------------Threshold of
Men =
5.2 mA
1.1 mA
12 mA
perception
Women =
3.5 mA
0.7 mA
8 mA
-----------------------------------------------------------------------------------------------------------------------Painful, but
Men =
62 mA
9 mA
55 mA
voluntary muscle Women =
41 mA
6 mA
37 mA
control maintained
-----------------------------------------------------------------------------------------------------------------------Painful, unable
Men =
76 mA
16 mA
75 mA
to let go of wires Women =
51 mA
10.5 mA
50 mA
-----------------------------------------------------------------------------------------------------------------------Severe pain,
Men =
90 mA
23 mA
94 mA
difficulty
Women =
60 mA
15 mA
63 mA
breathing
-----------------------------------------------------------------------------------------------------------------------Possible heart
Men =
500 mA
100 mA
fibrillation
Women =
500 mA
100 mA after 3 seconds
------------------------------------------------------------------------------------------------------------------------
However, we ignored the resistance of the battery itself, as
well as the very small resistance of the connecting wires.
If the battery is fresh, its internal resistance is small and can
can often
be neglected.
As the battery is used, its internal resistance gets larger.
The voltage of the battery, 1.5
V, is called the electromotive
force ε: the increase in
potential energy per unit
charge provided by the
chemical reactions in the
battery.
Loop equation:
ε =IR
If we know the resistance of a given portion of a circuit and
the applied voltage, we can calculate the current through that
portion of the circuit.
For example, consider a
1.5-V battery connected to a
light bulb with a resistance of
20 ohms.
If the resistance of the
battery itself is negligible, the
current can be found by
applying Ohm’s Law:
I = 1.5 V / 20 Ω
= 0.075 A
= 75 mA
If the internal resistance of the battery is 5 Ω, then the total
R = Rbattery + Rbulb
resistance of the circuit is:
= 5 Ω + 20 Ω
= 25 Ω
Then the total current in the circuit is: I = ε / R
= 1.5 V / 25 Ω
= 0.06 A = 60 mA
And the voltage difference
across the light bulb is:
∆V = I R
= (0.06 A)(20 Ω)
= 1.2 V
If we measure the voltage
difference across the battery or
the light bulb, we will get 1.2 V.
5
If we disconnect the bulb and measure the voltage across the
battery terminals, we will get 1.5 V again.
As a battery gets older, its internal resistance gets larger.
The total resistance of the circuit increases and reduces the
current flowing through the circuit.
As the current gets smaller, the bulb gets dimmer.
In a dead battery, the internal resistance has become so large
that the battery can no longer
produce a measurable current.
A good voltmeter does not
draw much current, so it can
still measure approximately the
electromotive force of the
battery, even if the battery has
too much internal resistance to
produce a measurable current.
In a series combination of resistances, each resistance
contributes to restricting the flow of current around the
loop.
The total series resistance of the combination Rseries is the sum of the
individual resistances:
Rseries = R1 + R2 + R3
A common mistake is to think the current gets used up in passing
through the resistances in a series circuit.
The same current must pass through each component much like the
continuous flow of water in a pipe.
Series and Parallel Circuits
In a series circuit,
circuit, there are no points in the circuit where
the current can branch into secondary loops.
It is the voltage that changes as the current flows through the
circuit.
All the elements line up on a single loop.
The current that passes through one element must also pass through
through
the others.
Voltage decreases by Ohm’
∆V = I R
Ohm’s Law:
as the current passes through each resistor.
The total voltage difference across the combination is the
sum of these individual changes.
If two light bulbs are connected in series with a battery, the current
current
will be less than with a single bulb, because the total series resistance
resistance
is larger.
The bulbs will glow less brightly.
6
Two resistors are connected in series with a
battery as shown. R1 is less than R2.
Which of the two resistors has the greater
current flowing through it?
a)
b)
c)
d)
R1
R2
Both
Neither
The current is the same in each,
since it is a series circuit.
In the circuit shown, the 11-Ω resistance is the
internal resistance of the battery and can be
considered to be in series with the battery and
the 99-Ω load. What is the current flowing
through the 99-Ω resistor?
a)
b)
c)
d)
e)
0.1 A
0.3 A
0.9 A
3A
10 A
Rseries = 9 Ω + 1 Ω = 10 Ω
Iseries
V
3V
= total =
= 0.3 A
Rseries 10 Ω
Two resistors are connected in series with a
battery as shown. R1 is less than R2.
Which of the two resistors has the greatest
voltage difference across it?
a)
b)
c)
d)
R1
R2
Both
Neither
The voltage difference is greater across R2. According to
Ohm's Law, V = IR, so for the same current, the larger
the resistance the greater the potential difference.
In the circuit shown, the 11-Ω resistance is
the internal resistance of the battery and can
be considered to be in series with the battery
and the 99-Ω load. What is the voltage
across the
9-Ω resistor?
a)
b)
c)
d)
e)
0.1 V
0.3 V
1.0 V
2.7 V
3.0 V
since I9Ω = Iseries :
V9Ω = IseriesR9Ω = (0.3 A)(9 Ω) = 2.7 V
7
Three resistors are connected to a 66-V
battery as shown. The internal resistance of
the battery is negligible. What is the
current through the 1515-Ω resistance?
a)
b)
c)
d)
e)
0.1 A
0.15 A
0.4 A
1.5 A
4.0 A
Rseries = 15 Ω + 20 Ω + 25 Ω = 60 Ω
I15Ω = Iseries
Does this same current flow through the 2525Ω resistance?
V
6V
= total =
= 0.1 A
Rseries 60 Ω
a)
b)
c)
Yes. Since all the resistors are in series, the same current
must flow through all three. There is no other path for the
current through the 15-Ω resistor to follow, except to go
through the 20-Ω resistor and then the 25-Ω resistor.
What is the voltage difference across the
2525-Ω resistance?
a)
b)
c)
d)
e)
0.1 V
2.5 V
6V
25 V
60 V
Yes.
No.
It depends on
various things.
In a parallel circuit,
circuit, there are points at which the current
can branch or split up into different paths.
The flow divides and later rejoins.
The total crosscross-sectional area the current (or water) flows through is
increased,
increased, therefore decreasing the resistance to flow:
flow:
1
1
1
1
= +
+
Rparallel R1 R2 R3
since I25Ω = Iseries :
V25Ω = IseriesR25Ω = (0.1 A)(25 Ω) = 2.5 V
8
In a parallel combination of resistances, the voltage
difference across each resistance is the same, since they are
all connected between the same two points.
The currents can be different, since they divide: they add to give
give the
total current through the combination.
A portion of the total current flows through each branch.
Two 1010-Ω light bulbs are connected in
parallel to one another, and this combination
is connected to a 66-V battery. What is the
total current flowing around the loop?
a) 0.6 A
b) 1.2 A
c) 6 A
d) 12 A
1
1
1
= +
Rparallel R1 R2
=
1
1
+
10 Ω 10 Ω
=
2
1
=
10 Ω 5 Ω
e) 60 A
I=
=
ε
R
6V
5Ω
= 1.2 A
Parallel combinations
decrease the resistance
and increase the amount
of current that will flow.
Increased current causes
the bulbs to burn more
brightly than in a series
circuit but also depletes
the batteries more quickly.
The energy available from
the batteries is the same in
either case.
How much current passes through each light
bulb?
a) 0.6 A
b) 1.2 A
c) 6 A
d) 12 A
I=
=
e) 60 A
∆V
R
6V
10 Ω
= 0.6 A
Rparallel = 5 Ω
9
Three identical resistors, each 24 Ω, are
connected in parallel with one another as shown.
The combination is connected to a 1212-V battery
whose internal resistance is negligible.
What is the equivalent resistance of this parallel
combination?
a)
b)
c)
d)
e)
0.0417 Ω
0.125 Ω
8Ω
24 Ω
72 Ω
What is the total current through the
combination?
a)
b)
1
Rparallel
=
c)
1
1
1
+
+
R1 R2 R3
=
1
1
1
+
+
24 Ω 24 Ω 24 Ω
=
3
1
=
24 Ω 8 Ω
d)
e)
0.5 A
1.0 A
1.2 A
1.5 A
12 A
I total =
Vtotal
12 V
=
= 1.5 A
Rparallel 8 Ω
Rparallel = 8 Ω
How much current flows through each resistor in
the combination?
a)
b)
c)
d)
e)
0.5 A
1.0 A
1.2 A
1.5 A
12 A
The resistors are identical and are in parallel,
so the same current must flow through each resistor.
The total current is plit into three identical parts :
1
1
I one 24-Ω resistor = I total = (1.5 A ) = 0.5 A
3
3
In the circuit shown, R3 is greater than R2,
and R2 is greater than R1. ε is the
electromotive force of the battery whose
internal resistance is negligible. Which of
the three resistors has the greatest current
flowing through it?
a)
b)
c)
d)
e)
R1
R2
R3
R1 and R2 are equal,
and greater than R3
They are all equal
R3 has the greatest current since the current in it is the
sum of the currents in R1 and R2.
10
A voltmeter measures the voltage difference between two
points in a circuit, or across an element in a circuit
It is inserted in parallel with the element whose voltage difference
is being measured.
A voltmeter should have a large resistance, so that it does not divert
much current from the component whose voltage is being measured.
measured.
In the circuit shown, the circle with a V in
it represents a voltmeter. Which of the
following statements is correct?
a)
b)
c)
The voltmeter is in the
correct position for
measuring the voltage
difference across R.
No current will flow
through the meter, so it
will have no effect.
The meter will draw a
large current.
The correct statement is (a). A voltmeter is a highresistance device connected in parallel with whatever circuit
element it is desired to measure the voltage across.
An ammeter measures the electric current flowing through a
point in a circuit.
It is inserted in series into the circuit whose current is being
measured, so that all the current flows through it.
An ammeter should have a small resistance, so that its effect on the
current is small.
small.
If you place an ammeter directly across the terminals of a battery,
battery,
you could damage the meter and the battery.
In the circuit shown, the circle with an A in
it represents an ammeter. Which of the
following statements is correct?
a)
b)
c)
The meter is in the
correct position for
measuring the current
through R.
No current will flow
through the meter, so it
will have no effect.
The meter will draw a
significant current from
the battery.
The answer is (c). An ammeter is a low-resistance device
and is to be placed in series in the circuit, just as a flowmeter is placed in a fluid circuit.
11
Electric Energy and Power
Similarly, in an electric circuit energy is supplied by a battery,
battery,
which draws its energy from the potential energy stored in its
chemical reactants.
Energy is supplied to a waterwater-flow system by the pump, which
increases the gravitational potential energy of the water by lifting
lifting
it up to a higher tank.
The battery increases the potential energy of electric charges as
as it
moves positive charges toward the positive terminal and negative
charges toward the negative terminal.
When we provide an external conducting path from the positive to
the negative terminal, charge flows from points of higher potential
potential
energy to points of lower potential energy.
As the water flows down
through pipes to a lower
tank, gravitational potential
energy is transformed into
kinetic energy of the
moving water.
Once the water comes to
rest in the lower tank, the
kinetic energy is dissipated
by frictional or viscous
forces which generate
heat.
Energy source → potential energy → kinetic energy → heat
Since voltage is potential energy per unit charge, multiplying a voltage
difference by charge yields energy.
Since current is the rate of flow of charge, multiplying a voltage
voltage
difference by current yields power, the rate of energy use.
The power supplied by a source must equal the power dissipated in
in the
resistances.
As potential energy
is lost, kinetic
energy is gained by
the electrons.
This kinetic energy
is converted to heat
by collisions with
other electrons and
atoms.
What is the power dissipated in a 2020-Ω
light bulb powered by two 1.51.5-V
batteries in series?
a)
b)
c)
P = εI
= ∆VI;
d)
∆V = IR ⇒
P = (IR)R = I 2 R
εI = I R
e)
0.15 W
0.45 W
3.0 W
6.67 W
60 W
ε = ε +ε
1
2
=3V
R = 20 Ω
ε = IR
ε
I= =
R
3V
= 0.15 A
20 Ω
P = εI = I 2 R = (0.15 A) 2 (20 Ω) = 0.45 W
2
check :
P = εI = (3 V)(0.15 A) = 0.45 W
12
The ease with which electric power can be transmitted over
considerable distances is one of its main advantages over
other forms of energy.
The source of the energy might be gravitational potential energy of
water, chemical potential energy stored in fossil fuels, or nuclear
nuclear
potential energy stored in uranium.
Power plants all use electric generators that convert mechanical
kinetic energy produced by turbines to electric energy.
These generators are the source of the electromotive force.
The unit of energy commonly used to discuss electric energy
is the kilowattkilowatt-hour,
hour, which is a unit of power (the kilowatt)
multiplied by a unit of time (an hour).
The kilowattkilowatt-hour is a much larger unit of energy than the
joule, but it is a convenient size for the amounts of electrical
energy typically used in a home.
How much does it cost to light a 100100-watt
light bulb for one day? Assume an average
rate of cost of 10 cents per kilowattkilowatt-hour.
a) 0.24 cents
b) 2.4 cents
c) 24 cents
d) $2.40
e) $24
Energy used = power x time
Cost = energy used x rate of cost
= (100 W)(24 hr)
= (2.4 kWh)(10 cents / kWh)
= 2400 Wh
= (2.4 kWh)(10 cents / kWh)
= 2.4 kWh
= 24 cents
1 kilowatt equals 1000 watts
1 hour = 3600 seconds
1 kilowattkilowatt-hour equals 3.6 million joules
Alternating Current and
Household Circuits
The current we draw from a wall outlet is alternating
current (ac)
ac) rather than direct current (dc).
dc).
Direct current implies that the current flows in a single
direction from the positive terminal of a battery or power
supply to the negative terminal
Alternating current continually reverses its direction -- it
flows first in one direction, then in the other, then back again.
again.
In North America the ac goes through 60 cycles each second
(60 Hz).
Hz).
60 Hz means that the
direction of the
current is alternated
every 1/60 seconds
(0.0167 s).
13
Certain sources of electricity naturally produce voltages alternating in
polarity, reversing positive and negative over time. Either as a voltage
switching polarity or as a current switching direction back and forth, this
“kind” of electricity is known as Alternating Current (AC):
How AC
electricity is
generated:
One might wonder why anyone would bother with such a
thing as AC. It is true that in some cases AC holds no
practical advantage over DC. In applications where
electricity is used to dissipate energy in the form of heat,
the polarity or direction of current is irrelevant, so long as
there is enough voltage and current to the load to produce
the desired heat (power dissipation). However, with AC it
is possible to build electric generators, motors and
power distribution systems that are far more efficient
than DC, and so we find AC used predominately across
the world in high power applications.
Transformers change voltage:
A transformer adjusts the voltage of an ac circuit
up or down as needed for a particular application.
Transformers are
seen on utility
poles, at electrical
substations, and as
voltage adapters for
electrical devices.
The ability to use
generators and
transformers mean
that alternating
current is
convenient for
large-scale power
production and
distribution.
The ratio of the number of turns in the primary coil to
the voltage on the primary coil is equal to the ratio of
the number of turns on the secondary coil to the
induced voltage in the secondary coil:
N1
N
= 2
∆V1 ∆V2
N 
∆V2 = ∆V1 2 
 N1 
14
War of the currents (circa 1880)
George Westinghouse and Thomas Edison became
adversaries because of Edison's promotion of direct
current for electric power distribution instead of the more
easily transmitted alternating current (AC) system invented
by Nikola Tesla and promoted by Westinghouse. Unlike
DC, AC could be stepped up to very high voltages with
transformers, sent over thinner and less expensive wires,
and stepped down again at the destination for distribution
to users.
The plot of electric current as a function of time for an
alternating current is a sinusoidal curve.
curve.
The average value of an ordinary alternating current is zero.
The power dissipated in a resistance is proportional to the square
square of
the current.
The effective current or rms current is obtained by squaring the
current, averaging this value over time, and taking the square root
root of
the result.
The effective current Ieff is 0.707 times the peak current Ipeak.
If we plot the voltage across an electrical outlet as a function
of time, we get another sinusoidal curve.
The effective value of this voltage is typically between 110 and 120
volts in North America.
The standard household power supplied in this country is 115 volts,
volts,
60 hertz ac.
Household circuits are wired in parallel so that different appliances
appliances
can be added to or removed from the circuit without affecting the
the
voltage available.
Root mean square, or quadratic mean, is a statistical measure of
the magnitude of a varying quantity.
It is the square root of the mean of the squares of the values.
15
Household circuits are wired in parallel so that different
appliances can be added to or removed from the circuit
without affecting the voltage available.
As you add more appliances, the total current drawn increases,
because the total effective resistance of the circuit decreases when
resistances are added in parallel.
Since too large a current could cause the wires to overheat, a fuse or
circuit breaker in series with one leg of the circuit will disrupt
disrupt the
circuit if the current gets too large.
Appliances with larger power requirements (stoves, clothes dryers,
dryers,
etc) are usually connected to a separate 220220-V line.
The axons can be as long as a meter or more, starting perhaps in the spinal cord and
terminating in your foot or hand.
Synapse is a junction between nerve endings.
The change in voltage along an axon of a nerve cell is transmitted very differently than that
in a metal wire: the flow of charges occurs perpendicularly to the axon rather than along its
length.
Outside:
axon
Na+,
Inside: Cl-
K+
RESTING potential:
inside – outside =
– 70 mV
When the axon is stimulated by an
electrical signal or other disturbance,
the membrane allows positively
charged sodium ions to rush through
the membrane, creating an ACTION
POTENTIAL.
This action potential will move down
the axon, until it is transmitted to
another neuron at a synapse or to a
muscle cell.
For longer axons, the speed of
propagation can be as high as 150 m/s.
For a person of average height, the
signal can reach the toes from the
brain in a hundredth of a second.
What does
lightning
have in
common...
... with hair on
a dry winter
day?
16
Effects of Electric Charge
Hair seems to have a mind of its own when combed on
a dry winter day.
What causes the hairs to repel one another?
Why does a piece of
plastic refuse to leave your
hand after you peeled it off
a package?
Why do you get a slight
shock after walking across
carpet and touching a light
switch?
Human skin
Leather
Rabbit’s fur
Quartz
Silk
The triboelectric effect is a
type of contact
electrification. The polarity
and strength of the charges
produced differ according to
the materials, surface
roughness, temperature,
strain, and other properties.
All these phenomena involve different materials rubbing against one
another.
Electrostatic effects can be demonstrated by rubbing plastic or glass rods with
different furs or fabrics.
Small wads of dry, paperlike material called pith balls are light enough to be
strongly influenced by electrostatic forces.
When a plastic rod, vigorously rubbed with cat fur, is brought near
near the pith
balls, at first the pith balls are attracted to the rod like bits
bits of iron to a magnet.
After contacting the rod, the pith balls
dance away from the rod.
They are now repelled by the rod and
also by each other.
A repulsive force must be acting between the two pith balls after
after
they have been in contact with the rod.
Perhaps the balls have received something (call it electric charge)
charge) from the
rod that is responsible for the force we observe.
This charge was somehow generated by rubbing the rod with the cat
cat fur.
The force that is exerted by one stationary charge on another is called the
electrostatic force.
force.
Amber
Rubber balloon
Resins
Plastic wrap
Ebonite
Thus, it is not very
predictable, and only broad
generalizations can be
made. Amber, for example,
can acquire an electric
charge by friction and
separation with a material
like wool. This property, first
recorded by Thales of
Miletus, suggested the word
"electricity", from the Greek
word for amber, ēlektron.
17
Experiments with different materials indicate that there are two
types of charge.
An electroscope consists of two metallicmetallic-foil leaves suspended from
a metal post inside a glassglass-walled container.
If the foil leaves are uncharged, they will hang straight down.
If a charged rod is brought in contact with the metal ball on top,
top, the leaves
immediately spread apart and stay apart, even if the rod is removed.
removed.
If an object of the same charge as the
original rod is later brought near the
metal ball, the leaves will spread
farther apart.
An object with the opposite charge will
make the leaves come closer together.
A larger charge produces a larger
effect.
Like charges repel each other,
and unlike charges attract
each other.
Franklin’s model comes surprisingly close to our modern view.
When objects are rubbed together, electrons may be
transferred from one object
to the other.
Electrons are small, negatively
charged particles present in all
atoms and, therefore, in all
materials.
A negatively charged object has
a surplus of electrons, and a
positively charged object has a
shortage of electrons.
The atomic or chemical properties
of materials dictate which way the
electrons flow when objects are
rubbed together.
Benjamin Franklin introduced the names positive and negative for
the two types of charge.
He also proposed that a single
fluid was being transferred
from one object to another
during charging.
A positive charge resulted from
a surplus of the fluid, and a
negative charge resulted from
a shortage of the fluid.
Franklin arbitrarily proposed
that the charge on a glass rod
when rubbed with silk be called
positive.
Like charges repel each other,
and unlike charges attract
each other.
Respond to the following student
statement:
"A positively charged object is an
object which has an excess of
positive electrons."
Like charges repel each other,
and unlike charges attract
each other.
18
On two occasions, the following charge interactions between balloons
A, B and C are observed. In each case, it is known that balloon B is
charged negatively. Based on these observations, what can you
conclusively confirm about the charge on balloon A and C for each
situation.
positive
Balloons X , Y and Z are suspended from strings as shown at the
right. Negatively charged balloon X attracts balloon Y and balloon
Y attracts balloon Z.
Balloon Z __________________. (List all that apply)
a. may be positively charged
b. may be negatively charged
negative
c. may be neutral
d. must be positively charged
e. must be negatively charged
positive
f. must be neutral
positive
Can you charge an object without
actually touching it with another
charged object?
Conductors and Insulators
Different materials behave differently in the
presence of electrostatic forces.
Charge can readily flow through conductors:
conductors:
Materials that do not ordinarily permit charge to flow are
insulators:
insulators:
metals, like copper, silver, iron, gold; our bodies
Charging by induction
involves the conducting
property of metals:
plastic; glass; ceramics; other nonmetallic materials
Charge flows much more readily through several miles of
copper wire than through the few inches of insulating
ceramic material.
Semiconductors are intermediate between a good
conductor and a good insulator.
Their importance to modern technology is enormous.
Charge a plastic rod with cat fur
and bring the rod near a metal
ball mounted on an insulating
post.
The electrons in the metal ball
are repelled by the negative rod.
There is a negative charge
buildup on the side opposite
the rod, and a positive charge
on the near side.
19
Can you charge an object without
actually touching it with another
charged object?
To charge the ball by
induction,
induction, now touch the ball
with your finger on the side
opposite the rod.
The negative charge flows from
the ball to your body, since it is
still repelled by the negative
rod.
If you now remove your finger
and then the rod, a net positive
charge is left on the ball.
When an oil tanker car has arrived at its destination, it prepares to
empty its fuel into a reservoir or tank. Part of the preparation
involves connecting the body of the tanker car with a metal wire to
the ground. Suggest a reason for why is this done.
As fuel is pumped from the tanker car to a reservoir, charge
can quickly build up as the fluid flows through the hoses.
This static charge can create sparks capable of igniting the
fuel. By connecting the body of the tanker car to the ground,
the static charge can be transferred to the ground. A metal
wire is used since metals are conductive and allow charge to
flow through them.
Charging by induction illustrates the mobility of
charges on a conducting object such as the metal ball.
The process will not work with a glass ball.
Charging by induction is an important process in machines used
for generating electrostatic charges, and in many other practical
practical
devices.
It also explains some of the phenomena associated with lightning
storms.
20
Why are insulators attracted to
charged objects?
Since the negatively charged surface is closer to the rod
than the positively charged surface, it experiences a
stronger electrostatic force.
Recall that the pith balls were attracted
to the charged rod before they were
charged themselves.
Electrons are not free to move in the
insulating material of the pith balls.
However, within each atom or
molecule, charges can move.
Each atom becomes an electric
dipole:
dipole: the center of the negative
charge is slightly displaced from the
center of the positive charge.
The material is polarized.
polarized.
Polarization explains why small bits of paper or
styrofoam are attracted to a charged object such as a
sweater rubbed against some other material.
Electrostatic precipitators used to remove particles from
smoke in industrial smoke stacks use this property.
Polarized particles are attracted to charged plates in the
precipitator, removing them from the emitted gases.
The overall effect is that the pith ball is attracted to the charged
charged
rod, even though the net (total) charge on the pith ball is zero.
zero.
After the ball comes in contact with the charged rod, some of the
the
charge on the
rod is transferred to the
pith ball.
The pith ball is then
positively charged like the
rod, and so is repelled by
the rod.
True or False:
When an object becomes
polarized, it acquires a charge
and becomes a charged object.
Answer: False
When an object becomes polarized, its center of
positive charge becomes separated from its
center of negative charge. Overall, there are just
as many positive charges as negative charges;
the object has a balance of charges and is
therefore neutral.
21
Coulomb’s Law
Charging by INDUCTION:
You don’t need to actually touch an
object to another charged one to
induce charges. This works for
CONDUCTORS only.
The electrostatic force between two charged objects
is proportional to the quantity of each of the charges
and inversely proportional to the square of each
distance between the charges.
But an INSULATORS also seem to
be attracted by charged objects.
This is because of
F=
POLARIZATION:
kq1q2
r2
in units of coulombs (C)
Coulomb' s constant k = 9 ×10 9 N ⋅ m2 /C 2
The Electric Field
How do the charges exert forces on each other,
when they are not even touching?
The concept of an electric field describes how one
charge affects the space around it, which then exerts a
force on another charge.
The electric field at a given point in space is the electric
force per unit positive charge that would be exerted on a
charge if it were placed at that point.
E=
F
q
It is a vector having the same direction as the force on a
positive charge placed at that point.
22
What is lightning?
Most thunderclouds generate a separation of charge
resulting in a net positive charge near the top and a net
negative charge near the bottom.
The charge separation produces strong electric fields in the
cloud as well as between the cloud and earth.
Since moist earth is a reasonably good conductor, a positive
charge is induced on the surface of the earth below the
cloud.
The electric field generated can be several thousand volts per
meter;
meter; the potential difference between the cloud’s base and the
earth can easily be several million volts!
This creates an initial flow of charge (the “leader
”) along a path that
“leader”)
offers the best conducting properties over the shortest distance.
distance.
The leader ionizes some of the
atoms in the air along that path.
The following strokes all take
place along this same path in
rapid succession.
The heating and ionizing
produce the lightning we see.
The thunder (sound waves) is
produced at the same time, but
takes longer to reach us since
sound travels slower than light.
TRUE or FALSE:
The presence of lightning rods on top of buildings prevents a cloud with
a static charge buildup from releasing its charge to the building.
Answer: FALSE
Contrary to a commonly held belief, a lightning rod does not serve to
prevent a lightning bolt. The presence of the rod on the building can
only serve to divert the charge in the bolt to the ground through a low
resistance pathway and thus protect the building from the damage
which would otherwise result.
TRUE or FALSE:
If you place a lightning rod on top of your home but failed to ground it,
then it is unlikely that your home would be struck by lightning.
Answer: False
The presence of an elevated lightning rod would serve to draw charge
from the cloud to the ground. In the event of a lightning strike, a bolt
would likely select a path from the cloud that ultimately connects to the
rod. If the rod is not grounded, then the charge would likely pass through
the home during its journey to the ground. The intense heat associated
with the lightning bolt would cause severe damage and possibly cause an
explosion or a fire. In the end, it would have been better to not even
have installed a lightning rod than to have installed one that is not
grounded.
23
The strangeness of the space surrounding a charged object is often
experienced first hand by the use of a Van de Graaff generator. A
Van de Graaff generator is a large conducting
sphere which acquires a charge as electrons are
scuffed off of a rotating belt as it moves past sharp
elongated prongs inside the sphere. The buildup of static
charge on the Van de Graaff generator is much greater than that on a
balloon rubbed with animal fur or an aluminum plate charged by
induction. On a dry day, the buildup of charge becomes so great that
it can exert influences on charged balloons held some distance away.
If you were to walk near a Van de Graaff generator and hold out your
hand, you might even notice the hairs on your hand standing up. And
if you were to slowly walk near a Van de Graaff generator, your
eyebrows might begin to feel quite staticy. The Van de Graaff
generator, like any charged object, alters the space surrounding it.
Other charged objects entering the space feel the strangeness of that
space. Electric forces are exerted upon those charged objects when
they enter that space. The Van de Graaff generator is said to create
an electric field in the space surrounding it.
The simplest circuit protection device is the fuse. A fuse is just a
thin wire, enclosed in a casing, that plugs into the circuit. When a
circuit is closed, all charge flows through the fuse wire -- the fuse
experiences the same current as any other point along the circuit.
The fuse is designed to disintegrate when it heats up above a
certain level -- if the current climbs too high, it burns up the wire.
Destroying the fuse opens the circuit before the excess current can
damage the building wiring.
The problem with fuses is they only work once. Every time you blow
a fuse, you have to replace it with a new one. A circuit breaker does
the same thing as a fuse -- it opens a circuit as soon as current
climbs to unsafe levels -- but you can use it over and over again.
A circuit breaker is an automatically-operated electrical switch
designed to protect an electrical circuit from damage caused by
overload or short circuit. Unlike a fuse, which operates once and
then has to be replaced, a circuit breaker can be reset (either
manually or automatically) to resume normal operation. Circuit
breakers are made in varying sizes, from small devices that
protect an individual household appliance up to large switchgear
designed to protect high voltage circuits feeding an entire city.
Non-conducting materials lack mobile charges, and so resist the
flow of electric current, generating heat. In fact, all materials offer
some resistance and warm up when a current flows.
A conductor of a given material and volume (length x cross-sectional
area) has no real limit to the current it can carry without being
destroyed as long as the heat generated by the resistive loss is
removed and the conductor can withstand the radial forces. This effect
is especially critical in printed circuits, where conductors are relatively
small and close together, and inside an enclosure: the heat produced,
if not properly removed, can cause fusing (melting) of the tracks.
Since all conductors have some resistance, and all insulators will carry
some current, there is no theoretical dividing line between conductors
and insulators. However, there is a large gap between the
conductance of materials that will carry a useful current at working
voltages and those that will carry a negligible current for the purpose
in hand, so the categories of insulator and conductor do have practical
utility.
A key application of transformers is to reduce the current before
transmitting electrical energy over long distances through wires. Most
wires have resistance and so dissipate electrical energy at a rate
proportional to the square of the current through the wire. By
transforming electrical power to a high-voltage, and therefore low-current
form for transmission and back again afterwards, transformers enable the
economic transmission of power over long distances. Consequently,
transformers have shaped the electricity supply industry, permitting
generation to be located remotely from points of demand. All but a
fraction of the world's electrical power has passed through a series of
transformers by the time it reaches the consumer.
Transformers are some of the most efficient electrical 'machines', with
some large units able to transfer 99.75% of their input power to their
output.
24
How do magnets work?
Magnets and the Magnetic Force
What is the
Earth’s
magnetic field?
Is the
magnetic force
similar to the
electrostatic
force?
As you probably already know, magnets attract metallic items made
made of
iron or steel, but not silver, copper, aluminum, or most nonmetallic
nonmetallic
materials.
The three most common magnetic elements are the metals iron, cobalt,
cobalt, and
nickel.
Magnets also attract or repel each other depending on how they are
are
aligned.
The northnorth-seeking end of a magnet wants to point north, and it is called the
north magnetic
pole.
pole.
The southsouth-seeking end wants
to point south, and it is called
the south magnetic pole.
pole.
We are generally more familiar with magnetic forces than with
electrostatic forces.
forces.
Is there a relationship between electrical effects and magnetism?
magnetism?
Like the gravitational force and the electrostatic force, this force
force acts even
when the objects are not touching one another.
Maxwell discovered that the electrostatic force and the magnetic force are
really just different aspects of one fundamental electromagnetic force.
force.
Our understanding of that relationship has led to numerous
inventions such as electric motors, electric generators,
transformers, etc.
The force that two poles exert on one another varies with
distance or pole strength.
The magnetic force between two poles decreases with the square
of the distance between the two poles, just as the electrostatic force
does.
Some magnets are stronger than others; the force is directly
proportional to the pole strength of the magnets involved.
Like poles repel one
another, and
unlike poles attract one
another.
25
A magnet always has at least two poles: a magnetic dipole.
dipole.
Breaking a magnetic dipole in half results in two smaller magnetic
magnetic
dipoles.
We cannot get just one magnetic north or south pole by itself:
magnetic monopoles do not exist.
Magnetic field lines produced by a magnetic dipole form a pattern
similar to the electric field lines produced by an electric dipole.
dipole.
A magnetic dipole tends to line up with an externally produced
magnetic field just as an electric dipole tends to line up with an electric
field.
Both dipoles experience a torque due to the force from the externally
externally produced
field.
This is why iron filings line up with the field lines around a magnet.
magnet.
Electric field lines originate on positive charges and terminate on negative
charges.
Magnetic field lines form continuous loops: they emerge from the north pole
and enter through the south pole, pointing from the north pole to
to the south pole
outside the magnet.
Inside the magnet, they point from the south pole to the north pole.
pole.
Is the Earth a magnet?
The north (north
(north--seeking)
seeking) pole of a compass needle points toward the
Earth’s “North Pole.”
The magnetic field
produced by the Earth can
be pictured by imagining a
large bar magnet inside the
Earth.
Since unlike poles attract,
the south pole of the Earth’s
magnet must point in a
northerly direction.
The axis of the Earth’s
magnetic field is not aligned
exactly with the Earth’s axis
of rotation.
30 to 60 micro teslas
1 gauss = 10-4 tesla
26
Magnetic Effects of Electric
Currents
Oersted discovered that a compass needle was
deflected by a currentcurrent-carrying wire.
With the wire oriented along a northnorth-south line, the compass
needle deflects away from this line when there is current
flowing in the wire.
The magnetic field produced by the current is perpendicular
to the direction of the current.
The magnetic field lines produced by a straight, currentcurrentcarrying wire form circles centered on the wire.
The rightright-hand rule gives the direction of the field lines: with the
thumb in the direction of the current, the fingers curl in the
direction of the field lines produced by that current.
The effect gets weaker as
the compass is moved
away from the wire.
The right-hand rule
First version with a closed hand: just
the thumb pointing at the direction of
the current; the other fingers show
the direction of the magnetic field.
Two parallel currentcurrent-carrying wires exert an attractive
force on each other when the two currents are in the same
direction.
The force is proportional to the two currents (I
(I1 and I2) and
inversely proportional to the distance r between the two wires:
F 2 k ′I1I2
=
l
r
where k ′ = 1×10−7 N/A2
One ampere (A) is the amount of current
flowing in each of two parallel wires
separated by a distance of 1 meter that
produces a force per unit length on each
wire of 2 x 10-7 N/m.
Two long parallel wires carry currents of 5 A
and 10 A in opposite directions as shown.
What is the direction of the magnetic field
produced by the 55-A wire at the position of
the 1010-A wire?
a)
b)
c)
d)
e)
f)
Perpendicular to the plane of the
page and into the page
Perpendicular to the plane of the
page and out of the page
Upward
Downward
Inward toward the other wire
Outward away from the other wire
Perpendicular to plane of page
and into page
27
The right-hand rule
Second version:
If the index finger of the
right hand points in the
direction of the velocity of
the charge (or the electric
current), and the middle
finger in the direction of the
magnetic field, then the
thumb indicates the
direction of the magnetic
force acting on a positive
charge.
Two long parallel wires carry currents of 5 A
and 10 A in opposite directions as shown.
What are the directions of the forces on each
wire?
a)
b)
c)
d)
e)
The wires exert an attractive
force on each other.
The wires exert a force repelling
each other.
Each wire exerts a force on the
other in the direction of the other
wire’
wire’s current (the red arrows
shown).
Each wire exerts a force on the
other in the direction opposite to
the other one’
one’s current.
The wires exert no force on each
other.
The wires repel each other.
Magnetic forces are exerted by magnets on other magnets, by magnets
on currentcurrent-carrying wires, and by currentcurrent-carrying wires on each other.
The force exerted by one wire on the other is attractive
when the currents are flowing in the same direction and
repulsive when the currents are flowing in opposite
F = IlB
directions.
The magnetic force exerted on a moving charge of an electric current
current is
perpendicular to both the velocity of the charges and to the magnetic
magnetic field.
This force is
proportional to the
quantity of the charge
and the velocity of the
moving charge and to
the strength of the
magnetic field:
F = qvB
For this relationship to be valid, the velocity must be
perpendicular to the field.
This actually defines the magnetic field as the force per unit
charge and unit of velocity:
units: 1 tesla (T) = 1 N/A⋅
N/A⋅m
If the index finger of the
right hand points in the
direction of the velocity of
the charge, and the middle
finger in the direction of the
magnetic field, then the
thumb indicates the
direction of the magnetic
force acting on a positive
charge.
B=
F
qv ⊥
Question 11, 12
28
Magnetic Effects of Current
Loops
The magnetic field produced by a current loop is
identical to one produced by a short bar magnet (a
magnetic dipole).
In fact, in an external magnetic field, a current loop will
experience a torque just as a bar magnet would.
When a currentcurrent-carrying wire is bent into a circular
loop, the magnetic fields produced by different
segments of the wire add to produce a strong field near
the center of the loop.
Consider a rectangular loop:
Each segment of the rectangular loop is a straight wire.
The force on each segment is given by F=IlB
F=IlB..
Using the rightright-hand rule, you can verify that the loop will tend to rotate in the
the
direction indicated.
The forces on the two ends of
the loop produce no torque
about center of the loop,
because their lines of action
pass through the center of the
loop.
The forces on the other two
sides combine to produce a
torque that tends to line up the
plane of the loop perpendicular
to the magnetic field.
29
This torque is also the basis of operation for electric motors.
motors.
The current must reverse directions every half turn to keep the coil
turning.
This can be achieved by using alternating current, or by using a
reversing direction of dc current with a split ring commutator.
commutator.
The magnetic field produced by a coil of wire will be
stronger than one produced by a single loop carrying the
same current.
One
design for a simple dc
motor consists of a wirewound rotor mounted on an
axle between the pole faces
of a permanent magnet.
The split ring causes the
current to reverse directions
every half turn, thus keeping
the coil turning the same
direction.
Can we utilize the similarities between a
currentcurrent-carrying coil of wire and a
magnet?
Faraday’s Law: Electromagnetic
Induction
•By winding a coil around a
steel needle or nail, the
magnetic field produced is
enhanced.
•The nail then behaves like
a magnet that is stronger
than most natural magnets.
•This is an electromagnet.
The magnetic field produced by each loop all add together.
The resulting field
strength is proportional
to the number of turns
N that are wound on
the coil.
The torque on the coil,
when placed in an
external magnetic field,
is also proportional to
both the current and
the number of turns in
the coil.
We have seen that an electric current produces a magnetic field.
Can magnetic fields produce electric currents?
Faraday tried, at first unsuccessfully, to detect a current in a coil
as a result of a current in a nearby coil.
The primary coil was connected to a battery to produce a current.
The secondary coil was connected to a galvanometer,
galvanometer, a device to detect
magnitude and direction of current.
30
With coils of about 200 feet of copper wire, Faraday noticed a very
very
brief deflection of a galvanometer when the current in the primary
primary coil
was first started or when it was interrupted.
The galvanometer deflected one way when the primary was first connected
connected to
the battery and the opposite direction when the contact was broken.
broken.
No current was detected in the secondary coil when there was a secondary
secondary
current in the primary coil.
The changing current in the primary coil implies a changing
magnetic field.
The electric current in the secondary coil implies that there is an
electric field being induced.
Faraday also detected a current in a coil of wire when a magnet
was moved into or out of the center of the coil.
An electric
current is only
induced in the
secondary coil
when there is a
changing
current in the
primary.
An electric
field is
produced
when there is
a changing
magnetic field.
Magnetic flux (Φ) is a measure of how much magnetic
field is passing through a loop of wire.
It is at a maximum when the field lines are perpendicular to the
plane of the loop, and it is zero when the field lines are parallel
parallel to
the plane of the loop.
For a coil of N
loops, the flux
through the coil is
equal to the flux
through one loop,
multiplied by the
number of loops:
The galvanometer deflected one way when the magnet was being inserted
inserted
and the opposite direction when it was being withdrawn.
No current was detected when the magnet was not moving.
Faraday’s Law
A voltage (electromotive force) is induced in a circuit
when there is a changing magnetic flux passing
through the circuit.
The induced voltage is equal to the rate of change of
the magnetic flux:
∆Φ
=
t
ε
Φ = NBA
This process is called electromagnetic inductance.
inductance.
31
Suppose that the magnetic flux through
a coil of wire varies with time as shown.
Where does the induced voltage have its
largest magnitude?
a)
b)
c)
d)
e)
From 0 s to 1 s
At 1 s
From 1 s to 3 s
At 3 s
From 3 s to 5 s
From 0 to 1s the flux is changing the
most rapidly and during this time the
induced voltage will be the largest.
Lenz’s Law
The direction of the
induced current
generated by a
changing magnetic flux
produces a magnetic
field that opposes the
change in the
original magnetic
flux.
flux.
Managing the flow of traffic
How is this wire loop involved in detecting the presence of your car?
When your car is located over the loop, the steel in the frame of your car increases
the magnetic field being produced by the current in the coil. The effect is similar to
that of placing a piece of iron inside the coil of an electromagnet. The presence of
the iron strengthens the magnetic field.
32
Joseph Henry noticed that the spark or shock
obtained when an electromagnet was connected to a
battery was larger than one obtained by touching the
terminals of the battery with an uncoiled wire.
The changing magnetic flux through a coil of wire
produced when the coil is connected or disconnected
from the battery produces an induced voltage in the same
coil.
The induced current in the coil opposes the changing
magnetic flux.
This phenomenon is called selfself-inductance.
inductance.
The flux changes continuously from a maximum value
in one direction, to zero, to a maximum value in the
opposite direction.
The induced voltage depends on the rate of change of
the flux.
When the flux is
increasing the fastest,
the voltage is a
maximum; when the
flux is decreasing the
fastest, the voltage is
a maximum in the
other direction
(negative).
Generators and Transformers
A generator converts mechanical energy to electrical
energy by electromagnetic induction and produces an
alternating current.
current.
A simple generator consists of
a coil of wire that generates an
electric current when turned
between the pole faces of
permanent magnets.
The coil’s rotation causes the
magnetic flux through the coil to
change continuously.
It is this changing flux that
produces a current in the coil.
A transformer adjusts the voltage of an ac circuit
up or down as needed for a particular application.
Transformers are
seen on utility
poles, at electrical
substations, and as
voltage adapters for
electrical devices.
The ability to use
generators and
transformers mean
that alternating
current is
convenient for
large-scale power
production and
distribution.
33
The ratio of the number of turns in the primary coil to
the voltage on the primary coil is equal to the ratio of
the number of turns on the secondary coil to the
induced voltage in the secondary coil:
N1
N
= 2
∆V1 ∆V2
If you need 12 volts to run an appliance, using the
power provided at the wall socket with 120 volts, you
need a stepstep-down transformer with ten times as many
turns in the primary coil as in the secondary coil.
If you need higher
voltages than the
120 volts provided,
you need a stepstep-up
N1
N2
transformer
= with
more∆V
turns
on 2the
∆V
1
secondary than on
the primary.
N 
∆V2 = ∆V1 2 
 N1 
N 
∆V2 = ∆V1 2 
 N1 
Can a transformer be used, as shown in
the diagram below, to step up the
voltage of a battery?
a)
b)
c)
Yes
No
Impossible to tell
from this figure
High voltages are desirable for longlong-distance transmission of
electrical power.
No, it will not work as shown in the
diagram. If one contact of the battery
and the primary were to be
continuously opened and closed, this
would produce a variable flux and then
the transformer would work.
The higher the voltage, the lower the current needed to transmit a
given amount of power.
Minimizing the current minimizes the heat lost to resistive heating
heating
(P=I2R).
Transmission voltages as high as 230 kV = 230,000 V are not
unusual.
Transformers at electrical substations reduce the voltage to 7200
7200
volts for inin-town distribution.
Transformers on utility poles or underground lower this voltage from
220 to 240 volts for entry into buildings.
This can be used as is for stoves, dryers, etc., or lowered to 110
110 volts
for common household circuits.
Direct current is occasionally used to transmit power over
long distances, as it does not lose energy by radiation of
electromagnetic waves like alternating current does.
34
When current flows through a coil,
does it become a magnet?
Where are the poles in the coil?
If now you insert a coil in a magnetic
field, what is going to happen?
In other words, how will the coil
interact with the permanent magnetic
field?
Another design for your DC motor
magnet
magnet
35
Your group CAN WRITE a lab
report ONLY IF it presents a
STEADILY RUNNING MOTOR.
The group with the fastest
running motor will receive the
prize of 10 points added to
this current lab report (grade
will cannot exceed 100).
Contest will happen towards the end of the class.
36