Electric Current
Chapter 34
Flow of Charge
When the ends of an electric conductor are at
different potentials, charge flows from one end
to another
- just like heat due to temperature
difference
- just like water flows downhill
(difference in gravitational potential
energy)
Flow of Charge
• This difference in potential is often referred to
as potential difference or voltage
• Voltage (V) is measured in volts (V).
• You can think of the source of voltage like a
pump.
Voltage Sources
• Supplies the voltage for any circuit.
• Acts like an “electric pump”.
• Examples:
– Batteries
– Generators
– Wall plug
Voltage Sources
Types of Current
There are two types of current:
AC – alternating current
- electrons flow first in one direction and then in the opposite
direction
- this happens by alternating the polarity of the generator or
voltage source
- the voltage of AC in America is normally 120 volts
DC – direct current
– electrons flow in a single direction
– a battery in a circuit produces DC because
the terminals of the battery always have the
same signs.
Electric Current
• Potential difference causes a flow of charge
(electrons) in a conductor
• We call this electric current (I)
• Current is measured in amperes (A)
Charge
Current =
time
I = q/t
A car’s starter motor draws 50 A. How much
charge flows if the motor runs for 0.75 s?
Given:
I = 50A t = 0.75s
Unknown:
q (charge)
Equation:
q = It
Substitute:
q = (50A)(0.75s)
Solve:
q = 37.5 C
How long does it take for 52 C to pass through a
wire carrying a current of 8.0 A?
Given:
q = 52 C I = 8.0 A
Unknown:
t (time)
Equation:
t = q/I
Substitute:
t = 52 C/8.0 A
Solve:
t = 6.5 s
Lab 20 - Current
What will happen to
bulbs 1 and 2 when
you disconnect
the wires at
various points?
Consensus:
Current requires a closed loop
New Terms:
Complete Circuit – all components are on the same
closed loop
Lab 21 - Conductivity
What type of object, when
inserted into the loop,
will allow the
two test bulbs
to light?
New Term:
Continuous Conducting Path
Test Circuit
Consensus:
Current requires a closed loop
made entirely of conductors.
Lab 22 – Conductivity 2
Clips
What parts of a socket and
bulb are conductors and
which are insulators?
Base
Fig 2. Clip – side view
Plates
Filament
Glass
Threaded
Section
Tip
Black Ring
Consensus:
Pieces that are metal
are conductors. Parts
like the base or the glass
are insulators.
Worksheet 1
In this circuit, which bulb lights
first?
D) They all light at the same
time.
Worksheet 1
Worksheet 1
Which bulbs will light?
D) Neither bulb
The battery is not in the
holder and the wires must
be connected to both the
positive and negative
terminals of the battery.
Worksheet 1
Which bulbs will light?
A) Bulb A
Bulb B is not part of the loop
Worksheet 1
Worksheet 1
4. Which bulbs will light?
C) Both bulbs
They are both on a complete circuit.
Worksheet 1
CLOSED LOOP
has
No
breaks
Bulbs will
is made of
B. conductors
A. light
made of
C. metals
Zinc
Copper
Aluminum
Worksheet 1
B. Open switch
OPEN LOOP
Breaks
or
nonconducting path
A. Not light
C. Insulators
nonmetals
D. Styrofoam
E. Plastic
F. Rubber
Worksheet 1
Worksheet 1
Power
Where have we seen “power”
before?
It was the rate work was done
(energy was transferred).
P = W/t
All power is measured in
Watts (W).
Electric Power
Electric power is similar. It’s the rate electrical
energy is transferred into another form.
P = IV
Electric power is often measured in the kilowatt,
because the watt is so small.
Electric Power
What do you know of that is electrical and has a
watt measurement on it?
Electric Energy
Electric energy can be calculated the same as any
other form.
E = Pt
Electric energy, however, is measured in kilowatt-hours
(kWh)
Your parents’ energy bill is measured in kilowatt-hours.
Usually, electric companies charge 7-12 cents per
kWh.
Cost = E(Rate)
How much power is used by a calculator that
operates on 8 volts and 0.1 ampere? If it is
used for two hours, how much energy does it
use?
Given:
V = 8 V I = 0.1 A
Unknown:
P (power)
Equation:
P = IV
Substitute:
P = (0.1 A)(8 V)
Solve:
P = 0.8 W
How much power is used by a calculator that
operates on 8 volts and 0.1 ampere? If it is
used for two hours, how much energy does it
use?
Given: V = 8 V I = 0.1 A P = 0.8 W t = 2 hr
Unknown:
E (energy)
Equation:
E = Pt
Substitute:
E = (0.8 W)(2 hr)
Solve:
E = 1.6 Wh
E = .0016 kWh
Will a 1200-watt hair dryer operate on a 120-volt
line if the current is limited to 15 amperes by a
safety fuse?
Given:
P = 1200 W
V = 120 V
Unknown:
I (current)
Equation:
I = P/V
Substitute:
I = 1200 W/120 V
Solve:
I = 10 A
Can you operate the hair dryer? What about two
hair dryers?
How much energy is expended in lighting a 100watt bulb for 30 minutes?
Given:
P = 100 W
t = 30 min
Unknown:
E (energy)
Equation:
E = Pt
Substitute:
E = (100 W)(0.5 hr)
Solve:
E = 50 Wh
E = 0.05 kWh
How much does it cost to operate a 100-watt
lamp continuously for one week if the power
utility rate is 10 cents per kilowatt-hour?
Given:
P = 100 W
t = 1 week
Unknown:
E (energy)
Equation:
E = Pt
Substitute:
E = (100 W)(168 hr)
Solve:
E = 16800 Wh
E = 16.8 kWh
How much does it cost to operate a 100-watt
lamp continuously for one week if the power
utility rate is 10 cents per kilowatt-hour?
Given:
E = 16.8 kWh Rate = 10c/kWh
Unknown:
C (cost)
Equation:
C = E(rate)
Substitute:
C = (16.8 kWh)(10c/kWh)
Solve:
C = 168 cents
C = $1.68
Meters
Ammeter
- measures current
- set in the loop
- symbol:
Voltmeter
- measures voltage
- set in “next to” the
loop
- symbol:
Meters
What type of meter is this?
What is the reading on this meter?
Ammeter
0.4 A
Meters
What type of meter is this?
What is the reading?
Voltmeter
6.5 V
Meters
What is the reading on this meter?
330 mA
or 0.33 A
Meters
What type of meter is this?
What is the reading?
Voltmeter
10 V
Lab 23 - Circuits
A
B
V
What is the relationship
between voltage and
current?
Consensus:
As voltage increases, current increases.
New Terms:
Resistance
Electric Resistance
• Current depends not only on the amount of
voltage impressed upon it but on the amount of
resistance in the conductor.
• Different conductors offer different amounts of
resistance
• Resistance is measured in ohms (W)
Ohm’s Law
• Georg Simon Ohm, a German physicist, tested
different wire circuits to see what effect
resistance had on the current.
• He discovered that:
Voltage
Current = --------------Resistance
We call this Ohm’s Law!
Ohm’s Law
Ohm’s Law can also be written using symbols:
I = V/R
A triangle can help when solving problems with
Ohm’s Law.
What is the resistance of an electric frying pan
that draws 12 amperes of current when
connected to a 120-volt circuit?
Given:
I = 12 A V = 120 V
Unknown:
R (resistance)
Equation:
R = V/I
Substitute:
R = 120 V/12 A
Solve:
R = 10 W
How much current is drawn by a lamp that has a
resistance of 100 ohms when a voltage of 50
volts is impressed across it?
Given:
R = 100 W
V = 50 V
Unknown:
I (current)
Equation:
I = V/R
Substitute:
I = 50 V/100 W
Solve:
I = 0.5 A