Lecture 25
Chapter 23
Electric Current
Resistance
Ohm’s Law
Circuits
24-Oct-10
Electric Current (I)
Just as water current is flow of water molecules,
electric current is the flow of electric charge.
In circuits, electrons make up the flow of charge.
Unit of Current: 1 Ampere = 1 Coulomb/s=1A=1Amp
ON
OFF
24-Oct-10
Direct (Electric) Current -- DC
Direct Current
• Current always flows in same general direction
• Charge moves due to a voltage difference
produced by a battery, DC generator, or solar cell.
• Amount of current may vary, but current never
reverses direction.
Alternating Current (AC)
• Current reverses direction every so often. In US power
system current goes through a set of two reversals 60
times per second (60Hz.)
• Charge moves due to a alternating voltage difference
produced by an ac generator (alternator).
• Commercial electric power throughout the world uses ac.
As we will see later, “transformers” can step ac from an
alternator up to very high voltage for transmission over
great distances with small heat losses, then step it back
down to lower voltage for safety where energy is
consumed.
Direct & Alternating Current
Direct current (DC) is
current that flows in
only one direction.
Alternating current (AC)
is current that flows
back and forth with
alternating direction.
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Ammeter
Ammeter measures electrical current.
Due to charge
conservation, same
current into and out
of light bulb.
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Light Bulb
Ammeter
Ammeter
Ammeter readings
the same.
Battery
CURRENT
Current increases
as the voltage
increases.
Voltage Sources
Charges flow only when they are “pushed” or “driven.” A
sustained current requires a suitable pumping device to
provide a difference in electric potential (a voltage).
Aluminum
Lemon
Copper
Simple Chemical Battery
Simple Mechanical Generator
24-Oct-10
Chemical Battery
Batteries separate
positive and
negative charges by
using a chemical
reaction.
Chemical potential
energy is converted
into electric
potential energy.
Rechargeable Battery
Eventually the battery’s chemicals are consumed
unless the reaction can be reversed by passing
a current into the battery.
Automobile battery
is recharged while
the gasoline engine
is running since the
engine powers a
generator that
produces a
recharging current.
Starting the car
Engine running
Electric Current
A battery uses chemical reactions to produce a
potential difference between its terminals. It
causes current to flow through the flashlight
bulb similar to the way the person lifting the
water causes the water to flow through the
paddle wheel.
Direction of Electric Current
The direction of current flow – from the positive
terminal to the negative one – was decided
before it was realized that electrons are
negatively charged. Therefore, we show current
flowing around a circuit in the direction a
positive charge
would move;
electrons move
the other way.
Battery
Electrical Resistance (R)
Current depends not only on the
voltage but also on the
electrical resistance of
whatever the current is flowing
through. For a “wire” -The wider the wire is, the less the
resistance.
A short wire offers less resistance
than a long wire.
Metals offer less resistance than
other materials.
More water flows through a
thick hose than through a
thin one connected to a
city's water system (same
water pressure).
Jumper cables use thick wires so as
to minimize the electrical resistance.
The cables are usually copper wire.
24-Oct-10
Ohm’s Law
The relation between the current flowing through a
device, the voltage across the device, and
resistance of the device is Ohm’s law,
(Current) =
(Voltage)
(Resistance)
I = V/R
Ampere is unit of current; symbol is A
Volt is unit of voltage; symbol is V
Ohm is unit of resistance; symbol is Ω
24-Oct-10
Ohm’s Law
CHECK YOURSELF
When you double the voltage in a simple electric circuit,
you double the
A.
B.
C.
D.
current.
resistance.
Both A and B.
Neither A nor B.
Ohm’s Law
CHECK YOUR ANSWER
When you double the voltage in a simple electric circuit,
you double the
A.
B.
C.
D.
current.
resistance.
Both A and B.
Neither A nor B.
Explanation:
This is a straightforward application of Ohm’s law.
Current =
voltage
resistance
Example: Car Headlamp
A 12V car battery is attached to a headlamp and 2
A of current flows. What is the resistance of the
headlamp? .
Ammeter
+12V
24-Oct-10
CURRENT
Headlamp
Example: Car Headlamp
A 12V car battery is attached to a headlamp and 2
A of current flows. What is the resistance of the
headlamp? .
Ohm’s Law: I = V/R
Headlamp
= (12V)/(2 A) = 6 Ω
Ammeter
How much current flow
would there be if two
batteries were put
together to get a
voltage of 24V ?
+-
CURRENT
R = V/I
12V
24-Oct-10
Current in Nerves & Muscles
Nervous systems in animals
use electrical currents to
signal the contraction and
relaxation of muscles.
Frog leg jumps when electrical
current passes through it.
Conduction in Human Heart
The most important
electrical signal in our
body is the periodic signal
that contracts and relaxes
our heart muscle to pump
blood.
SA
AV
Current and Electric Shock
The damaging effects of shock are the result
of current passing through the body.
From Ohm's law,
current depends on
voltage and on
electrical resistance.
When dry, skin’s
resistance around
100,000 Ω.
Resistance drops as
low as 100 Ω when
wet and salty.
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Effects of Electric Shock on Human Body
Current (A)
Effect
0.001
Can be felt
0.005
Is painful
0.010
Causes involuntary muscle
contractions (spasms)
0.015
Causes loss of muscle control
0.070
If through the heart, serious
disruption; probably fatal if current
lasts for more than 1 s
Check Yourself
If your resistance is 100,000 Ω, what is the
current passing through you when you
touch an electric socket (120 volts)?
What if your resistance is only 100 Ω?
24-Oct-10
Check Yourself
If your resistance is 100,000 Ω, what is the
current passing through you when you
touch an electric socket (120 volts)?
I = V/R = (120 V)/(105 Ω) = 0.0012 A
What if your resistance is only 100 Ω?
Current will be 1000 times larger; 1.2 A
24-Oct-10
Shock Prevention: Grounding Appliances
Third prong is “safety” ground;
connects case of appliance to
“ground”
Ohmic Heating
As current flows through a
resistance, heat is
produced. Flowing
electrons strike atoms in
a conductor, heating the
material.
24-Oct-10
Toaster
Rear Window Defogger
Ohmic heating evaporates fog on car window.
24-Oct-10
Demo: Ohmic Cooking
An electric current running through a
hot dog generates enough heat to
cook it.
24-Oct-10
Electric Energy and Power
When a charge q moves across a voltage
difference V, its potential energy changes:
∆PE = qV
Therefore, the electric power involved is
P=
PE qV
q
=
= V = IV
∆t
∆t ∆t
I is current in Amperes; V is voltage difference in Volts
Electric Power
Power is rate at which energy is delivered.
Power = (Voltage) x (Current)
For example,
(100 Watts) = (120 Volts) x ( 5/6 Ampere)
Commercial power
voltage in the US is
120 Volts.
In Europe voltage
is 240 Volts.
24-Oct-10
Physics 1 (Garcia) SJSU
Check Yourself
A light bulb draws 60 W of power when connected
to a 120 V electric power outlet. What is the
current flowing through the bulb?
What is the electrical resistance of the bulb?
How much energy does the bulb use in one
second?
24-Oct-10
Check Yourself
A light bulb draws 60 W of power when connected
to a 120 V electric power outlet. What is the
current flowing through the bulb?
P = IV so I = P/V = (60 W)/(120 V) = 0.5 A
What is the electrical resistance of the bulb?
I = V/R so R = V/I = (120 V)/(0.5 A) = 240 Ω
How much energy does the bulb use in one
second?
P = ∆E/(∆t) so ∆E = P•(∆t) = (60 W)•(1 s) = 60J
24-Oct-10
Electric Circuit
• Make electric circuit by connecting voltage
source and resistive object(s) together in a loop
with metal wires that are (nearly) perfect
conductors
Resistance
+
(light bulb)
9V
Battery
Wires
(voltage source) (assumed to be perfect conductors)
• This is a closed circuit -- there is a continuous
path through conducting material from the +
terminal to the - terminal of the voltage source
Circuit
Electric Circuit
• Any closed path around which current (electrons) can flow.
• A complete circuit has a voltage source and one or more
resistances (such as light bulbs), connected by “ideal”
wires. There may be a switch to open and close the circuit.
• “Ideal” wires have no resistance.
Water circuit.
Electric circuit.
Circuit Diagram
“Ideal” wires
Could be light bulb,
heater, etc.
“Ideal” wires
+ -
Key Points of Lecture 25
• Electric Current
• DC & AC
• Voltage Sources
• Resistance
• Ohm’s Law
• Ohmic Heating
• Electric Power
• Circuits
z Before Wednesday, read Hewitt Chap. 23 (second half).
z Homework #17 due by 11:00 PM Tuesday Oct. 26.
z Homework #18 due by 11:00 PM Friday Oct. 29.