Download Electric Current (Chap. 23)

Chapter 23
Electric Current
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Current
Water flows from the
reservoir of higher
pressure to the reservoir
of lower pressure; flow
stops when the pressure
difference ceases.
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Water continues to flow
because a difference in
pressure is maintained with the
pump.
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Electric Current
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.
ON
OFF
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Demo: 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
always the same.
Galvanometer is an ammeter
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with both positive
negative
Power
Supply
(adjustable
voltage)
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
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Simple Mechanical Generator
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Chemical Battery
Batteries separate
positive and
negative charges by
using a chemical
reaction.
Chemical potential
energy is converted
into electrical
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
Electrical Resistance
Current depends not only on
the voltage but also on the
electrical resistance of the
conductor.
water flows through a thick
The wider the wire is, the More
hose than through a thin one
connected to a city's water
less the resistance.
system (same water pressure).
A short wire offers less
resistance than a long wire.
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Jumper cables use thick wires so as
to minimize the electrical resistance.
Jacob’s Ladder
Electrical resistance through air is greater
for a larger gap.
Biblical Jacob’s Ladder
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Physics Jacob’s Ladder
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Ohm’s Law
Relation between current, voltage, and
resistance is Ohm’s law,
(Current) =
(Voltage)
(Resistance)
Ampere is unit of current; symbol is A
Volt is unit of voltage; symbol is V
Ohm is unit of resistance; symbol is 
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Lab: Ohm’s Law
Measuring voltage, current, and resistance
in simple circuits to verify Ohm’s law.
Ammeter
Battery
Resistor
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Galvanometer
Demo: Resistance of Water
Pure water has very high resistance;
impurities, such as salt, lower resistance.
When salt dissolves
the sodium and
chlorine atoms are
charged (ions).
These mobile
charges carry the
current in the water.
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CURRENT
Light Bulb
Ammeter
Salty
Water
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Power
Supply
Nervous System
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.
Without a constant flow of
blood the brain can suffer
permanent damage.
SA
AV
Conduction in Human Heart
The normal electrical conduction in the heart
allows the impulse that is generated by
the sinoatrial (SA) node of the heart to be
propagated to (and stimulate) the
myocardium (muscle of the heart).
When the myocardium is stimulated, it
contracts, pumping blood in the body.
As the electrical activity is spreading
throughout the atria, it travels via
specialized pathways, known as
internodal tracts, from the SA node to the
Atrioventricular (AV) node.
The AV node functions as a critical delay in
the conduction system. Without this
delay, the atria and ventricles will contract
at the same time, and blood won't flow
effectively from the atria to the ventricles.
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SA
AV
Electric Shock
The damaging effects of shock are the result
of current passing through the body.
Effects of Electric Shock on Human Body
From Ohm's law, current
depends on the voltage and
on electrical resistance.
Current (A)
Effect
0.001
Can be felt
0.005
Is painful
When dry, skin’s resistance
around 100,000 .
0.010
Causes involuntary muscle
contractions (spasms)
0.015
Causes loss of muscle control
Resistance drops as low as
100  when wet and salty.
0.070
If through the heart, serious
disruption; probably fatal if
current lasts for more than 1 s
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Check Yourself
If your resistance is 100,000 , what is the
current passing through you when you
touch an electric socket (120 volts)?
What if your resistance is only 100 ?
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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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DC vs. AC
Easy to produce small
DC currents using
batteries, which also
have low voltages.
For major power lines,
less ohmic heating if
high voltage AC
current is used
instead of DC.
Electrons in Conductors
Electrons in a wire are
in constant, rapid, but
random motion.
With direct current the
electrons slowly drift
down the wire.
With alternating current
the electrons slosh
back and forth.
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Without
Voltage
With
Voltage
Demo: Resistance & Temperature
Resistance in a material goes
down when the material is
cooled because the electrons
don’t bump into atoms often.
CURRENT
Without
Voltage
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With
Voltage
With
Voltage
& Cold
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Battery
Ammeter
increases when wire cooled
Ohmic Heating
Flowing electrons
strike atoms in a
conductor, heating
the material.
Toaster
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Rear Window Defogger
Ohmic heating evaporates fog on car window.
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Demo: Ohmic Cooking
An electric current running through a
hot dog generates enough heat to
cook it.
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Electric Power
Power is rate at which energy is delivered.
Power = (Voltage) x (Current)
For example,
(100 Watts) = (120 Volts) x ( 5/6 Ampere)
Line voltage in the
US is 120 Volts.
In Europe voltage
is 240 Volts.
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Check Yourself
A kilowatt is 1000 watts, and a kilowatt-hour is the
amount of energy consumed in one hour at the
rate of 1 kilowatt.
If electric energy costs 5 cents per kilowatt-hour,
what does it cost to operate a 100-watt light bulb
for 10 hours?
What does it cost to run a 1200 watt microwave for
10 minutes?
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Lab: Simple Electric Circuits
Combining circuit
elements (battery,
wires, bulbs, resistors,
etc.) in different
arrangements can
give different results.
Can predict results by
tracing current and
voltage differences.
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These three circuits are equivalent
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Series Circuit
Same current passes through each element.
Disconnect one of the bulbs and the circuit is broken (other bulbs go out).
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Parallel Circuit
Same voltage on each bulb; current splits
through each branch.
Disconnect one of the bulbs and the other bulbs stay light with same brightness.
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Physics 1 (Garcia) SJSU
Check Yourself
How do the brightnesses of the identical
light bulbs compare?
Which bulb draws the most current?
What happens if bulb A is unscrewed?
What happens if bulb C is unscrewed?
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Overloading a Circuit
More appliances added to a
parallel circuit, the more
current flows.
A large current can cause
significant ohmic heating in
the wires, which is a fire
hazard.
Protect against overloading a
circuit by adding a fuse.
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Fuses & Circuit Breakers
Fuse is designed to melt (due to ohmic
heating) when current is too large.
Circuit breaker does same job without
needing replacement; flip the switch to
reconnect.
Fuse
Circuit
Breaker
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Physics 1 (Garcia) SJSU
Check Yourself
If a 1200 watt hair dryer is connected to a
120 Volt line, how much current will it
draw?
How many hairdryers can you operate
before blowing a 30 amp fuse?
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