Chapter 23 Electric Current

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Chapter 23Electric 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.
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.

Current increases as the voltage increases. Due
to charge conservation, same current into and out
of light bulb.
Light Bulb
CURRENT
Power Supply
Ammeter readings always the same.
(adjustable voltage)
Galvanometer is an ammeter with both positive
negative
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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 potentiala voltage.

Aluminum
Lemon
Copper
Simple Chemical Battery
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.

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Rechargeable Battery

• Eventually the batterys 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
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Electrical Resistance

• Current depends not only on the voltage but also
  on the electrical resistance of the conductor.
• The wider the wire is, the less the resistance.
• A short wire offers less resistance than a long
  wire.

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.
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Jacobs Ladder

• Electrical resistance through air is greater for
  a larger gap.

Biblical Jacobs Ladder
Physics Jacobs Ladder
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Ohms Law

• Relation between current, voltage, and resistance
  is Ohms law,
• (Current)
• Ampere is unit of current symbol is A
• Volt is unit of voltage symbol is V
• Ohm is unit of resistance symbol is ?

(Voltage)
(Resistance)
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Lab Ohms Law

• Measuring voltage, current, and resistance in
  simple circuits to verify Ohms law.

Ammeter
Galvanometer
Battery
Resistor
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Demo Resistance of Water

• Pure water has very high resistance impurities,
  such as salt, lower resistance.

Light Bulb
When salt dissolves the sodium and chlorine atoms
are charged (ions). These mobile charges carry
the current in the water.
CURRENT
Power Supply
Salty Water
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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.
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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
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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.

SA
AV
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Electric Shock

• The damaging effects of shock are the result of
  current passing through the body.

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
From Ohm's law, current depends on the voltage
and on electrical resistance. When dry, skins
resistance around 100,000 ?. Resistance drops as
low as 100 ? when wet and salty.
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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.

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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.

Without Voltage
With Voltage
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Demo Resistance Temperature
Liquid Nitrogen
Resistance in a material goes down when the
material is cooled because the electrons dont
bump into atoms often.
CURRENT
Battery
With Voltage Cold
Without Voltage
With Voltage
Ammeter
Current increases when wire cooled
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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.

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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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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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?