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Chapter 23 Electric Current. 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. Electric Current.

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Chapter 23 electric current l.jpg

Chapter 23Electric Current

Physics 1 (Garcia) SJSU

Current l.jpg

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.

Physics 1 (Garcia) SJSU

Electric current l.jpg
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.



Physics 1 (Garcia) SJSU

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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




Ammeter readings always the same.



Galvanometer is an ammeter with both positive & negative

Physics 1 (Garcia) SJSU

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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 potential—a voltage.




Simple Chemical Battery

Simple Mechanical Generator

Physics 1 (Garcia) SJSU

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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 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

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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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Jacob’s Ladder

Electrical resistance through air is greater for a larger gap.

Biblical Jacob’s Ladder

Physics Jacob’s Ladder

Physics 1 (Garcia) SJSU

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Ohm’s Law

Relation between current, voltage, and resistance is Ohm’s 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 



Physics 1 (Garcia) SJSU

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Lab: Ohm’s Law

Measuring voltage, current, and resistance in simple circuits to verify Ohm’s law.





Physics 1 (Garcia) SJSU

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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.






Physics 1 (Garcia) SJSU

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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.



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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.



Physics 1 (Garcia) SJSU

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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

From Ohm's law, current depends on the voltage and on electrical resistance.

When dry, skin’s resistance around 100,000 .

Resistance drops as low as

100  when wet and salty.

Physics 1 (Garcia) SJSU

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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 ?

Physics 1 (Garcia) SJSU

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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.

Physics 1 (Garcia) SJSU

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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.





Physics 1 (Garcia) SJSU

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Demo: Resistance & Temperature



Resistance in a material goes down when the material is cooled because the electrons don’t bump into atoms often.





& Cold






Current increases when wire cooled

Physics 1 (Garcia) SJSU

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Ohmic Heating

Flowing electrons strike atoms in a conductor, heating the material.


Physics 1 (Garcia) SJSU

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Rear Window Defogger

Ohmic heating evaporates fog on car window.

Physics 1 (Garcia) SJSU

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Demo: Ohmic Cooking

An electric current running through a hot dog generates enough heat to cook it.

Physics 1 (Garcia) SJSU

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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.

Physics 1 (Garcia) SJSU

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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?

Physics 1 (Garcia) SJSU

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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

Physics 1 (Garcia) SJSU

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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).

Physics 1 (Garcia) SJSU

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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.

Physics 1 (Garcia) SJSU

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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?

Physics 1 (Garcia) SJSU

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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.

Physics 1 (Garcia) SJSU

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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.




Physics 1 (Garcia) SJSU

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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?

Physics 1 (Garcia) SJSU