Chapter 34Electric Current

And

And

Power

Electric Circuits:1. Electric circuits transfer energy. 2. Electrical energy is converted into light, heat, sound, mechanical work, etc. 3. The by product of any circuit is always heat.

Potential difference:difference in voltage between

the ends of a conductor.

When the ends of an electric conductor are at different

electric potentials, charge flows from one end to the other.

The greater the difference, the greater the "push" behind the

electrons.

The charge will continue to flow until both ends reach a common

potential.

Eel uses chemical activity to maintain a potential difference between head and tail of about 600 volts.

Potential difference causes charge in water to flow from head to tail.

Current:The number of coulombs per second which travel around a circuit each second is called the current, symbolized by I.

Current  (I) is measured in Andre M. Ampere (1775-1836)

amperes.

One ampere = One coulomb per second.

Electrons in metal wire and light bulb filament are pushedaround the circuit by the voltage source.

This battery pushes electrons in one direction only:

1 amp = 1 C/sec

The Coulomb Unit

6.24 billion billion electrons is about  1 C.                

DC current (direct current) a steady flow of current in one

direction

DC is produced when stored electric potential is tapped.

Batteries and solar cells have electric potential.

Inside the Dry Cell Battery

The source of the voltage  is chemical energy.

Batteries Set up Electric Fields  between their Terminals

If a wire is connected between the terminals,in which direction do the electrons move?

AC current (alternating current)

direction of current flow changes many times a second. AC

electricity in the U.S. is 60 cycle electricity. This means that the direction of energy

flow changes 60 times every second.

AC is produced by a generator

Alternating Voltage Sources             

Current surges first one way, then the other through the toasterheating filament, changing direction 60

times per second.

The amount of charge that flows in a circuit depends on the

voltage provided by the voltage source.

Current also depends on the resistance that the conductor offers to the flow of charge –

electric resistance.

Resistors: Resistors reduce the flow of electrons in an electrical circuit.

Resistance is measured in ohms.

Resistance of a solid conductor depends upon: 1. nature of the material 2. length of the conductor 3. cross-sectional area of the conductor 4. temperature

Not all materials are created equal in terms of their conductive ability. Some materials are better conductors than others and offer less resistance to the flow of charge. Silver is one of the best conductors, but is never used in wires of household circuits due to its cost. Copper and aluminum are among the least expensive materials with suitable conducting ability to permit their use in wires of household circuits.

Material

Resistivity(ohm•meter)

Silver1.59 x 10-8

Copper1.7 x 10-8

Gold2.4 x 10-8

Aluminum2.8 x 10-8

Tungsten5.6 x 10-8

Iron10 x 10-8

Platinum11 x 10-8

Lead22 x 10-8

The longer the wire, the more resistance that there will be. There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire. More collisions means more resistance.

Wider wires have a greater cross-sectional area. Water will flow

through a wider pipe at a higher rate than it will flow through a narrow pipe; this can be attributed to the

lower amount of resistance which is present in the wider pipe. In the

same manner, the wider the wire, the less resistance that there will be

to the flow of electric charge.

electric power:

(symbol is P; SI unit is watt)

-the rate of doing electrical work

Power • Power = current x voltage

Units:Watts = amperes x volts

• Example:

Voltage = 120 volts

What is the plannedcurrent?

Current = Power / voltage             = 100 / 120             = 0.825 amperes

Ohm’s lawfor a given resistance, the

potential difference is proportional to the current flow

Ohm's Law:  I = V/R   

Filament provides resistance to the flow of electrons.

Georg Simon Ohm (1787-1854)

Calculating V from Ohm's Law        

Ohm's Law:  V = I R

V = (6 amperes)(3 ohms)

    = 18 volts

 Calculating I from Ohm's Law  

Ohm's Law:  I = V / R

I = (12 volts) / 3 ohms

  = 4 amperes

Calculating R from Ohm's Law    

Ohm's Law: R = V / I

R = (36 volts) / (6 amperes)

    = 6 ohms

Resistance, Ohm's Law, and Short Circuits

Why isn't this birdshocked?

What if the right footof the bird were moved

to the back wire?

Safety in Electricity

Other end of ground plug is connected to the appliance cover.

The wall jack which receives the ground plug is connected to the ground, so any charge leaking onto the appliance will drain to ground.

Appliance without Short   

 

Improperly Grounded Appliance with Short  

One milliampere:    tingling sensationTen milliamperes:   nerves and muscles overloaded200 milliamperes:   potentially fatal; heart fibrillation500 -1000 milliamperes:  not necessarily fatal; heat will restartOne ampere or more:  burn alive

Grounded Appliance

House Wiring and Fuses 

Appliances are connected in parallel.

When ribbon carries too much current,it melts, interrupting the current.

We use fuses to stop overloading of circuits.

Why?

It is easier to replace a fuse then a circuit.

Contains a wire that will melt when too much current flows.

For larger circuits, Circuit Breakers are also used.-A switch that flips

open when too much current flows.