Electric Current And Power Chapter 34 Electric Current And Power

Electric Circuits: 1. Electric circuits transfer energy. 2 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 pushed around the circuit by the voltage source. This battery pushes electrons in one direction only:

1 amp = 1 C/sec The Coulomb Unit 6 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 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 toaster heating 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.

Resistance is measured in ohms. Resistors: Resistors reduce the flow of electrons in an electrical circuit. Resistance is measured in ohms.

Resistance of a solid conductor depends upon: 1 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.

Silver 1.59 x 10-8 Copper 1.7 x 10-8 Gold 2.4 x 10-8 Aluminum Material Resistivity (ohm•meter) Silver 1.59 x 10-8 Copper 1.7 x 10-8 Gold 2.4 x 10-8 Aluminum 2.8 x 10-8 Tungsten 5.6 x 10-8 Iron 10 x 10-8 Platinum 11 x 10-8 Lead 22 x 10-8

The longer the wire, the more resistance that there will be 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 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 planned current? Current = Power / voltage              = 100 / 120              = 0.825 amperes

Ohm’s law for a given resistance, the potential difference is proportional to the current flow

Ohm's Law: I = V/R Georg Simon Ohm (1787-1854) 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 bird shocked? What if the right foot of 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 sensation Ten milliamperes:   nerves and muscles overloaded 200 milliamperes:   potentially fatal; heart fibrillation 500 -1000 milliamperes:  not necessarily fatal; heat will restart One 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 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 For larger circuits, Circuit Breakers are also used. -A switch that flips open when too much current flows.