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Chapter 34. Electric 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. - PowerPoint PPT Presentation
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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.
When the ends of an electric conductor are at different
electric potentials, charge flows from one end to the other.
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:
DC is produced when stored electric potential is tapped.
Batteries and solar cells have electric potential.
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.
Current surges first one way, then the other through the toasterheating filament, changing direction 60
times per second.
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.
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 Law: I = V/R
Filament provides resistance to the flow of electrons.
Georg Simon Ohm (1787-1854)
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.
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
House Wiring and Fuses
Appliances are connected in parallel.
When ribbon carries too much current,it melts, interrupting the current.
It is easier to replace a fuse then a circuit.
Contains a wire that will melt when too much current flows.