Presentation on theme: "Electric Current And Power"— Presentation transcript:
1 Electric Current And Power Chapter 34Electric CurrentAndPower
2 Electric Circuits: 1. Electric circuits transfer energy. 2 Electric Circuits: 1. Electric circuits transfer energy Electrical energy is converted into light, heat, sound, mechanical work, etc The by product of any circuit is always heat.
3 Potential difference: difference in voltage between the ends of a conductor.
4 When the ends of an electric conductor are at different electric potentials, charge flows from one end to the other.
9 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.
10 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 ( ) amperes One ampere = One coulomb per second.
11 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:
12 1 amp = 1 C/sec The Coulomb Unit 6 1 amp = 1 C/sec The Coulomb Unit billion billion electrons is about 1 C.
13 DC current (direct current) a steady flow of current in one direction
14 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.
15 Inside the Dry Cell Battery The source of the voltage is chemical energy.
16 Batteries Set up Electric Fields between their Terminals If a wire is connected between the terminals, in which direction do the electrons move?
17 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.
20 Current surges first one way, then the other through the toaster heating filament, changing direction 60 times per second.
21 The amount of charge that flows in a circuit depends on the voltage provided by the voltage source.
22 Current also depends on the resistance that the conductor offers to the flow of charge – electric resistance.
23 Resistance is measured in ohms. Resistors: Resistors reduce the flow of electrons in an electrical circuit.Resistance is measured in ohms.
24 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
25 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.
26 Silver 1.59 x 10-8 Copper 1.7 x 10-8 Gold 2.4 x 10-8 Aluminum MaterialResistivity(ohm•meter)Silver1.59 x 10-8Copper1.7 x 10-8Gold2.4 x 10-8Aluminum2.8 x 10-8Tungsten5.6 x 10-8Iron10 x 10-8Platinum11 x 10-8Lead22 x 10-8
27 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.
28 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.
29 electric power: (symbol is P; SI unit is watt) -the rate of doing electrical work
30 Power Power = current x voltage Units: Watts = amperes x volts Example: Voltage = 120 volts What is the planned current? Current = Power / voltage = 100 / 120 = amperes
31 Ohm’s law for a given resistance, the potential difference is proportional to the current flow
32 Ohm's Law: I = V/R Georg Simon Ohm (1787-1854) Filament provides resistance to the flow of electrons.Georg Simon Ohm ( )
33 Calculating V from Ohm's Law Ohm's Law: V = I RV = (6 amperes)(3 ohms) = 18 volts
34 Calculating I from Ohm's Law Ohm's Law: I = V / RI = (12 volts) / 3 ohms = 4 amperes
35 Calculating R from Ohm's Law Ohm's Law: R = V / IR = (36 volts) / (6 amperes) = 6 ohms
36 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?
37 Safety in ElectricityOther 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.
39 Improperly Grounded Appliance with Short One milliampere: tingling sensation Ten milliamperes: nerves and muscles overloaded 200 milliamperes: potentially fatal; heart fibrillation milliamperes: not necessarily fatal; heat will restart One ampere or more: burn alive