2 Flow of Charge Charge flows when there is a potential difference When the ends of an electric conductor are at different electric potentials, charge flows from one end to the otherThe flow of charge will continue until both ends reach a common potentialRecall that heat flows through a conductor when a difference in temperature exists between its ends. Heat flows from the end of higher temperature to the end of lower temperature. When both ends reach the same temperature, the flow of heat ceases.Charge flows in a similar way.
3 Electric Current (I) current = charge time I = Δq Δt Electric current = the flow of electric chargee- carry the charge through the circuit because they are free to move throughout the atomic networkElectric current is measured in amperes (A)ampere = the flow of 1 coulomb of charge per secondWhen the rate of flow of charge past any cross section is 1 cou lomb (6.24 billion billion elec- trons) per second, the current is 1 ampere.When electrons flow in a wire, the number enter- ing one end is the same as the number leaving the other. So we see that the net charge of the wire is normally zero at every moment.current = chargetimeI = ΔqΔt
4 What is the net charge of a current carrying wire (as pictured above)? A current-carrying wire has a net electric charge of zero because the number of electrons entering one end is the same as the number leaving the other.
5 Current (I) vs. Voltage (V) Charges flow when there is a potential difference (V)A voltage source provides a potential differenceEx: batteries, generatorsCharges flow through a circuit because of an applied voltage across the circuitVoltage (V) causes current (I)If I increase the potential difference (voltage), will the current…(a) increase or(b) decrease?Increase! V α I
6 Resistance (R) We now know: But… Amount of charge that flows in a circuit depends on the voltage provided by the voltage source (V α I)But…The current also depends on the resistance (R) that the conductor offers to the flow of charge—the electric resistanceResistance is measured in ohms (Ω)
7 Resistance (R) of a wire depends on: Conductivity of the material used in the wireBetter conductors have less resistanceThickness of the wireThick wires have less resistance than thin wires.Length of the wireLonger wires have more resistance than short wiresTemperatureFor most conductors, increased temperature means increased resistanceThe longer an extension cord, the greater its resistance, and the more energy is dissipated in it, with less available for the electric drill.Why will an electric drill operating on a very long extension cord not rotate as fast as one operated on a short cord?
8 What do we know? I α V I α 1/R I = V/R How is current (I) related to voltage (V)?I α VHow is current (I) related to resistance (R)?I α 1/RCombining these findings…I = V/R
9 Ohm’s LawThe current in a circuit is directly proportional to the voltage impressed across the circuit, and is inversely proportional to the resistance of the circuitI = V/RV = I*RUnits:V = volts (V)I = amps (A)R = ohms (Ω)
10 Examples V = IR V = 2R*_I V = 2R * ½ I The current is halved V = IR If the voltage impressed across a circuit is constant but the resistance doubles, what change occurs in the current?V = IRV = 2R*_IV = 2R * ½ IThe current is halvedHow much voltage is required to make 2 amperes flow through a resistance of 8 ohms?V = IRV = 2 A * 8 ΩV = 16 volts
11 Ohm’s Law and Electric Shock The damaging effects of electric shock are the result of current passing through the bodyBody’s resistance = 100 ohms if you’re soaked with salt water to about 500,000 ohms if your skin is very dry
12 Why don’t birds get fried? Every part of the bird’s body is at the same high potential as the wire, so the bird feels no effectsFor the bird to receive a shock, there must be a difference in potential between one part of its body and another partJane falls from a bridge and manages to grab onto a high-voltage power line, halting her fall. Will she be ok??Suppose you fall from a bridge and manage to grab onto a high- voltage power line, halting your fall. So long as you touch nothing else of different potential, you will receive no shock at all. Even if the wire is thousands of volts above ground potential and even if you hang by it with two hands, no charge will flow from one hand to the other. This is because there is no appreciable difference in electric potential between your hands.If, however, you reach over with one hand and grab onto a wire of different potential, ZAP!!As long as Jane touches nothing else of different potential, she will receive no shock at all.Even if the wire is thousands of volts above ground potential and even if she hangs by it with two hands, no charge will flow from one hand to the other.If, however, you reach over with one hand and grab onto a wire of different potential, ZAP!!
13 Source of e- in a circuit The source of electrons in a circuit is the conducting circuit material itselfElectrons DO NOT come from:electric outlets in the walls of homespower linesPower utilities do not sell electrons. They sell energy. You supply the electrons.When you are jolted by an AC electric shock, the electrons making up the current in your body originate in your body.Electrons do not come out of the wire and through your body and into the ground; energy does.
14 Electric Power P = V*I P = I2*R A charge moving in a circuit expends energyThis may result in heating the circuit or in turning a motorElectric power (P) = the rate at which electrical energy is converted into another form such as mechanical energy, heat, or lightMeasured in Watts (W)Electric power is equal to the product of current and voltageP = V*IP = I2*R
15 Example P = V*I P = 120 V*1.20 A P = 144 W Calculate the power supplied to an electric blanket that carries 1.20 A when connected to a 120-V outlet.P = V*IP = 120 V*1.20 AP = 144 W
17 Electric Circuits Circuit = Any path along which electrons can flow For a continuous flow of electrons, there must be a complete circuit with no gapsA gap is usually provided by an electric switch that can be opened or closed to either cut off or allow electron flow
19 In SeriesTotal resistance to current in the circuit is the sum of the individual resistances along the circuit pathRtotal = R1 + R2 + R3Current passing through each electric device is the sameI1 = I2 = I3Vtotal = I*RtotalOhm’s Law applies across each individual deviceV1 = I*R1 V2 = I*R2 V3 = I*R3Sum of the voltage drops across the individual devices is equal to the total voltage supplied by the sourceVtotal = V1 + V2 + V3
20 Example 1: SeriesFind the total resistance of the three resistors connected in series.
21 Example 2: SeriesWhat is the current through the battery?R = 4 Ω
22 Example 3: SeriesFind the resistance of the unknown resistor, R.
24 In ParallelOverall resistance of the circuit is less than the resistance of any one of the branches1/Rtotal = 1/R1 + 1/R2 + 1/R3Voltage is the same across each deviceV1 = V2 = V3Total current in the circuit equals the sum of the currents in its parallel branchesItotal = I1 + I2 + I3Ohm’s Law applies across each individual deviceV = I1*R1 V = I2*R2 V = I3*R3
25 Example 1: ParallelFind the total resistance of the three resistors connected in parallel.
26 Example 2: ParallelFind the current through the 2 ohm resistor.
27 Example 2: ParallelThree resistors are connected in parallel. If placed in a circuit with a 12-volt power supply. Determine the equivalent resistance, the total circuit current, and the voltage drop across and current in each resistor.
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