# Electricity and Magnetism Topic 5.2 Electric Circuits

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• Slide 1
• Electricity and Magnetism Topic 5.2 Electric Circuits
• Slide 2
• Electromotive Force Defining potential difference Defining potential difference The coulombs entering a lamp have electrical potential energy; The coulombs entering a lamp have electrical potential energy; those leaving have very little potential energy. those leaving have very little potential energy. There is a potential difference (or p.d.) across the lamp, because the potential energy of each coulomb has been transferred to heat and light within the lamp. There is a potential difference (or p.d.) across the lamp, because the potential energy of each coulomb has been transferred to heat and light within the lamp. p.d. is measured in volts (V) and is often called voltage. p.d. is measured in volts (V) and is often called voltage.
• Slide 3
• The p.d. between two points is the electrical potential energy transferred to other forms, per coulomb of charge that passes between the two points. The p.d. between two points is the electrical potential energy transferred to other forms, per coulomb of charge that passes between the two points.
• Slide 4
• Resistors and bulbs transfer electrical energy to other forms, but which components provide electrical energy? Resistors and bulbs transfer electrical energy to other forms, but which components provide electrical energy? A dry cell, a dynamo and a solar cell are some examples. A dry cell, a dynamo and a solar cell are some examples. Any component that supplies electrical energy is a source of electromotive force or e.m.f. Any component that supplies electrical energy is a source of electromotive force or e.m.f. It is measured in volts. It is measured in volts. The e.m.f. of a dry cell is 1.5 V, that of a car battery is 12 V The e.m.f. of a dry cell is 1.5 V, that of a car battery is 12 V
• Slide 5
• A battery transfers chemical energy to electrical energy, so that as each coulomb moves through the battery it gains electrical potential energy. A battery transfers chemical energy to electrical energy, so that as each coulomb moves through the battery it gains electrical potential energy. The greater the e.m.f. of a source, the more energy is transferred per coulomb. In fact: The greater the e.m.f. of a source, the more energy is transferred per coulomb. In fact: The e.m.f of a source is the electrical potential energy transferred from other forms, per coulomb of charge that passes through the source. The e.m.f of a source is the electrical potential energy transferred from other forms, per coulomb of charge that passes through the source. Compare this definition with the definition of p.d. and make sure you know the difference between them. Compare this definition with the definition of p.d. and make sure you know the difference between them.
• Slide 6
• Internal Resistance
• Slide 7
• The cell gives 1.5 joules of electrical energy to each coulomb that passes through it, The cell gives 1.5 joules of electrical energy to each coulomb that passes through it, but the electrical energy transferred in the resistor is less than 1.5 joules per coulomb and can vary. but the electrical energy transferred in the resistor is less than 1.5 joules per coulomb and can vary. The circuit seems to be losing energy can you think where? The circuit seems to be losing energy can you think where?
• Slide 8
• The cell itself has some resistance, its internal resistance. The cell itself has some resistance, its internal resistance. Each coulomb gains energy as it travels through the cell, but some of this energy is wasted or `lost' as the coulombs move against the resistance of the cell itself. Each coulomb gains energy as it travels through the cell, but some of this energy is wasted or `lost' as the coulombs move against the resistance of the cell itself. So, the energy delivered by each coulomb to the circuit is less than the energy supplied to each coulomb by the cell. So, the energy delivered by each coulomb to the circuit is less than the energy supplied to each coulomb by the cell.
• Slide 9
• Very often the internal resistance is small and can be ignored. Very often the internal resistance is small and can be ignored. Dry cells, however, have a significant internal resistance. Dry cells, however, have a significant internal resistance. This is why a battery can become hot when supplying electric current. This is why a battery can become hot when supplying electric current. The wasted energy is dissipated as heat. The wasted energy is dissipated as heat.
• Slide 10
• Resistance Combinations
• Slide 11
• Resistors in series
• Slide 12
• The diagram shows three resistors connected in series The diagram shows three resistors connected in series There are 3 facts that you should know for a series circuit: There are 3 facts that you should know for a series circuit: the current through each resistor in series is the same the current through each resistor in series is the same the total p.d., V across the resistors is the sum of the p.d.s across the separate resistors, so: V = V l + V 2 + V 3 the total p.d., V across the resistors is the sum of the p.d.s across the separate resistors, so: V = V l + V 2 + V 3 the combined resistance R in the circuit is the sum of the separate resistors the combined resistance R in the circuit is the sum of the separate resistors
• Slide 13
• R = R l + R 2 + R 3 R = R l + R 2 + R 3 Suppose we replace the 3 resistors with one resistor R that will take the same current I when the same p.d. V is placed across it Suppose we replace the 3 resistors with one resistor R that will take the same current I when the same p.d. V is placed across it
• Slide 14
• Slide 15
• This is shown in the diagram. Let's calculate R. This is shown in the diagram. Let's calculate R. We know that for the resistors in series: We know that for the resistors in series: V = V l + V 2 + V 3 V = V l + V 2 + V 3 But for any resistor: p.d. = current x resistance (V = I R). But for any resistor: p.d. = current x resistance (V = I R). If we apply this to each of our resistors, and remember that the current through each resistor is the same and equal to I, we get: If we apply this to each of our resistors, and remember that the current through each resistor is the same and equal to I, we get: IR = IR l +IR 2 +IR 3 IR = IR l +IR 2 +IR 3 If we now divide each term in the equation by I, If we now divide each term in the equation by I, we get: we get: R = R 1 + R 2 + R 3 R = R 1 + R 2 + R 3
• Slide 16
• Resistors in parallel
• Slide 17
• We now have three resistors connected in parallel: We now have three resistors connected in parallel: There are 3 facts that you should know for a parallel circuit: There are 3 facts that you should know for a parallel circuit: the p.d. across each resistor in parallel is the same the p.d. across each resistor in parallel is the same the current in the main circuit is the sum of the currents in each of the parallel branches, so: the current in the main circuit is the sum of the currents in each of the parallel branches, so: I = I 1 + I 2 + I 3 I = I 1 + I 2 + I 3 the combined resistance R is calculated from the equation: the combined resistance R is calculated from the equation:
• Slide 18
• Suppose we replace the 3 resistors with one resistor R that takes the same total current I when the same p.d. V is placed across it. Suppose we replace the 3 resistors with one resistor R that takes the same total current I when the same p.d. V is placed across it.
• Slide 19
• Slide 20
• This is shown in the diagram. Now let's calculate R. This is shown in the diagram. Now let's calculate R. We know that for the resistors in parallel: We know that for the resistors in parallel: I = I 1 +I 2 +I 3 I = I 1 +I 2 +I 3 But for any resistor, current = p.d. = resistance (I = V/R ). But for any resistor, current = p.d. = resistance (I = V/R ). If we apply this to each of our resistors, and remember that the If we apply this to each of our resistors, and remember that the p.d. across each resistor is the same and equal to V, p.d. across each resistor is the same and equal to V, we get:V/R=V/R 1 + V/R 2 + V/R 3 we get:V/R=V/R 1 + V/R 2 + V/R 3 Now we divide each term by V, to get: Now we divide each term by V, to get: 1/R=1/R 1 + 1/R 2 + 1/R 3 1/R=1/R 1 + 1/R 2 + 1/R 3
• Slide 21
• You will find that the total resistance R is always less than the smallest resistance in the parallel combination. You will find that the total resistance R is always less than the smallest resistance in the parallel combination.
• Slide 22
• Circuit Diagrams You need to be able to recognize and use the accepted circuit symbols included in the Physics Data Booklet You need to be able to recognize and use the accepted circuit symbols included in the Physics Data Booklet
• Slide 23
• Ammeters and Voltmeters In order to measure the current, an ammeter is placed in series, in the circuit. In order to measure the current, an ammeter is placed in series, in the circuit. What effect might this have on the size of the current? What effect might this have on the size of the current? The ideal ammeter has zero resistance, so that placing it in the circuit does not make the current smaller. The ideal ammeter has zero resistance, so that placing it in the circuit do