Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the When the balls have to be squeezed bulb or a resistor, they cause the wires through a thin section of the pipe, they to heat up. will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout. Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the When the balls have to be squeezed bulb or a resistor, they cause the wires through a thin section of the pipe, they to heat up. will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout. Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the When the balls have to be squeezed bulb or a resistor, they cause the wires through a thin section of the pipe, they to heat up. will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout. Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the When the balls have to be squeezed bulb or a resistor, they cause the wires through a thin section of the pipe, they to heat up. will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout. Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the When the balls have to be squeezed bulb or a resistor, they cause the wires through a thin section of the pipe, they to heat up. will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout. Actual Analogy Electrons flowing in a circuit. Balls flowing through a hollow pipe. The cell pushes the electrons in the wire. An engine pushes the balls in the pipe. When the electrons go through the When the balls have to be squeezed bulb or a resistor, they cause the wires through a thin section of the pipe, they to heat up. will make the pipe hot. When the cell runs out the current stops flowing. When the engine runs out of fuel it will stop pushing the balls. The flow stops. Measuring the current is counting the number of Coulombs passing a certain point in the circuit each second. Watching the number of balls passing a certain point of the pipe each second The current is the same everywhere in the circuit. The no. of balls entering the narrow strip is equal to no. of balls exiting it. The speed of the balls is constant throughout. Adding another cell will increase the current. Adding another motor will speed up the balls in the pipe. Adding a resistor will cause the current in the whole circuit to be reduced. Current is still the same everywhere. The current in P is equal to the current in Q. Adding an obstruction in the pipe will make the balls go slower. The rate of flow of the balls at is the same as the rate at Q. Adding another cell will increase the current. Adding another motor will speed up the balls in the pipe. P Q Adding a resistor will cause the current in the whole circuit to be reduced. Current is still the same everywhere. The current in P is equal to the current in Q. Adding an obstruction in the pipe will make the balls go slower. The rate of flow of the balls at P is the same as the rate at Q. The sum of the currents entering a point is equal to the sum of the currents leaving that point. The number of balls entering any junction is the same as the number of balls exiting that junction. Lesson 12 Circuit rules 1. What are the rules for series and parallel circuits? 2. What are the principles behind these rules? 3. How do we use the rules in circuits? Circuit rules Kirchhoff’s first law This states that the total current flowing into a point is equal to the current flowing out of that point. In other words, the charge does not leak out or accumulate at that point. Charge that flows away must be replaced. Conservation of charge principle. 3A 2A 1A 1.50A 0.50A 0.75A 1.25A Circuit rules Kirchhoff’s second law This states that around any closed loop in a circuit, the sum of the potential differences (voltages) across all components is zero. Any charge that starts and ends up at the same point must have gained as much energy as it lost. Conservation of energy principle. the electrons gain 12V at the battery Write down the readings you would expect on the ammeters in the circuits below. a Draw a circle around the voltmeter that is measuring the emf of the cell. b Is the motor or the resistor transferring the most energy? c What is the potential difference across the lamp? Write the missing potential differences and currents on these circuit diagrams. Write the missing potential differences and currents on these circuit diagrams. Show your working for any calculations. (Hint: work out your answers in the order of the letters.) Calculate the potential difference supplied by the cell in this circuit. Calculate the current in this circuit. Calculate the missing resistance in this circuit. Circuit rules Both Kirchhoff’s laws lead to the following rules: Series circuit rules Rule 1: In a series circuit the current is the same all the way round the circuit. Rule 2: In a series circuit the voltage is shared between the components in the circuit in ratios of their resistances. the higher resistance components will get a bigger share of the voltage. Parallel circuit rules Rule 1: In a parallel circuit the current splits up to go down different routes. The total current through the cell is the sum of all the separate currents Rule 2: In a parallel circuit the voltage is the same across each route. Rule 3:In a parallel circuit the current through a route depends on the resistance. If it has a big resistance only a small current will flow along it. If all the resistors in the circuit are 10 Ω resistors: a what is the potential difference between points 2 and 4? b what is the potential difference between points 5 and 7? c what is the potential difference between points 2 and 5? d The current at point 1 is 0.6 A. What is the current at point 3? e What is the current at point 6? The resistors in the circuit have the following values: R1 = 10 Ω, R2 = 20 Ω, R3 = 15 Ω, R4 = 15 Ω a What is the potential difference between points 2 and 4? b What is the potential difference across R1? c What is the potential difference across R2? d What is the potential difference across R3? e How does the current at point 3 compare with the current at point 6? R1 and R2 are both 10 Ω so the total resistance in that branch is 20 Ω. R3 and R4 are both 20 Ω. a What is the potential difference across points 2 to 4? b What current is flowing between points 2 and 4?
