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KS3 Physics 9I Energy and Electricity 1 of 20 34 © Boardworks Ltd 2005 2004 Contents 9I Energy and Electricity Measuring current Measuring voltage Energy in circuits Summary activities 1 2 of 20 34 © Boardworks Ltd 2005 2004 Measuring current The unit of measure for current is the amp, which has the symbol A. Current is measured using a device called an ammeter. In a circuit diagram, an ammeter is shown by the symbol A . When measuring the current through a component, the ammeter is always connected in series (in the same loop) with that component. A 1 3 of 20 34 © Boardworks Ltd 2005 2004 Experiment 1: Current in series circuit Circuit 1 1 A R1 A 2 1. Set up the circuit as shown above. 2. Measure the current using the ammeter at positions 1 and 2. 1 4 of 20 34 © Boardworks Ltd 2005 2004 Experiment 1: Current in a series circuit Circuit 2 1 A A R1 A 3 R2 2 1. Add another resistor into the circuit (R2) and another ammeter after it. 2. Now measure the current using the ammeter at positions 1, 2 and 3. 1 5 of 20 34 © Boardworks Ltd 2005 2004 Experiment 1: Current in a series circuit – results Circuit 1 Circuit 2 1 A 1 A R1 A 2 A 3 R1 A R2 2 Circuit 1 results: Circuit 2 results: Current at position 1 = Current at position 1 = Current at position 2 = Current at position 2 = Current at position 3 = 1 6 of 20 34 © Boardworks Ltd 2005 2004 Experiment 1: Current in a series circuit – summary Circuit 1 Circuit 2 A A R1 A A R1 A R2 Circuit 1 The current at different positions in the circuit, before and after the resistor, was the _______. Current is _____ used up by the components in the circuit. Circuit 2 Increasing the number of components in the circuit ________ the current. The current at all points in a series circuit is the _______. same / same / decreased / not 1 7 of 20 34 © Boardworks Ltd 2005 2004 What is a parallel circuit? A parallel circuit is one which contains a point (a junction) where the current can split (point A) or join (point B). This means that there is more than one path around the circuit. A 1 8 of 20 34 B © Boardworks Ltd 2005 2004 Measuring current in a parallel circuit 1. Place the ammeter, in turn, at positions 1, 2, 3 and 4. A4 A1 A2 A3 2. Record the ammeter readings in the table. Ammeter Current (A) A1 A2 A3 A4 1 9 of 20 34 © Boardworks Ltd 2005 2004 Current in a parallel circuit For a parallel circuit, the current that leaves the cell is the same as the current that returns to the cell. A4 A1 A2 A1 = A4 A3 The current does not get used up by the circuit, just the energy that the electrons are carrying. 1 10ofof20 34 © Boardworks Ltd 2005 2004 Current in a parallel circuit The current splits up at the first junction and then joins together at the second junction. A4 A1 A2 A3 The following is always true for this type of parallel circuit: A1 = (A2 + A3) = A4 If the bulbs are identical then the current will split evenly. If the bulbs are not identical, then the current will not split evenly. 1 11ofof20 34 © Boardworks Ltd 2005 2004 Contents 9I Energy and Electricity Measuring current Measuring voltage Energy in circuits Summary activities 1 12ofof20 34 © Boardworks Ltd 2005 2004 Measuring voltage Voltage is the amount of push and is measured in ‘volts’ which has the symbol V. Voltage is measured using a device called a voltmeter. In a circuit diagram, a voltmeter is given the symbol V . When measuring the voltage across a component, the voltmeter is always connected in parallel with (or across) the component. This is still a series circuit. V1 The voltage supplied by the battery is shared between all the components in a series circuit. V2 V3 1 13ofof20 34 © Boardworks Ltd 2005 2004 Experiment 2: Voltage in a series circuit Circuit 1 V R1 V 1. Set up the circuit as shown above. 2. Connect the voltmeter across the power supply (battery) and measure the supply voltage. 3. Then connect the voltmeter across the resistance (R) and measure this voltage. 1 14ofof20 34 © Boardworks Ltd 2005 2004 Experiment 2: Voltage in a series circuit Circuit 2 V R1 R2 V1 V2 1. Add another resistor (R2) to the circuit as shown. 2. Connect the voltmeter across the power supply (battery) and measure the supply voltage. 3. Then measure the voltage across each of the resistor. 1 15ofof20 34 © Boardworks Ltd 2005 2004 Experiment 2: Voltage in a series circuit – results Circuit 1 Circuit 2 V R1 R1 R2 V V1 V2 Circuit 2 results: Circuit 1 results: 1 16ofof20 34 V Voltage (supply) = V Voltage (supply) = V Voltage (R1) V Voltage (R1) = V Voltage (R2) = V = © Boardworks Ltd 2005 2004 Experiment 2: Voltage in a series circuit – summary Circuit 1 V Circuit 2 V R R1 R2 V V1 V2 The current is the _______ of electricity around the circuit. The _________ is the amount of push. When two components were put into Circuit 2, the voltage of the supply was the _______ as Circuit 1. However, the voltage across R1 __________ . The voltage across both components in Circuit 2 added to be equal to the ________ voltage. supply / decreased / voltage / flow / same 1 17ofof20 34 © Boardworks Ltd 2005 2004 Make your own series circuit 1 18ofof20 34 © Boardworks Ltd 2005 2004 Measuring voltage in a parallel circuit Connect up this circuit and measure, in turn, the voltage at V1, V2 and V3. Record your results in the table. V1 Voltmeter V2 Voltage(V) V1 V2 V3 V3 What do you notice about the results? How can you explain this? 1 19ofof20 34 © Boardworks Ltd 2005 2004 Make your own parallel circuit 1 20ofof20 34 © Boardworks Ltd 2005 2004 Experiment 3: Cells in a series circuit Circuit 1 V R A V 1. Set up the circuit as shown above. 2. Connect the voltmeter across the power supply (battery) and measure the supply voltage. Then measure the voltage across the resistance. Also measure the current. 1 21ofof20 34 © Boardworks Ltd 2005 2004 Experiment 3: Cells in a series circuit Circuit 2 V R A V 1. Add an additional battery to the circuit. 2. Connect the voltmeter across the power supply and measure the supply voltage. Then measure the voltage across the resistance. Also measure the current. 1 22ofof20 34 © Boardworks Ltd 2005 2004 Experiment 3: Cells in a series circuit – results Circuit 1 Circuit 2 V R A V 1 23ofof20 34 R V A V Circuit 1 results: Circuit 2 results: Supply voltage = Supply voltage = Voltage R = Voltage R = Current = Current = © Boardworks Ltd 2005 2004 Experiment 3: Cells in a series circuit – summary Circuit 1 Circuit 2 V R A V R V A V Delete the wrong answer: Increasing the number of cells increases/decreases the current that flows in the circuit. The current/voltage depends on the current/voltage. 1 24ofof20 34 © Boardworks Ltd 2005 2004 Current and voltage – key ideas Current In a series circuit, the current is the same in all parts of the circuit. In a parallel circuit, the current splits up and recombines when the branches of the circuit meet up. (The sum of the current in the branches equals the total current.) The current depends on the voltage in any circuit. Voltage In a series circuit, the supply voltage is shared between the components. (The sum of the voltage across each component is the same as the total supply voltage.) In a parallel circuit, the voltage across each component is the same as the supply voltage. 1 25ofof20 34 © Boardworks Ltd 2005 2004 Contents 9I Energy and Electricity Measuring current Measuring voltage Energy in circuits Summary activities 1 26ofof20 34 © Boardworks Ltd 2005 2004 Energy transfer in circuits Energy cannot be created or destroyed. In all devices and machines, including electric circuits, energy is transferred from one type to another. When this circuit is connected, chemical energy stored in the battery is transferred via electrical energy to heat and light energy in the bulbs. The total amount of heat and light energy is the same as the amount of chemical energy lost from the battery. 1 27ofof20 34 © Boardworks Ltd 2005 2004 Energy transfer in electrical circuits 5 J transferred to bulb as light energy chemical energy from battery (e.g. 100J) 95 J transferred to heat energy of bulb Most of the energy from the battery does not produce light – most of it is wasted as heat! 1 28ofof20 34 © Boardworks Ltd 2005 2004 Calculating energy efficiency The efficiency of an energy transfer can be calculated using this formula: %Efficiency = useful energy output total energy input x 100 This bulb converts 200 J of chemical energy form battery into 10 J of useful light energy: Efficiency of bulb = 10 x 100 200 ( ) = 5% 1 29ofof20 34 © Boardworks Ltd 2005 2004 What’s the energy transfer? Batteries can power many electrical devices. What sort of energy is electrical energy transferred into in these electrical devices? 1 30ofof20 34 © Boardworks Ltd 2005 2004 Contents 9I Energy and Electricity Measuring current Measuring voltage Energy in circuits Summary activities 1 31ofof20 34 © Boardworks Ltd 2005 2004 Glossary current – The flow of electricity, measured in amps (A). efficiency – A measure of how much energy is changed from one form to another. potential difference – The amount of ‘push’ or electrical energy there is in a circuit, measured in volts (V). power – The amount of energy that an electrical device uses per second, measured in watts (W). power station – A place where an energy resource is transformed into electrical energy. transfer – To move energy from one place to another. transform – To change energy from one type to another. voltage – Another name for ‘potential difference’. 1 32ofof20 34 © Boardworks Ltd 2005 2004 Anagrams 1 33ofof20 34 © Boardworks Ltd 2005 2004 Multiple-choice quiz 1 34ofof20 34 © Boardworks Ltd 2005 2004