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Instructions: Read the summary, play the game and take the test! Game: http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/circ uitsact.shtml Current and voltage in electrical circuits Electrical circuits can be represented by circuit diagrams. The various electrical components are shown by using standard symbols in circuit diagrams. Components can be connected in series, or in parallel. The characteristics of the current and potential difference (voltage) are different in series and parallel circuits. Circuit symbols You need to be able to draw and interpret circuit diagrams. Standard symbols The diagram below shows the standard circuit symbols you need to know. Open Switch Closed Switch Lamp Cell Battery Voltmeter Resistor Fuse Ammeter Variable resistor Thermistor Light emitting diode (LED) Diode Circuit diagrams Light dependent resistor (LDR) Two things are important for a circuit to work: There must be a complete circuit There must be no short circuits To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp, and back to the battery. To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, there is a short circuit and the lamp will not light. Work out which of these four lamps will light when the switch is closed. Move your mouse over a circuit, then left-click to check your answer: Series and parallel connections You should know the difference between series and parallel connections in circuits. Series connections Components that are connected one after another on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same. Two lamps connected in series The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not light. Parallel connections Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel. Two lamps connected in parallel The circuit diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still light. Which of the circuits below are connected in series, and which are connected in parallel? Move your mouse over each circuit to check your answers: Electric current Electric current is a flow of electric charge. No current can flow if the circuit is broken - for example, when a switch is open. Click on the animation to see what happens to the charges when the switch is opened or closed: Measuring current Current is measured in amperes (which is often abbreviated toamps or A). The current flowing through a component in a circuit is measured using an ammeter. This must be connected in series with the component. Calculating current The size of an electric current is the rate of flow of electric charge. You can calculate the size of a current using this equation: I=Q÷t I is the current in amperes (amps), A Q is the charge in coulombs [coulomb: The unit of electric charge, symbol 'C'. One coulomb is the amount of charge transferred by a current of 1 ampere in 1 second. ], C t is the time in seconds, s For example, what is the current if 20 C of charge passes in 5 s? Current = 20 ÷ 5 = 4 A Potential difference (voltage) A potential difference, also called voltage [voltage: The potential difference of a cell, electrical supply or electric component. It is measured in volts, 'V'. ], across an electrical component is needed to make a current [current: Moving electric charges, for example, electrons moving through a metal wire. ] flow through it. Cells [cell (physics): A component that uses chemical reactions to supply electrical energy. ] or batteries often provide the potential difference needed. Measuring potential difference Potential difference is measured in volts, V. The potential difference across a component in a circuit is measured using a voltmeter. This must be connected in parallel with the component. Calculating potential difference The potential difference between two points in an electric circuit is the work done when a coulomb of charge passes between the points. You can calculate the size of a potential difference using this equation: V=W÷Q V is the potential difference in volts, V W is the work done (energy transferred) in joules, J Q is the charge in coulombs, C For example, what is the potential difference if 48 J of energy is transferred when 4 C of charge passes? Potential difference = 48 ÷ 4 = 12 V Cells and circuits You should know what happens to the potential difference and current when the number of cells in a circuit is changed. Potential difference A typical cell produces a potential difference of 1.5 V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5 V cells are connected in series in the same direction, the total potential difference is 3.0 V. If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow. Current When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result. What will happen to the reading on the ammeter in the simulation below when the number of cells is increased? Series circuits You should know the characteristics of the current and potential difference in series circuits. Current When two or more components are connected in series, the same current flows through each component. Check your understanding of this by answering the questions about the circuit below: Potential difference When two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply. Check your understanding of the voltages across components connected in series using this activity: Electricity Print Current and voltage in electrical circuits - Test 1. What component does this circuit symbol represent? a) Diode b) LDR c) LED 2. What component does this circuit symbol represent? a) Fuse b) Variable resistor c) Thermistor 3. What component does this circuit symbol represent? a) Battery b) Cell c) Open switch 4. When measuring the potential difference across a component, you need: a) A voltmeter connected in series b) A voltmeter connected in parallel c) A potentiometer connected in parallel 5. There are three light bulbs connected to the same battery. If one of the bulbs broke: a) All the bulbs would stop working b) The other two bulbs would break as well c) The other bulbs would carry on working 6. Which of these ammeters is connected correctly? a) A b) B c) C 7. What is the other ammeter reading in this circuit? 0.0 A 0.1 A 2.0 A 8. What is the reading on the ammeter labelled A1 in this circuit? 0.1 A 0.2 A 0.3 A 9. What is the current if 12 C of charge passes in 3 s? 36 A 4A 0.25 A 10. What is the potential difference if 24 J of energy is transferred when 6 C of charge passes? 4V 0.25 V 144 V