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AQA GCSE Physics 2-5 Current Electricity. GCSE Physics pages 170 to 183. October 28 th 2010. AQA GCSE Specification. CURRENT ELECTRICITY 12.6 What does the current through an electrical circuit depend on? Using skills, knowledge and understanding of how science works:

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## AQA GCSE Physics 2-5 Current Electricity

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**AQA GCSE Physics 2-5Current Electricity**GCSE Physics pages 170 to 183 October 28th 2010**AQA GCSE Specification**CURRENT ELECTRICITY 12.6 What does the current through an electrical circuit depend on? Using skills, knowledge and understanding of how science works: • to interpret and draw circuit diagrams using standard symbols. • to apply the principles of basic electrical circuits to practical situations. Skills, knowledge and understanding of how science works set in the context of: • Current-potential difference graphs are used to show how the current through a component varies with the potential difference across it. • The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor. • Potential difference, current and resistance are related by the equation: potential difference = current × resistance • The resistance of a component can be found by measuring the current through, and potential difference across, the component. • The resistance of a filament lamp increases as the temperature of the filament increases. • The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction. • The resistance of a light-dependant resistor (LDR) decreases as light intensity increases. • The resistance of a thermistor decreases as the temperature increases (ie knowledge of negative temperature coefficient thermistor only is required). • The current through a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component. • The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected). • For components connected in series: – the total resistance is the sum of the resistance of each component – there is the same current through each component – the total potential difference of the supply is shared between the components. • For components connected in parallel: – the potential difference across each component is the same – the total current through the whole circuit is the sum of the currents through the separate components.**Electric circuits**An electric current will only flow if there is a complete, unbroken electric circuit, that contains a power supply. A circuit diagram uses a standard set of symbols to show how electrical components are connected together.**Circuit symbols**cell a cell is required to push electrons around a circuit battery a battery consists of two or more cells wire wires should always been drawn as straight lines wire junction switch a switch enables the current in a circuit to be turned on or off**A**V indicator often a light bulb – this is used to show whether or not a circuit is on old symbol – the indicator symbol is now used light bulb ammeter measures electric current in amperes (A) voltmeter measures potential difference in volts (V)**fixed resistor**a resistor is used to limit the current in a circuit variable resistor thermistor a device whose resistance decreases with temperature light dependent resistor (LDR) a device whose resistance decreases with brightness**diode**a diode only allows current to flow in one direction (indicated by the arrow) light emitting diode (LED) a diode that emits light when it allows the flow of electric current fuse a fuse is designed to melt and so break an electric circuit when too much electric current flows heater a device used to convert electrical energy to heat**Electric current flow**Electric current flows from the POSITIVE terminal of a power supply around a circuit to the NEGATIVE terminal. The longer thinner line of the symbol for a cell is the positive terminal. In the circuit above the diode is aligned so that it allows current to flow through the radio.**A**Complete ammeter heater resistor diode indicator thermistor cell LDR**Question**Draw a circuit diagram for the torch shown below.**Electric circuitsNotes questions from pages 170 & 171**• For an electric circuit to work, what must be true? • Copy Figure 2 on page 170. • Copy and answer questions (a), (b) and (c) on pages 170 and 171. Include the diagrams with questions (b) and (c). • Copy the Key Points on page 171. • Answer the summary questions on page 171.**In text questions:**So current passes through it and through the lamp. Two cells, a switch and a heater. No. Summary questions: 1. Cell, switch, indicator, fuse. 2. (a) The diode arrow should be pointing to the right. (b) A variable resistor. Electric circuits ANSWERS**a resistor**circuit symbol for a resistor Resistance Resistance is the opposition that an electrical device has to the flow of electrical current. All devices have some resistance. A resistor is a device that has a particular resistance.**Resistance equation**resistance = potential difference current Where: potential difference is in volts (V) current is in amperes (A) resistance is in ohms (Ω)**Also:**potential difference = current x resistance and: current = potential difference resistance potential difference current resistance**Circuit used for measuring the resistance of an indicator**lamp Measuring Resistance The resistance of a component can be found by measuring the current through, and potential difference across, the component.**Question 1**Calculate the resistance of a lamp if a potential difference of 12V causes a current of 3A to flow through the lamp. resistance = potential difference current = 12V / 3A resistance = 4 ohms (4Ω)**Question 2**Calculate the resistance of a heater if a potential difference of 230V causes a current of 200mA to flow through the heater. resistance = potential difference current = 230V / 200mA = 230V / 0.200A resistance = 1150 Ω**Question 3**Calculate the potential difference across a resistance of 40Ω when a current of 5A is flowing. potential difference = current x resistance = 5A x 40Ω potential difference = 200V**Question 4**Calculate the current flowing through a wire of resistance of 8Ω when a potential difference of 12V is connected to the wire. current = potential difference resistance = 12V / 8Ω current = 1.5A**Answers**Complete: 5 Ω 200 V 6 A 16 Ω 20 A 120 V**Current-potential difference graphs**These are used to show how the current through a component varies with the potential difference across it. The circuit opposite could be used to obtain a current-potential difference graph of a wire.**Typical results:**The current-potential difference graph of a resistor at a constant temperature The graph is a straight line through the origin. The graph shows that the current through a resistor is directly proportional to the potential difference across the resistor.**Choose appropriate words to fill in the gaps below:**An electric ________ will only flow around a circuit if there are no ______ in the circuit. All components have __________. The greater the resistance the ________ is the current for the same applied potential difference. Resistance is measured in ______. A current – potential difference graph for a ________ is a straight line through the _______. This shows that the current through the resistor is ___________ to the applied potential difference. current gaps resistance smaller ohms resistor origin proportional WORD SELECTION: smaller resistor proportional gaps origin resistance ohms current**ResistanceNotes questions from pages 172 & 173**• Copy Figure 1 on page 172 and explain the placement of the two meters. • Explain what causes a conductor to have resistance. • Copy the equation for resistance on page 172 along with the units used and the symbolic version of the equation. • Copy and answer question (a) on page 172. • What is Ohm’s law? • Sketch the current-potential difference graph of a wire and explain how this graph shows that the wire obeys Ohm’s law. • Copy and answer question (b) on page 173. • Copy the Key Points on page 173. • Answer the summary questions on page 173.**In text questions:**8.0 Ω 10 Ω Summary questions: 1. (a) The diagram should be the same as Figure 2 on page 173 but it does not need to have the variable resistor. (b) 8.0 Ω 2. W = 6.0Ω; X = 80V; Y = 2.0A; Z = 24Ω Resistance ANSWERS**Filament lamp**The resistance of a filament lamp increases as the temperature of the filament increases. Reversing the potential difference (negative values on the graph) reverses the direction of the electric current but does not change the shape of the curve.**Diode**The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.**Thermistor**The resistance of a thermistor decreases as the temperature increases. The higher temperature line therefore has a greater slope than the lower temperature case.**Light dependent resistor (LDR)**The resistance of a light-dependant resistor decreases as light intensity increases. The bright light line therefore has a greater slope than the dim light case.**Choose appropriate words to fill in the gaps below:**The resistance of a filament lamp _________ when the lamp comes on and the filament rises in ___________. A ______ only allows electric current to flow one way. The allowed direction is shown by the _______ on its circuit symbol. The ________ of a thermistor decreases if its temperature is increased. The resistance of a LDR _________ if the _______level is increased. increases temperature diode arrow resistance decreases light WORD SELECTION: temperature arrow decreases increases diode resistance light**More current-potential difference graphs Notes questions**from pages 174 & 175 • Copy Figure 1 on page 174 and explain why the resistance of a filament lamp varies with potential difference. • Copy Figure 2 on page 174 and explain how the resistance of a diode varies with potential difference. • Copy and answer questions (a) and (b) on page 174. • Copy Figure 4 on page 175 and explain how the resistance of a thermistor varies with temperature. • Copy Figure 5 on page 175 and explain how the resistance of a light dependent resistor varies with brightness. • Copy and answer question (c) on page 175. • Copy the Key Points on page 175. • Answer the summary questions on page 175.**In text questions:**(i) 5.0 Ω; (ii) 10 Ω It decreases The resistance is constant. Summary questions: (a) thermistor (b) diode (c) filament lamp (d) resistor 2. (a) 15 Ω (b) The ammeter reading increases because the resistance of the thermistor decreases. More current-potential difference graphs ANSWERS**3A**3A 3A 3A 3A 3A 3A Series circuits Circuit components are said to be connected in series if the same electric current passes through each of them in turn. The cell and the two lamps are in series with each other and so the same electric current passes through all of them.**Using cells in series to make a battery**With 1.5V cells: The total potential difference (voltage) provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected). 3V battery 4.5V battery 1.5V battery**Total resistance = R1 + R2**Resistors in series The total resistance is the sum of the resistance of each component.**2V**3Ω 5Ω Question Total resistance = R1 + R2 = 3Ω + 5Ω Total resistance = 8Ω current = potential difference total resistance = 2V / 8Ω Current = 0.25 A Calculate the total resistance and the current flowing in the circuit below.**Vs**V2 V1 Vs = V1 + V2 Voltages in series circuits The total potential difference of the supply is shared between the components.**Vs**V2 V1 Question Complete the table opposite for the various settings of the variable resistor if the power supply voltage, Vs is 3V.**Vs**current V1 V2 Complete the table for the circuit below:**Choose appropriate words to fill in the gaps below:**When components are _________ together in series they will all have the same _________ flowing through each of them. The total voltage of a ________ made up of cells connected in _______ is equal to the sum of the cell ________ differences provided that the cells are all connected the ______ way around. The total resistance of __________ connected together in series is equal to the ______ of the individual resistances. connected current battery potential series same resistors sum WORD SELECTION: same sum connected series potential battery resistors current**Series circuitsNotes questions from pages 176 & 177**• Copy and explain the two circuit rules for series circuits. Draw Figures 1 and 2 on page 176. • Copy and answer questions (a) and (b) on page 176. • Explain what happens when cells are connected together in series. • Copy Figure 4 on page 177 and explain what happens when resistors are connected in series. • Copy and answer questions (c) and (d) on page 177. • Copy the Key Points on page 177. • Answer the summary questions on page 177.**In text questions:**0.12 A 0.4 V 1.1 V 5 Ω Summary questions: (a) The same as (b) Less than 2. (a) (i) 12 Ω (ii) 3.0 V (b) 3V / 12Ω = 0.25 A (c) P = 0.5V; Q = 2.5V Series circuitsANSWERS**Parallel circuits**The potential difference (voltage) across each component connected in parallel is the same. The voltmeter reading for component X, V1 will be the same as the voltmeter reading for component Y, V2.**5A**5A 3A 3A 2A 2A Currents in parallel circuits The total current through the whole circuit is the sum of the currents through the separate components.**Question 1**What are the advantages of connecting two lamps in parallel rather than in series to a power supply? • When connected in parallel: • the lamps are brighter than when connected in series • the lamps can be controlled individually with switches • one lamp will continue working even if the other does not**6A**2A A2 A1 A3 Question 2 Calculate the currents measured by ammeters A1, A2 and A3 in the circuit below. A1 = 2A A2 = 4A A3 = 6A**12V**6Ω 4Ω Question 3 Calculate the currents through the 6Ω and 4Ω resistors in the circuit below. Both resistors will have the same potential difference = 12V current = potential difference resistance For the 6Ω resistor: =12V / 6Ω current through 6Ω resistor = 2A For the 4Ω resistor: =12V / 4Ω current through 6Ω resistor = 3A**6V**A R 2Ω Question 4 Both resistors will have the same potential difference = 6V For the 2Ω resistor: current = potential difference resistance = 6V / 2Ω = 3A As the total current, measured by the ammeter, is 5A there must be 2A flowing through resistor R. resistance = potential difference current =6V / 2A resistance of R = 3Ω Calculate the resistance of R in the circuit below if the ammeter reads 5A.**Choose appropriate words to fill in the gaps below:**When _________ are connected in parallel to each other they will have the same _________ difference across each of them. The total __________ flowing to a group of conductors connected in __________ to each other will be equal to the ______of the currents flowing through each of the individual components. The ________ current will flow through the component with least ____________. components potential current parallel sum greatest resistance WORD SELECTION: greatest parallel sum potential components current resistance

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