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1 of 40 © Boardworks Ltd 2009 2 of 40 © Boardworks Ltd 2009 Controlling current and voltage 3 of 40 © Boardworks Ltd 2009 Resistance revision 4 of 40 © Boardworks Ltd 2009 Variable resistors A variable resistor, also known as a rheostat, allows the resistance of a circuit to be varied. slider thick bar coil variable resistor variable resistor symbol A variable resistor has two potential paths for current: one along a short, thick bar; another along a thin long coil. The slider is a mobile point of contact between these two routes, and its position determines the path of the current. 5 of 40 © Boardworks Ltd 2009 How do variable resistors work? 6 of 40 © Boardworks Ltd 2009 Ohm’s Law Ohm’s Law links current, voltage and resistance: voltage (V) = current (I) × resistance (R) Volts (V) Amps (A) Ohms (Ω) Ohm’s Law explains why resistance helps to control the current and voltage in a circuit. Any changes in resistance will have a knock-on effect on both the current and voltage. A formula triangle can be used to rearrange this equation. × 7 of 40 © Boardworks Ltd 2009 Ohm’s Law practice questions A filament lamp has a current of 20 A running through it, with a potential difference of 100 V across it. What is the resistance of the filament in the bulb? V 100 V V=I×R R= = = 5Ω I 20 A Calculate the current flowing through a 230 V kettle element which has a resistance of 57.5 Ω. V 230 V V=I×R I= = = 4A R 57.5 Ω 8 of 40 © Boardworks Ltd 2009 Voltage–current graphs are a simple plot of voltage, on the x-axis, against current, on the y-axis. Ohm’s Law tells us that the gradient of a V–I graph can be used to calculate resistance: change in current gradient = change in voltage current (A) Voltage–current graphs voltage (V) voltage resistance = current 1 Therefore: resistance = gradient with these axes. Voltage–current graphs can vary greatly in form depending on the properties of the substance conducting electricity. 9 of 40 © Boardworks Ltd 2009 Calculating resistance from line graphs Calculate the resistance of these copper and nichrome wires. copper current (A) 4 4 2 2 0 0 0 5 10 15 voltage (V) 1 resistance = gradient copper: 20 nichrome 0 10 15 voltage (V) 20 change in current gradient = change in voltage gradient = 2 ÷ 5 = 0.4 nichrome: gradient = 2 ÷ 10 = 0.2 10 of 40 5 R = 1 ÷ 0.4 = 2.5 Ω R = 1 ÷ 0.2 = 5 Ω © Boardworks Ltd 2009 Different types of V–I graph Such variation in resistance leads to a curved V–I graph. A light bulb has a curved graph: it warms up as more electricity passes through it, increasing resistance. current (A) While a resistor produces a constant resistance, and thus a straight line graph, other components show a variation in resistance at different levels of current and voltage. voltage (V) Such components are non-Ohmic – they do not obey Ohm’s Law. 11 of 40 © Boardworks Ltd 2009 V–I graphs for diodes A diode is a component that stops current flowing in one direction, but allows it to flow readily in the other, providing it is over a certain voltage. current (A) What would a V–I graph for a diode look like? voltage (V) 12 of 40 © Boardworks Ltd 2009 V–I graphs for different components 13 of 40 © Boardworks Ltd 2009 Calculating resistance from curves 14 of 40 © Boardworks Ltd 2009 Ohm’s Law summary 15 of 40 © Boardworks Ltd 2009 16 of 40 © Boardworks Ltd 2009 Understanding voltage Voltage is an electrical pushing force. The voltage of a cell describes how much electrical potential energy it gives the electrons: this pushes them around a circuit. When voltage is measured across a component it records the difference in electrical potential energy between the two sides of the component. This is also known as the potential difference. Thus the voltmeter reading of 4 V tells us that there is 4 V more electrical potential energy on one side of the resistor than the other. 17 of 40 4.0 © Boardworks Ltd 2009 Controlling voltage Imagine your alarm clock’s battery is flat. It requires 4 V to run successfully, but you only have a 6 V battery. This will overload its circuitry. However, you do have a selection of fixed resistors. How can these resistors help you to run the alarm clock from the battery without damaging it? 18 of 40 © Boardworks Ltd 2009 Series resistors and potential difference If two resistors are connected in series with a power supply, then the voltage is shared out between them. 6V 2.0 4.0 10Ω 20Ω The voltage is divided between components in proportion to their resistance. Thus the larger resistor has a larger share of the power supply voltage. 19 of 40 © Boardworks Ltd 2009 What is a potential divider? VIN This principal can be used to power the alarm clock. 6V The clock itself has a resistance of 20 Ω. When placed in series with a 10 Ω resistor, the battery’s voltage is split between the resistor and clock in a 2:1 ratio. 2V 4V 10 Ω 20 Ω The voltage across the resistor is 2 V, while the voltage across the clock is 4 V. The clock can now run safely. A circuit that splits the voltage between components, to produce a specific output voltage, is a potential divider. 20 of 40 © Boardworks Ltd 2009 Drawing potential dividers A potential divider uses series resistance to produce an output voltage (VOUT) that differs to the input voltage (VIN). Potential dividers are drawn in a slightly different way to other circuits. V IN The distance between the horizontal lines represents the potential difference between different parts of the circuit. This arrangement is designed to visually demonstrate the change in potential difference across the resistors. 6V R1 10 Ω VOUT 4V R2 0V 20 Ω 0V potential divider diagram 21 of 40 © Boardworks Ltd 2009 Fixed output potential dividers 22 of 40 © Boardworks Ltd 2009 The potential divider equation The output voltage (VOUT) of a potential divider depends on the size of the resistors, and also the input voltage (VIN). VOUT can be calculated using the potential divider equation: VIN R2 VOUT = VIN × (R + R ) 1 2 R1 VOUT VOUT and VIN are measured in volts (V). R2 R1 and R2 are measured in ohms (Ω). 0V 23 of 40 0V © Boardworks Ltd 2009 Potential divider questions Calculate the output voltage, VOUT, for this potential divider. R2 VOUT = VIN × (R1 + R2) 10 V R1 60 = 10 × 15 + 60 VOUT 60 = 10 × 75 = 10 × 0.8 15Ω R2 0V 60 Ω 0V = 8V 24 of 40 © Boardworks Ltd 2009 Potential divider questions Calculate the output voltage, VOUT, for this potential divider. VOUT = VIN × R2 (R1 + R2) 10V R1 300 = 10 × 75 + 300 VOUT 300 = 10 × 375 = 10 × 0.8 75 Ω R2 0V 300 Ω 0V = 8V 25 of 40 © Boardworks Ltd 2009 Variable resistors in potential dividers 26 of 40 © Boardworks Ltd 2009 Potential dividers with a variable output If a variable resistor is used in a potential divider, VOUT becomes variable. If R1 is a variable resistor… VIN R1 VOUT is low when the resistance of R1 is high. VOUT R2 0V 0V R1 has a high proportion of the resistance, and thus a high proportion of the voltage. 27 of 40 © Boardworks Ltd 2009 Potential dividers with a variable output What happens when R2 is a variable resistor? VIN R1 In this arrangement, the relationship between the resistance of the variable resistor and VOUT inverts. VOUT R2 0V 0V VOUT is high when resistance of R2 is high. R2 has a high proportion of the resistance and thus a high proportion of the voltage is at VOUT. 28 of 40 © Boardworks Ltd 2009 Potential divider summary 29 of 40 © Boardworks Ltd 2009 30 of 40 © Boardworks Ltd 2009 Semiconductors A semiconductor is a material which has electrical properties somewhere between an insulator, such as wood, and a conductor, such as iron. Semiconductors are usually made from silicon. Uses for semiconductors include: computer processor light dependent resistor light emitting diode Some semiconductors are able to vary their conductivity in response to changes in temperature or light intensity. 31 of 40 © Boardworks Ltd 2009 LDRs: light and resistance The resistance of a light dependent resistor (LDR) is not fixed. It is dependent on the intensity of incident light. resistance (k) An LDR has a high resistance in the dark but a low resistance in the light. light intensity 32 of 40 LDR symbol The graph shows how the resistance of an LDR decreases as the light intensity increases. This means that LDRs can be used in light sensing circuits, because their output is dependent on the light conditions. © Boardworks Ltd 2009 Thermistors: temperature and resistance The resistance of a thermistor varies depending on temperature. resistance (k) It has a high resistance when cold but a low resistance when hot. temperature (°C) 33 of 40 thermistor symbol This is unusual, as resistance normally increases with increasing temperature. Thermistors are useful in the sensor circuits of a thermostat, as their output varies with temperature fluctuations. © Boardworks Ltd 2009 How do semiconductors work? 34 of 40 © Boardworks Ltd 2009 Semiconductors as sensors 10 V The combination of a potential divider and a thermistor creates a temperature sensor. The thermistor’s resistance will vary with temperature, resulting in a VOUT that is temperature dependent. R1 VOUT R2 0V thermistor 0V To produce a light sensor, replace the thermistor with an LDR. If the thermistor is in the R2 position, VOUT will be high at low temperatures, as the thermistor’s resistance will be high relative to R1. How will VOUT change if the thermistor is in the R1 position? 35 of 40 © Boardworks Ltd 2009 Comparing conductors and semiconductors 36 of 40 © Boardworks Ltd 2009 37 of 40 © Boardworks Ltd 2009 Glossary 38 of 40 © Boardworks Ltd 2009 Anagrams 39 of 40 © Boardworks Ltd 2009 Controlling current and voltage quiz 40 of 40 © Boardworks Ltd 2009