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Transcript
REVISION NOTES OHM’S LAW NATIONAL 5 A water pump can push water through a pipe. If the pipe is large, the water flows easily and quickly. If the pipe is narrow, it is harder to push the water along the pipe. In the same sort of way, a battery pushes electric charge around a circuit. We can measure how easily the charge moves round by the idea of resistance. The higher the resistance, the more difficult it is to get charge to flow. This is summed up by Ohm’s Law: V=IxR V = voltage (potential difference) I = current R = resistance Resistance is measured in ohms (). It is quite common to use kilo-ohms (k) and megaohms (M). Resistance measures how much a circuit opposes the current. Example A current of 200 mA passes through a 500 resistor. Calculate the potential difference (voltage) across the resistor. I = 200 mA = 0.2 A R = 500 W V=? V = IR = 0.2 x 500 Confirming Ohm’s Law This circuit is used to measure the current in a resistor at different voltages. If we plot a graph of voltage (p.d.) against current, we get something like this: The gradient of this graph gives the value of the resistance (as R = V/I) = 100 V REVISION NOTES OHM’S LAW NATIONAL 5 Resistance and temperature Many components that we put in circuits are ‘ohmic’ – this means that they obey Ohm’s Law: V = IR However, some things are ‘non-ohmic’ – they do not obey Ohm’s Law. A good example is filament of an old-fashioned light bulb. When the bulb is switched on, a current starts to pass through the filament. This heats up the filament. As the temperature of the filament increases, its resistance also increases. This graph on the left shows an ohmic component, like an ordinary resistor. The voltage is directly proportional to the current. The gradient of the graph gives the resistance. The gradient doesn’t change, so the resistance doesn’t change. The graph on the right shows a non-ohmic component, like a bulb filament. See how the graph gets steeper as the current increases and the filament gets hotter. The steeper the gradient, the higher the resistance. Some components behave in the opposite way. The resistance of a thermistor reduces as the thermistor gets hotter. This graph shows how the temperature of a thermistor affects its resistance.