Used normally, an oscilloscope plots a graph of voltage (y-axis) against time (x-axis). EMF and internal resistance (4). Familiarity with the oscilloscope so that they can get a stable trace and adjust settings. 16-19. The starting point for the theory can be either Kirchhoff’s second law or conservation of energy in the circuit (the same thing really) but a general discussion based on the circuit diagram below should use a variety of approaches. The same power would be delivered by a DC with values I and V if: This links the DC equivalent values to AC peak values. Teaching Guidance and point out that both I and V change sign together, so power is always positive but varies over the cycle, having a maximum value of IpVp and a minimum of zero. The Institute is a charity registered in England and Wales (no. 293851) and Scotland (no. If the terminals are shorted (e.g. Student experiment: Measuring internal resistance and EMF (45 minutes) Student questions: Practice questions (30 minutes) Discussion: More about the practical importance of internal resistance (10 minutes) Discussion: Deriving an equation. It’s better to turn the headlamps off when starting the car. Or will you have to include one or two CRO-based experiments in a circus? Since the starter motor has a low resistance it demands a very high current (say 60 A). It is usually 5 M Ω . They could also measure the current drawn from the cell, although this is less important. The terminal voltage of the cell is equal to only its EMF when no current is drawn (no lamps attached). Once students have completed this (and earlier) electrical topics, they should have a clear understanding of electric circuits. The point is that a sinusoidal AC supply of peak value Vp delivers the same average power as steady DC of value Vp√ 2 . Start by looking at the available CROs. Remind them of the internal resistance r. This limits the cell to a maximum (short-circuit) current of: We can use this to prevent EHT supplies giving the user an unpleasant shock. This should also make it clear that the average power is less than IpVp . This resistor can be by-passed when necessary. Students learn to use an oscilloscope to measure voltages. A typical EHT supply (up to 5000 V) is protected by a 5 M Ω  resistor so the maximum current if shorted is just 1 mA. The simplest is to measure terminal voltage (V) and current (I) and to plot V against I. There are two experiments here, in which students determine the EMF E and internal resistance r of cells – one involving a potato cell (leading to a high internal resistance) and one involving a normal C cell (much lower internal resistance). One way to deal with this is to connect a large resistance in series with the output (positive) terminal. Episode 121-1: Internal resistance of power supplies (Word, 30 KB). There are two experiments here, in which students determine the EMF E and internal resistance r of cells – one involving a potato cell (leading to a high internal resistance) and one involving a normal C cell (much lower internal resistance).

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