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How We Load Power Supplies

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All sources have an EMF. EMF is the open terminal voltage of the battery. All sources have a certain amount of internal resistance. Perfect batteries have 0 internal resistance. A car battery has very low internal resistance so is almost perfect

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The car battery has a very low internal resistance. This means that it can give out the heavy current needed by a starter motor. You can see the heavy wires leading to the starter motor

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V E V V R I This time we find that the terminal voltage goes down to V. It has been lost due to the internal resistance which heats the battery up. Emf = Useful volts + Lost volts In code: E = V + v

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V E R I r The cell is now a perfect battery in series with an internal resistor, r. Internal resistance

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o Emf = voltage across R + voltage across the internal resistance E = V +v o We also know from Ohms Law that V = IR and v = Ir, so we can write: E = IR + Ir E = I(R + r) or E = V + Ir

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V E R I r o We adjust the variable resistor so we can record a range of voltages and currents. o We use the switch to avoid flattening the battery, and preventing the variable resistor from getting too hot. o We plot the results on a graph.

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Current (A) P.d. (V) The graph is a straight line, of the form y = mx + c. We can make the equation for internal resistance V = -rI + E. There are three features on the graph that are useful: The intercept on the y-axis tells as the emf. The intercept on the x- axis tells us the maximum current the cell can deliver when the p.d. is zero, i.e. a dead short circuit. The negative gradient tells us the internal resistance.

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