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EMF and Internal Resistance

How We Load Power Supplies. EMF and Internal Resistance. Key Ideas. 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.

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EMF and Internal Resistance

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  1. How We Load Power Supplies EMF and Internal Resistance

  2. Key Ideas • 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

  3. Sources of Low Internal Resistance 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

  4. E V V I V R Internal Resistance 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

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

  6. Emf = voltage across R + voltage across the internal resistance • E= V + v • We also know from Ohm’s Law that V = IR and v = Ir, so we can write: • E = IR + Ir • E = I(R + r) or E = V + Ir

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

  8. P.d. (V) Current (A) 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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