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Illustration 30.3: Current and Voltage Dividers



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This Illustration shows two different configurations of resistors connected to a battery (voltage is given in volts, current is given in amperes, and resistance is given in ohms). Restart.

Start with the voltage divider animation. The circuit shows an ideal battery supplying current to a 100-Ω resistor in series with a variable resistor, RA. When the resistance of RA is equal to 100 Ω (the value of the top resistor), the voltage is equally divided across the two resistors. As you increase RA, what happens? What happens as you decrease RA? The current from the battery also changes in this process; however, notice that the current through the top resistor and RA are always equal. This is because the current through the top resistor must also go through RA.

Now try the current divider animation. The 100-Ω resistor is now in parallel with RA. When the resistance of RA is equal to 100 Ω (the value of the fixed resistor), the current is equally divided between the two branches of the circuit. As you increase RA, what happens? What happens as you decrease RA? The current from the battery also changes in this process, but the voltage across the two resistors is the same because we have assumed that the battery is capable of supplying a large amount of current and because the wires are assumed to have negligible resistance. If you added a third resistor in parallel with the other two, the current from the battery would increase since the battery needs to supply current to that resistor (with the same voltage drop) as well.

Many students think that if you add additional resistors to circuits such as the ones just seen, the current from the battery must decrease (for the voltage to stay the same). Notice, however, that resistors added in parallel increase the total current from the battery, while resistors added in series reduce the total current from the battery.

Illustration authored by Anne J. Cox.

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