A potentiometer is essentially a resistor with three contacts, one of which is m

Question

A potentiometer is essentially a resistor with three contacts, one of which is mobile. It acts as a variable resistor, since changing the position of the mobile contact called the "wiper" (denoted by an arrow) changes the amount of the resistor through which current must pass. The circuit to the right consists of one 71.5 V battery, one fixed resistor, and one potentiometer. In addition, one of the wires is grounded (at zero potential). 2380 The resistive material inside the potentiometer has a resistance of 375 Ohms per millimeter. To what distance x should the wiper be moved so that 14.8 milliamps of current pass through point B? Number 1.75 With the wiper in this position, what is the potential at point B? Number

Explanation / Answer

i)

What is the total circuit resistance that results in a current of 14.8 mA when a voltage of 71.5 volts is applied across it?

Ohm's Law gives us:
r = v/i = (71.5 volts / 0.0148 amps) = 4831.1 ohms

This is made up of a fixed resistor of 2380 ohms in series with the unknown resistance of the potentiometer (Rp).

Resistances in series just add. So 2380 + Rp = 4831.1
Potentiometer resistance = (4831.1 - 2380) = 2451.1 ohms

(2451.1 ohms / 375 ohms/mm) = 6.54 mm

ii)

What is the potential difference across a 2380 ohm resistor when 14.8 mA flows through it?

Ohm again: v = i * r = (0.0148 amp * 2380 ohms) = 35.224 volts


The top end of the fixed resistor is grounded, so it is at a potential of 0 V.
It is also shown as connected to the positive terminal of the battery.

So if the p.d. across the fixed resistor is 35 V, point B must be at a potential of - 35 volts.

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