Potential Dividers

Question 1

What does a potential divider consist of?

Answer 1

A potential divider consist of 2 or more resistors in series with each other and with a source of fixed potential difference. The pd of the source is divided between the components in the circuit, as they are in series with each other.

Question 2

A potential divider can be used:

Answer 2

• To supply a pd which is fixed at any values between 0 and the source pd.
• To supply a variable pd.
• To supply a pd that varies with a physical condition such as temperature or pressure.

Question 3

The equation V₁/V₂ = R₁/R₂ shows that:

Answer 3

The ratio of the pds across each resistor is equal to the resistance ratio of the two resistors.

Question 4

Current through the resistors

Answer 4

V₀/R₁+R₂

Question 5

Pd across resistor 1

Answer 5

V₁ = IR₁ = V₀R₁ / R₁+R₂

Question 6

Pd across resistor 2

Answer 6

V₂ = IR₂ = V₀R₂ / R₁+R₂

Question 7

How to supply a variable pd?

Answer 7

1. The source pd is connected to a fixed length of uniform resistance wire.
2. A sliding contact on the wire can then be moved along the wire, giving a variable pd between the contact one end of the wire.
3. A uniform track of a suitable material may be used instead of a resistance wire. The track maybe linear or circular.

Question 8

A variable potential divider can be used: as a simple audio volume control to change the loudness of the sound from a loudspeaker.

Answer 8

The audio signal pd is supplied to the potential divider in place of a cell or battery. The variable output pd from the potential divider is supplied to the loudspeaker.

Question 9

A variable potential divider can be used: to vary the brightness of a light bulb between zero and normal brightness.

Answer 9

In contrast with using a variable resistor in series with the light bulb and the source pd, the use of potential divider enables the current through the light bulb to be reduced to zero. If a variable resistor in series with the light bulb had been used, there would be a current through the light bulb when the variable resistor is at maximum resistance.

Question 10

What does a temperature sensor consist of?

Answer 10

A potential divider made using a thermistor and a variable resistor.

Question 11

When the temperature of the thermistor is constant:

Answer 11

The source pd is divided between the thermistor and the variable resistor. By adjusting the variable resistor, the pd across the thermistor can then be set at any desired value.

Question 12

When the temperature of the thermistor changes:

Answer 12

• Its resistance changes so the pd across it changes. E.g. if the variable resisor adjusted so the pd across the thermistor at 20 is exactly half the source pd.
• If the temperature is then raised, its resistance falls, so the pd across it falls.

Question 13

A light sensor uses a:

Answer 13

Light dependent resistor and a variable resistor.

Question 14

The pd across the LDR changes when the incident light intensity changes.

Answer 14

• If intensity increases, the resistance of the LDR falls and the pd across the LDR falls.
• If intensity decreases, the resistance of the LDR rises and the pd across the LDR rises.