period 2

Open Ended

How can we get an output of 3.0V from a battery of e.m.f of 6V?

• When two resistors are connected in series, the potential difference across the power source is shared between them

• A potential divider is a circuit which splits potential difference from a power source, so only a fraction goes to a component

Potential divider

where
R2: the resistance of the component over which the output is taken
R1: the second component divider
Vin: the p.d across the two component

Potential divider equation

Open Ended

Determine the range of Vout for the circuit in the figure as the variable resistor R2 is adjusted over its full range from 0 Ω to 40 Ω (assume the supply of e.m.f. 10 V has negligible internal resistance

• Variable and sensory resistors are used in potential dividers to vary the output voltage

• This could cause an external component to switch on or off e.g. a heater switching off automatically when its surroundings are at room temperature

Variable resistance components

- The resistance of an LDR varies with light intensity:The higher the light intensity, the lower the resistance and vice versa

- An LDR circuit is often used for street and security lights

LDR and thermistors in a circuit

- The resistance of a thermistor varies with temperature: The hotter the thermistor, the lower the resistance and vice versa

- A thermistor circuit is used in fire alarms, ovens and digital thermometers

The potential difference Vout from a resistor in a potential divider circuit is proportional to its resistance

Multiple Choice

What happens to the p.d through resistor R and the thermistor when the temperature of the thermistor decreases?

• A

• B

• C

• D

Open Ended

Calculate Vout when the light intensity is 60lux