# Potential Dividers & Variable Resistance(Edexcel International A Level Physics)

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Joanna

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Physics

## Potential Dividers & Variable Resistance

#### The Potentiometer

• A potentiometer is a single component which can act as a potential divider.
• It consists of a coil of wire with a sliding contact
• A variable output voltage can be varied by moving a slider along the component

A potentiometer is a type of variable resistor

• The circuit symbol is drawn as an arrow next to the resistor, to represent the sliding contact

• The sliding contact has the effect of separating the potentiometer into two parts
• Each part will have different resistances
• Therefore output voltage will change

Moving the slider (the arrow in the diagram) changes the resistance (and hence potential difference) of the two parts of the potentiometer

• If the slider in the above diagram is moved upwards, the resistance of the lower part will increase and so the potential difference across it will also increase
• Therefore, the variable resistor obtains a maximum or minimum value for the output voltage
• If the resistance is 3 Ω:
• Maximum voltage is when the resistance is 3 Ω
• Minimum voltage is when the resistance is 0 Ω

#### Thermistors & Light Dependent Resistors (LDRs)

• Sensory resistors are used in potential dividers to vary the output voltage
• This could cause an external component to switch on or off
• For example, a heater switches off automatically when its surroundings are at room temperature
• Examples of the variable sensory resistors used are thermistors and light-dependent resistors (LDRs)

LDR and thermistor in a potential divider circuit with a fixed resistor R

• The voltmeter in both circuits is measuring Vout
• From Ohm’s law V = IR, the potential difference Vout from a sensory resistor in a potential divider circuit is proportional to its resistance
• If an LDR or thermistor's resistance decreases, the potential difference through it also decreases
• If an LDR or thermistor's resistance increases, the potential difference through it also increases
• Since the total potential difference of the components must be equal to Vin:
• If the potential difference of the sensory resistor decreases then the potential difference across the other resistor in the circuit must increase
• If the potential difference of the sensory resistor increases then the potential difference across the other resistor in the circuit must decrease

The resistance of an LDR...

• Varies with light intensity
• The higher the light intensity, the lower the resistance
• The lower the light intensity, the higher the resistance
• Therefore:
• If light intensity increases, Vout across the LDR will decrease because resistance has decreased
• If light intensity decreases, Vout across the LDR will increase because resistance has increased
• An LDR circuit is often used for street and security lights
• When light intensity falls, Vout increases and so this can provide the voltage required to turn on a lamp

The resistance of a thermistor...

• Varies with temperature
• The hotter the thermistor, the lower the resistance
• The cooler the thermistor, the higher the resistance
• Therefore:
• If temperature increases, Vout across the thermistor will decrease because resistance has decreased
• If temperature decreases, Vout across the thermistor will increase because resistance has increased
• A thermistor circuit is used in fire alarms, ovens and digital thermometers
• When temperature falls, Vout increases and so this can provide the voltage required to turn on a heater

#### Worked example

A potential divider consists of a fixed resistor R and a thermistor connected in series.Which row of the table describes what happens to the potential difference across resistor R and the thermistor when the temperature of the thermistor decreases?

Step 1: Consider Ohm's Law

• Due to Ohm’s Law (V = IR), both the resistor and thermistor are connected in series and have the same current I
• If resistance R increases, the potential difference across the thermistor also increases

Step 2: Consider the electrical voltages rule

• In series, the input potential difference is shared equally amongst the components due to the electrical voltages rule
• Therefore, since the potential difference across the thermistor increases, the potential difference across the resistance R must decrease

• This is row D

#### Exam Tip

A potentiometer can also be used as a variable resistor if only two of the three possible terminals are used.

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### Author:Joanna

Joanna obtained her undergraduate degree in Natural Sciences from Cambridge University and completed her MSc in Education at Loughborough University. After a decade of teaching and leading the physics department in a high-performing academic school, Joanna now mentors new teachers and is currently studying part-time for her PhD at Leicester University. Her passions are helping students and learning about cool physics, so creating brilliant resources to help with exam preparation is her dream job!