2.45 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 and Light Dependent Resistors (LDRs) in Potential Divider Circuits

• 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 switching 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 V_out

• From Ohm’s law V = IR, the potential difference V_out 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

• 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, V_out across the LDR will decrease because resistance has decreased
  • If light intensity decreases, V_out across the LDR will increase because resistance has decreased
• An LDR circuit is often used for street and security lights
  • When light intensity falls, V_out 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, V_out across the thermistor will decrease because resistance has decreased
  • If temperature decreases, V_out across the thermistor will increase because resistance has decreased
• A thermistor circuit is used in fire alarms, ovens and digital thermometers
  • When temperature falls, V_out increases and so this can provide the voltage required to turn on a heater

• Since the total potential difference of the components must be equal to V_in:
  • 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 

ANSWER:  D

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

   Step 3: State the answer

• • This is row D