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 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

• 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 decreased
• 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 decreased
• 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

• 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

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