AQA A Level Physics复习笔记5.3.4 Potential Divider Circuits

• When two resistors are connected in series, through Kirchhoff’s Second Law, the potential difference across the power source is divided between them
• Potential dividers are circuits which produce an output voltage as a fraction of its input voltage
• Potential dividers have two main purposes:
  • To provide a variable potential difference
  • To enable a specific potential difference to be chosen
  • To split the potential difference of a power source between two or more components

   

• Potential dividers are used widely in volume controls and sensory circuits using LDRs and thermistors
• Potential divider circuits are based on the ratio of voltage between components. This is equal to the ratio of the resistances of the resistors in the diagram below, giving the following equation:

Potential divider diagram and equation

• The input voltage Vin is applied to the top and bottom of the series resistors
• The output voltage Vout is measured from the centre to the bottom of resistor R2
• The potential difference V across each resistor depends upon its resistance R:
  • The resistor with the largest resistance will have a greater potential difference than the other one from V = IR
  • If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share

   

• In potential divider circuits, the p.d across a component is proportional to its resistance from V = IR

• 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

   

• Sensory resistors used are Light Dependent Resistors (LDRs) and thermistors

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

• The voltmeter in both circuits is measuring Vout
• Recall that 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
• 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
• From Ohm’s law V = IR, the potential difference Vout from a 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 p.d of the components must be equal to Vin, if the p.d of the sensory resistor decreases then the p.d of the other resistor in the circuit must increase and vice versa

ANSWER:  D

• 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
• In series, the potential difference is shared equally amongst the components. Their sum equals the e.m.f of the supply (Kirchhoff’s second law)
• If the potential difference across the thermistor increases, the potential difference across the resistance R must decreases, to keep the same overall total e.m.f
• This is row D