Action and use of circuit components
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Voltage and Resistance Relationship
- For a constant current through a circuit, the p.d. across a conductor increases as its resistance increases
- from Ohm's law, V = IR - if I is constant and R increases, then V must increase too
- This principle is what makes potential dividers work, producing different output voltages from different resistances
The Potential Divider
Extended Only
- A potential divider (or voltage divider) is a circuit that produces an output voltage that is a fraction of the input voltage
- two resistors, R_1 and R_2, are connected in series across the input voltage V_in
- the output voltage V_out is taken across one of the resistors
The input voltage is divided between R₁ and R₂ in proportion to their resistance
- Formula:
- (R_1/R_2) = (V_1/V_2)
- The ratio of the resistances equals the ratio of the p.d.s across them
- the larger resistance always gets the larger share of the voltage
Worked Example: Potential Divider Calculation
Museli connects a 6 V supply across two resistors in series, R_1 = 4 Ω and R_2 = 2 Ω, and wants to find the p.d. across each resistor.
- Step 1: use the ratio. (V_1/V_2) = (R_1/R_2) = (4/2) = 2
- Step 2: so V_1 = 2V_2, and the p.d.s must add up to the supply voltage. V_1 + V_2 = 6 V
- Step 3: substitute and solve. 2V_2 + V_2 = 6 3V_2 = 6 V_2 = 2 V
- Step 4: find V_1. V_1 = 2 × 2 = 4 V
Variable Potential Divider
Extended Only
- A variable potential divider uses a variable resistor (potentiometer) to give a continuously adjustable output voltage
- the input voltage is connected across the whole resistor
- a sliding contact (wiper) moves along the resistor, and the output voltage is taken between one end and the wiper
- moving the wiper changes the output voltage anywhere from 0 to V_in
Sliding the wiper along the potentiometer varies the output voltage continuously
Applications
- Volume controls - adjust the level of an audio signal
- Dimmer switches - control lamp brightness
Sensor Circuits
- Replacing one of the fixed resistors in a potential divider with a thermistor or LDR makes the output voltage respond to temperature or light
- Thermistor (in series with a fixed resistor):
- cold → thermistor resistance is high → larger share of the voltage is across the thermistor
- hot → thermistor resistance is low → smaller share of the voltage is across the thermistor
- LDR (in series with a fixed resistor):
- dark → LDR resistance is high → larger share of the voltage is across the LDR
- bright light → LDR resistance is low → smaller share of the voltage is across the LDR
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