Retrieval Practice
Circuits & Potential Dividers — AQA A-Level Physics
Q1. State Kirchhoff's first law and the conservation law it follows from.
- At any junction, the total current entering equals the total current leaving.
- It follows from the conservation of charge.
Q2. State Kirchhoff's second law and the conservation law it follows from.
- Around any closed loop, the sum of the e.m.f.s equals the sum of the p.d.s.
- It follows from the conservation of energy.
Q3. Write down three equations for electrical power.
P = IV, P = \(I^{2}\) R, P = \(V^{2}\) / R.
Q4. What is the equation for total resistance of resistors in series?
- R_total = R_1 + R_2 + R_3.
- Total resistance is always greater than the largest individual resistor.
Q5. What is the equation for total resistance of resistors in parallel?
- 1/R_total = 1/R_1 + 1/R_2 + 1/R_3.
- Total resistance is always less than the smallest individual resistor.
Q6. Write the potential divider equation and state what each symbol represents.
- V_out = (R_2 / (R_1 + R_2)) x V_in.
- V_out is the output p.d. across R_2, V_in is the supply voltage, R_1 and R_2 are the two series resistors.
Q7. In a potential divider with an NTC thermistor as R_2, what happens to V_out across R_2 when temperature increases?
- V_out decreases.
- As temperature rises, thermistor resistance falls, so it takes a smaller share of V_in.
Q8. In a potential divider with an LDR as R_2, what happens to V_out across R_2 when light intensity increases?
- V_out decreases.
- As light intensity rises, LDR resistance falls, so it takes a smaller share of V_in.
Q9. How does a potentiometer differ from a fixed potential divider?
- A potentiometer uses a single variable resistor with a sliding contact.
- It can vary V_out continuously from 0 V to V_in, whereas a fixed potential divider gives only one output voltage.
Q10. Convert 1 kWh to joules and state what a kWh measures.
- 1 kWh = \(3.6 \times 10^{6}\) J.
- A kilowatt-hour is a unit of energy equal to the energy transferred by a 1 kW device in 1 hour.