Learning objectives
- Use Ohm’s law in numerical problems.
- Compare series and parallel connections.
- Calculate electrical power and energy.
- Explain common household circuit choices.
Current, voltage and resistance
Electric current is the rate of flow of charge. Its SI unit is the ampere. Potential difference is the work done per unit charge and is measured in volts.
Resistance opposes current. For many conductors at constant temperature, Ohm’s law states that potential difference is proportional to current: V = IR.
Series and parallel circuits
In series, the same current flows through each component and the total resistance is the sum of individual resistances. If one component breaks, the whole path is interrupted.
In parallel, each branch has the same potential difference. Household appliances are connected in parallel so they can operate independently and receive the rated voltage.
Power and energy
Electrical power is the rate at which electrical energy is converted. P = VI, and equivalent forms include P = I²R and P = V²/R.
Electrical energy equals power multiplied by time. The commercial unit is the kilowatt-hour. One kilowatt-hour is the energy used by a 1 kW device operating for one hour.
Practice questions with explanations
Try each question before opening the answer. The explanation shows the reasoning, not only the final response.
Q1. What is electric current?
Answer: The rate of flow of electric charge.
Explanation: Current equals charge divided by time.
Q2. What is the SI unit of resistance?
Answer: The ohm (Ω).
Explanation: One ohm allows one ampere when one volt is applied.
Q3. State Ohm’s law.
Answer: At constant temperature, V = IR.
Explanation: Potential difference is proportional to current for an ohmic conductor.
Q4. Find current when V = 12 V and R = 4 Ω.
Answer: 3 A.
Explanation: I = V/R = 12/4.
Q5. Find resistance when V = 9 V and I = 0.5 A.
Answer: 18 Ω.
Explanation: R = V/I = 9/0.5.
Q6. What is the equivalent resistance of 2 Ω and 3 Ω in series?
Answer: 5 Ω.
Explanation: Series resistances add directly.
Q7. Why are household appliances connected in parallel?
Answer: They receive the same voltage and operate independently.
Explanation: Switching off one branch does not open the others.
Q8. Calculate power for a 230 V appliance drawing 2 A.
Answer: 460 W.
Explanation: P = VI = 230 × 2.
Q9. How much energy does a 1.5 kW heater use in 2 hours?
Answer: 3 kWh.
Explanation: Energy = power × time = 1.5 × 2.
Q10. Why does a fuse wire melt during excessive current?
Answer: Heating increases as current rises.
Explanation: The thin fuse wire melts and opens the circuit before damage occurs.