Current Electricity
Q1. Define electric current.
A. Electric current is the rate of flow of electric charges through a conductor. Formula: I = Q / t
Q2. What is the SI unit of electric current?
A. Ampere (A)
Q3. Define 1 ampere.
A. If 1 coulomb of charge flows through a conductor in 1 second, the current is said to be 1 ampere.
Q4. What is the direction of electric current?
A. Conventional current flows from the positive terminal to the negative terminal of a cell.
Q5. State Ohm’s law.
A. At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends. V = I × R
Q6. What is resistance?
A. Resistance is the property of a conductor to oppose the flow of electric current. SI unit: ohm (Ω)
Q7. Define 1 ohm.
A. A conductor has resistance of 1 ohm if 1 ampere current flows through it when the potential difference is 1 volt.
Q8. On what factors does resistance of a conductor depend?
A. Resistance depends on:
1. Length (R ∝ l)
2. Cross-sectional area (R ∝ 1/A)
3. Material
4. Temperature
Q9. Write the formula for resistance in terms of resistivity.
A. R = ρ × (l / A), where ρ is resistivity.
Q10. What is resistivity?
A. Resistivity is the resistance of a conductor of unit length and unit cross-sectional area. SI unit: ohm-metre (Ω·m)
Q11. Define conductance.
A. Conductance is the reciprocal of resistance. G = 1 / R
Q12. What is a series circuit?
A. In a series circuit, components are connected end-to-end, so the same current flows through each component.
Q13. Write the formula for total resistance in series.
A. R_total = R₁ + R₂ + R₃ + …
Q14. What is a parallel circuit?
A. In a parallel circuit, components are connected across the same two points, so the voltage is the same across each branch.
Q15. Write the formula for total resistance in parallel.
A. 1 / R_total = 1 / R₁ + 1 / R₂ + 1 / R₃ + …
Q16. Why is parallel connection preferred in household wiring?
A. Because each appliance gets the same voltage and can be operated independently.
Q17. What is electric power?
A. Electric power is the rate at which electric energy is consumed or produced. P = V × I
Q18. State the SI unit of electric power.
A. Watt (W)
Q19. Define 1 watt.
A. A device has power of 1 watt if it consumes 1 joule of energy per second.
Q20. Write the formula for power in terms of resistance.
A. P = I² × R or P = V² / R
Q21. What is electrical energy?
A. Electrical energy is the work done by an electric current in a given time.
Q22. Write the commercial unit of electrical energy.
A. Kilowatt-hour (kWh)
Q23. Convert 1 kWh to joules.
A. 1 kWh = 1000 × 3600 = 3.6 × 10⁶ joules
Q24. Why is copper used for electrical wiring?
A. Because copper has low resistivity and high conductivity.
Q25. Why is tungsten used in bulb filaments?
A. Because tungsten has high melting point and high resistivity.
Q26. Why are fuse wires made of tin-lead alloy?
A. Because it has low melting point, so it melts easily and breaks the circuit in case of overload.
Q27. What is short circuiting?
A. When live and neutral wires come into direct contact, causing large current flow, it is called short circuiting.
Q28. What is overloading?
A. When current in a circuit exceeds the safe limit due to many appliances being used at once.
Q29. Why are electric switches connected in the live wire?
A. To ensure the circuit is completely cut off from the power supply when switched off.
Q30. State the difference between direct current (DC) and alternating current (AC).
A. In DC, current flows in one direction only; in AC, current changes direction periodically.