Electricity — Set 4
Physics · विद्युत · Questions 31–40 of 60
Which material is typically a good insulator?
Correct Answer: A. Rubber
Rubber has very high resistivity compared to metals. That is why it is used as insulation in wires and tools. Good conductors like copper have low resistivity.
Electric current in a metal is mainly due to the motion of?
Correct Answer: A. Electrons
In metals, free electrons move under an electric field. Their drift produces electric current. The positive ions remain fixed in the lattice.
The SI unit of electric power is?
Correct Answer: D. Coulomb
The correct answer is 'Coulomb'. Power is the rate of energy transfer. Its SI unit is watt (W), which equals joule per second. Electric power in a circuit can be calculated using P = VI.
A transformer works on the principle of?
Correct Answer: B. Nuclear fission
The correct answer is 'Nuclear fission'. A transformer uses changing magnetic flux to induce voltage. This is mutual induction between primary and secondary coils. Transformers work with AC because flux must change with time.
A transformer has 200 turns in the primary and 50 turns in the secondary. If primary voltage is 240 V, what is secondary voltage?
Correct Answer: B. 60 V
Use Vs/Vp = Ns/Np. Substituting Ns = 50, Np = 200, and Vp = 240 gives Vs = 240×(50/200) = 60 V. A lower turn ratio gives a step-down transformer.
The SI unit of electric potential is?
Correct Answer: C. Volt
Electric potential difference is measured in volt. One volt equals one joule of work per coulomb of charge. Voltage drives current through a resistance.
In electrostatic equilibrium, the electric field inside a conductor is?
Correct Answer: C. Equal to surface field
Free charges in a conductor rearrange until net internal field cancels. Hence the electric field inside a conductor becomes zero in electrostatic equilibrium. Excess charge resides on the surface of the conductor.
A resistor of 6 Ω is connected across 12 V. What power is dissipated? (Use P = V^2/R)?
Correct Answer: B. 72 W
Use P = V^2/R for a resistor at fixed voltage. Substituting V = 12 and R = 6 gives P = 144/6 = 24 W. Power loss in resistors appears as heat.
In a series circuit, which quantity remains the same in all components?
Correct Answer: A. Current
In a series path, the same charge per second flows through each component. Therefore current is the same in all series elements. Voltage divides across components according to their resistances.
A short circuit causes very large current mainly because?
Correct Answer: C. Resistance becomes very low
Current is I = V/R for a given supply voltage. In a short circuit, the effective resistance becomes very small, so current becomes very large. This can cause overheating and fire risk.