Free Steady Current MCQs with Answers

1. Electric current is defined as

• A. the charge flowing per unit time past a point in the circuit
• B. the energy delivered per unit charge
• C. the resistance per unit length
• D. the potential difference per unit resistance only

Explanation: One ampere is one coulomb per second, so current measures rate of flow of charge. Energy per unit charge is potential difference, measured in volts, and confusing the two is the commonest error in this chapter. Conventional current is taken as the direction positive charge would move, which is opposite to the actual drift of electrons in a metal.

2. Ohm's law states that, at constant temperature, the current through a conductor is

• A. inversely proportional to the potential difference across it
• B. directly proportional to the potential difference across it
• C. independent of the potential difference
• D. proportional to the square of the potential difference

Explanation: A graph of current against voltage for an ohmic conductor is a straight line through the origin whose gradient is the reciprocal of the resistance. The condition of constant temperature matters, since a filament lamp heats up as the current rises and its graph curves. Semiconductor diodes are non ohmic for a different reason, conducting in one direction only.

3. Household appliances are connected in parallel rather than in series so that

• A. each receives the full mains voltage and can be switched independently
• B. the total resistance is increased
• C. the current through each is the same
• D. less current is drawn overall

Explanation: In parallel every appliance sees the same 220 V and switching one off leaves the others unaffected, whereas a series chain would fail entirely if one element broke and would share the voltage between them. The trade off is that the total current rises as more appliances are switched on, which is what the fuse or breaker guards against. Parallel connection lowers the total resistance rather than raising it.

4. A fuse in a circuit is a

• A. thick wire of low melting point placed in parallel with the appliance
• B. thin wire of low melting point placed in series with the appliance
• C. high resistance placed in parallel with the supply
• D. device that raises the current to a safe value

Explanation: Being in series, the whole current passes through it, and being thin with a low melting point it melts and breaks the circuit if that current exceeds the rated value. It must be fitted in the live wire so that the appliance is isolated from the supply when it blows. A fuse in parallel would carry no current and protect nothing.

5. Kirchhoff's first law, applied at a junction, is a statement of the conservation of

• A. energy
• B. charge
• C. momentum
• D. resistance

Explanation: The total current entering a junction equals the total leaving, because charge cannot accumulate at a point in a steady circuit. The second law, that the sum of the emfs around a loop equals the sum of the potential drops, expresses conservation of energy instead. Together they solve any network that series and parallel rules cannot.

6. An ammeter must be connected in series and should have

• A. very high resistance
• B. very low resistance, so it hardly alters the current it measures
• C. infinite resistance
• D. the same resistance as the circuit

Explanation: Since the whole current flows through it, any appreciable resistance would reduce that current and falsify the reading, so an ideal ammeter has zero resistance. A voltmeter is the opposite case: connected in parallel, it should have very high resistance so it draws almost no current from the branch it is measuring. Connecting an ammeter in parallel across a supply is close to a short circuit and can destroy it.

7. In a metal, the drift velocity of the electrons that make up a current of a few amperes is typically

• A. close to the speed of light
• B. about 1000 m per second
• C. of the order of a millimetre per second
• D. exactly zero

Explanation: Electrons drift extremely slowly because there are so many of them, roughly 10 to the 28 per cubic metre, so a small average velocity still delivers a large current. A lamp lights instantly not because the electrons travel fast but because the electric field is established along the wire at nearly the speed of light and sets them all moving at once. Their random thermal speeds are far higher than the drift speed.