Free Current Electricity 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. The resistance of a uniform wire is

• A. proportional to its length and to its cross sectional area
• B. proportional to its length and inversely proportional to its cross sectional area
• C. independent of its dimensions
• D. inversely proportional to its length

Explanation: R equals rho L over A, so a longer wire offers more resistance and a thicker one less, which is why thick cables are used for heavy currents. The constant rho is the resistivity, a property of the material rather than of the sample. Doubling the diameter quarters the resistance, because area depends on the square of the radius.

4. A wire is stretched so that its length doubles while its volume stays constant. Its resistance becomes

• A. double
• B. half
• C. four times
• D. unchanged

Explanation: Constant volume means doubling the length halves the cross sectional area, and since resistance is proportional to length over area, the effect is two multiplied by two, giving four times the original. Answering double accounts for the length change only. This is a favourite examination trick precisely because the area change is easy to overlook.

5. The SI unit of resistivity is

• A. ohm
• B. ohm metre
• C. ohm per metre
• D. volt per ampere

Explanation: Rearranging R equals rho L over A gives rho equal to RA over L, whose units are ohm times metre squared divided by metre, that is ohm metre. The plain ohm, equivalently the volt per ampere, is the unit of resistance itself. Resistivity is a material property, so copper has the same resistivity whether it is a thin wire or a thick bar.

6. As the temperature of a metal conductor rises, its resistance

• A. increases, because the lattice ions vibrate more and impede the electrons
• B. decreases, because the electrons move faster
• C. stays constant
• D. falls to zero

Explanation: More vigorous lattice vibrations scatter the drifting electrons more often, shortening the mean free path and raising resistance, which is why a filament lamp's resistance is much higher when lit. Semiconductors behave in the opposite way, since heating releases many more charge carriers and resistance drops sharply. That contrast is the basis of the thermistor.

7. The temperature coefficient of resistivity for a semiconductor such as silicon is

• A. positive
• B. zero
• C. negative
• D. always exactly one

Explanation: Heating a semiconductor promotes far more electrons across the energy gap than it adds scattering, so the number of charge carriers rises steeply and resistivity falls, giving a negative coefficient. Metals have a positive coefficient because their carrier number is already fixed and only scattering changes. A thermistor exploits the negative case to measure temperature.

8. Three resistors of 2, 3 and 5 ohms are connected in series. The total resistance is

• A. 10 ohms
• B. 0.97 ohms
• C. 1.03 ohms
• D. 30 ohms

Explanation: Series resistances simply add, giving 10 ohms, and the same current passes through each while the voltages divide in proportion to the resistances. In parallel the reciprocals add, which for these values gives about 0.97 ohms, always less than the smallest resistor. Recognising which arrangement the question describes is half the work.

9. Two 6 ohm resistors are connected in parallel. Their combined resistance is

• A. 12 ohms
• B. 6 ohms
• C. 3 ohms
• D. 0.33 ohms

Explanation: For two equal resistors in parallel the result is simply half of one of them, so 3 ohms. The general rule is that the reciprocals add, and the total is always smaller than the smallest branch because the current has more than one path available. Adding to get 12 ohms is the series answer.

10. The electromotive force of a cell is

• A. the same as the potential difference across its terminals when supplying current
• B. the energy supplied by the cell per unit charge driven round the whole circuit
• C. the force exerted on the electrons in newtons
• D. always smaller than the terminal potential difference

Explanation: Despite its name, emf is an energy per unit charge measured in volts, not a force at all. It exceeds the terminal potential difference whenever current flows, because some energy is dissipated inside the cell against its internal resistance. Only on open circuit, with no current, do emf and terminal voltage become equal.