Free Current Electricity MCQs with Answers

21. The reciprocal of the resistivity of a material is called its:

• A. Impedance
• B. Conductivity
• C. Admittance
• D. Reactance

Explanation: Conductivity measures how readily a material carries current and is the reciprocal of resistivity, with units of siemens per metre. Admittance is the reciprocal of impedance and reactance is the opposition offered by a capacitor or inductor, both of which belong to circuit analysis rather than to material properties. Copper has a high conductivity and rubber a very low one.

22. Terminal voltage of a cell equals its EMF only when:

• A. No current flows
• B. Current is maximum
• C. Internal resistance is infinite
• D. Load resistance is zero

Explanation: The lost volts inside the cell are the current multiplied by the internal resistance, so only when the current is zero, that is on open circuit, does the terminal voltage rise to the full emf. This is why a high resistance voltmeter across an unloaded cell reads its emf. Short circuiting the cell does the opposite, making the current maximum and the terminal voltage nearly zero.

23. If the length of the conductor is made 4 times its original length, its resistance becomes

• A. Quarter
• B. Half
• C. Zero
• D. 4 times

Explanation: Resistance is proportional to length for a given cross section and material, so quadrupling the length quadruples the resistance. Note that if the wire were stretched to four times its length at constant volume, the area would fall to a quarter and the resistance would rise sixteenfold, which is the harder variant of this question. Here only the length is stated to change.

24. The unit of temperature coefficient of resistivity is

• A. 1/C
• B. 1/K
• C. 1/A
• D. 1/s

Explanation: The coefficient is the fractional change in resistivity per unit rise in temperature, so the resistivity units cancel and only reciprocal temperature remains, giving per kelvin. Because a kelvin and a degree Celsius are the same size, per degree Celsius is numerically identical, which is why the first option is such a close distractor. Reciprocal ampere and reciprocal second belong to quite different quantities.