All Free Physics MCQs with Answers

131. Electric potential at a point is

• A. a vector measured in newtons per coulomb
• B. the work done per unit positive charge in bringing it from infinity to that point
• C. the force per unit charge
• D. the charge stored per unit voltage

Explanation: Potential is a scalar measured in volts, so potentials from several charges are added arithmetically with their signs rather than as vectors, which makes it far easier to handle than field intensity. Force per unit charge is the field, and charge per unit voltage is capacitance. Only differences in potential have physical meaning, which is why infinity is chosen as the zero.

132. The work done in moving a charge between two points on the same equipotential surface is

• A. maximum
• B. zero
• C. negative
• D. equal to qV

Explanation: Work equals charge multiplied by the potential difference, and on an equipotential surface that difference is zero, so no work is done however long the path. It follows that field lines always meet equipotential surfaces at right angles, since any component along the surface would do work. The surface of any conductor in equilibrium is an equipotential.

133. An electron volt is the energy gained by an electron when it moves through a potential difference of

• A. one volt
• B. one joule
• C. 1000 volts
• D. one coulomb

Explanation: Since energy is charge multiplied by potential difference, one electron volt equals 1.6 times 10 to the minus 19 joules, the magnitude of the electronic charge in coulombs. The unit is convenient because joules are absurdly large for single particles. Electron volts and their multiples are the standard currency of atomic and nuclear physics.

134. Capacitance is defined as

• A. the charge stored per unit potential difference across the capacitor
• B. the energy stored per unit charge
• C. the charge multiplied by the voltage
• D. the potential difference per unit charge

Explanation: C equals Q over V, measured in farads, and it depends only on the geometry of the plates and the dielectric between them, not on how much charge happens to be stored. A farad is a very large unit, so practical capacitors are rated in microfarads or picofarads. Doubling the voltage doubles the charge stored and leaves the capacitance unchanged.

135. The capacitance of a parallel plate capacitor is increased by

• A. increasing the plate separation
• B. decreasing the plate area
• C. increasing the plate area and inserting a dielectric
• D. increasing the applied voltage

Explanation: Capacitance is proportional to the area and to the relative permittivity of the dielectric, and inversely proportional to the separation, so larger closer plates with a dielectric between them store more charge per volt. Applied voltage does not appear in the expression at all, which is the trap in the last option. A dielectric also allows a higher working voltage before breakdown.

136. Three capacitors are connected in parallel. The total capacitance is

• A. the sum of the individual capacitances
• B. less than the smallest of them
• C. the reciprocal of the sum of the reciprocals
• D. always equal to one of them

Explanation: Parallel capacitors share the same voltage and their plate areas effectively add, so the capacitances add directly. In series the reciprocals add, giving a total smaller than the smallest capacitor. This is exactly the opposite of how resistors behave, which is why the two are so often confused.

137. The energy stored in a charged capacitor is given by

• A. QV
• B. half CV squared
• C. CV
• D. half QC

Explanation: The factor of one half appears because the voltage rises from zero to V as the capacitor charges, so the average voltage during the process is half the final value. The expression can equally be written as half QV or as Q squared over 2C. This stored energy is what a camera flash releases in a few milliseconds.

138. A dielectric inserted between the plates of a charged, isolated capacitor causes the potential difference to

• A. increase
• B. decrease, because the capacitance rises while the charge is fixed
• C. stay the same
• D. become zero

Explanation: With the capacitor disconnected the charge cannot change, and since V equals Q over C, raising the capacitance must lower the voltage. The dielectric molecules polarise and set up a field opposing the original one, which is the physical reason for the drop. If instead the capacitor stayed connected to a battery, the voltage would be fixed and the charge would rise.

139. Two identical charges are placed a fixed distance apart. The point where the resultant field is zero lies

• A. midway between them
• B. at one of the charges
• C. outside the pair, beyond the smaller charge
• D. nowhere, since the field is never zero

Explanation: For like charges of equal magnitude the two fields are equal and opposite at the midpoint, so they cancel there. If the charges were unlike, no point between them could give cancellation because both fields would point the same way, and the null point would lie outside the pair on the side of the smaller charge. Working out which case applies is the first step in any such question.

140. Charge on a charged conductor resides

• A. uniformly throughout its volume
• B. on its outer surface, concentrating most where the surface is sharply curved
• C. at its centre
• D. only where it was first placed

Explanation: Mutual repulsion drives the charges as far apart as possible, so they collect on the outside, and the surface charge density is highest at points and edges. This concentration produces the very strong field that ionises the surrounding air at a sharp point, which is the principle of the lightning conductor. Inside the metal the field remains zero.