Free Electric Potential MCQs with Answers

1. 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.

2. 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.

3. 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.

4. The relationship between uniform electric field intensity E and potential difference V across a separation d is

• A. E equals V times d
• B. E equals V divided by d
• C. E equals d divided by V
• D. E equals V plus d

Explanation: For a uniform field, such as that between parallel plates, the intensity is the potential gradient, so a 100 V supply across plates 2 mm apart gives a field of 50,000 V per metre. This is why the unit volt per metre is equivalent to newton per coulomb. In a non uniform field the same relationship holds only as a local rate of change.

5. Two protons are brought closer together. The electrostatic potential energy of the pair

• A. increases, because work must be done against their repulsion
• B. decreases
• C. stays constant
• D. becomes negative

Explanation: Like charges repel, so pushing them together requires work and that work is stored as potential energy, which is why the system flies apart if released. For unlike charges the energy is negative and becomes more negative as they approach, since the force does the work instead. The sign convention takes zero energy to be infinite separation.