Electric and magnetic fields practice questions.

The electrostatic force between two point charges is F. The separation between them is tripled while the charges remain unchanged. State the new force.

• F/9
• F/3
• 3F
• 9F

Answer: F/9

Coulomb's law is an inverse-square law: F ∝ 1/r², so tripling the separation reduces the force by a factor of 3² = 9.

Two parallel plates separated by 4.0 mm are connected to a 200 V supply. Calculate the electric field strength between the plates.

• 8.0 × 10² V m⁻¹
• 2.0 × 10⁴ V m⁻¹
• 5.0 × 10⁴ V m⁻¹
• 5.0 × 10¹ V m⁻¹

Answer: 5.0 × 10⁴ V m⁻¹

For a uniform field E = V/d = 200 / (4.0 × 10⁻³) = 5.0 × 10⁴ V m⁻¹. A common error is forgetting to convert mm to m.

A uniform electric field points due east. Determine the direction of the electric force acting on an electron placed in this field.

• East
• North
• South
• West

Answer: West

The field direction is defined by the force on a positive charge; the electron is negative, so the force on it is opposite to the field, i.e. due west.

Which statement best describes the magnetic field pattern produced by a long straight wire carrying a constant current?

• Straight field lines parallel to the wire
• Concentric circles around the wire, in planes perpendicular to it
• Field lines directed radially straight outward from the wire in every direction
• A uniform field pointing in the direction of the current

Answer: Concentric circles around the wire, in planes perpendicular to it

The field forms closed circular loops centred on the wire; their direction is given by the right-hand grip rule and the field weakens with distance from the wire.

Electric charge is quantised. Which value CANNOT be the total charge of an isolated object? (e = 1.6 × 10⁻¹⁹ C)

• 1.6 × 10⁻¹⁹ C
• 3.2 × 10⁻¹⁹ C
• 2.4 × 10⁻¹⁹ C
• 4.8 × 10⁻¹⁹ C

Answer: 2.4 × 10⁻¹⁹ C

Charge only occurs in whole-number multiples of e; 2.4 × 10⁻¹⁹ C would correspond to 1.5e, which is impossible for an isolated object.

Two equal positive point charges are held a fixed distance apart. State the electric field strength at the midpoint between them.

• Zero
• Maximum along the line joining the charges
• Directed towards one of the charges
• Equal to kQ/r², where r is the full separation

Answer: Zero

At the midpoint the two fields have equal magnitude but point in opposite directions, so they cancel exactly and the resultant field is zero.