Fields practice questions.

Two point charges of +1.0 μC each are separated by 0.10 m in vacuum (k = 8.99e9). What is the magnitude of the Coulomb force between them?

• 0.90 N
• 9.0 N
• 0.090 N
• 90 N

Answer: 0.90 N

F = kq₁q₂/r² = 8.99e9 × (1.0e-6)² / (0.10)² = 8.99e9 × 1e-12 / 0.01 ≈ 0.90 N.

A magnetic field of 0.50 T passes perpendicularly through a flat coil of area 0.020 m². What is the magnetic flux through one turn?

• 0.010 Wb
• 0.10 Wb
• 0.025 Wb
• 10 Wb

Answer: 0.010 Wb

Φ = BA cosθ = 0.50 × 0.020 × cos0° = 0.010 Wb.

An ideal transformer steps 230 V down to 23 V. If the secondary has 50 turns, how many turns are on the primary?

• 5 turns
• 500 turns
• 50 turns
• 230 turns

Answer: 500 turns

Np/Ns = Vp/Vs = 230/23 = 10, so Np = 10 × 50 = 500 turns.

For a satellite in a circular orbit around a planet, the gravitational attraction of the planet provides the

• centripetal force
• tangential driving force
• frictional force
• upward lift force

Answer: centripetal force

Gravity acts towards the centre of the orbit, providing exactly the centripetal force needed to keep the satellite moving in a circle at constant speed.

The SI unit of gravitational field strength is

• N
• N kg⁻¹
• J kg⁻¹
• m s⁻¹

Answer: N kg⁻¹

g = F/m, so the unit is newtons per kilogram (equivalent to m s⁻²).

The gravitational potential a distance r from a point mass M is V = −GM/r. As r increases towards infinity, the gravitational potential

• decreases towards −∞
• increases towards zero
• stays constant
• steadily becomes a large positive value

Answer: increases towards zero

The potential is negative and defined to be zero at infinity. As r grows, −GM/r becomes a smaller negative number, i.e. it increases towards zero.