Topic 4.2: Electric and Magnetic Fields Questions

A small test charge of 1.5 × 10⁻⁹ C is placed at a point in an electric field and experiences a force of 3.6 × 10⁻⁵ N.

Calculate the electric field strength at that point.

State the shape of the magnetic field lines around a long straight current-carrying wire.

Two long parallel plates are held a few centimetres apart, the left plate positive and the right plate negative.

Sketch the pattern of the electric field lines in the gap between the plates, and explain one feature of your sketch that shows the field is uniform.

A positively charged rod is brought close to, but not touching, an isolated neutral metal sphere.

While the rod is held in place the sphere is briefly earthed and the earth connection is then removed; finally the rod is taken away.

Two identical point charges, a fixed distance apart, exert a force F on each other.

One of the charges is then replaced by a charge half as large, with the separation unchanged.

State the new force on each charge, in terms of F.

Outline how charging by contact differs from charging by induction, including the sign of the final charge in each case relative to the charged object first brought up.

Two parallel metal plates are connected to a battery so that the upper plate is positive and the lower plate is negative.

State what is meant by a uniform electric field, and state the direction of the field between the plates.

State what is meant by the electric field strength at a point.

Small oil drops are sprayed into the top of a vertical tube and fall through it.

As a drop passes through the spray nozzle it picks up charge by friction.

Two point charges, q₁ = 3.0 × 10⁻⁶ C and q₂ = 4.0 × 10⁻⁶ C, are 0.25 m apart.

Two small spheres carry charges of q₁ = −6.0 × 10⁻⁶ C and q₂ = +9.0 × 10⁻⁶ C and are held 0.50 m apart.

Calculate the magnitude of the electric force between them and state whether it is attractive or repulsive.

(k = 8.99 × 10⁹ N m² C⁻².)

Two point charges exert a force of 12 N on each other.

One of the charges is then halved and, at the same time, the separation between the charges is doubled.

Deduce the new force between the charges.

A long straight vertical wire carries a steady current upwards.

Using the right-hand grip rule, describe the direction of the magnetic field at a point directly to the EAST of the wire.

Two long parallel wires each carry a current of 10 A in the same direction and are separated by 0.050 m.

Estimate the additional separation needed to halve the force per unit length between them, and state whether the wires would still attract.

Two small charged spheres, q₁ = 5.0 × 10⁻⁶ C and q₂ = 8.0 × 10⁻⁶ C, are held 0.40 m apart.

A proton (charge +1.6 × 10⁻¹⁹ C) starts from rest at one plate and is accelerated through a potential difference of 1.8 kV to the other plate.

A single point charge of −8.0 × 10⁻⁹ C is fixed in a vacuum.

Determine the magnitude of the electric field strength at a point 0.20 m from the charge and state the direction of the field there.

(k = 8.99 × 10⁹ N m² C⁻².)

A uniform electric field of strength 2.5 × 10⁴ V m⁻¹ is needed between two parallel plates that are connected to a 750 V supply.

Determine the separation required between the plates.

A small point charge of +4.0 × 10⁻⁹ C is fixed in a vacuum.

Calculate the electric field strength at a point 0.20 m from the charge, state its direction, and explain how the field strength would change if the point were moved to 0.40 m from the charge.

(k = 8.99 × 10⁹ N m² C⁻².)

Two long parallel wires are 0.080 m apart.

They carry currents of 6.0 A and 9.0 A in opposite directions.

Determine the force per unit length on each wire and state whether the wires attract or repel.

(μ_{0} = 4π × 10⁻⁷ T m A⁻¹.)