Topic 4.2: Electric and Magnetic Fields Questions

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.

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

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 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 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.

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.

(a) Identify the sign of the charge left on the sphere.

(b) State whether contact between the rod and the sphere occurred at any stage.

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.

Two equal positive point charges, each +5.0 × 10⁻⁹ C, are fixed 0.50 m apart in a vacuum.

Calculate the electric field strength that EACH charge produces at the midpoint, then determine the magnitude of the resultant electric field at the midpoint and explain your reasoning.

(k = 8.99 × 10⁹ N m² C⁻².)

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) State its kinetic energy on arrival in kiloelectronvolts.

(b) Calculate this kinetic energy in joules.

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 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.

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

(a) Determine the magnitude of the electric force between them.

(b) The separation is then increased to three times this value. Determine the new force. (k = 8.99 × 10⁹ N m² C⁻².)

Two horizontal parallel plates are separated by a gap of 0.025 m and connected to a 500 V supply.

(a) Calculate the uniform electric field strength between the plates.

(b) A small charge of +4.0 × 10⁻⁹ C is placed in the gap; calculate the electric force on it.

(c) State, with a reason, how the force would change if the charge were moved to a point nearer the negative plate.

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.)

An ion of charge +2e (where e = 1.6 × 10⁻¹⁹ C) is released from rest at the positive plate and accelerated across two parallel plates with a potential difference of 350 V.

(a) State the kinetic energy gained by the ion in electronvolts.

(b) Calculate this kinetic energy in joules.

(c) The plates are 0.070 m apart; calculate the uniform electric field strength between them.

Two long parallel wires are 0.15 m apart and carry currents of 8.0 A and 12 A in the same direction.

Calculate the force per unit length on each wire, state whether the wires attract or repel, and explain how the force on the 8.0 A wire compares with the force on the 12 A wire.

(μ_{0} = 4π × 10⁻⁷ T m A⁻¹.)

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.

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.