Topic 4.2: Electric and Magnetic Fields Questions

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 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.

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.

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

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 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.)

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.

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.)

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.

In the theory of electromagnetism, the speed of light in a vacuum c is related to the permeability of free space μ₀ and the permittivity of free space ε₀ by c = 1/√(μ₀ε₀).

What is the product μ₀ε₀ expressed in fundamental SI units?

Two small positive charges are fixed on a horizontal line.

A charge q₁ = +4.0 μC is at point X, and a charge q₂ = +1.0 μC is at point Y, a distance of 0.30 m to the right of X.

Point P lies on the line between them, 0.10 m to the right of X (and so 0.20 m to the left of Y).

Using Coulomb's constant k = 9.0 × 10⁹ N m² C⁻², what is the magnitude and direction of the resultant electric field at P?

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

A plastic rod is rubbed with a dry cloth and becomes negatively charged.

Two small positive point charges lie on a straight horizontal line in a vacuum. A charge of +3.0 μC is fixed at point X, and a charge of +6.0 μC is fixed at point Y, with X and Y separated by 0.30 m. Point P lies on the line between the charges, 0.10 m from X (and therefore 0.20 m from Y).

Using k = 9.0 × 10⁹ N m² C⁻², what is the magnitude of the resultant electric field strength at P?

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

Two identical small conducting spheres, P and Q, are held a fixed distance apart in a vacuum.

Each sphere carries the same charge, and they repel each other with an electrostatic force of magnitude F.

Sphere Q is now briefly earthed so that its charge is reduced to exactly half its original value, while the charge on P and the separation are unchanged.

What is the magnitude of the electrostatic force now acting on sphere P?