Electromagnetism

The Earth’s aurora is an atmospheric effect visible in the polar regions.

Electrons in the upper atmosphere initially move vertically downwards along one of the Earth’s magnetic field lines. They are accelerated by an electric field.

These electrons collide with oxygen atoms causing atomic electrons to be excited to higher energy levels. The atomic electrons then return to lower energy levels, releasing photons of green light, red light and ultraviolet.

The accelerated electrons are accelerated by electric fields parallel to the magnetic field lines.

The length of the acceleration region is 20 000 km and the electric field value is 0.2 mV / m. The electric field can be assumed to be uniform.

Calculate the kinetic energy gained by the electrons as they pass through the electric field, in joules.

State and explain the direction of the electric field required to accelerate the electrons downwards.

A simplified energy level diagram for atomic oxygen is shown below.

Determine the wavelength of the green light emitted.

Fully justify your answer.

The horizontal current closest to the Earth produces a magnetic field which can be detected at ground level.

With reference to Fig. 1.1, state and explain the direction of this magnetic field.

Suggest two ways that the horizontal current may affect the atmosphere.

The diagram below shows the top-view of a long current-carrying wire.

The direction of the current in the wire is into the plane of the paper.

Draw at least three field lines to indicate the magnetic field pattern around this wire.

The diagram below shows two long vertical current-carrying wires X and Y.

The direction of the current in each wire is the same.

Explain why wire Y experiences a force and deduce the direction of this force.

State Faraday’s law of electromagnetic induction.