OCR A Level Physics A Unified Physics June 2025 question paper H556/03

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.

A student carries out an experiment to measure the frequency, f, of the mains electricity supply, using the apparatus shown in the diagram.

The wire passes between the poles of a magnet. It is held in tension by the weight W of the mass hanger. The fixed bridge and the movable bridge are separated by distance L.

Mains frequency current is passed through the wire. The student adjusts the movable bridge until the wire vibrates at its fundamental frequency. The weight W and the length L are recorded. This is repeated for several different weights.

Explain how a stationary wave is formed on the wire.

It can be shown that the fundamental frequency f₀ of a wire of length L under tension W is given by the equation

where is the mass per unit length of the wire, and has a value of 8.0 × 10^–4 kg m^–1 ± 2%.

Suggest why the magnetic flux density, B, of the magnet does not appear in this equation.

Suggest a method to determine the mass per unit length, , of the wire.

The graph shows the results of the experiment.

The student draws a best fit line on the graph. The gradient of the best fit line is 2.81 N^1/2m^–1.

Draw a worst acceptable line on the graph.

Use your worst acceptable line to calculate the percentage uncertainty in the gradient of the best fit line.

gradient = 2.81 ±........ N^1/2m^–1%

Calculate a value for the mains frequency, f, including a percentage uncertainty.

f =........ Hz ±........ %

Evaluate the accuracy of your value for f, given that the true value is 50.0 Hz.

This question is about comparing two methods for storing energy.

A proposed method for storing energy uses trucks powered by electric motors.

The trucks carry sand up a mountain, storing surplus energy as gravitational potential energy.

When the energy needs to be released, they carry the sand back down the mountain, converting the gravitational potential energy into chemical energy stored in a battery.

Explain how an electrical generator in the truck could use electromagnetic induction to convert gravitational potential energy into chemical energy stored in a battery.

A second proposed method for storing energy uses a tank of wax.

The wax is heated using an electrical heater, storing surplus energy as internal energy of the wax. The wax increases in temperature and melts as it is heated.

When the energy needs to be released, the wax is allowed to cool, converting the internal energy of the wax into thermal energy.

Explain, in terms of the particles in the wax, how energy is stored when it is heated.

An electroscope is an instrument used to measure electric charge.

The diagram shows one type of electroscope.

The diagram below shows the needle of the electroscope when it is negatively charged. The centre of mass, X, of the needle has been marked on the diagram.

Suggest why the centre of mass of the needle is a short distance below the pivot.

A student charges the electroscope by connecting the metal plate to the negative terminal of a high voltage power supply.

They record the deflection angle of the needle for a range of values of the voltage measured using a voltmeter.

Their results are shown in the graph below.

The student concludes that the voltmeter has a zero error of 300 V.

Explain their reasoning.

The capacitance of the electroscope is 20 pF.

Estimate the number of electrons stored on the electroscope when the deflection angle is 45°.

To control insect populations, sterilised male insects can be released into the wild.

They mate with females, who then produce no offspring, reducing the future population.

Sterilisation can be carried out using ionising radiation. The insects are placed in a canister about 10 cm across and then deliberately exposed to a known dose of ionising radiation.

Suggest why exposing an insect to ionising radiation may cause it to become infertile.

Both gamma rays and X-rays can be used to sterilise the insects.

Suggest why alpha radiation is not suitable for irradiating large numbers of insects.

Gravitational waves are disturbances in space-time that are produced by violent astrophysical events.

Define strain.

The maximum strain shown on the graph is approximately 10^–21. (ii)

Calculate the force that would produce the same strain in a copper wire of diameter 2 mm.

Young modulus of copper = 130 GPa

By considering the answer to (ii), suggest why it is very difficult to detect the strain caused by gravitational waves.

In this part of the question you will use Newtonian physics to estimate the power output of the black hole collision.

The diagram shows two identical black holes, each of mass M, orbiting each other with period T.

Prior to colliding, the black holes move at speed V in identical circular orbits of diameter D about the central point C.

Write down an expression for the gravitational force, F, acting between the black holes in terms of M, D and the universal gravitational constant, G.

The total orbital energy, E, of the black holes is the sum of their kinetic energy and gravitational potential energy. Show that E is given by the expression:

E = k (²G²M⁵)^1/3 (1/T^2/3)

where k is a numerical constant.

By considering times B and D, use your answer to (iii) to estimate the power radiated as gravitational waves in this time interval.