OCR A Level Physics B (Advancing) Scientific literacy in physics June 2022 question paper H557/02

This question is about the experiment to measure the charge on an electron performed by American physicists Robert Millikan and Harvey Fletcher in the early years of the twentieth century.

Consider the forces on an oil drop falling at terminal velocity through air, as shown in Fig. 1.1.

Ignoring any upthrust forces, state why the drag force D must be equal and opposite to the weight W of the drop when it is falling at terminal velocity.

The drag force is given by the equation D = 6_airrv where r is the radius of the oil drop and v is the terminal velocity. The symbol _air represents the viscosity of air; this is a measure of the resistance that air has to the motion of objects in it.

Show that the terminal velocity of a drop of mass 1.8 10^–15 kg is about 7 10^–5 m s^–1. The buoyancy of air may be ignored.

density of oil = 8.7 10² kg m^–3

viscosity of air = 1.8 10^–5 Pa s

The oil drop is given a negative charge and falls into a region of uniform electric field, as shown in the diagram.

Draw six lines on Fig. 1.2 to represent the uniform field between the plates. Assume

that the oil drop does not distort the field.

When Millikan and Fletcher performed this experiment with many oil drops, they found that the charge on the drop was always a multiple of about 1.6 10^–19 C (to two significant figures). Why does this suggest that there is a fundamental quantity of electric charge and how does the modern model of the structure of hadrons suggest that 1.6 10^–19 C may not be the smallest quantity of charge that a particle can possess?

The core is made of sheets of iron which have been laminated (separated by thin sheets of electrically insulating material). The layers of iron and insulator are all parallel to the plane of Fig. 2.1a and stacked as shown in Fig. 2.1b.

Explain why iron is used in the core and why laminating the core improves the performance of the generator.

The permeance of a magnetic circuit can be compared to the conductivity of an electrical circuit. Suggest and explain a change to the generator in Fig. 2.1 which would increase the permeance of the magnetic circuit.

The emf across each turn of the coil is given by the equation

= 2f_maxsin 2ft

Use the equation and data from Fig. 2.2 and part (b) (i) to calculate the maximum emf induced across the coil.

This question is about using the change in resistance of a wire to measure strain.

Strain gauges use extremely thin wire to give a measurable change in resistance for a small increase in strain.

A strain gauge has the structure shown in Fig. 3.1. It has 14 strands of wire connected by thicker strips of negligible resistance.

A student places the gauge in a potential divider circuit as shown. The gauge has a resistance of 310 when it is not stretched or compressed. This is called its unstrained resistance.

The student had a choice of three resistors for the strain potential divider; 100 , 470 and 2000 . Suggest and explain why they chose the 470 resistor.

Show that the p.d. across the resistor when the gauge is not under strain is about 3.6 V

The p.d. across the resistor drops to 3.57 V when the gauge experiences a tensile force. Calculate the resistance of the gauge in this case.

This question is about the force on a current-carrying wire in a magnetic field.

A student sets up an experiment to measure the field strength between a pair of magnets as shown in Fig. 4.1a. A stiff copper wire experiences a force when it carries a current. The magnetic field when no current is flowing through the wire is represented in Fig. 4.1b.

Fig. 4.1a
Fig. 4.1b

The reading on the electronic balance changes when the current in the wire changes. Explain why this is an example of Newton’s Third Law of motion.

0.040 m of the copper wire is within the uniform magnetic field which acts perpendicularly to the wire.

The reading on the balance changes by 1.4 gram when the wire carries a current of 3.1 A.

Calculate the strength B of the magnetic field.

The student records the balance reading for a range of current values. She plots a graph of balance reading (y-axis) against current (x-axis). Explain how she can use the graph to find a value for the field strength. Describe why this can give a more reliable result than finding the mean value from a number of estimates of field strength using a range of current values.

A wire of length 0.90 m carries an alternating current of frequency 50 Hz. The tension in the wire can be changed using weights as indicated in Fig. 4.2.

The Earth’s magnetic field exerts a force on the current carrying wire. Explain why this makes the wire oscillate at frequency of 50 Hz.

Use the equation for centripetal force and the equation giving the gravitational force between two bodies, to show that the constant k in Kepler’s relationship is equal to Make each step of your reasoning clear.

Ganymede is the largest moon of the planet Jupiter. It orbits Jupiter at a distance of 1.07 10⁹ m. The period of the orbit is 6.18 10⁵ s. Use Kepler’s relationship to show that the mass of Jupiter is about 2 10²⁷ kg.

The spacecraft fired motors to slow it down before it entered orbit with Jupiter. Suggest and explain why this was necessary.

Fig. 6.1 shows how the displacement of an oscillator varies with time.

State how the graph shows that the oscillator is undamped.

Draw a curve on Fig. 6.1 showing how the velocity of the oscillator varies with time.

Label this line ‘v’.

Draw a curve on Fig. 6.1 showing how the acceleration of the oscillator varies with time.

Label this line ‘a’.

When the model is run over many more iterations, the graph in Fig. 6.5 is produced.

The model incorrectly predicts that the amplitude of the oscillation increases over time. A student suggests that the model shows that the energy of the oscillator increases by a constant factor during each oscillation. Perform calculations to test this suggestion, stating your conclusion. Explain your method and reasoning.

This question is about the movement of energy through the radiative zone (lines 53 – 60).

Use the equation given in the article (line 58) to estimate the number of steps required for a photon to make a displacement of 3.5 10⁸ m through the radiative zone. Use this value to calculate the total distance travelled by a photon as it passes through the radiative zone and show that this suggests that radiation takes about 100 000 years to travel through the zone.

distance between photon-electron interactions, x = 1.2 10^–4 m

1 year = 3.2 10⁷ s

This question is about the aurorae (lines 15 – 26).

Explain the mechanism that produces light from aurorae and calculate the energy associated with photons of the green light of wavelength 558 nm that is often seen in aurorae.

Consider the situation represented in Fig. 8.2.

An electron is travelling at 5.0 10⁵ m s^–1 at an angle of 25° to the direction of the magnetic field of strength 40 T (Fig. 8.2a). It forms a spiral of radius r = 0.029 m (Fig. 8.2b).

By considering horizontal and vertical components of its velocity, calculate the vertical distance d the electron travels between each full rotation around the field line.

Suggest and explain how the spiral might change as the electron moves nearer to the pole, considering the strengthening field and the energy transferred to particles in the atmosphere.

9* This question is about the slow rate of fusion in the Sun (lines 33 – 47).

The temperature of the core of the Sun is 1.5 10⁷ K. Assuming that two protons need to come to within 10^–14 m of each other to fuse, show that, on average, protons in the core do not have enough kinetic energy to fuse.

Explain, using calculations, why the Boltzmann factor for the process shows that fusion of any two protons at this temperature is extremely unlikely and that it cannot account for the 3.7 10³⁸ protons which fuse in the Sun each second.

The total mass of the Sun is 2 10³⁰ kg.

This question is about the Parker Solar Probe mission (lines 114 – 138).

Use your answer to (a) and Stefan’s Law (line 125) to estimate the temperature of a body in thermal equilibrium at 1.4 10¹⁰ m from the Sun.

Explain why missions investigating the Sun such as the Parker Solar Probe can have an importance beyond scientific discovery.