OCR GCSE Physics B (21st Century) Paper 4 June 2018 question paper

This question is about using an LDR (light-dependent resistor) to measure light intensity.

The resistance R of an LDR varies with illuminance (the amount of light energy per unit area hitting a surface) as shown in the graph.

Use the graph to estimate the change in resistance of the LDR when the illuminance increases from 10 lux to 70 lux.

Change in resistance =........ k Ω

The LDR is connected in series with a fixed resistor of resistance 10 k Ω and a 4.5 V battery.

The total resistance at 30 lux is 22 000 Ω.

Calculate the current in the circuit.

Calculate the potential difference across the fixed 10 k Ω resistor when the illuminance is 30 lux.

Describe, without any calculations, how the potential difference across the fixed resistor will change when the illuminance increases from 30 lux to 100 lux.

Sarah carries out an experiment to measure the specific latent heat of vaporisation of water. She does this by finding the energy needed to evaporate a known mass of water.

The apparatus she uses is shown in Fig. 2.1.

Using this apparatus, Sarah takes these readings.

Table 2.1

* Sarah is not happy with her results.

Is Sarah right?

What could Sarah do to get an accurate value of the specific latent heat of vaporisation of water from her experiment?

Sarah’s book has this information about vaporisation of two liquids.

Suggest why it takes more energy to evaporate 1 gram of water than it does to evaporate 1 gram of alcohol.

There is a film about an astronaut named Mark Watney. He is left alone on the planet Mars. He has to use science to stay alive until he can be rescued.

Mars is a cold planet, and Watney has a radioactive thermal generator. This contains radioactive plutonium-238 which emits alpha-particles, giving an isotope of uranium.

Complete the radioactive decay equation for plutonium-238.

........

The plutonium emits 1.6 × 10¹⁵ alpha particles every second, each with an energy of 9.0 × 10^–13 J.

The energy released is all transferred to the internal energy of the generator.

Show that the input power of the generator is about 1500 W.

Watney uses the generator to heat up water for a bath. He heats 100 kg of water from 20 °C to 37 °C.

Show that it takes more than an hour (3600 s) for his bath to warm up using his 1500 W generator.

You can assume that all the input energy to the generator is transferred to the internal energy of the water.

specific heat capacity of water = 4200 J/kg °C

To be rescued, Watney needs to drive a vehicle to a site 3200 km away. The vehicle is powered by batteries of capacity 18 kWh.

Watney knows that the vehicle can travel at 25 km/hour using 5 kW of power from the batteries to do this. When the batteries are discharged Watney has to wait until the next day to continue. He has solar panels to recharge the batteries after a day’s travel.

Use these data to calculate the smallest number of days it would take to drive to his destination.

Give one reason why it would actually take longer than the time calculated in (b)(i).

This question is about a measurement of the speed of sound in air that Isaac Newton made over 300 years ago.

At Newton’s college in Cambridge there was a long outdoor corridor where clapping his hands would give a loud echo a fraction of a second later.

Newton measured the distance from where he stood to the reflecting wall as 64 m.

To measure the time, he made a very tiny pendulum – a weight swinging on a thin cotton thread – and adjusted the length until one to-and-fro swing of this pendulum matched the time between the clap and the echo.

This happened when the length L of the pendulum was 4.6 cm (0.046 m).

Newton showed that the time of one swing, T, was given by the equation:

T^{2 =} kL

L = the length of the pendulum k = 4.02 s²/m.

Calculate the swing time T of his 0.046 m pendulum.

Use Newton’s data to calculate the speed of sound. Distance to wall = 64 m

Newton’s calculated value for the speed of sound was low when compared with the speed found by modern measurements.

Explain which of Newton’s measurements (distance or time) was likely to be the least accurate.

Explain why Newton’s value for the speed was too low.

Alex makes this calculation to find the energy.

Kinetic energy gained by the trolley = gravitational potential energy store it had at the top of the slope. = mass × g × height = 800 g × 10 N/kg × 20 cm = 160 J

The value for the energy calculated by Alex is too large.

Identify mistakes that Alex has made in his measurements and in his calculation.

Describe how Alex should have done this experiment to get a more accurate value for the kinetic energy of the trolley.

Alex carries out this experiment for a range of kinetic energy values.

Table 5.1 shows his results.

Some of these data are plotted on the graph in Fig. 5.2.

State the reason why Alex was right to plot a point at the origin, (0,0).

Plot the three remaining points on the graph in Fig. 5.2 and draw an appropriate best fit

Two of Alex’s friends make comments about the graph in Fig. 5.2.

Discuss the statements made by Mia and Kai.

Explain, using ideas of momentum and force, how air molecules exert pressure on the wall.

As you go further from the Earth’s surface, the atmospheric pressure gets smaller.

Explain this in terms of the movement of the molecules at different heights.

Solar farms are large power stations made up from many photovoltaic (PV) panels. They have

A large solar farm in England has a total area of 216 000 m² covered by PV panels. (a) Every square metre of the solar panels receives about 1000 W of power from the Sun during each day.

The panels have an efficiency of energy transfer of 15%.

Calculate the daily average electrical power produced by the solar farm.

Give your answer in MW (1 MW = 1 000 000 W).

* Many people are not in favour of solar farms.

Discuss what Jane has said about solar farms and gas-burning power stations.

This question is about the radioactive isotope americium-241, which is found in smoke detectors.

The graph shows how the activity of a sample of americium-241, with an initial activity of 800 counts per second, would change with time.

Use the graph to obtain an estimate of the half-life of americium-241. Show your working on the graph.

Americium-241 decays by emitting alpha-particles.

A smoke detector is not a hazard in your house. They usually last 10 years.

Explain why a smoke detector should be disposed of carefully when it no longer works.

In schools, the decay of radioactive isotopes such as americium-241 can be modelled by a game rolling many dice. Each dice has 1 chance in 6 of showing a ‘six’ each time.

In a typical game, 100 dice are rolled onto a table.

The number showing six spots on the top are removed and counted.

The remaining dice are rolled again, and the process continued.

The results are put into a table. The following is an example for one game.

Each ‘roll number’ stands for an equal interval of time.

What does the number of dice remaining at any roll stand for?

What does the number of sixes taken out in any roll stand for?

Explain why the data in this table suggest that the half-life is about 4 ‘rolls’ but that it’s not possible to be exact.

This question is about the refraction of waves.

The diagram below is a section through part of the Earth.

Following an earthquake at X, earthquake waves travel through the Earth. Two wave paths, A and B, are shown on the diagram.

Look at path A.

The wave direction changes suddenly at the place ringed.

Explain what this shows about the speed of the earthquake wave as it moves from the mantle into the core.

Look at path B.

The wave direction changes continuously.

Explain what this shows about the speed of the earthquake waves in the mantle at different depths below the Earth’s surface.