OCR A Level Physics B (Advancing) Practical skills in physics November 2020 question paper H557/03

This question is about finding the specific thermal capacity of a liquid. An electrical immersion heater connected to a supply unit is placed inside a calorimeter containing a liquid at room temperature, 17 °C. The calorimeter, shown in Fig. 1.1, has insulated lid, walls and base. The mass of the liquid is 200 ± 5 g. The temperature of the liquid is measured using a digital thermometer at 30-second intervals for 6 minutes.

Suggest an improvement, with a reason, to the experimental design.

Draw a circuit diagram to represent the apparatus used to calculate the power of the

The following readings were obtained:

potential difference V = 10.5 ± 0.2 V current I = 2.3 ± 0.2 A.

Calculate the power supplied to the heater per unit mass of the liquid. Include the uncertainty in the value.

power per unit mass =........ ±........ W kg^–1

Fig. 1.2 shows the temperature–time graph obtained.

Explain the shape of the graph in Fig. 1.2.

Draw a straight line of best fit through the initial linear section of Fig. 1.2.

Show that the relationship between the gradient of the linear section and the specific thermal capacity, c, takes the form

where Z is a constant.

Use the linear part of Fig. 1.2 to find the specific thermal capacity of the liquid.

=........ J kg^–1 K^–1 c

This question is about calibrating a light meter to measure the intensity of light emitted by a torch.

Light intensity is measured in W m^–2, and can be calculated using the equation (a)

where E is the radiant energy passing through a surface in time t and A is the cross-sectional area of the surface.

The beam from a torch, shown in Fig. 2.1, has a conical shape. At a distance, x, the intensity of light is L. Show that at a distance of 2x, the intensity will be reduced to .

An LDR is placed in a potential divider circuit as shown in Fig. 2.2.

Explain why the output reading on the voltmeter will increase as light intensity increases.

* Use the calibration plots in this question to find the distance of the torch from the light meter in the following two cases:

• the voltmeter in Fig. 2.2 reads 5.0 V

• the voltmeter in Fig. 2.2 reads 5.0 V when the torch shines on it in a room with a constant level of background illumination equal to the intensity from the source at a distance of 25.0 cm.

Make your method clear.

Explain how a calibration curve yielding the intensity of the light from the torch in the scenario with background illumination would differ from Fig. 2.4, the calibration curve of the light

incident on the light meter with no background illumination.

The speed of sound can be measured using the resonance of air in a tube. A loudspeaker is placed above the open end of a hollow glass tube with the other end immersed in water as shown in Fig. 3.1.

The loudspeaker is connected to a signal generator set at a frequency of 480 Hz with an audible volume. The height of the column of air is changed by moving the glass tube up and down, until maximum volume of sound is heard.

Explain how a standing wave is formed in the tube.

The gradient of the line is 83 m s^–1. Calculate the speed of sound in air. Calculate the (i) percentage uncertainty in your result by taking a ‘worst-fit’ line from the graph.

speed of sound =........ m s

The end correction is approximately equal to 0.3 D, where D is the diameter of the tube. What is the approximate diameter of the tube used in this experiment?

This is a question about transformers. A student performs an experiment to investigate transformers using the apparatus shown in Fig. 4.1.

The student connects an oscilloscope as shown in Fig. 4.1, to measure the input voltage to the primary coil of the transformer. The screen of the oscilloscope is shown in Fig. 4.2:

The oscilloscope has the following settings: 500 mV / div on the vertical scale and 2.5 ms / div on the horizontal scale. Use Fig. 4.2 to calculate the following values for the input coil:

peak potential difference V

frequency f

A secondary coil is added to the right-hand side of the C-core shown in Fig. 4.1.

Fig. 4.3 shows the p.d.s across the primary and secondary coils. The scale settings are unchanged.

The student suggests that the turns ratio is

Use Fig. 4.3 to explain the student’s reasoning.

The actual ratio of turns for the transformer is . Suggest and explain a reason for this difference.

The current in the primary coil is I₁ and the current in the secondary coil is I₂. Show that the maximum current in the secondary coil is given by the relationship

State any assumptions you make.

The student alters the turns ratio to . N₂ is now 200 turns. The peak output emf is 2.0 V. The frequency of the supply is unchanged.

Estimate the maximum flux through the secondary coil.

The student now uses a different set of C-cores which have a greater overall length.

Complete the diagram by adding two lines which represent the flux in the core at the instant shown.

State the effect of increasing the length of the magnetic circuit of the transformer.

Another student has a faulty set of apparatus such that the C-cores do not fully meet on one side as shown in Fig. 4.4.

Explain how a small air gap affects the magnetic circuit in Fig. 4.4 and why the core is made of vertically stacked laminated sheets rather than a solid block of iron or

horizontally stacked laminated sheets as shown in Fig. 4.5.