# Physics– P03 June 1988

CARIBBEAN EXAMINATIONS COUNCIL

SECONDARY EDUCATION CERTIFICATE

EXAMINATION

PHYSICS

Paper 03

General Proficiency

1 3/4 hours

In addition to the 1 3/4 hours, candidates are allowed 15 minutes of reading time. No writing must be done during the 15 minute period.

1. Attempt question 1 and any THREE of the other questions.
2. ALL WORKING MUST BE SHOWN in this booklet, since marks will be awarded for correct steps in calculations.
3. The use of calculators is allowed.
4. Mathematical tables are provided.
5. Use the following data where required: DO NOT TURN THIS PAGE UNTIL YOU ARE TOLD TO DO SO

Section I

You must answer the question in this section. No more than 1/2 hour should be spent on this question.

1. A student measured the variation of potential difference, V, across two components A and B with the current, I, passing through them. Plots of V against I produced the following graphs: (i) Read off and tabulate SIX pairs of values of V and I from Graph x.

(6 marks)

(ii) Plot the resistance against current for component A.

(8 marks)

(iii) What can you deduce from the graph you have plotted about the resistance of component A?

(2 marks)

(iv) Give ONE possible physical interpretation of the intercept on the V axis of Graph X.

(2 marks)

(v) Calculate the resistance of component B when V= 0.5, 2.7 V and 5.0 V. Suggest the nature of component B.

(8 marks)

(vi) Discuss whether either of the components obeys Ohm’s law.

(4 marks)

Total 30 marks

Section II

Answer any THREE questions in this section.

2. (a) a bubble of air has a volume of 10 cm3  at the bottom of a pond of depth h. On reaching the surface, where the pressure is atmospheric the volume is 12cm. If the temperature of the pond is constant, find h.

(7 marks)

(b) Explain, with the aid of a diagram, how the transmission of hydraulic pressure can provide a force large enough to stop a motor car. Make it particularly clear how a small applied force can be magnified into a large one.

(8 marks)

(c) Use Archimedes’ principle to explain how a submarine can both float and sink.

(5 marks)

Total 20 marks

3. (a) A small scale demonstration solar cooker is to be set up for exhibition. Explain, with the aid of a diagrams, why it would be better if the mirror were parabolic rather than circular in cross section.

(6 marks)

(b) A small can containing 100 g of water is to be hung at the focus of the parabolic mirror to demonstrate to visitors how the cooker could boil water. How could you prepare the can to make it suitable for this purpose?

If the heat energy received per second by the can is 500 J, estimate how long it would take for the temperature of the water to be raised from 30°C to 100°C.

(6 marks)

(c) In practice, the time is longer than predicted by the calculation in (b) above. Suggest TWO possible reasons why this is so.

(4 marks)

(d) In rural areas of some parts of the Caribbean, it is quite common to see crops being dried on roof tops and at the edge of tarred roads. Mention TWO reasons why these methods of drying crops may be very effective.

(4 marks)

Total 20 marks

4. (a) Discuss Newton’s and Young’s views about the nature of light. Describe, briefly, Young’s experiment to support his views.

(8 marks)

(b) Describe a laboratory experiment similar to that performed by Young, which could be used to show that sound is transmitted as a wave.

(3 marks)

(c) Two students perform an experiment to measure the speed of sound. One student stands 45 m from a wall and claps at a steady rate. Her partner counts 40 intervals between claps in 20 s. Echoes return only midway between successive claps. What value does this experiment give for the speed of sound in air?

(4 marks)

State THREE possible sources of error in the experiment. In your view, which of these errors is likely to be the most serious? Give a reason for your choice.

(5 marks)

Total 20 marks

5. What important principle is involved in the operation of the pin-hole camera?

(1 mark)

(b) A simple pin-hole camera, 20 cm long, is used to view a bright object 2 cm high, placed 50 cm in front of the pin-hole. By means of a scale drawing on graph paper, show how the image is formed, and find the magnification of the image.

(5 marks)

(c) If the camera were only 10 cm long, how big would the image be?

(2 marks)

(d) With the aid of ray diagrams, describe the effect of using a large hole instead of a pin-hole.

(4 marks)

(e) In order to form a visible image on the screen of a pin-hole camera, a bright object is required. To get a good image of a non-luminous object, a student cuts a circular hole in the front of his camera and fits a converging lens of focal length 15 cm in the hole. He now finds he can get a clear image on the screen, which is 20 cm from the lens, only when the object is at a certain fixed distance away.

On graph paper, draw a ray diagram to scale to show how the image is formed. Find the distance from the object to the lens.

(8 marks)

Total 20 marks

6. (a) Draw and label a diagram of a dry cell (torch cell) and describe the function of EACH of its main parts.

(6 marks)

(b) When the circuit shown below is switched on, the voltmeter reading changes from 3.0 V to 2.5 V. Explain why this happens. (2 marks)

(c) A radio has a battery of four 1.5 V dry cells in series which delivers a current of 0.5 A. The battery runs ‘flat’ after about after about six hours of continuous use. If EACH dry cell costs \$1.50, estimate the cost of one kilowatt-hour of electrical energy from these dry cells. Compare your estimate with the likely cost of mains electricity.

(7 marks)

(d) Some low voltage d.c. radios can be operated from the a.c. mains. Name TWO extra devices that must be built into these radios and explain why each is necessary.

(5 marks)

Total 20 marks

7. (a) With clear diagrams for EACH step in the process, describe how a metal sphere on an insulating stand might be charged by induction, using a negatively charged polythene rod.

(5 marks)

Explain, in terms of electron flow, how the sphere acquires a charge, and state the sign of the charge.

(3 marks)

(b) When a conducting sphere, fixed on an insulating stand, is raised to a high potential, a spark passes between it and a nearby earthed sphere. (See diagrams below) (i) Explain how the air between the two spheres is able to pass a current.

(3 marks)

(ii) The microammeter registers an average current of 40µA as a charge of 2.0 µC passes through it.  For how long did the current flow?

(3 marks)

(c) There is a potential difference of 5kV between the cathode and anode of a discharge tube. An electron leaves the cathode and travels toward the anode. Find the work done on the electron and hence calculate the speed of the electron when it reaches the anode.

(6 marks)

Total 20 marks

END OF TEST

Tell a friend