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L.35

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PRE-LEAVING CERTIFICATE EXAMINATION, 2015

PHYSICS – ORDINARY LEVEL

TIME – 3 HOURS

Answer three questions from Section A and five questions from Section B.

N.B. Relevant data are listed in the Formulae and Tables booklet, which is available

from the Superintendent.

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2015.1 L.35 2/12 page 2 of 12

SECTION A (120 marks)

Answer three questions from this section. Each question carries 40 marks.

1. An experiment was set up to investigate the relationship between the acceleration, a, of an

object and the force applied, F, to it. The acceleration was measured when a number of different forces were applied to an object.

The following results were recorded during the experiment.

F (N) 0.3 0.4 0.5 0.6 0.7 0.8

a (m s–2) 0.62 0.83 1.03 1.24 1.45 1.65

(i) Draw a labelled diagram of the apparatus used in the experiment. (9) (ii) What measurements would have been taken to calculate acceleration? (9) (iii) Draw a graph, on graph paper, of F on the X-axis against a on the Y-axis. (12) (iv) What can you tell from the graph about the relationship between the acceleration of an

object and the force applied to it? (6) (v) Name one force, other than the applied force, whose effect needs to be minimised in this

experiment. (4) 2. A student carried out an experiment to measure the focal length of a concave mirror. The

student placed an illuminated object in front of the mirror and measured the object distance, u, from the mirror and also measured the image distance, v. The procedure was repeated for different values of u.

The table shows the measurements recorded by the student.

u (cm) 20 30 40

v (cm) 37 23 19

(i) Draw a labelled diagram showing how the student set up this experiment. Indicate the

distances u and v on your diagram. (15) (ii) Explain how the image distance, v, was measured. (6) (iii) Calculate the focal length, f , of the concave mirror using the student’s data. (12) (iv) When the student placed the object a distance of 10 cm from the mirror, an image could

not be located. Why could the student not locate an image? (7)

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3. A student carried out an experiment to measure the speed of sound in air. The following is an extract from her report.

“I held a 1 above a 2 that was held in a graduated cylinder filled

with water. I adjusted the length of the 2 above the water until a loud sound

occured. I recorded the following data:

X

Y ” (i) Copy the student’s report into your answer book and replace the spaces 1 and 2 with the

names of the missing pieces of apparatus. (6) (ii) What data would the student have recorded at X and Y in her report? (9) (iii) Explain how the wavelength, , of the sound wave would have been approximated by the

student using one of the measurements X or Y. (6) (iv) How were the data X and Y used to approximate the speed of sound in air? (6) (v) The student took one more measurement to improve the accuracy of the

wavelength value. (a) What did the student measure? (b) Name the instrument with which it was measured. (7) (vi) State two other possible sources of error that may lead to an inaccurate result. (6) 4. In an experiment to verify Joule’s law, a heating coil was placed in an insulated calorimeter of

water. A current, I, flowed in the water for a certain length of time and the rise in temperature, Δθ, of the water was measured. The procedure was repeated for different values of I.

(i) Draw a labelled diagram of the apparatus used in this experiment. (15) (ii) State three quantities in this experiment that should be kept constant, for each value of I,

to ensure a more accurate result. (9) (iii) (a) Sketch a graph that could be used to verify Joule’s law. On your graph, label the

vertical (Y-axis) and horizontal (X-axis) axes with the appropriate quantities. (b) How does your graph verify Joule’s law? (16)

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SECTION B (280 marks)

Answer five questions from this section. Each question carries 56 marks.

5. Answer any eight of the following parts (a), (b), (c), etc. (a) State Newton’s third law of motion. (b) Which of the following is the unit of momentum? N m–2 kg m s–1 N m m s–2 (c) How is the Celsius scale of temperature

related to the Kelvin scale? (d) Why is it not possible to polarise sound waves? (e) What physical quantity is measured in decibels? (f) A capacitor has a capacitance of 0.0025 F. Calculate its charge

when it is connected to a 6 V battery. (g) Explain the distinction between radial circuits and ring circuits. (h) A transformer has 100 turns in the primary coil and 2000 turns in the secondary coil.

Calculate the output voltage if 12 V is connected across the primary coil. (i) What is the photoelectric effect? (j) Which of the following scientists is associated with discovering radioactivity and also has

the unit for the rate of radioactive decay named after him? Becquerel Rutherford Röntgen Einstein

(8 × 7)

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6. (a) State the principle of conservation of energy. (9)

(b) A man of mass 90 kg (including his parachute) takes a parachute jump from a helicopter when it is hovering in a fixed position 1400 m above the ground. He pulls the parachute chord when he is 1000 m above the ground.

Find (i) the potential energy of the man when he leaves the helicopter (ii) the potential energy of the man when he pulls the parachute chord. (9) (iii) Assuming that he has not been affected by air friction before the parachute is

opened, what is the kinetic energy gained by the man between leaving the helicopter and opening the parachute? (6)

(iv) Calculate the speed of the man when the parachute is opened. (9) (v) Draw a diagram showing three forces acting on the man or the parachute

canopy after the parachute is opened. (15) (vi) If instead the man jumped from an aeroplane, explain why he would not land

directly underneath the aeroplane. (8) (acceleration due to gravity, g = 9.8 m s–2)

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7. What is meant by latent heat? What is the difference between latent heat of fusion and latent heat of vaporisation? (18) A person pours water of mass 500 g at a temperature of 15 C into an ice cube tray. The ice

cube tray is then placed in a freezer. The ice at 0 C in the freezer is then lowered to –18 C.

Calculate how much energy is required (i) to reduce the temperature of the water from 15 °C to 0 C (ii) to convert the water at 0 C to ice at 0 C (iii) to reduce the temperature of the ice at 0 °C to –18 °C. (21) (specific heat capacity of water, 4180 J kg–1 K–1; specific latent heat of fusion of ice,

3.3 × 105 J kg–1; specific heat capacity of ice 2200 J kg–1 K–1) If the power rating of the freezer is 250 W and assuming all the electrical energy of the freezer

is removing heat from the water, how long will it take for the water at 15 C to become ice at –18 C? (11)

What change of state occurs during perspiration?

(6)

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8. When light passes through a diffraction grating, firstly, it undergoes diffraction at the grating and then at various positions away from the grating both constructive and destructive interference are observed.

(i) Explain the underlined terms. (12) (ii) An interference pattern can be observed when a diffraction grating is used with a

spectrometer. Draw a labelled diagram of a spectrometer when it is used for this purpose. (12)

(iii) What is observed in the spectrometer when constructive interference occurs? (6) A light source that contains two different wavelengths (one green and one red) is placed in

front of a spectrometer. A diffraction grating, with distance between adjacent slits of 2.5 × 10–6 m, is placed on the turntable.

(iv) If the first order fringe for the green line is observed at an angle of 12 from the

central fringe, calculate the wavelength of the green light. (12) (v) If the wavelength of the red light is 6.32 × 10–7 m, calculate the angle of the

first order fringe from the central fringe. (10) (vi) What term is given to the phenomenon where light is split into different colours? (4) 9. (i) Explain the term resistance and name an instrument which measures it. (9) (ii) Define electromotive force (emf) and name an instrument which measures it. (9) (iii) Name two sources of emf. (6)

A

2 Ω

4 Ω

12 V

The circuit diagram shows a 4 Ω and a 2 Ω resistor connected in parallel with a 12 V battery. Calculate (iv) the total resistance of the circuit (v) the current flowing through the ammeter A (vi) the current flowing through the 2 Ω resistor. (24) (vii) Why does the resistance of a metal bar decrease when it is placed in a refrigerator? (8)

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2015.1 L.35 8/12 page 8 of 12

10. Cathode ray tubes were first produced in the late 1800s and were used in the identification of a fundamental particle. Cathode ray tubes were widely used in televisions and computer monitors up to the year 2000. Since then, flat screen technology has become dominant in these applications.

(i) What are cathode rays? (6) (ii) Name the Irish scientist who named the particle discovered using electrons. (3) (iii) Give one other application of a cathode ray tube. (4) (iv) Draw a labelled diagram showing the main parts of a cathode ray tube. (15) (v) What is thermionic emission? Explain how thermionic emission is applied to the

production of cathode rays in a cathode ray tube. (15) (vi) How are cathode rays deflected in a cathode ray tube? (6) (vii) State two energy conversions that take place in a cathode ray tube. (7)

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2015.1 L.35 9/12 page 9 of 12

11. Read this passage and answer the questions below. A freely suspended magnet always points in the

North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South. The shape of the Earth’s magnetic field resembles that of a bar magnet of length one-fifth of the Earth’s diameter buried at its centre.

The south pole of the Earth’s magnet is in the

geographical north because it attracts the north pole of the suspended magnet and vice versa. Thus, there is a magnetic South pole near the geographical North, and a magnetic North pole near the geographical South. A freely suspended magnet does not show exact geographical South and North because the magnetic axis and geographical axis of the Earth do not coincide.

It is now believed that the Earth’s magnetism is due to the magnetic effect of current

which is flowing in the liquid core at the centre of the Earth. Thus, the Earth is a huge electromagnet.

The vertical plane passing through the axis of a freely suspended magnet is called

magnetic meridian. The direction of Earth's magnetic field lies in the magnetic meridian and may not be horizontal. The vertical plane passing through the true geographical North and South (or geographical axis of Earth) is called geographical meridian. The angle between the magnetic meridian and the geographic meridian at a place is called magnetic declination at that place.

(Adapted from ‘howmagnetswork.com’)

(a) Why does a freely suspended magnet align in the North–South direction, when not in the

presence of any other magnet? (b) Why does a magnet not point towards the geographical North pole? (c) What is meant by magnetic declination? (d) Draw a diagram showing the shape of the Earth’s magnetic field. (e) The Earth’s magnetic field is believed to be due to a current flowing in the liquid core of

the Earth. Sketch the shape of the magnetic field around a current flowing through a straight current-carrying conductor.

(f) How has the Earth’s magnetic field been used in ships to help them navigate? (g) The magnetic flux density of the Earth’s magnetic field at a particular point on the Earth

is 3.1 × 10–5 T. What is the magnetic flux cutting through an area of 1000 m2 at that position?

(h) A straight wire of length 100 m is carrying a current of 100 A. It is placed at right angles

to the Earth’s magnetic field, at the same position on the Earth as indicated in part (g). Calculate the force on the wire.

(8 × 7)

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2015.1 L.35 10/12 page 10 of 12

12. Answer any two of the following parts (a), (b), (c), (d). (a) Explain what is meant by the moment of a force. (6) Copy the diagram below and show all the forces acting on the horizontal plank of the

seesaw. (8)

Person A Person B

If person B is 40 cm from the fulcrum and has a weight of 950 N and person A has

a weight of 450 N, how far apart are they on the seesaw? (14) (b) Optical fibres are widely used in telecommunications. They transfer signals over large

distances using total internal reflection.

(i) Describe an experiment to demonstrate total internal reflection. (12) (ii) Give one advantage of using optical fibres instead of electrical copper wires

in telecommunications. (4) (iii) If the refractive index of the material of the optical fibre is 1.42, what is the

smallest angle to the normal, i.e. the critical angle, at which the light can strike the surface of the fibre and still transmit the signal?

What would happen to the light signal if the angle at which it strikes the surface

is less than the critical angle? (12)

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(c) A large amount of charge builds up on the outside metal surfaces of aeroplanes when in flight. Pointed rods behind the wings allow some of the charge to be removed during flight. The remainder of the charge is removed after landing before fuel is supplied to the plane.

(i) What causes this large build-up of charge? (6) (ii) Copy the diagram below and show how the charge would be distributed on the

aeroplane. (6)

(iii) Why would having pointed rods attached to the wings help remove charge

from the plane? (6) (iv) How could the remainder of the charge be removed when the plane is on

the ground? Why is it important that the charge is removed before fuel is added? (10)

(d) Nuclear fusion is the reaction that has been powering the Sun for the last 5 billion years.

(i) What is nuclear fusion? (6) (ii) Name the two gases which are involved in nuclear fusion in the Sun. (6) (iii) Explain how Einstein’s theory of relativity (E = mc2) is important in explaining

how nuclear fusion provides so much energy from the Sun. (9) (iv) State two advantages of using nuclear fusion as a source of energy instead of

nuclear fission. (7)

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