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SIR M A REFALO CENTRE FOR FURTHER STUDIES ANNUAL EXAM 2002 A LEVEL - PHYSICS 2ndyear TIME ALLOWED: 2 HRS

Data: speed of light in vacuum. c = 3 x108 m/s, Planck's constanth = 6.6 x 10-34 Js

rest mass of an electron mo = 9.1 x 10-31 kg.

Section A. Attempt all questions.

1. A metal wire of length L and area of cross-section A is fixed at one end and hangsvertically with a load W attached to its free end. The wire is found to stretch by anamount x.a) Give expressions in terms of L, W and x , one in each case, fori) stress , ii) strain , iii) the Young modulus E of the metal.b) The wire has length 2.5 m. A tensile stress of 6.4 x 107 Pa is applied. The Youngmodulus of the metal is 1.1 x 1011 Pa. Calculate the strain and the extension of the wire.c) Suggest, with a reason

i) whether a 30 cm rule would be a suitable measuring instrument for the extension,ii) what would happen to the extension if the wire were to be replaced by another wireof the same dimensions made of a metal with a smaller Young modulus. Assume thatthe load remains the same and that the wire does not exceed the elastic limit.

2. A radiant heater consists of four heating elementsas shown in the adjacent circuit. Each heatingelement is made of wire of length 0.60 m and cross-sectional area 4.0 x 10-7 m2 . The wire has resistivity8.0 x 10-5m at room temperature. i) Show that theresistance of one heating element at room

temperature is 120 .ii) Calculate the total resistance at room temperatureof the radiant heater.iii) Calculate the power output from the heater whenit is connected to a 230 V supply.iv) Calculate the energy output of the heater in kWh if it is kept on for 8 hours.v) The wire in one of the elements of the heater breaks. State and explain the effect onthe total resistance and on the power output of the heater.

3. a) An insulated zinc plate has a negative charge. When ultra-violet radiation isincident on the plate, the plate rapidly loses its negative charge. Explain what is

happening to cause this effect.b) Einstein analyzed the phenomenon described above and derived the equation

hf = + mv2 .

State what each of the three terms in the equation represent.c) i) Briefly describe how the wave nature of electrons may be demonstrated.ii) Use E =hf and E = mc2 to derive an expression for the wavelength of a photon interms of its linear momentum. Now calculate the de Broglie wavelength of electronstraveling at a speed of 5.0 x 107m/s.

4. The diagram shows some of the energy levels for atomichydrogen.i) Copy the diagram and add arrows to it showing all the singletransitions which would ionize the atom.ii) Why is the level labelled -13.6 eV called the ground state?

0.00 eV- 1.51 eV

- 3.39 eV

- 13.6 eV

Power supply

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iii) Do some calculation to identify the transition which would result in the emission oflight of wavelength 660 nm. Mark this transition on your copy of the diagram.

5.

a) Calculate the speed of the wave.b) If the load is doubled and the frequency is kept constant at 60 Hz, how many loopswill there be on the string? Give reasons.c) If the frequency is increased, calculate the next resonating frequency assuming theload remains constant.

6. The grid enables different nuclei to be represented byplotting the number of neutrons N against the number ofprotons Z in nucleus. The arrow shows a nucleus Xdecaying to a nucleus Y.i) What type of radioactivity decay is taking place?ii) Write a nuclear equation for this decay. Include all theinformation possible.iii) Copy the grid and add another arrow to the grid torepresent what happens if nucleus Y subsequently decaysby - emission to nucleus W.iv) Mark a point P on the grid that could represent the

nucleus of an isotope of X.

SECTION B: Choose TWO questions.

7. a) A capacitor is marked '2200F 15V" .i) Explain what is meant by a capacitance of 2200F.ii) What is the significance of the 15V marking?

b) An egg time is designed toproduce a sound when an egg

has been boiled for a sufficienttime. The time which elapsesbefore the alarm sounds iscontrolled by the circuit shown.The circuit is operated from a6.0 V cell of negligibleinternal resistance.

The time is set by means of the variable resistor R.The capacitor is charged by moving the two-way switch to position S1 for a short time.The timing is then started automatically when the two-way switch is moved to position

S2. An alarm rings when the potential difference between terminals X, Y reaches 2.0 V.i) In one setting the time constant of the circuit when the capacitor is discharging is 3.0minutes. Sketch a graph to show how the p.d. between the terminals X, Y varies withtime for the first 6.0 minutes after the switch moves to the position S2.ii) The discharge of a capacitor is given by

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V = V0 e- t/(RC)

Calculate how long after timing begins will the alarm start to sound for the setting inpart (i)?iii) Calculate the resistance of the variable resistor when the time constant is 3.0minutes.

iv) The system is designed to measure cooking times up to 5.0 minutes. Determine themaximum value of the resistance R that is needed.v) State how a suitable capacitor would be connected to increase the measurablecooking time.

8.

SECTION C:Choose ONE question.

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10. a) The fate of the Universe is thought to depend on the mean density of matter inthe Universe.i) State what is meant by the critical mean density o of matter in the Universe.ii) The figure below shows possible curves for the variation with time of the size of theUniverse.Label the curvecorresponding too .

iii) Suggest one reason

why the ultimate fate ofthe Universe is not yetknown.

b)