The Diagram Shows Some Energy Levels, in Ev, of an Atom

SKH LAM WOO MEMORIALSECONDARY SCHOOL

F.7 Physics Mock Examination (2004-2005)

Paper I

Mark: /120

Name: ______( )

Instructions:

1. Date of examination: 3/2/2005

Time allowed: 3 hours
Answer all questions
Unless otherwise stated, take

5. Total mark of this paper is 120, with marks distributed as:

Question / 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10
Mark / 13 / 13 / 12 / 14 / 13 / 8 / 10 / 12 / 12 / 13

crane

The figure below a platform mounted on a spring used to measure the mass of a lorry. The vehicle and its contents are put onto platform by a crane. The platform is then made to oscillate vertically and the mass is found from a measurement of the natural frequency of oscillation.

(a)(i) During measurements, the lorry is dropped from the crane suddenly from a height of

3m onto the platform and the spring under the platform is compressed by 1m. It is known that the mass of the lorry and its contents is 5000 kg. Find the spring constant.

(2 marks)

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(ii) Show that the platform will perform SHM and show that period of oscillation is given by. (4 marks)

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(iii) Find the amplitude of oscillation. (2 marks)

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(iv) If the frequency of oscillation of the platform with the lorry on it is 1 Hz. Find

the mass of the platform. (2 marks)

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(b)The driver is required to turn off the vehicle engine while the measurement is taking place.

The driver of the lorry in part (a)(iv) fails to do this and slowly increases the frequency of his vehicle from 0.5 Hz to about 4 Hz when the measurement is in progress and the platform is free to move. Describe and explain how the amplitude and frequency of the platform vary at this frequency increase occurs. You should sketch a graph to support your answer. (3 marks)

Graph:

Explanation:

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2. The data in the table below is from the Apollo 11 space fight (which resulted in the first manned moon landing). The velocity of the spacecraft is given for various distances from the centre of the Earth. During the period to which this data applied there was no rocket propulsion and the total mass of the spacecraft was m = 44  103 kg.

Distance from centre of Earth
r/106 m / Velocity
v/ms-1
11.0 / 8406
26.3 / 5374
54.4 / 3653
95.7 / 2619
169.9 / 1796
209.2 / 1532
240.6 / 1356

At the distances from the Earth’s centre given in the table the influence of the moon’s gravitational field on the motion of the spacecraft is negligible, and you should ignore it when answering the following questions:

(a)Why does the velocity of the spacecraft decrease? (1 mark)

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(b)Using the data in the above table, determine the change in gravitational potential energy of the spacecraft as it moves from a distance of 11.0  106 m to a distance of 240.6  106 m from the Earth’s centre. (2 marks)

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(c)When the data from the table is used to plot the kinetic energy of the spacecraft as a function of its distance from the Earth’s centre, the graph in Figure a is obtained (the kinetic energy for r = 11  106 m has not been plotted since it falls outside the scale chosen). (Given radius of the earth, gravitational constant ).

(i)Show that the slope of the line = GMm, Mis Earth’s mass, and m is the total mass of the spacecraft. Hence find the mass of the earth.

(ii)What is the physical significance of the intercept on the kinetic energy axis?

(iv) If the value of y-intercept is 0.751,determine the initial take-off speed of the spacecraft.

(7 marks)

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(d)(i) Define the term “parking orbit”.

(ii) Use the data given in part (b) and (c), find the distance of the parking orbit

form the centre of the earth, if the spacecraft circles in the parking orbit

instead of traveling to moon.

(3 marks)

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3. Two students decide to investigate the optical spectrum of helium from a

low-pressure helium discharge tube as shown in Figure 3.1.

In order to measure the wavelengths in the spectrum, the students use a diffraction

grating having 300 lines per mm and they set up the grating and two meter rules, P

and Q, on a bench as shown in Figure 3.2, which may NOT be drawn into scale

with the Figure 3.1.

(a)In Figure 3.2, show where you think the discharge tube should be placed. Indicate clearly its orientation and its alignment. (Circuit is not required). (2 marks)

(b)Explain briefly the three factors affecting the choice of the distance of the discharge tube from the grating. (3 marks)

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The students find that both the first and the second order spectra produced by the grating are clearly visible. They decide to make measurements on the second order.

(c)Why do you think they choose the second order rather than the first order. ?

(1 mark)

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Theoretically the higher-order spectra can be observed although they are less bright.

(d)Find the theoretical maximum order of spectra that can be observed for blue light of wavelength 449 nm. (2 marks)

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(e)What measurement(s) must the students take in order to calculate the wavelengths of the spectral lines. (1 mark)

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The students measure the wavelengths of three spectral lines. The values obtainedare shown below.

Blue line / Green line / Yellow line
Wavelength / nm / 449 / 506 / 592

(f)Sketch the first and second order spectra of blue, green and yellow lines produced by the grating in the space below. Label significant features.

(3 marks)

A microwave transmitter directs waves towards a metal plate. When a microwave detector is moved along a line normal to the transmitter and the plate, it passes through a sequence of equally spaced maxima and minima of intensity.

(a) Explain how these maxima and minima are formed. (3 marks)

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(b) The detector is placed at a position where the intensity is a minimum. When it is moved a

distance of 144 mm it passes through nine maxima and reaches the ninth minimum from the starting point. Calculate

(i) the wavelength of the microwaves,

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(ii) the frequency of the microwave transmitter.

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(3 marks)

(c)(i) In another occasion the same transmitter moves instead and the detector rests at

certain position. There are still alternative strong and weak signals being

detected. Explain why.

(ii) For a detector to detect a time interval of s between successive strong

signals, how fast should the transmitter move? Is this speed available in school

laboratory?

(7 marks)

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5. A coil is connected to a centre zero ammeter, as shown in Fig.a. A student drops a magnet so that it falls vertically and completely through the coil.

Fig. a

It is given that the coil has 100 turns and its cross-sectional area is 30 .

The magnitude of average magnetic field strength in the coil varies with time as:

B field strength /0.001 Tesla

Fig. b

Time /0.01s

(a)(i) Maximum induced e.m.f. in the coil. Show your work.

(ii) If the coil has a resistance of 2 Ohm. Find the maximum power delivered to

the coil.

(iii) Sketch the induced e.m.f. in the coil against time in Fig. b.

( 7 marks)

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(b)If the coil were not present the magnet would accelerate downwards at the acceleration due to gravity. State and explain how its acceleration in the student’s experiment would be affected, if

(i) as it entered the coil,

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(ii) as it left the coil.

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(4 marks)

(c) Suppose the student forgot to connect the ammeter to the coil, therefore leaving the circuit incomplete, before carrying out the experiment. Describe and explain what difference this would make to your conclusions in part (b). (2 marks)

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6. A square wave signal of frequency 50 Hz spends equal time high and low. The signal is applied to the input of a resistor-capacitor network and the output waveform is viewed using an oscilloscope as shown in Figure a. The trace seen on the oscilloscope is shown in Figure b.

Figure a

Figure b

(a) Describe how the circuit in Figure a leads to the output waveform shown in Figure b. (4 marks)

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(b) The resistor R has a resistance of 10 kΩ. Estimate a value for the capacitance of C.

time base setting = 2 ms/division (4 marks)

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7. The circuit shows an ideal operational amplifier used as voltagecomparator.

The voltage Vin is steadily increased from 0V. Calculate Vin when the sign of Vout changes from negative to positive. (2 marks)

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(b) (i) The op-amp is required to operate an LED. Add to the circuit an LED and its limiting resistor so that the LED lights when Vin is less than the value calculated in part (a).

(ii) Explain why the LED functions in the position you have drawn it.

(iii) Calculate the minimum value for the limiting resistor with the LED. Assume that the LED has a voltage drop of 2.0V across it when emitting and a maximum current of 25mA through it.

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(5 marks)

of the LDR is shown below.

(lux is the unit for measuring light intensity)

Determine the light intensity at which the LED switches.(3 marks)

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8.One mole of helium gas undergoes a cycle ABCA in which its pressure, P and

temperature T are indicated in the P-T diagram in the figure 3.1.

Given: Universal gas constant = 8.31 J mol-1 K-1

Avogadro constant = 6.02 x 1023 mol-1

Assume that helium gas behaves as an ideal gas.

(a)(i)Find the volume of the gas at state A.(2 marks)

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(ii)Calculate the root mean square speed of the helium gas at state A.

(2 marks)

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(iii)Do you expect the value calculated in (ii) would be higher or lower, if the gas is oxygen gas of same mole? Explain briefly. (3 marks)

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(b)(i)Draw a relevant P-V graph in the space below corresponding to the

cycle ABCA.(3 marks)

(ii)Calculate the net work done by the gas in the cycle ABCA.(2 marks)

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9. The diagram shows some energy levels, in eV, of an atom.

Figure a

Photons of specific wavelengths are emitted from these atoms when they are excited by collisions with electrons.

(a) Explain

(i) what is meant by the process of excitation,

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(ii) why the emitted photons have specific wavelengths.

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(5 marks)

(b) One of the emitted photons has an energy of .

(i) Calculate the wavelength of this photon.

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(ii) Determine which transition is responsible for this emitted photon.

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(iii) Draw an arrow on the energy level diagram on Figure ato show the transition responsiblefor the emission of a photon with the shortest wavelength.

(7 marks)

10. (a) (i) Define the Young modulus for a material. (1 mark)

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(ii) Explain what is meant by the elastic limit for a wire. (1 mark)

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(b) A wire supported at its upper end, hangs vertically. The table shows readings obtained when stretching the wire by suspending masses from its lower end.

(i)Plot a graph of load against extension on the grid provided.

(ii) Indicate on your graph the region where Hooke’s law is obeyed and explain your reason.

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(iii) The unstretched length of the wire is 1.6 m and the area of cross-section . Calculate the value of the Young modulus of the material.

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(7 marks)

(c) (i) By considering the work done in stretching a wire, show that the energy stored is given by ½ Fe, where F is the force producing an extension e.

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(ii) Calculate the energy stored in the wire in part (b) when the extension is 4.0mm.

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(4 marks)

End of paper I