pre-leaving certiﬁ cate examination, 2012 triailscrúdú...

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PHYSICS — HIGHER LEVEL

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TIME: 3 HOURS_______________

Answer three questions from section A and fi ve questions from section B.

*P16*

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Pre-Leaving Certifi cate Examination, 2012Triailscrúdú na hArdteistiméireachta, 2012

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SECTION A (120 marks)

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

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1. A student carried out an investigation in the laboratory to verify the laws of equilibrium for a set of co-planar forces acting on a uniform horizontal metre stick. The weight of the metre stick was found to be 1.2 N. The experimental arrangement is illustrated below. Note that all the forces are acting perpendicular to the metre stick.

Using the information in the above diagram

(i) Show that the vector sum of the forces on the metre stick is zero. (6)

(ii) Show that the sum of the moments of the forces about an axis through the zero mark of the metre stick is zero. (9)

(iii) By choosing an axis through any point on the metre stick other than zero show that the sum of the moments of the forces is zero. (9)

State the conclusion that can be drawn from the results of part (i) and (ii) above. (4)

What change would you make to your calculations if the 5.2 N force was not acting perpendicular to the metre stick. (6)

Show with the help of a diagram how the measured upward forces could be applied to the metre stick. (6)

5.2 N 3.2 N

14.6 7416 85

5 N 2.2 N

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2. In an experiment to measure the specifi c latent heat of fusion of ice a student added melting ice to warm water in a copper calorimeter. The following data was obtained.

Mass of calorimeter……………………………………….. = 60 g Mass of calorimeter and warm water……………………… = 96 g Mass of ice………………………………………………… = 6.5 g Initial temperature of warm water…………………………. = 28oC Final temperature of water and melted ice………………… = 14oC Temperature of ice…………………………………………. = 0oC

Using the given data calculate the value of the specifi c latent heat of fusion of ice. (18)

What is the benefi t of the room temperature being 21oC for the experiment? (6)

If the ice was at –5oC initially, how much extra heat would be needed to change it to water at 14oC. Where might this heat come from? (9)

If the student was unaware that the ice was initially at –5oC how would the value obtained for the specifi c latent heat of fusion of the ice be different? (7)

(specifi c heat capacity of copper = 390 J kg–1K–1; specifi c heat capacity of ice = 2,100 J kg–1K–1; specifi c heat capacity of water = 4,200 J kg–1K–1.)

3. A student carried out an experiment to investigate the relationship between the natural frequency of a stretched string and its length. A sonometer was used and the string was subjected to a constant tension of 80 N throughout the investigation. A set of tuning forks was used to supply the forced frequencies applied to the string. The values of frequency f and the values of length l were noted as follows.

f / Hz 256 288 320 341 384 480 512

l / cm 80 71 64 60 53 43 40

Explain with the help of a diagram how the data above was obtained. (12)

Plot a suitable graph on graph paper to show the relationship between the natural frequency of the stretched string and its length. (12)

Using the graph:

(i) Calculate the length of the string corresponding to a frequency of 300 Hz.

(ii) Calculate the value of the mass per unit length of the string. (12)

Why would higher frequency tuning forks lead to less accuracy in this experiment? (4)

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4. In an experiment to calculate the resistivity of a wire a student measured the diameter of the wire fi ve times at different points along the wire. The values noted were 0.7 mm, 0.68 mm, 0.73 mm, 0.71 mm and 0.68 mm. When fully closed the micrometer used to measure the diameter was reading 0.06 mm. The resistance of the wire was noted as 3.2Ω and the length of the wire was 82 cm.

What was the purpose of measuring the diameter fi ve times? (6)

What is the reading of the micrometer shown in the picture above if the scale is calibrated in mm? (4)

Calculate the average value of the diameter of the wire from the above data. (6)

Calculate the value of the resistivity of the wire. (12)

What precautions should you take to improve the accuracy (i) in the measurement of the length of the wire and (ii) in the measurement of the resistance of the wire? (8)

Why do energy companies need to know the resistivity of the wire they use to transmit electrical energy? (4)

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SECTION B (280 marks)

Answer fi ve questions from this section.Each question carries 56 marks.

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5. Answer any eight of the following parts (a), (b), (c), etc.

(a) A cyclist exerts a force of 50 N on each of the two handlebars of a bicycle and the distance between the handlebars is 50 cm. Calculate the torque on the handlebars. (7)

(b) State Hooke’s law. (7)

(c) The sound intensity level in a room was increased by 6 dB. What was the corresponding increase in sound intensity? (7)

(d) Calculate the critical angle for a transparent material of refractive index 1.8. (7)

(e) State the three factors upon which the capacitance of a parallel capacitor depends. (7)

(f) Explain why a ring circuit for electrical sockets in the home reduces the risk of the wires overheating while carrying current. (7)

(g) A refrigerator uses a heat pump. What is the principle of a heat pump? (7)

(h) Calculate the emf induced in a coil of 5,000 turns if the magnetic fl ux changes from 6 Wb to 2 Wb in a time of 4 ms. (7)

(i) Which of the three types of radiation ,α β or γ is the most ionising? (7)

(j) Give the quark composition of the proton. or Explain the difference between a diode in forward bias and a diode in reverse bias. (7)

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6. State (i) the principle of the conservation of energy (ii) the principle of the conservation of momentum. (12)

A sphere of mass 2 kg is attached to a string of length 3 m. The sphere is pulled to one side so that the string makes an angle of 600 to the vertical. The sphere is released and it swings down and hits a stationary block of mass 10 kg on a smooth horizontal table. The sphere stops moving on impact and the block moves to the right.

Calculate: (i) the speed of the sphere at the moment of impact (9) (ii) the speed of the block immediately after impact. (9)

The speed of the block is rounded to the nearest whole number. The motion of the block is now recorded with a data logger. The following is noted: The block moves at a uniform speed for 3 seconds. The block then accelerates uniformly at 2 m s–2 for 5 seconds. The block then moves at a uniform speed for half a minute. The block then decelerates to rest in 8 seconds.

(i) Draw a velocity-time graph to represent the motion of the block. (9)

(ii) Use the velocity-time graph to calculate the distance travelled by the block while being analysed by the data logger. (9)

(iii) Draw a diagram to show the surface along which the block might have moved to record the data above. What problem might be encountered by the data logger as it tracks the motion of the block? (8)

(g = 9.8 m s–2)

3 m

3 m

60o

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7. State the laws of refl ection. (6)

Show by means of ray diagrams (i) the formation of a virtual image in a concave mirror and (ii) the formation of an image in a convex mirror. (18)

Why does a dentist use a concave mirror to view a cavity in a tooth rather than a plane mirror? (6)

Why is a convex mirror used as the rear view mirror in a car? (6)

An object of height 5 cm is placed 90 cm in front of a convex mirror of focal length 30 cm. Calculate the position and size of the image formed. (12)

Most torches contain a concave refl ector. Explain with a diagram the purpose of this refl ector. (8)

8. Defi ne electric fi eld strength and give its unit. (6)

Why does charge reside on the outside of a conductor? Outline an experiment to demonstrate that charge resides on the outside of a conductor. (15)

Point action is due to the accumulation of charges at a sharp point on a conductor. Explain how point action contributes to the operation of a lightning conductor placed on the roof of a tall building. (9)

It is generally considered unwise to play golf on a day when thunder storms are expected. Give a reason based on electricity theory why this is so. (5) A metal sphere of diameter 30 cm has a charge of 3 mC spread evenly on its surface. Calculate: (i) the electric fi eld strength at the surface of the sphere (ii) the electric fi eld strength 7 cm from the surface (iii) the force on a 4 Cμ point charge placed 7 cm from the surface of the sphere. (21)

(Assume that the sphere is located in air and that the permittivity of air is the same as the permittivity of free space).

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9. What are x-rays and name the scientist credited with their discovery. (12)

Outline with the help of a labelled diagram how x-rays are made. Comment on the suitability of a metal that may be used to produce the x-rays. (12)

Give a brief description as to what happens inside an atom when x-rays are made. (6)

X-rays are used to examine bone structure in the human body. Distinguish between the intensity of an x-ray beam and the penetrating power of an x-ray beam. (8)

Give two other uses for x-rays apart from examining bone structure. (6)

Calculate the minimum wavelength of x-rays produced when the accelerating voltage is 80,000 V. (12)

10. Answer either part (a) or part (b).

(a) In 1932 Cockcroft and Walton carried out an experiment in which they bombarded lithium with high energy protons. The nuclear reaction resulted in the production of two α-particles.

(i) Outline with a labelled diagram the details of their experimental arrangement. (9)

(ii) Write a nuclear equation for the reaction. (6)

(iii) How did they detect the α-particles that were produced? (6)

(iv) Calculate the energy released in the reaction. (12)

(mass of proton = 1.6730 × 10–27 kg; mass of lithium nucleus = 1.1646 × 10–26

kg;

mass of α-particle = 6.6443 × 10–27 kg; speed of light, c = 3.00 × 108

m s–1)

The success of Cockcroft and Walton resulted in other research scientists improving the design of particle accelerators. One type of accelerator uses very strong magnetic fi elds to force charged particles to travel in circular motion. Having been accelerated to very high speeds the particles are often arranged to collide. These collisions may result in the making of new particles.

Give the name of three new particles and state their charge. (6)

Why does a high speed charged particle travel in a circular motion when it enters a magnetic fi eld? (5) Electrons enter a magnetic fi eld travelling with a speed of 9.2 × 107

m s–1.

The magnetic fl ux density of the magnetic fi eld is 3 kT. Calculate the radius of the circular orbit that the electron follows. Assume that the electron travels perpendicular to the direction of the magnetic fi eld. (12)

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(b)

(i) The diagram above shows the various parts of an a.c. generator. Why is a voltage induced in the coil? (6)

Draw a graph of the induced voltage as a function of time. (6)

State three factors that determine the magnitude of the induced voltage. (9)

(ii) An induction coil was invented by N. Callan, an Irish scientist working in Maynooth University.

Draw and label a diagram of an induction coil. (12)

Give a use for an induction coil. (6)

(iii) Transformers are used to increase the voltage of an a.c. supply.

Why do transformers work for a.c. and not for d.c. (6)

Energy companies transmit electricity at very high voltages even though these high voltages are dangerous. What is the reason for the high voltages?

(5) Give two factors that lead to energy losses in transformers. (6)

Sinusoidalvoltage output

The mechanical energy input toa generator turns the coil in themagnetic fi eld.

A voltage proportional tothe rate of change of thearea facing the magneticfi eld is generated in the coil. This is an exampleof Faraday’s law.

11. Read the following passage and answer the accompanying questions.

While preparing for a school debate on the advantages and disadvantages of nuclear power a student wrote down the following points.

Advantages: • Almost zero emissions (very low greenhouse gas emissions). • The cost of importing a fossil fuel like oil is very high. • A small amount of matter creates a large amount of energy. • A lot of energy is generated from a single power plant. • A nuclear aircraft carrier can circle the globe continuously for 30 years on its original fuel while a diesel fuelled carrier has a range of only about 3000 miles before having to refuel.

Disadvantages: • Nuclear plants are more expensive to build and maintain. • Proliferation concerns – breeder reactors yield products that could potentially be stolen and turned into an atomic weapon. • Waste products are dangerous and need to be carefully stored for long periods of time. The spent fuel is highly radioactive and has to be carefully stored for many years or decades after use. This adds to the costs. • Nuclear power plants can be dangerous to its surroundings and employees. It would cost a lot to clean in case of spillages. • There exist safety concerns if the plant is not operated correctly or conditions arise that were unforeseen when the plant was developed, as happened at the Fukushima plant in Japan; the core melted down following an earthquake and tsunami the plant was not designed to handle despite the world’s strongest earthquake codes.

(a) What is the difference between nuclear fi ssion and nuclear fusion? (7)

(b) Why is nuclear fusion not used at present to generate energy for commercial use? (7)

(c) Nuclear fi ssion reactors can use enriched uranium as a fuel. What is enriched uranium? (7)

(d) What is the purpose of the moderator in a thermal nuclear reactor? (7)

(e) A nuclear reactor is just a big kettle. What is the justifi cation for such an apparently strange comment? (7)

(f) Calculate the decay constant for a substance whose half- life is 2 hours. (7)

(g) In the following nuclear reaction the mass of each deuterium nucleus has a value of 3.344 × 10–27

kg and the mass of the helium nucleus is 6.646 × 10–27

kg. E is the

energy released in the reaction. Calculate the value of E. (c = 3 × 108 m s–1)

12

12

24H H He E+ → + (7)

(h) Give two sources of background radiation. (7)

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12. Answer any two of the following parts (a), (b), (c), (d).

(a) State Newton’s universal law of gravitation. (6)

To a reasonably good approximation the earth orbits the sun in a circular orbit. Given that the mass of the sun is 2 1030 kg, that the radius of the orbit is 1.5 1011 m and that G = 7.6 10–11 N m2 kg–2.

Calculate (i) the linear speed of the earth (ii) the angular speed of the earth (iii) the period of the orbit. (18)

Explain why the mass of the earth is not needed for these calculations. (4)

(b) Explain with the help of a diagram the Doppler effect. (10)

A train’s whistle emits a continuous note of frequency 640 Hz as it approaches a person standing near the track. To the person the frequency appears to be 720 Hz. Calculate the speed of the train. (10)

Explain in terms of both frequency and wavelength how you can tell whether a distant star is travelling to or from the earth by examining the light from the star. (8) (speed of sound in air = 340 m s–1)

(c) What is (i) current (ii) resistance? (6)

Using the diagram below calculate: (i) the total resistance of the fi ve resistors (ii) the total current fl owing from the 12 V battery (iii) the current fl owing through the 2 Ω resistor. (22)

(d) Name the Irish scientist who gave the electron its name in the nineteenth century. (6)

Distinguish clearly between thermionic emission and photoelectric emission in relation to the electron. (6)

Give a brief outline including diagrams of how you might demonstrate either thermionic emission or photoelectric emission in the laboratory. (10)

Why did Einstein’s explanation of photoelectric emission cause quite a sensation when it was fi rst published? (6)

5 Ω2 Ω

12 V

4 Ω

6 Ω10 Ω

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pre-leaving certiﬁ cate examination, 2012 triailscrúdú...

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