ST. DENIS SEBUGWAWO S.S GGABA

UACE

INTERNAL SEMINAR OCTOBER 2013

PHYSICS II

SECTION A

1. (a) What is meant by the term radii of curvature as applied to a converging lens?

(1mk)

(b) (i) Show that the focal length, f, of a converging lens is given by:

1

𝑓= (𝜇 − 1)

1

𝑟1+

1

𝑟2

where 𝜇 is the refractive index of the material of the lens and r1 and r2 are the radii of

curvature of the surfaces of the lens. (5mks)

(ii) A biconvex lens of radius of curvature 24cm is placed on a liquid film on a

plane mirror. A pin clamped horizontally above the lens coincides with its

image at a distance of 40cm above the lens. If the refractive index of the

liquid is 1.4, what is the refractive index of the material of the lens?

(6mks)

(c) Describe, with the aid of a labeled diagram, the functions of the essential parts of a

photographic camera. (4mks)

(d) A projector is required to project slides which are 5.0cm square onto a screen which

is 5.0m square. If the focal length of the projection lens is 0.1m, what should be the

distance between the screen and the slide? (4mks)

2. (a) What is meant by the following terms as applied to a telescope?

(i) Magnifying power (1mark)

(ii) eye-ring (1mark)

(b) (i) Draw a ray diagram to show the formation of the final image by an

astronomical telescope in normal adjustments. (3 marks)

(ii) With the aid of the diagram in (b) (i), derive an expression for the magnifying

power of an astronomical telescope in normal adjustment. (4marks)

(iii) Give the disadvantage of the telescope in (b) (i) when used to view distant

objects on earth. Describe how the telescope can be modified to overcome

this adjustment. (4marks)

(c) Find the separation of the eye piece and objective of an astronomical

telescope of magnifying poower20 and in normal adjustment, if its eye piece

has a focal length of 5cm. (4marks)

(d) State three advantages of reflecting telescope over a refracting telescope.

(4marks)

3. (a) (i) State the laws of refraction. (2mks)

(ii) What is meant by critical angle? (1mk)

(b) Monochromatic light is incident at an angle of 38o on the glass prism of refractive

index 1.50. The emergent light glazes the surface of the prism as shown:

(i) Calculate the angle of refraction, r. (2mks)

(ii) Find the critical angle, c, for the glass-air interface. (3mks)

(iii) Find the refractive angle, A, of the prism. (2mks)

(c) (i) For a ray of light passing through a prism, what is the condition for minimum

deviation to occur? (1mk)

(ii) Describe how you would measure the minimum deviation, D, of a ray of light

passing through a glass prism. (6mks)

(iii) If the angle of minimum deviation in c(ii) above, is 41o for a glass prism of

refractive angle 60o find the refractive index of glass. (3mks)

4. (a) define the terms principal focus and power of a lens (2marks)

(b) Derive the relation between the focal length, f, object distance, u, and image distance,

v, for a thin lens. (7marks)

(c) A thin converging lens, P, of focal length 10cm and thin diverging lens Q, of focal

length 15cm are placed coaxially 50cm apart. If an object, 0, is placed 12cm from P,

on the side remote from Q,

(i) Find the position, nature and magnification of the final image. (7 marks)

(ii) Sketch a ray diagram to show the formation of the final image

(d) Explain why lenses of narrow aperture are preferred to lenses of wide aperture in

optical instruments.

SECTION B

5. (a) (i) What is meant by polarized light? (1 mark)

(ii) Describe how polarized light can be produced. (2 marks)

(iii) Sketch the time variation of electric and magnetic vectors in plane polarized

light waves. (2 marks)

(b) Two coherent sources a distance, s, apart produce light of wave length, , which

overlap at a point on a screen a distanced, from the sources to form an interference

pattern.

(i) What is meant by coherent sources? (2 marks)

(ii) Show that the fringe width, , is given by D

S

(4 marks)

(iii) if 𝜆 =5.46x10-7m, S= 5x10-5m and D = 0.3m, find the angular position of the

first dark fringe on the screen. (4 marks)

(c) (i) what is meant by diffraction of light ? (2marks)

(ii) Light of wave length 6x10-7mis incident on a diffracting grating with 500

lines per cm. Find the diffraction angle for the first order image.

(3marks)

6. (a) Explain the terms wavelength and wave front as applied to wave motion. (2)

(b) (i) Define the term resonance. (1)

(ii) Describe how you would determine the velocity of sound in air using a

resonance tube. (5)

(c) Explain with the aid of suitable diagrams the terms fundamental note and overtone as

applied to a vibrating wire fixed at both ends. (5)

(d) A stretched wire of length 0.75m, radius 1.36 mm and density 1380kg m-3 is clamped

at both ends and plucked in the middle. The fundamental note produced by the wire

has the same frequency as the first overtone in a pipe of length 0.15 m closed at one

end.

(i) Sketch the standing wave pattern in the wire. (1)

(ii) Calculate the tension in the wire (6)

(The speed of sound along the stretched wire is 𝑇 𝛿 where T is the tension

in the wire and 𝛿 the mass per unit length. Speed of sound in air = 330ms-1).

7. (a) Distinguish between transverse and longitudinal waves. (2mks)

(b) The displacement y given to a wave travelling in the x-direction at time t is:

𝑦 = 𝑎𝑠𝑖𝑛2𝜋 𝑡

0.1−𝑥

2.0 𝑚

Find

(i) the velocity of the wave. (4mks)

(ii) the period of the wave. (1mk)

(c) (i) What is meant by the Doppler effect? (1mk)

(ii) A police car sounds a siren of 1000Hz as it approaches a stationary observer.

What is the apparent frequency of the siren as heard by the observer if the

speed of sound in air is 340ms-1? (3mks)

(iii) Give one application of the Doppler Effect. (1mk)

(d) (i) Describe the motion of air in a tube closed at one end and vibrating in its

fundamental mode. (3mks)

(ii) A cylindrical pipe of length 29cm is closed at one end. The air in the pipe

resonates with a tuning fork of frequency 860 Hz sounded near the open and

of the tube. Determine the mode of vibration and find the end correction.

(5mks)

8. (a) What is meant by the terms;

(i) Constructive interference, and

(ii) Destructive interference, as applied to two sources of light. (3mks)

(b) In Young’s double slit experiment state what happens to the fringes.

(i) when the source is moved near the slits. (1mk)

(ii) when separation of the slits is changed? (2mks)

(c) In Young’s experiment, an interference pattern in which the tenth bright fringe was

2.4cm from the centre of the pattern was obtained. The distance between the slits and

the screen was 2.0m while the screen separation was 0.34mm. Find the wavelength of

the light source. (3mks)

(d) (i) What is polarized light? (1mk)

(ii) Explain the polarization of light by reflection at a glass surface and by

scattering. (8mks)

(iii) The polarizing angle for light in air incident on a glass plate is 57.5o. What is

the refractive index of the glass? (2mks)

SECTION C

9. (a) (i) State ohms law (1mks)

(ii) State any two limitations of ohms law (2mks)

(b) A resistor 𝑅 is connected across a 100V supply. A voltmeter of resistance 1500𝑜𝑕𝑚𝑠

is connected between the centre of the resistor and one side of the supply. Determine

𝑅 if the voltmeter reading is 35V. (6mks)

(c) (i) State two principle uses of a resistor in a circuit. (2mks)

(ii) What is the voltage drop across and current in each of the resistors

𝑅1 through 𝑅5 in the fig shown. (7mks)

(iii) How much power is dissipated in 𝑅4 ? (2mks)

10. (a) Outline the principle of operation of a slide wire potentiometer. (4mks)

(b) With the aid of a circuit diagram, describe how to calibrate an ammeter. (6mks)

(c) In figure below a cell, X, of negligible internal resistance, has e.m.f of 2V. AB is a

uniform slide wire of length 100cm and resistance 50Ω. With both switches K1 and

K2 open, the balance length AD is 90cm. when K2 is closed and K1 is left open, the

balance length changes to 75cm.

Calculate

(i) The e.m.f of cell Y. (2mks)

(ii) The internal resistance, 𝑟, of Y. (3mks)

(iii) The balance length when both K1 and K2 are closed. (3mks)

(d) Explain two advantages of a potentiometer over a moving coil voltmeter. (2mks)

11. (a) (i) What is meant by internal resistance of a cell? (1mk)

(ii) State three factors under which the internal resistance of a cell depends

(2mks)

(a) In the parallel circuit shown, the current through the resistor 𝑥 is 2A and the total

current taken from the supply mains is . Find

(i) the value of the unknown resistance . (5mks)

(ii) The pd across the circuit (1mk)

(iii) the current in each resistor and (1mk)

(iv) the total resistance of the circuit. (3mks)

(c) When a coil X connected across the left hand gap of a meter bridge is heated to a

temperature of 30℃, the balance point is found to be 52.5cm from the left hand end

of the slide wire. When the temperature is raised to 100℃, the balance point is

54.6cm from the left end. Find the temperature coefficient of resistance of X.

(7mks)

12. (a) Sketch the field lines between two oppositely large parallel metal plates across which

a p.d is applied. (1mk)

(b) Describe how you would investigate the factors which affect the capacitance of a

parallel plate capacitor. Use sketch diagrams to illustrate your answer. (7mks)

(c) Explain how lightning can cause severe damage to buildings. Name one device that

can be used to prevent such damage and explain how it operates. (6mks)

(d) Find the total energy stored in the capacitors between point X and Y in the given

network. (6mks)

FC 13

FC 21

FC 22

FC 24

FC 25 V8

X

Y

(e) A 20μF capacitor is charged to 40V and then connected across an uncharged 60μF

capacitor. Calculate the potential difference across the 60μF capacitor. (4mks)

13. (a) (i) State the law of conservation of charge. (1mk)

(ii) Explain how an object gets charged by rubbing. (3mks)

(b) Explain the following observation

(i) a hand held metal sheet cannot be charged by rubbing (2 marks)

(ii) A comb passed through one’s dry hair attracts a small bit of paper.

(2 marks)

(iii) Metals are conductors but rubber is an insulator (2 marks)

(c) Two metal spheres A and B are supported on insulating stands and placed in contact.

A glass rod, charged positively, is held close to sphere A. the spheres are then

separated while the glass rod is in place.

(i) State the charge acquired by each of the spheres. (1mk)

(ii) Sketch the electric field pattern between the spheres. (2mks)

(iii) Explain how the p.d between the spheres changes as the spheres as moved

further apart. (2mks)

(d) A small charged sphere of mass 𝑚 = 1.5𝑔 is suspended on a light string in the

presence of an electric field of magnitude 4.0 × 105𝑁𝐶−1 and directed horizontally.

The mass is in equilibrium when the string makes an angle of 30° with the vertical.

What is the charge on the sphere? (5 marks)

m

30

E

(c) Describe, with the aid of a labeled diagram, the application of corona discharge in a

van der Graaf generator. (6mks)

14. (a) (i) What is the difference between electric potential and electric potential

energy? (3 marks)

(ii) The diagram below shows two point charges −1.2𝜇𝐶 𝑎𝑛𝑑+ 1.0𝜇𝐶 separated

at a distance of 0.6𝑚. Point O is the midpoint of the two charges. Calculate

the electric potential at point 𝑃 which is 1.2𝑚 vertically above O. (7marks)

C2.1 C0.1

m3.0 m3.0

P

m2.1

2q1q

15. (a) What is a dielectric constant? (2mks)

(b) (i) Explain the effect of a dielectric placed between the plates of a charged

capacitor. (6mks)

(ii) Give two other uses of dielectrics in capacitors. (2mks)

(c) Explain what would happen if a conductor instead of a dielectric was placed

between the plates of a capacitor. (2mks)

(d) A 2μF capacitor that can just withstand a p.d of 5000V uses a dielectric. With a

dielectric constant 6 which breaks down if the electric field strength in it

exceeds 4.0 × 107𝑉𝑚−1. Find the:

(i) Thickness of the dielectric, (2mks)

(ii) Effective area of each plate (2mks)

(iii) Energy stored per unit volume of dielectric. (3mks)

(The capacitance, C = 휀𝑜휀 ,𝐴

𝑑; where A is the surface area; d is the distance

between the plates and 휀𝑜 , is the dielectric constant).

SECTION D

16. (a) State the laws of electromagnetic induction (2mks)

(b) (i) Explain how eddy current are produced. (3mks)

(ii) Explain briefly two applications of eddy currents. (6mks)

(iii) Two parallel wires each of length 75cm are placed 1.0cm apart. When the

same current is passed through the wires, a force of 5.0x10-5N develops

between the wires. Find the magnitude of the current. (4mks)

(c) With the aid of a diagram, describe how the magnetic flux density, B, between the

poles of a strong magnet may be measured. (5mks)

17. (a) (i) What is meant by the root-mean-square value of an alternating current?

(1mk)

(ii) Describe, with the aid of a labeled diagram, the structure and mode of

operation of repulsion type moving iron motor. (5mks)

(b) A sinusoidal alternating voltage 𝑉 = 𝑉𝑜𝑆𝑖𝑛𝜔𝑡, is connected across a pure

capacitance 𝐶. Derive an expression for the capacitive reactance of repulsion type

moving iron motor. (5mks)

(c) A transformer connected to an ac supply of peak voltage 240v, is to supply a peak

voltage of 9v to a mini lighting system of resistance 5Ω.

Calculate

(i) The ratio of the primary to the secondary turns. (1mk)

(ii) The rms current supplied by the secondary. (2mks)

(iii) The average power delivered to the lighting system. (2mks)

(d) (i) Explain why the voltage of the electric generated at Owen Falls Dam has to

be stepped up to about 132 kV for transmission places in the western region,

and then stepped d for general use. (3mks)

(ii) Give any two power losses in a transformer state how they are minimized.

(3mks)

18. (a) State Lenz’s law of electromagnetic induction (2mks)

(b) A coil of 100 turns and area 2x10-2m2 lies in a magnetic field of flux density 3x10-3T

and rotates uniformly at 100 revolutions per second about an axis perpendicular to the

magnetic field as shown.

Calculate:

(i) The emf induced when the plane of the coil makes 60o with B. (3mks)

(ii) The amplitude of the induced emf. (2mks)

(c) (i) What is meant by the back emf in a motor? (1mk)

(ii) State the major difference between a dc motor and a dc dynamo. (2mks)

(iii) What are eddy currents? Give one example where eddy currents are useful

and one example where they are a nuisance. (3mks)

(d) A dc motor has an armature resistance of 1Ω and is connected to a 240-v supply. The

armature current taken by the motor is 10A. Calculate;

(i) the back emf in the armature. (2mks)

(ii) the mechanical power developed by the motor. (2mks)

(iii) the efficiency of the motor. (3mks)

19. (a) (i) Define the unit of magnetic flux density. (2mks)

(ii) Give the expression for the force experienced by an electron moving at an

average velocity v1 in a wire placed at right angles to a magnetic field of

intensity B. (1mk)

(b) (i) With the aid of a labeled diagram describe the mode of operation of a moving

coil galvanometer. (6mks)

(ii) A rectangular coil of 100 turns is suspended in a uniform magnetic filed of

flux density 0.02T with the plane of the coil parallel to the field. The coil is

3cm high and 2cm wide. If a current of 50μA through the coil causes a

deflection 30o, calculate the torsional constant of the suspension. (3mks)

(c) (i) Define the ampere. (2mks)

(ii) Explain how the definition in c(i) above is used in the measurement of

current. (4mks)

(iii) Two parallel wires carrying currents of 5A and 3A respectively are 10cm

apart. If the wire carrying current of 5A is 50cm long, find the force exerted

on it. (3mks)

GOOD LUCK