Physics Paper 2 Questions and Answers - Form 3 Term 3 Opener Exams 2021

INSTRUCTIONS TO CANDIDATES

• This paper consists of Two sections A and B
• Answer ALL the questions in sections A and B in the spaces provided.
• All working MUST be clearly shown
• Mathematical tables and Electronic calculators may be used.

QUESTIONS

SECTION A (25MKS)

1. State one disadvantage of using a pin hole camera to take photographs (1mk)
2. Name two advantages which a lead accumulator has over a dry cell (2mks)
3. A girl observes her face in a concave mirror of a focal length 90cm. If the mirror is 70cm away, state two characteristics of the image observed. (2mks)
4. What property of light is illustrated by formation of shadows? (1mk)
5. Other than local action state another defect of a simple cell and explain how it reduces the current produced. (2mks)
6. The figure below shows a simple experiment using a permanent magnet and two metal bars X and Y.
                   
   State, with reason, which bar is a soft magnetic material. (2mks)
7. .A plain sheet of paper and a plane mirror both reflect light yet only the plane mirror forms images. Explain why the paper cannot form images (2mks)
8. The Figure below shows an ammeter used to measure current through the conductor. The student used the lower scale.
                     
   State the reading from the meter. (1mk)
9. Two similar razor blades are placed one on a wooden block and the other on a soft iron block as shown in the figure below
                         
   It was observed that the razor blade on the wooden block was attracted to the magnet while the other on the soft iron block was not. Explain. (2 marks)
10. The figure below shows a displacement - time graph for a wave. Determine its frequency.(3mks)
    
11. explain two reasons why in domestic electrical wiring bulbs are connected in parallel (2mks)              
12. explain two defects of a simple cell (2mks)
13. differentiate between hammering in magnetization and demagnetization (2mks)
14. describe how an electric bell works

SECTION B (55 Marks)
Answer all questions in this section in the spaces provided

15.  
    1.  
       1. State Ohm’s law (1mk)
    2. A piece of red-hot charcoal is brought close to the cap of a negatively charged electroscope, Explain what is observed. (3mks)
    3. A cell of e.m.f E and internal resistance r is used to pass a current through various resistors R, Ohms and the values of current recorded in the table below.
       

       (Ohms)	1.6	2.1	2.5	3.6	5	8
       i(A)	1	0.8	0.7	0.5	0.37	0.24
       1/i(A−1)

       
       
       1. On the table record values of 1/i (1mk)
       2. Plot a graph of 1/i versus R and use it to determine E (8mks)
          
          
          
16.  
    1. state and explain what happens when one removes a nylon cloth off the body (2mks)
    2. describe an experiment to investigate the law of charges ( 4mks)
    3. state basic law of charges (1mk)
    4. explain why repulsion is the surest test for electrostatic charging (2mks)
    5.  a metal rubbed with a piece of cloth does not acquire charge (3mks)
       1. explain the phenomena
       2. what precaution need to be taken so that the rod acquires charge
    6. explain 3 dangers of electrostatics ( 3mks)
17.  
    1.  
       1. State three properties of electric field lines. (3mks)
       2. describe with explanations an experiment to show how the shape of a core affects the strength of an electromagnet (5mks)
18.  
    1. Define the following terms as used in curved mirrors.
       1. Principal focus (F). (1mk)
       2. Focal length (f) (1mk)
    2. By use of a ray diagram, show how a concave mirror may be used by dentist when extracting teeth. (3mks)
    3. You are provided with the following apparatus a white screen, metre rule and concave mirror, using the apparatus, describe an approximate method of determining the focal length of the mirror. (3mks)
    4. Show that the linear magnification M of a convex mirror is given by
       
       Where V is the image distance and f the focal length of the mirror. (3mks)
    5. The figure below represents an object, O, placed in front of a curved mirror.
           
       
       1. By drawing suitable rays, complete the diagram to show the position of the image. (3marks)
       2. State the characteristics of the image in (i) above (2mks)
19.  
    1. Differentiate between motor rule and Maxwell right hand grip rule (2mks)
    2. Define a solenoid (1mk)
    3. Describe how magnets should be stored (2mks)

MARKKINIG SCHEME

1.  
   • One picture can be taken at a time ;
   • It has long exposure time / picture of moving objects cannot be taken (ANY ONE ) ;
2.  
   • A lead accumulator give large current over a long time compared to a dry cell;
   • A lead acid accumulator can be recharged while dry cell cannot be renewed / recharged
3. Any two
   • Virtual ;
   • Erect / upright;
   • Enlarged ;
4. Light travels in a straight line
5. Polarisation.Hydrogen gas produced causes insulation at the cathode .It increases the internal resistance of the cell
6. None
   Each of them retained their magnetism
7. Image formation takes place with regula- reflection not with diffuse reflection
   Plane mirror-regular reflection
   Sheet of paper-irregular diffuse reflection 
8. 1.375 Ampres; 
9. Soft iron became induced magnet and attracts back the razor
   Wood is non-magnetic material hence does not attract the blade.
10. T = 2000 s ;
    f  = 1
          T
       =   1   
          2000
    = 5.0 x 10^-4 Hz 
11.  
    • Parallel connection ensures similar and constant voltage across all bulbs to fully operate. 
    • Bulbs connected in parallel can be independently connected or disconnected from the circuit and the other bulbs will continue to operate normally.
12.   
    • Polarization- Formation of a layer of hydrogen bubbles on the copper plate weaken the current
    • Local action- caused by presence of zinc of small impurities which set up tiny local cells at the zinc surface and bubles of hydrogen are given off.
13. Hammering a rod will either allow it to be slightly magnetized ( method of developing magnetic properties in a substance) if laid along a magnetic field that is North-South or demagnetized ( remove the magnetic property), if laid across the field lines, East-West
14. When an electric current flows through the coils, the electromagnet creates a magnetic field which pulls the armature tpwards it, causing the hammer to strike the bell.

SECTION B

15.  
    1.  
       1. For a metallic Conductor, the Current flowing through is directly proportional to the p.d across its ends provided temperature ( and other physical condition) are kept constant
    2. The leaf collapsed / Electroscope discharged 
       The red hot charcoal ionizes the air around which neutralizes 
       The negative charges on the electroscope
    3. Graph
       1. 
          

          i (A−1)1/1	1.25	1.43	2	2.7	4.17
          R (Ω) 1.6	2.1	2.5	3.6	5.0	8.0

       2.  
          
          
          

          Labelling both axes correctly with units
          Appropriate scale
          Uniform and covers More than half page
          Plotting 6 pts corrects 4-5 pts 0-3pts no mark
          Straight line, positive gradient and cutting y-axis at positive value ;
          Gradient = ¹/_E

          = 3.4 - 1.6
             6.5 - 2.8

          ¹/_E= 1.8
                  3.7

       3. ¹/_E = 0.4865
          E = 2.058 volts
          Accept 2 ± 0.05

16.   
    1. A crackling sound or a spark is produced as a result of the static charges ( negative)  produced by rubbing nylon material againct the body.
    2. when two different substances are rubbed together, an electrical charge is produced because one of them will give electrons to the other. If for example a balloon  is rubbed agaiinst hair electrons move from hair to the balloon. Because electrons are negatively charged, the balloon acquires a negative charge as the hair, with its loss of negative charges, now having a positive charge. sine the hair and balloon have opposite charges, unlike charges attract. Rubbing two balloons against hair or wool makes them electrically charged and once brought together the like charges causes them repel and the balloons move apart. 
    3. Like charges repel each other, unlike charges attract. two negative charges repel one another , while a positive charge attracts a negative charge.
    4. Repulsion is the sure test for electrostatic charging  to find whether an object is charged or not since only electrostatic force between two like charges will make them repel each other though the gravitational force would still try to attract them.
    5.  
       1. A metal rod is a conductor rod while the piece of cloth is made of insulating material. 
          Insulators prevent the electrons from moving and the charge remains static.
          Conductors cannot hold charge as electrons are moving through them.
       2. Avoid touching the metal rod. Use electrostatic induction to make a rod aqcuire charge.
    6.  
       • When you touch something with a large electrical charge the charge  flows through your body causing an electric shock
       • You may suffer from burns or have a heart attack
       • Pain or death from an electrical shock
       • Damage to property
17.  
    1.  
       1.  
          • They leave the surface of a charge body at right angles;
          • the electric field lines are directed away from positive charges and towards the negative charges;
          • The closer the field lines the stronger the field at that point,
       2. The moving clouds get charged through friction; 
          When a charged cloud pass over a lightning arrestor it induces on opposite charge in it; Because of the structure of the arrestor at the top (sharp spikes) it releases charges into the air which discharges the charged cloud as shown in the diagram; (1mark)        
          The arrestor acts as an easy way of discharging the clouds because it is a good conductor;
18.  
    1.  
       1. Point on principal axis to which all rays originally close and parallel to the principal axis converge to or from which they appear to diverge from after reflection from the mirror.
       2. Distance between the pole of the mirror and principal focus.
    2.  
                          
    3.  
       • Focus image of distant object on the screen.
       • Measure distance between screen and mirro
       • Repeat the procedure several times
       • Determine the average value of d
       • Focal length of mirror f = d
    4. For convex mirror, its formula is
       1 = 1 − 1       and   M =  V
       f     u     v                       u
       Multiply terms in  1 = 1 − 1  by V
                                 f     u     v
       v = v − 1
       f     u
       ∴ M = v + 1 
    5.  
       1. 
                 
          Ray parallel to principal focus diverged but seem to diverge from F ;
          Ray seeming to pass through C are undeviated ;
          Position of image
       2. Characteristics
          • Virtual
          • upright
          • between F and O
          • smaller 
19.  
    1. Maxwell right hand grip rule states that when a conductor is held in your right hand, such that the direction of the thums points the direction of the current and the curled fingers  gives the direction of the magnetic finger while the motor rule is for finding the direction of motion in an electric motor.
    2. A solenoid is type of electromagnet comprising a coil of wire, the housing and a moveable plunger (armature) that converts electrical energy into mechanical work.
    3. Store magnets in the attracting position and with a keeper to prevent them from becoming demagnetized. Ring magnets can be stack together as the magnet field of one will reinforce the other. 
       Store magnets in a dry place to avoid corrosion and keep them away from high temperatures.