Magnetic Effect of an Electric Current Questions and Answers - Physics Form 2 Topical Revision

Questions

1. The figure below shows a simple microphone in which sound waves from the person talking causes the cardboard diaphragm to vibrate.
   
   
   1. Explain how a varying current is induced in the coil when the diaphragm vibrates
   2. State two ways in which the induced current in (a) above can be increased
2. The figure below shows an electric generator. The points P and Q are connected to a Cathode Ray Oscilloscope (CRO).
   
   Sketch on the axes provided the graph of the voltage output as seen on the CRO, given that when t= 0, the coil is at the position shown in the figure.
   
3. An armature composed  of turns of insulated copper wire wound on a laminated soft- iron core is rotated in a magnetic field to generate an e.m.f. Use this information to answer the following questions.
   1. State two factors other than the speed of rotation that affect the magnitude of the e.m.f generated.
   2. State the reason why soft- iron is laminated.
4. The figure below shows a wire XY at right angle to a magnetic field. XY is part of a circuit containing a galvanometer.
   
   When XY is moved, the current flows in the direction shown. State the direction in which XY is moved.
5. The figure below shows an incomplete circuit of an electromagnet. Complete the circuit between X and Y by drawing the windings on the two arms of the core such that A and B are both North poles when switch S is closed. Indicate the direction of the current on the windings drawn.
   
6. State, with reasons, the material which you would consider most suitable for an electromagnet.
7. Explain two ways of demagnetizing a magnet.
8. State two ways of increasing the strength of an electromagnet.
9. An iron rod XY is placed inside a coil of wire. What type of magnetic pole is induced at the end x when the current flows through the coil?
   
10. An un insulated copper wire XY lies over the fixed wire A and B connected to a battery when the key in the circuit is closed, the wire XY experiences a force.
    
    
    1. In which direction does the wire XY- experience the force?
    2. How do you determine the direction in (i) above
    3. When is the force on the wire XY greatest?
    4. What is the effect of reversing both the magnetic field and direction of flow of current?
    5. State TWO factors by which the force on XY can be decreased
    6. Name an instrument which uses this effect
11. The figure below shows a D.C electric motor.
    
    
    1. Name the parts labeled X₁ and X₂ and state their functions.
    2. What is the purpose of parts labeled Y₁ and Y₂?
    3. When the  switch k is closed state the forces acting on the sides  of the coil and the direction of movement of the coil
    4. What can be done to increase the speed of rotation of the motor?
12. State FOUR reasons why the efficiency of an electric motor is always less than 100%
13. Give TWO practical applications of an electromagnet

Answers

1.  
   1. As the diaphragm vibrates, it causes the coil to move back and forth, in the magnetic field cutting the field lines. This causes varying current to be induced in the coil.  Varying current flows.
   2. - Increasing the number of turns in the coil.
      - Increasing the strength of the magnet.
2.  
   
   
3.  
   1. - Number of turns or number of coils.
      - Strength of magnetic field.
      - (Angle of inclination)
   2. To reduce eddy current in the armature.
4. Downwards (or into the paper)
5.  
   
6. Soft iron – Iron produces a stronger magnet but looses it magnetism quickly. Also it is more easily magnetized than steel.
7.  
   1. Heating the magnet strongly until it is red hot and then cooling it suddenly.
   2. Keeping it in a solenoid through which an alternating current is flowing when the current is still passing the magnet is slowly removed to a distance from the solenoid.
8. -Increasing the number of turns of the coil.
   -Increasing the strength of the current.
9. Solution A – South Pole
10.  
    1. Towards the right
    2. Using the Fleming’s left hand rule.
       First finger- Field
       Second finger- Current direction 
       Thumb – Motion
    3. -When the wire is positioned at right angle to the magnetic field.
       -It is minimum when the wire is parallel to the magnetic field.
    4. No effect
    5. - Reducing the amount of current.
       - Replacing the magnet with a weaker one.
11.  
    1. Commutator 
       -They are the split - rings which reverses the current in the coil every half-cycled and hence allow rotation to continue.
    2. They provide electric connection to the coil.
    3. -The lengths AB and CD of the coil are current carrying conductors in a strong magnetic field which is perpendicular to the wires.
       -Each side of the coil has force acting on it
       -AB has a downward force while CD experience and upward force.
       -Overall the coil experiences an anticlockwise motion.
    4. -Increasing the amount of current
       -Increasing the strength of magnetic field
       -Using more turns in the coil
       -Winding the coil on a soft iron core
12. • Energy loss due to friction between brushes and commutators.
    • Resistance of the coil
    • Energy loss due to heat during magnetization and demagnetization 
    • Flux leakage
13. - Electric bell
    - Telephone receiver