MAGNETIC EFFECTS OF ELECTRIC CURRENT - III : LONG ANSWERS

*INDEX-TOPIC SEARCH

GIVE ANSWER IN DETAIL

1. Explain the presence of magnetic field due to electric current.
   
   ANS:
   
   
   
   
   
   • Danish physicist H.C.Oersted was the first to observe the presence of magnetic field due to electric current during an experiment.
     
   • As shown in figure, a magnetic needle kept near a conducting wire connected in a circuit shows no deflection(figure 1) when there is no current flowing through the conductor.
     
   • When current is allowed to pass through the conductor, the needle shoes deflection(figure 2).
     
   • If the terminals of the battery are reversed(i.e. the direction of the current is reversed), the needle shows deflection in opposite direction.
     
   • These observations clearly show that when current passes through a conductor it behaves as a magnet,i.e. it possesses a magnetic field or a magnetic field is produced when current passes through a conductor.
     
   
   
   
2. EXPLAIN : SOLENOID
   
   ANS:
   
   
   
   
   • A coil of many turns of wire wrapped in the shape of a cylinder (which behaves as a magnet when current passes through it) is called a solenoid.
     
   • As shown in figure 1, if a solenoid has N turns, the magnetic field associated with it is N times stronger than the field due to one loop(turn).
     
   • This strength is due to the fact that the direction of current in each turn is the same and the magnetic field produced in each turn of the solenoid adds up, giving a very strong resultant field inside the solenoid.
     
   • If we compare figure 1 and figure 2, we find that the magnetic field of a solenoid is quite similar to that of a bar magnet.
     
   • Thus, one end of a solenoid behaves as north pole and the other as south pole.
     
   
   
   
3. DESCRIBE AN ACTIVITY TO DEMONSTRATE THE PATTERN OF MAGNETIC FIELD LINES AROUND A CURRENT-CARRYING STRAIGHT CONDUCTOR.
   
   ANS:
   
   
   • Take a cardboard and arrange it in a circuit as shown in figure by inserting a wire through the cardboard.
   • Sprinkle some iron filings on the cardboard and pass current through the wire.
   • Tap the cardboard gently. The iron filings will align themselves in a pattern of concentric circles around the wire.
   • The concentric circles represent the magnetic field lines produced due to the current passing through the wire.
   • The direction of the magnetic field can be obtained by placing a magnetic compass needle at a point on a circle. The north pole of the needle will point in the direction of the field. The arrows on the circles show the direction of the field.
   • If we reverse the direction of the current, the direction of the magnetic field also gets reversed.
   • We can use 'Right Hand Thumb Rule' to know the direction of the magnetic field associated with the current-carrying conductor. The rule states: Imagine that you are holding the current-carrying wire in your right hand such that the thumb is stretched along the direction of the current, then the fingers would wrap around the conductor in the direction of magnetic field.
     
   
   
   
4. DISTINGUISH BETWEEN ALTERNATING CURRENT AND DIRECT CURRENT.
   
   ANS:
   
   
   
5. EXPLAIN : ELECTRIC MOTOR
   
   ANS:
   
   
   When current passes through a conductor placed in a magnetic field, a force acts on the conductor tending to move it out of the field. This effect is the basis of the electric motor.
   
   As shown in the figure, a rectangular coil ABCD is free to rotate about its axis (the line with breaks). It is placed in a magnetic field. The coil is connected to a commutator XY. X and Y are two halves of a split ring. Brush P is in contact with X and brush Q is in contact with Y. When the current flows as shown, the coil will move in clockwise direction according to the Fleming's Left-hand rule. Since the current passing through the parts AD and BC of the coil is parallel to the magnetic field, they do not experience any force. Because the current passing through parts AB and CD of the coil is perpendicular to the magnetic field lines, the magnetic field will push CD down and AB up. When the coil completes half rotation, Y comes in contact with P and X with Q. The direction of current in AB (and CD) changes. This change pushes AB down and CD up. Thus, a continuous rotary motion of the coil is obtained. This is the basic principle of a d.c. motor.
   
   Electric motor is used to convert electrical energy into mechanical energy and thereby to run various machines.
   
   
6. EXPLAIN : ELECTRIC GENERATOR
   
   ANS:
   
   
   If a coil is rotated mechanically in a magnetic field, a current is generated in the coil. This current is called 'induced current' and the process is called 'electromagnetic induction'. Electromagnetic induction is exactly opposite of the motor concept.
   
   As shown in the figure, a rectangular coil ABCD is placed in a magnetic field. The ends of the coil are connected to two halves of a split ring of a commutator XY. The spindle (shown as broken line) supporting the coil is connected to a turbine. When the turbine is rotated mechanically, the coil rotates and a current is generated in it. The current is received at two terminals and supplied as electricity. The galvanometer shows deflection when the coil is rotated indicating the presence of electric current. The current produced in this way is direct current and the generator is called d.c.generator.
   
   
7. EXPLAIN FARADAY'S EXPERIMENT TO EXPLAIN ELECTROMAGNETIC INDUCTION OR EXPLAIN ELECTROMAGNETIC INDUCTION.
   
   ANS:
   • Make a loop of conducting wire and connect it to a galvanometer.
   • Keep a bar magnet near the loop as shown in figure (A). The indicator of the galvanometer shows no deflection indicating that there is no current in the coil.
   • Move the magnet quickly towards the loop as shown in figure (B). The galvanometer shows deflection indicating the presence of current.
   • Pull the magnet quickly away from the loop as shown in figure (C). The indicator shows deflection on the other side indicating that the current is now flowing in the opposite direction.
   • CONCLUSION : (i)When there is a relative motion between the magnet and the loop, voltage and hence current is induced in the conducting loop due to a change in the magnetic field linked with the loop.(ii) The induced current(voltage) in a coil depends on the number of turns in the coil and the rate of change in the magnetic field linked with the coil.
   • The above phenomenon is called electromagnetic induction and the coil is called the inductor.
     
   
   
   
8. EXPLAIN DOMESTIC WIRING WITH DIAGRAM.
   
   ANS :
   
   
   • The figure shows a schematic diagram of a common domestic circuit.
   • The current supplied for domestic use is alternating current(AC) with 50 Hz frequency.
   • A wire with red insulation cover is called 'live wire' or 'phase wire'. Another wire with black insulation cover is called 'neutral wire' or 'negative'.
   • The potential difference maintained between them is 220 V.
   • These wires enter our house through electric meter and fuse board.
   • Two separate lines of 5 A and 15 A are used for different appliances. The 5 A line is used for lamps, T.V., etc. while the 15 A line is used for appliances with higher power ratings like washing machine, geyser, etc.
   • A third wire with green insulation cover is called the 'earth wire' which is connected to a metal plate kept deep in the earth near the house.
   • The earth wire is used as protective measure against the leakage of current, especially from appliances with metallic body such as toaster, electric iron, refrigerator.
   • Parallel connections are used for domestic wiring.
     
   
   
   
9. EXPLAIN : DIRECT CURRENT AND ALTERNATING CURRENT.
   
   ANS:
   • The current flowing from the positive terminal to the negative terminal of a battery is called direct current(dc).
   • The magnitude of the voltage and the current do not change with time and the current flows in one direction only in case of d.c.
   • The graph of current(voltage) v/s time for direct current is a straight line parallel to the time axis.
   • If the current changes its direction between two terminals of the battery periodically then it is called alternating current(ac).
   • The frequency of alternating current is the number of times the current changes its direction in one second. In India, the frequency of a.c. is 50 Hz.
   • The graph of current(voltage) v/s time is a curve in case of a.c.
   • The advantages of using a.c. are :(i)It is easy to produce. (ii)It is relatively cheaper. (iii)The energy loss for long distance transmission is much less.
     
   
   
   
10. EXPLAIN : SAFETY FUSE
    
    ANS:
    
    
    • Safety fuse uses heating effect of electric current.
    • Fuse wire is a wire having low melting point. It is generally made from tin or an alloy of tin and copper.
    • Fuse wire is joined with metallic contacts on an insulation base.
    • If due to some reason, the current in the circuit increases suddenly, the fuse wire melts and breaks the circuit preventing the current going further.
    • Thus, current cannot pass through costly appliances like T.V., freeze, etc., and they are protected from being damaged. Also, probability of an accident or major damage is minimised.
      
    
    
    
11. DISTINGUISH BETWEEN ELECTRIC MOTOR AND ELECTRIC GENERATOR.
    
    ANS: