Magnetic effects of electric current

• Properties of magnetic field lines:

  • Originate from the North pole and end at the South pole outside the magnet
  • They are closed continuous lines
  • Density of the lines increases near the poles and decreases away from the poles
  • Lines never cross each other
• Like poles repel and unlike poles attract each other
• The region where magnetic field lines are crowded has relatively greater strength
• The magnetic poles of the Earth continuously change their position with time, known as magnetic reversal
• The angle of the horizontal plane between the geographic North (true North) and the magnetic North is known as magnetic declination
• Deflection of compass
• Right-hand thumb rule:

  • If one holds a current-carrying wire in the right hand such that the thumb is pointing in the direction of the current, then the direction in which the other finger encircles the wire will give the direction of the produced magnetic field lines around the wire
• Corkscrew rule:

  • If one drives a corkscrew in the direction of the current, then the direction in which the handle is turned is the direction of the magnetic field on the magnetic field lines
• The magnitude of the field lines produced by a circular loop at its centre is:

  • Directly proportional to the amount of current
  • Inversely proportional to the radius of the loop
• The strength of the magnetic field produced by a current-carrying solenoid:

  1. Is directly proportional to the number of turns in the solenoid

  2. Is directly proportional to the strength of the current in the solenoid

  3. Depends upon the nature of the core material
• A current-carrying rod experiences a force when placed between two poles of strong magnets. The direction of force exerted on the rod is related to the direction of the current
• Magnitude of magnetic force depends on:

  • F ∝ I (current flowing in the rod)
  • F ∝ B (Strength of magnetic field)
  • F ∝ l (length of the rod)
• Fleming’s left-hand rule:

  • Thumb points in the direction of the force acting on the conductor
  • Forefinger represents the magnetic field
  • Middle finger represents the current
• Electric motor:

  • Converts electrical energy into mechanical energy
  • Works based on the magnetic effect of current
  • A current-carrying rectangular coil starts rotating when placed in a magnetic field
• Commutator:

  • An electric device that reverses the direction of current in a circuit
  • The split ring acts as a commutator of the electric motor
• Electromagnetic induction:

  • Phenomenon of the generation of induced current in a conductor by changing the magnetic field or by moving a conductor in the magnetic field
  • Induced e.m.f. = change in magnetic flux in each turn × number of turns in the coil / time in which the magnetic flux changes
  • Direction of induced e.m.f. is given by Lenz's law
• Difference between A.C. and D.C.:

  Direct Current (D.C.):

  • Current of constant magnitude
  • Always flows in one direction
  • Can be obtained from cell or battery

  Alternating Current (A.C.):

  • Magnitude of current varies periodically with time
  • Direction of current reverses periodically
  • Can be obtained from A.C. generator
• Working of a transformer is based on the phenomenon of mutual induction
• Fleming’s right-hand rule:

  • Thumb points towards the motion of the conductor
  • Forefinger represents the magnetic field
  • Middle finger represents the direction of current
• Electric Generator:

  • Converts mechanical energy into electrical energy
  • Direction of the induced current in the generator is given by Fleming’s right-hand rule
• Domestic wiring:

  • Electricity is transferred through a pair of wires consisting of a red live wire (L) and a black neutral wire (N)
  • A green Earth wire (E) is also connected with the circuit
  • In India, 220 V potential is supplied through the live wire, while the neutral wire has ground potential of zero volts
• Switches:

  • Device connected in the live wire to turn 'ON' or 'OFF' the current in the circuit
  • Types of switches: Single pole switch, Double pole switch
• Fuse:

  • Important safety device for protecting the circuit due to short-circuiting or overloading
  • Fuse wire has low melting point, generally made of an alloy of lead and tin
  • Fuse wire is always connected in series with the live wire and its resistance is higher than that of copper wires
• Current rating of the fuse wire decides its thickness, with higher current rating leading to thicker wire