electromagnetism

• Hans Oersted observed in 1820 that when an electric current passes through a conducting wire, a magnetic field is produced around it
• Right Hand Thumb Rule:

  • If a current carrying conductor is held in the right hand with the thumb pointing in the direction of the current, the direction in which the fingers encircle the wire gives the direction of the magnetic lines of force around the wire
• Maxwell’s Corkscrew Rule:

  • If a corkscrew is screwed along a conductor in the direction of current, the direction in which the ends of the handle move gives the direction of lines of magnetic force
• The magnetic field produced by a current-carrying solenoid is similar to that produced by a bar magnet
• A solenoid is an insulated copper coil wound on a cylindrical cardboard, behaving like a magnet when a current flows through it
• In a current-carrying circular coil, when the direction of current is downward, the direction of magnetic lines of force is clockwise. When the direction of current is upward, the direction of magnetic lines of force is anticlockwise
• Polarity at the faces of a current-carrying loop:

  • Anticlockwise current develops north polarity
  • Clockwise current develops south polarity
• An electromagnet is a temporary strong magnet made of a solenoid with wire wound on a soft iron core
• Ways to increase the magnetic field of an electromagnet:

  • Increase the number of turns of winding
  • Increase the current through the solenoid
  • Decrease the air gap between two poles
  • Wind the coils on a soft iron core
• Electromagnets are used in:

  • Electrical appliances like electric bells, electric fans, telephones, electric generators
  • Magnetic separation and lifting heavy ferromagnetic loads
  • Scientific research to study magnetic properties of substances in a magnetic field
• A permanent magnet is made from steel or alnico and once magnetized, it does not lose its magnetism easily
• Ways to demagnetize an electromagnet:

  • Heating, rough handling, or hammering
  • Electrical method using alternating current
  • Self-demagnetization
• An electric bell is a common application of an electromagnet
• Fleming’s Left Hand Rule:

  • Thumb, first finger, and second finger of the left hand at right angles
  • First finger indicates the direction of the magnetic field, second finger indicates the direction of current, thumb indicates the direction of force on the conductor
• A DC motor converts electrical energy into mechanical energy by placing a current-carrying coil in a magnetic field
• Main parts of a DC motor:

  • Field magnet supplied by a permanent horseshoe magnet
  • Armature rotating between two concave cylindrical poles
  • Split rings or commutator rotating with the armature
  • Brushes made of carbon supplying current to the coil
• Speed of a motor depends on the strength of the magnetic field, current, and the number of turns in the armature coil
• The direction of rotation of a motor can be reversed by interchanging the terminals of the battery connected to the brushes
• Electromagnetic induction is the production of current in a closed coil due to relative motion between the coil and the magnetic field
• Fleming’s Right Hand Rule:

  • Forefinger, middle finger, and thumb of the right hand at right angles
  • Forefinger points in the direction of the magnetic field, thumb denotes the direction of motion of the conductor, middle finger points in the direction of induced current
• Lenz’s law states that the direction of induced emf always opposes the cause which produces it
• An AC generator or dynamo converts mechanical energy into electrical energy through electromagnetic induction
• Factors affecting the voltage produced by a generator:

  • Speed of the coil
  • Strength of the magnetic field
  • Number of turns on the coil
• A transformer increases or decreases the amplitude of an alternating emf and works on the principle of electromagnetic induction
• In a step-up transformer, low AC voltage changes to high AC voltage with more turns in the secondary coil than the primary coil
• In a step-down transformer, high AC voltage changes to low AC voltage with fewer turns in the secondary coil than the primary coil
• Factors affecting the emf induced in the secondary coil of a transformer:

  • Ratio of the number of turns in the secondary to primary coil
  • Magnitude of emf applied in the primary coil
• Energy losses in a transformer:

  • Heating in the coil
  • Eddy currents in the core
  • Hysteresis loss in the core
• Step-up transformers are used in power transmission at generating stations, televisions, wireless sets, X-ray tubes
• Step-down transformers are used in electric bells, radio-sets, power substations to step down voltage before distribution to consumers
• Electric charges can be positive or negative, while electric current is always considered as flowing from positive to negative.
• A moving charge produces an electric current, which generates a magnetic field around it.
• The magnetic field lines are always perpendicular to the direction of motion.
• The direction of the force on a charged particle depends on its velocity and the direction of the magnetic field.
• An electron with velocity v has a magnetic moment m = -ev/2c (where c is the speed of light).
• Magnetic moments have units of ampere-metres squared (Am^2) or joules per tesla (JT^-1).
• Magnetic moments have units of ampere-metres squared (Am^2) and are measured using a magnetometer.
• In a solenoid, the magnetic field inside is uniform and parallel to its axis.
• In a uniform magnetic field, the torque experienced by a dipole is given by T = mB sinθ, where B is the magnitude of the magnetic field and θ is the angle between the dipole axis and the field vector.
• A current loop can be considered as an equivalent magnetic dipole.