Magnetism

Properties of Magnets

• Magnets:
  • have a magnetic field around it
  • have two opposite poles (North and South) which exert forces on other magnets.
    Like poles repel and unlike poles attract
  • will attract magnetic materials by inducing (either permanently or temporarily) magnetism in them
  • will exert little or no force on a non-magnetic material
• The direction of an electric field at a point is the direction of the force on a positive charge

Induced Magnetism

• Magnets attract materials by inducing magnetism in them; the material becomes a magnet as well
• The side of the material facing the magnet will become the opposite pole as the magnet
• Materials that can be attracted by magnets, Ferrous materials, include iron, nickel, cobalt, etc

Magnetization Methods

• A piece of steel becomes permanently magnetized when placed near a magnet, but its magnetism is usually weak
• It can be magnetized more strongly by stroking it with one end of a magnet
• The most effective method is to place the material iin a solenoid and pass a large, direct current (d.c.) through the coil

Demagnetization Methods

• If a magnet is hammered, its atomic magnets are thrown out of line and it becomes demagnetized
• Heating a magnet to a high temperature also demagnetizes it
• The most efficient method is to place the magnet inside a solenoid connected to an alternating current (a.c.) supply

Experiment: Field Lines Around a Bar Magnet

• The magnetic field lines can be traced on a paper by a compass needle (a tiny needle)
• The compass needle is first placed near the north pole of the magnet
• The position of the poles of the needle are marked on paper
• Then the needle is moved to a new position such that the position of its south pole coincides with the previous position of its north pole
• This process is continued until the needle reaches the South pole
• By joining these points, we get to magnetic line of force
• Then it is placed at some other position near the North pole and the above procedures are repeated

Properties of Magnets

Properties of Magnets

Electromagnetic Effects

Electromagnetic Induction

• Wire passed across a magnetic field:
  
  • If a wire is passed across a magnetic field, a small EMF is induced
  • If the wire forms part of a complete circuit, the EMF makes a current flow, and this can be detected by using a galvanometer
  • The EMF induced is proportional to the rate at which the magnetic field lines are cut by the conductor
  • The induced EMF can be increased by:
    • moving the wire faster
    • using a stronger magnet
    • increasing the length of the wire in the magnetic field, for example, looping the wire through the field several times
  • The current and EMF direction can be reversed by:
    • moving the wire in the opposite direction
    • turning the magnet round so that the field direction is reversed
• Fleming’s Right-Hand Rule gives the current direction
  
• A bar magnet pushed into a coil
  
  • The induced EMF (and current) can be increased by:
    • moving the magnet faster
    • using a stronger magnet
    • increasing the number of turns in the coil
  • If the magnet is pulled away, the direction of the induced EMF (and current) is reversed
  • Using the South pole instead of the North pole reverses the direction of the induced EMF (and current)
  • If the magnet is held still, there is no EMF
  • An induced current always flows in a direction such that it opposes the change which produced it
  • When a magnet is moved towards a coil, the pole of the coil and magnet next to each other are the same
  • When the magnet is moved away, the poles are opposite (opposite pole attract)
  • The pole-type (north or south) is controlled by the direction in which the current is induced
  • The direction of the current is given by the right-hand grip rule
    
  • The fingers point in the conventional current direction and the thumb gives the North pole

A.C. Generator

• The coil is made of an insulated copper wire and is rotated by turning the shaft; the slip rings are fixed to the coil and rotate with it
• The brushes are two contacts which rub against the slip rings and keep the coil connected to the outside part of the circuit, usually made of carbon
• When the coil is rotated, it cuts the magnetic field lines, so an EMF is generated, which makes a current flow
• Each side of the coil travels upwards, then downwards, then upwards, etc., so the current flows backwards, then forwards, then backwards, etc., so it is an alternating current
• The current is maximum when the coil is horizontal, since the field lines are being cut at the fastest rate, and zero when the coil is vertical, since it is cutting NO field lines
• The EMF can be increased by:
  • increasing the number of turns on the coil
  • increasing the area of the coil
  • using a stronger magnet
  • rotating the coil faster

Transformers

• AC currents can be increased or decreased by using a transformer
• A transformer consists of a primary coil, a secondary coil and an iron core
• The iron core gets magnetized by the incoming current and this magnetism then creates a current in the leaving wire
  
• The power is the same on both sides (assuming 100% efficiency)
• To find the number of coils and the voltage:
  • ^{Output voltage}/_{Input voltage} = ^{Turns on output coil}/_{Turns on input coil}
    • ^V_P/_VS = ^N_P/_NS
  • Input voltage × Input current = Output voltage × Output current
    • V₁ × I₁ = V₂ × I₂
• When a magnetic field is changed across the primary coil by connecting it with A.C., an EMF induces across the secondary coil
• The iron core channels the alternating field through the secondary coil, inducing an alternating EMF across it
• A step-up transformer increases the voltage and a step-down transformer decreases it
• Transformers are used to make high voltage A.C. currents
• Since power, P, lost in a resistor is I² × R, having a lower current will decrease the power loss
• Since transmission cables are many kilometers long, they have a lot of resistance, so a transformer is used to increase the voltage and decrease the current to decrease power loss
• The advantages of high-voltage transmission:
  • less power loss
  • thinner, light, cheaper cables can be used since current is reduced

Electromagnetic Effect of a Current

• Increasing the current increases the strength of the field
• Increasing the number of turns of a coil increases the strength
• Reversing the current direction reverses the magnetic field direction (right-hand rule)
• Magnetic effect of current is used in a relay and a circuit breaker

Force on a Current Carrying Conductor

• If a current carry conductor is in a magnetic field, it warps the field lines
• The field lines form the magnet want to straighten out naturally
• This causes a catapult like action on the wire creating a force
  
• If you reverse the current, you will reverse the direction of the force
• If your reverse the direction of the field, you will reverse the direction of the force
• The direction of the force, current or magnetic field is given by Fleming’s Left-Hand Rule:
  

D.C. Motor

• When a current carrying coil is in a magnetic field, it experiences a turning effect
• A DC motor runs on a direct current
• The coil is made of insulated copper wire and is free to rotate between the poles of the magnet
• The commutator (split-ring) is fixed to the coil and rotates with it
• When the coil overshoots the vertical, the commutator changes the direction of the current through it, so the forces change direction and keep the coil turning
• The brushes are two contact which rub against the commutator and keep the coil connected to the battery, usually made of carbon
• The maximum turning effect is when the coil is horizontal
• There is no force when the coil is vertical but it always overshoots this position