Magnetism

• The poles of a magnet are the places where the magnetic forces are strongest.
• When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two opposite poles attract each other.
• Attraction and repulsion between two magnetic poles are examples of non-contact force.
• A permanent magnet produces its own magnetic field.
• An induced magnet is a material that becomes a magnet when it is placed in a magnetic field
• Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.
• The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.
• The force between a magnet and a magnetic material is always one of attraction.
• The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.
• The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.
• A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.
• When a current flows through a conducting wire a magnetic field is produced around the wire. 

  The strength of the magnetic field depends on the current through the wire and the distance from the wire.
• Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.
• The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases thestrength of the magnetic field of a solenoid . An electromagnet is a solenoid with an iron core.
• what is a solenoid ?

  wire wrapped into a coil
• right hand rule:

  using right hand point thumb in the direction of the current and curl your fingers

  the direction of your fingers is the direction of the (magnetic) field
• what is the motor effect?

  When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.
• Flemmings left hand rule :

  using left hand point your First finger in the direction of the Field

  point your seCond finger in the direction of the Current

  your thuMb will point in the direction of the force (Motion)
• factors that effect the size of the force on the conductor :

  • the magnetic flux density (how many field lines there are in a region)- this shows the strength of the magnetic field
  • the size of the current through the director
  • the length of the conductor thats in the magnetic field
• A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
• Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
• What type of current is sent through the coil in loudspeakers?

  Alternating current (AC)
• How does the coil in a loudspeaker interact with the magnet?

  The coil surrounds one pole of a magnet
• What happens to the coil when the current reverses?

  The force acts in the opposite direction
• What does the movement of the cone in a loudspeaker create?

  Variations in surrounding air pressure
• What is the relationship between the frequency of the sound wave and the AC frequency?

  The frequency of the sound wave equals AC frequency
• How can you alter the sound wave produced by a loudspeaker?

  By controlling the AC frequency
• What is the process of sound wave creation in loudspeakers?

  • AC sent through coil
  • Coil interacts with magnet
  • Current reverses, changing force direction
  • Cone vibrates, creating air pressure variations
  • Sound wave produced
• If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.
• An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.
• Changing the size of the induced pd

  • Increasing the speed of the movement - cutting more magnetic field lines in a given time
  • Increasing the strength of The magnetic field (so there are more field lines that can be cut)
• The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.

  Students should be able to:
• Alternators :

  • alternators rotate a coil in a magnetic field
  • Their construction is similar to a motor
  • As the coil (or magnet) spins a current is induced in the coil, this current changes direction every half turn
  • Instead of a split ring communicator, ac generators have slip rings and brushes so the contacts dont swap every half turn
  • This means they produce and alternating potential difference
• dynamos :

  • dynamos work in the same way as alternators apart from one important difference
  • they have a split ring communicator instead of slip rings
  • this swaps the connection every half turn to keep the current flowing in the same direction
• Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.
• microphones are essentially loud speakers in reverse

  • sound waves hit a flexible diaphragm that is attached to a coil of wire wrapped around a magnet
  • this causes the coil of wire to move in the magnetic field which generates a current
  • the movement of the coil (and so the generated current) depends on the properties of the sound wave (louder sounds make the diaphragm move further)
  • this is how mics convert the pressure variations of a sound wave into variations in current in an electric circuit
• A basic transformer consists of a primary coil and a secondary coil wound on an iron core.

  Iron is used as it is easily magnetised.
• The ratio of the potential differences across the primary and secondary coils of a transformer V pand V s depends on the ratio of the number of turns on each coil, n p andn s .
• In a step-up transformer Vs > V p

  In a step-down transformer V s < V p

  If transformers were 100% efficient, the electrical power output would equal the electrical power input.
• Vs × Is  = Vp × Ip

  Where V s × I s is the power output (secondary coil) and V p × I p is the power input (primary coil).

  power input and output, in watts, W