magnetism and electromagnetism

• • A magnetic field is defined as:

  The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel)
• Magnetic fields are invisible
• • The Law of Magnetism states that:
  • Two like poles (S and S or N and N) repel each other
  • Two unlike poles (S and N) attract each other
• • The attraction or repulsion between two magnetic poles is an example of a non-contact force
• • Magnetically soft materials (e.g. iron):
  • Are easy to magnetise
  • Easily lose their magnetism (temporarily magnetised)
• • Magnetically hard materials (e.g. steel):
  • Are difficult to magnetise
  • Do not easily lose their magnetism (permanently magnetised)
• • Permanent magnets are made out of magnetically hard materials
  • Electromagnets are made out of magnetically soft materials
  • This means that electromagnets can be made magnetic or non-magnetic as an when required
• • Magnetic field lines are used to represent the strength and direction of a magnetic field
  • The direction of the magnetic field is shown using arrows
  • The strength of the magnetic field is shown by the spacing of the magnetic field lines
  • If the magnetic field lines are close together then the magnetic field will be strong
  • If the magnetic field lines are far apart then the magnetic field will be weak
• • There are some rules which must be followed when drawing magnetic field lines. Magnetic field lines:
  • Always go from north to south (indicated by an arrow midway along the line)
  • Must never touch or cross other field lines
• Magnetic Field Around a Bar Magnet

  • The magnetic field is strongest at the poles
  • This is where the magnetic field lines are closest together
  • The magnetic field becomes weaker as the distance from the magnet increases
  • This is because the magnetic field lines are getting further apart
• Uniform magnetic field

  A magnetic field that has the same strength and direction at all points
• Producing a uniform magnetic field with two bar magnets


  1. Point opposite poles (north and south) of the two magnets a few centimetres apart

  2. A uniform magnetic field will be produced in the gaps between opposite poles
• Uniform magnetic field

  • Magnetic fields are always directed from North to South
  • There must be equal spacing between all magnetic field lines to show the field strength is the same at all points
  • There must be an arrow on each magnetic field line going from the north pole to the south pole to show the field is acting in the same direction at all points
• • Very few metals in the Periodic Table are magnetic. These include:
  • Iron
  • Cobalt
  • Nickel
• • Magnetic materials (which are not magnets) will always be attracted to the magnet, regardless of which pole is held close to it
• • To test whether a material is a magnet it should be brought close to a known magnet
  • If it can be repelled by the known magnet then the material itself is a magnet
  • If it can only be attracted and not repelled then it is a magnetic material
• • There are two types of magnets
  • Permanent magnets
  • Induced magnets
• • permanent magnets are made out of permanent magnetic materials, for example steel
  • A permanent magnet will produce its own magnetic field
  • It will not lose its magnetism
• • When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet.
  • This is called induced magnetism
  • When magnetism is induced on a material:
  • One end of the material will become a north pole
  • The other end will become a south pole
• • Magnetic materials will always be attracted to a permanent magnet
  • This means that the end of the material closest to the magnet will have the opposite pole to magnets pole closest to the material
• • When the magnetic material is removed from the magnetic field it will lose most/all of its magnetism quickly
• • he magnetic field is made up of concentric circles
  • A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
  • As the distance from the wire increases the circles get further apart
  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
• • Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field
• • If there is no current flowing through the conductor there will be no magnetic field
  • Increasing the amount of current flowing through the wire will increase the strength of the magnetic field
  • This means the field lines will become closer together
• Factors Affecting Field Strength

  • The strength of the magnetic fields field depends on:
  • The size of the current
  • The distance from the long straight conductor (such as a wire)
  • A larger current will produce a larger magnetic field and vice versa
  • The greater the distance from the conductor, the weaker the magnetic field and vice versa
• Electromagnets are used in electric motors, generators, transformers, relays, solenoids and loudspeakers.
• The explanation of the loudspeaker is very similar to the explanation of a motor, however direct current is used in a d.c motor and alternating current is used in a loudspeaker. You need to learn how both work.When explaining how a loudspeaker works remember to refer to the alternating current and the changing magnetic field that it creates.
• • he motor effect occurs:

  When a wire with current flowing through it is placed in a magnetic field and experiences a force
• • Reversing the direction of the current will also reverse the direction in which the forces are acting
  • As a result, the coil will continue to rotate
• • Reversing the direction of the current will also reverse the direction in which the forces are acting
  • As a result, the coil will continue to rotate
• Factors Affecting the D.C Motor

  • The speed at which the coil rotates can be increased by:
  • Increasing the current
  • Increasing the strength of the magnetic field
  • The direction of rotation of coil in the d.c motor can be changed by:
  • Reversing the direction of the current
  • Reversing the direction of the magnetic field by reversing the poles of the magnet
  • The force supplied by the motor can be increased by:
  • Increasing the current in the coil
  • Increasing the strength of the magnetic field
  • Adding more turns to the coil
• two devices that use the motor effect are headphonea and speakers
• • Magnetic forces are due to interactions between magnetic fields
  • Stronger magnetic fields produce stronger forces and vice versa
• • For a current carrying conductor, the size of the force exerted by the magnetic fields can be increased by:
  • Increasing the amount of current flowing through the wire
  • This will increase the magnetic field around the wire
  • Using stronger magnets
  • This will increase the magnetic field between the poles of the magnet
  • Placing the wire at 90o to the direction of the magnetic field lines between the poles of the magnet
  • This will result in the maximum interaction between the two magnetic fields
• If the two magnetic fields are parallel there will be no interaction between the two magnetic fields and therefore no force produced
• Remember that the magnetic field is always in the direction from North to South and current is always in the direction of a positive terminal to a negative terminal.
• • he direction of the force (aka the thrust) on a current carrying wire depends on the direction of the current and the direction of the magnetic field
  • All three will be perpendicular to each other
  • This means that sometimes the force could be into and out of the page (in 3D)
• • EM induction is when:

  A voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it
• Factors Affecting the Induced Potential Difference

  • The size of the induced potential difference is determined by:
  • The speed at which the wire, coil or magnet is moved
  • The number of turns on the coils of wire
  • The size of the coils
  • The strength of the magnetic field
• 3. The size of the coils:

  • Increasing the area of the coils will increase the potential difference induced
  • This is because there will be more wire to cut through the magnetic field lines