Knowledge-17 MF

• A magnetic field is a region of space in which objects with magnetic properties experience a force.
• magnetic fields can be represented with field lines , also known as lines of magnetic flux, where :

  • the arrows in a field line show the direction of a field always going from north to south.
  • the separation of field lines indicates the strength of the field.
• Right hand rule :

  Where the fingers curl in the direction of current and the thumb points towards the north pole.
• Right hand rule:

  For a current carrying straight wire , the thumb points in the direction of the current, and the fingers coil in the direction of the field.
• Magnetic field strength is also know as flux density , symbol B.
• Magnetic flux density is defined as the force per unit length per unit current on a current carrying conductor at right angles to the direction of the magnetic fields.
• Magnetic flux density is measured in tesla (T).
• Magnetic flux is measured in weber turns (Wb).
• Magnetic flux is defined as the number of flux lines passing through an area A at right angles to the field the flux passes through.
• For a coil of wire with a cross-sectional area A and N turns :

  • magnetic flux linkage = BAN
• The unit of magnetic flux linkage is weber turns
• If a magnetic field is at an angle 0 to the normal at the face of the coil magnetic flux linkage through the coil is:

  • magnetic flux linkage = BAN cos0
• A current carrying wire in a magnetic field will experience a force , this force is known as the motor effect.
• The size of the force F in N of a wire length L in m that is perpendicular to the field lines is :

  • F = BIL
• The direction of force is found using Flemings left hand rule, where :

  • the thumb points in the direction of the force
  • the first finger points in the direction of the magnetic field from north to south.
  • second finger points in the direction of current
• The force on a charged particle moving at right angles to a magnetic field is:

  • F = BQv
• Since a charge particle moving in a magnetic field experiences a centripetal force :

  • mv^2 / r = BQv
  • r =mv /BQ
• A cyclotron is a circular particle accelerator which can be used to produce high energy beams for uses such as radiation therapy.
• Cyclotron:

  1. charge particles fired into a dee near centre of cyclotron
  2. magnetic field forces a charged particle to follow circular path out of first dee
  3. a p.d between the dees accelerates charged particles across the gap
  4. particle is moving faster so its circular path has bigger radius in second dee
  5. p.d across gaps reverses so the particle accelerates again
  6. particle leaves the cyclotron
• A fixed frequency alternating p.d can be used to accelerate the particle between the dees as the time taken to travel round each dee is independent of speed.
• deriving Frequency of alternating p.d :

  • r = mv / BQ
  • r = pie r / v = m pie /BQ
  • V = BQR / m
  • T = 2m pie /BQ
  • 1/t = BQ /2 pie m
• A wire will feel a magnetic field if the field is perpendicular to the current-carrying wire.

  Evidence is F= BIL
• Fleming's left hand rule:

  Thumb - Force

  First finger - Field

  Second finger - current
• Magnetic flux density is measured in teslas and is the flux per metre.
• A charge particle moving through a field feels a force perpendicular to the field when moving perpendicular to field lines or along field lines.
• F = BQv
• The force applied to a particle in motion is perpendicular to its motion creating circular motion.
• Cyclotrons use both an electric and magnetic field.
• How do cyclotrons work?

  • a cyclotron is made up of 2 semi-circular electrodes called 'dees' with a magnetic field applied perpendicular to the dees and an alternating pd applied between the dees.
  • each dee is a metal electrode with opposite charges , this creates an electric field in the gap between the two dees. this accelerates the particles.
  • The magnetic field causes the particles to move in circular motion , which allows it to gain speed whilst minimising space. As they speed up the radius of their motion increases , until it breaks free tangential to one of the dees.
• Magnetic flux is the flux density multiplied by the area.
• flux linkage is the number of turns cutting the flux at one time.
• Faradays law states that the induced e.m.f is directly proportional to the rate of change of magnetic flux linkage.
• Lenz's law states the direction of induced e.m.f is such as to oppose the change that induces it.
• When a straight conductor moves through a magnetic field the electrons experience a force pushing them to one end of the conductor creating emf. The rod obeys faradays law , it is changing flux as it moves through the field hence emf is induced.
• how to transformers work:

  1. A primary coil wrapped around an iron core with an alternating pd creates and alternating magnetic field.
  2. this magnetic field induces an EMF in a secondary coil also wrapped around the core. This creates current in the secondary coil.
• In a transformer an alternating current is produced as an emf is induced by changing magnetic field.
• transformers are used because by changing the number of coils , the transformers can be used to increase the voltage and reduce the current when transporting power. the voltage is then dropped again locally to ensure safe usage in households.
• what equation links number of coils in a transformer and their voltage?

  Ns / Np = Vs / Vp
• what is transformer effiecency?


  the ratio of power output to the power input.

  = IsVs / IpVp
• in a step up transformer the secondary coil has more coils.