7 Electric and magnetic fields

• Explain the variation of e.m.f with time

  • Induced EMF from the change in magnetic flux linkage
  • The greater the rate of change of flux linkage the larger the induced emf
  • After the south pole reaches the coil the flux linkage decreases
  • as the south pole of the magnet passes through the coils the induced emf is negative
  • downward speed of magnet increases
  • EMF is zero after leaving coil
• Describe how the device can be used a s both a generator and an electric motor

  In a generator, a coil has to rotate

  • there is a rate of change of magnetic flux linkage
  • which induces an emf

  In a motor, a current is provided to coil

  • Force on sides of coil that are perpendicular to magnetic field
  • coil keeps rotating as the force provides a moment
• Electric Potential, Potential difference

  ΔV = kQ/r

  ΔV = E/Q

  ΔV = E.D

  Amount of energy required to move a positive test charge from infinity to a point in a e field
• E field between parallel plates

  E = Δ V/ D
• Equipotentials

  Lines along which the electric potential remains the same
• Electric field

  A region of space in which a charged particle experiences a force
• All charged particles create their own electric fields.

  These fields exert an electrostatic force(Fe) on other charged particles
• Electric field strength

  E = F/Q

  E = Δ V/D

  E = KQ/r^2

  
  

  Force per unit charge acting on a positive test charge at that point in a electric field
• Coulomb's Law

  F = KQq/r^2

  
  

  Force between 2 charges are directly proportional to the product of their charges

  and are inversely proportional to the square of the distance between them (inverse square law)
• Describe an E field with a positive point charge

  Radial field, arrows pointing outwards
• Describe an E field with a negative charge

  Radial field, arrows pointing inwards
• Electric potential energy

  U = KQq/r

  U = Δ V.q

  U = kinetic energy
• What do field lines show

  Field lines show the direction of acceleration and the direction of force on a charge
• When field line is straight?

  The charge will follow the field line

  + charge to - charge
• When field line is curved?

  Acceleration will change its direction

  but the velocity is not in the direction of acceleration
• Calculate charge on full charged capacitor

  Q = CV
• Calculate energy stored by fully charged capacitor

  1/2 CV^2

  1/2 QV
• Capacitance

  CV^-1
• Electric field

  Region of space in which a charged particle experiences a force
• Potential energy

  
  

  E = Q . Δ V

  1/2mv^2 = QV
• Explain why current rises to a maximum then decreases

  • Current as circuit is complete or due to battery
  • Coil rotates
  • Coil changes the magnetic flux linkage
  • Which induces an EMF according to Faradays law
  • The new EMF opposes the original EMF according to Lenz law
  • So the faster the coil rotates the larger this EMF and the smaller the current

  
• E fields vs G fields (6)

  • G fields are regions in which a mass experiences a force due to its mass
  • E fields are regions in which a charge experiences a force due to its charge
  • Both types of field have an infinite range
  • In each field , force varies as inverse square
  • Force between masses are always attractive however force between charges can be attractive and repulsive
  • Force between unit charges at given seperation is much stronger than force between unit masses at same seperation
• Explain why the kettle heats up when there is an alternating current in the coil. (6)

  • Alternating current produces a magnetic field
  • So this is a alternating magnetic field
  • Change in magnetic flux linkage due to alternating current
  • An EMF is induced
  • Current circulates in the kettle
  • Kettle heats up as energy is dissipated by the heating effect of current
• Explain why electrons are deflected into a parabolic path when a potential difference is applied between A and B

  • Electrons will experience a force in a electric field
  • They will experience a vertical acceleration
  • Horizontal component remains constant
• High frequency with a AC supply

  Increase rate of change in magentic flux linkage

  leads to increased induced emf in pe
• Ac supply is switched on coil connected to an ac current. Pan is placed on induction hob, why the pan gets very hot

  • Current produces a magnetic field
  • ac leads to changing magneitc field through pan
  • induces an emf
  • Leads to current in the pan, it is a conductor
• Show that the velocity in a e field question

  E = F/Q

  F = EQ

  Wd = F.d

  W = 1/2mv^2
• Explain why alternating current in coil causes the cone to oscillate with the freq of ac

  • Current in coil causes a magnetic field
  • Current is alternating so field changes direction with current
  • Field interacts with magnetic field so coil experieces a force