electromagnetism

Question 1

electromagnetic induction

Answer 1

• when you move a wire perpendicularly to a magnetic field you induce a current in the wire
• move wire up and downwards and a current flows in the wire but only when the wire is actually moving
• move wire back down and a current flows in the opposite direction
• this is called electromagnetic induction (or the dynamo effect)
• you get a bigger induced current by :
  1) moving the wire quicker
  2) using stronger magnets
  3) using more coils of wire

dynamo effect: motion + magnets = current
motor effect: current + magnets = motion

Question 2

transformers

Answer 2

• transformers use electromagnetic induction to change voltages
• a step up transformer:
• voltage increases
• voltage ratio = turns ratio
  Vp/Vs = Np/Ns
  N= number of turns
  V= voltage
  p= primary
  s= secondary
• a step down transformer:
• voltage decreases
• transformers only work using a.c as they need a changing magnetic field in order to induce the voltage in the secondary

Question 3

power and current in transformers

Answer 3

• if a transformer is 100 % efficient
  power in = power out
  P= IV
• if the voltage decreases the current must increase in the same ratio

Question 4

the national grid

Answer 4

• electricity is transferred around the country at very high voltages so that the current can be kept low and so less energy is wasted as heat
• step down transformers into factories and homes
• step up transformers into power stations