Physics - electromagnetism

• electric motor -> electrical current into mechanical force
• electric generator -> mechanical force into electrical current
• generators convert kinetic energy into electrical energy (by rotating a wire coil in a mag. field)
• What is electromagnetic induction?

  It is the process of generating an electric current in a conductor by exposing it to a changing magnetic field.
• What are the key components involved in electromagnetic induction?

  A conductor, a changing magnetic field, and the induced electric current.
• What does Faraday's Law state about induced electromotive force (EMF)?

  It states that the induced EMF in a closed circuit is directly proportional to the rate of change of magnetic flux through the circuit.

  
  

  induced V across a coil or wire moving in a mag. field depends on rate of change in flux
• Why is there a negative sign in Faraday's Law?

  The negative sign is due to Lenz's Law, which states that the induced current opposes the change that caused it.
• What factors affect the induced EMF?

  1. Rate of change of magnetic flux: Faster changes produce larger EMF.
  2. Number of turns in the coil: More turns increase the EMF.
  3. Strength of the magnetic field: Stronger fields produce larger EMF.
  4. Area of the coil: Larger area increases the magnetic flux.
  5. Angle between the magnetic field and the coil: Maximum EMF when perpendicular.
• What does Lenz's Law state?

  It states that the direction of an induced current is such that it opposes the change that caused it.

  
  

  induced V direction opposes the change in flux producing it
• induced voltage is equal to the rate of change in the flux through the coil
• In a D.C circut, the change in flux can be achieved by:

  • changing the mag. field strength, by turning the switch on and off for the circut generating the field and keeping the area constant
  • moving the coil in the field (which has a fixed strength) changing the area
  • moving the magnet of strength mag. field inside a coil
• an inductor will have an inductance when the current is being switched on and off and the coil will behave as a pure inductor in this case
• when the current is stable the inductor coil will behave as a pure resistor.
• the size of the max current on any inductor resistor current will depend on resistance (r)
• in any inductor, flux (Φ) is proportional to current (I)
• inductance of a coil (L) is determined by:

  1. N.O. turns in the inductor (each turn acts as a magnet)
  2. material of the coil
  3. the coil area
  4. the penetrability of the material inside the coil
• when a switch in the circuit is closed what happens to brightness of lamp:

  • current in the coil will change
  • a voltage will be induced across the coil as current changes from 0 to x (faradays law)
  • this induced voltage will oppose the change in current producing it (lenzs law)
  • this will stop the current from reaching max value directly but will make it increase exponentially with time
  • since the brightness of a lamp depends on current and voltage received. Brightness will increase gradually as current increases.
• induced voltage opposes -> Vs = VL + Vb => Vs is fixed so as Vb grows VL drops
• when induced voltage becomes zero, inductor acts as a resistor
• in an open circuit kirchoffs law doesn't apply
• VL gets high when a switch is open because the circuit becomes an open circuit which no longer obeys kirchoffs law.

  open circuit means R ≈ ∞

  T = L/R becomes very small and the change in flux drops to zero very quickly
• to produce the change in flux in d.c

  turn flux on and off by switching the circuit on and off

  turning the coil inside a magnetic field which changes area
• Np = N.O. turns of primary coil
• Ns = N.O. turns of secondary coil
• any change in flux in the primary coil will induce a voltage across the secondary coil depending on the ratio of the turns in the two coils. Np/Ns = Vp/Vs
• transformer step down -> primary has more turns

  transformer step up -> secondary has more turns

  isolating -> same number of turns
• a transformer is made up of two inductors with same or different N.O. of turns.

  Primary connected to A.C. source with Np turns

  Secondary connected to a circuit, gives output voltage across Ns turns
• For an ideal transformer there's no loss in input power (IpVp =IsVs)
• common iron core of inductors gets magnetised and links flux between two inductors
• change in current in primary coil produces induced voltage across secondary coil and induced current if secondary coil has a complete circuit
• change in flux through primary coil travels through common iron core and links with the secondary coil, providing a flux linkage.
• changing flux in the solid iron core will produce an opposing voltage across it and opposing current as it provides a closed loop. this current is called Eddy current which can be very large and can cause power to be lost as heat inside transformer
• Pheat usually depends on resistant and current, current has a large effect as it is squared
• Eddy current is reduced by laminating the iron core so instead of having a large current we will have smaller incomplete current which reduces the Pheat