Electromagnetic Induction

Electromagnetic Induction

1. induced emf is produced in a conductor due to a CHANGING magnetic field

   • factors
     • Faraday's Law: magnitude of induced emf is directly proportional to rate of change of magnetic fields in circuit
       • When north going into solenoid, there is a repulsion effect (north pole induced on near end of solenoid)
       • When north going out of solenoid, there is a attraction effect (south pole induced on near end of solenoid)
         • Lenz's Law: direction of induced emf in circuit is always such that the magnetic effect opposes the magnet's motion that produces it
     • number of coils (without increasing length of wire)
     • Strength of magnet
     • soft iron core
   • can use galvanometer to determine if induced emf is produced (if needle is deflected)
     • direction of deflection is inconsistent (will say in qns)
       • But different direction of current is different deflection (with the same galvanometer
         • different pole
         • different motion of magnet (in/out)

2. A.C. generator

   • instead of relying on motor effect to turn motor, an external force is used to rotate the coil
     • as coil turns, magnetic field changes hence there is induced emf
       • induced emf not constant (whether coil is horizontal/vertical position)
         • horizontal will hv maximum voltage as they will cut off magnetic field lines (most change)
           • if opposite direction of current, then induced emf if opposite direction
         • vertical position, it already cut off the magnetic field lines so there is 0 change and 0 voltage
           • voltage increases after vertical as magnetic field lines start to change again to horizontal position
   • instead of split ring, it uses slip ring that are always in contact with carbon brushes
     • direction of current changes every half rotation but constant
   • uses Fleming's Right Hand Rule instead of Left hand

3. Transformers

   • one powered coil of wire can induce emf on another unpowered coil of wire when wrapped around the same iron ring
   • change a high AC voltage to low AC voltage (or vice versa)
     • primary coil connected to ac voltage
       • varying magnetic field set up in soft iron core
         • emf produced in secondary coil (output voltage)
           • Turns Ratio: Vs/Vp = Ns/Np = Ip/Is
             • where s = secondary and p = primary
             • Step up transformer, no. of turns in secondary coil (Ns) is greater than turns in primary coil (Np)
               • hv to change voltage to minimise energy lost through thermal energy (I^2R) as well as allowing for thinner wires
             • Step down transformer, no. of turns in secondary coil (Ns) is lesser than turns in primary coil (Np)
       • ideal conditions: power at input = power at output
         • non-ideal: power at output / power at input x 100%

4. Cathode-Ray Oscilloscopes

   • to observe voltage waveforms in transformer
     • y-gain
       • varies the voltage across Y-plates of CRO to amplify vertical deflection of electron beam
         • essentially changing sensitivity of cro (changing scale of y axis)
     • Time Base
       • varies voltage across X-plates of CRO to adjust how quickly electron beam sweeps horizontally across the screen
         • essentially changing frequency
     • number of complete cycles given by: fy/fx, where fy = frequency of a.c. input (Hz), fx - frequency of time base (Hz)
   • to measure short intervals of time
     • just see the distance (in terms of no. of divisions) between the 2 pulses of sound then multiply by time base (time per division)

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