2.1.10 Faraday’s & lenz’s laws of em induction

Faraday’s Law Consider a conductor of length l (which is part of a complete circuit), cutting through a magnetic field with a flux density of B. There is an induced current I flowing in the circuit. The conductor will experience a force given by F = BIl An equal & opposite force must be applied to keep the conductor moving and if the conductor is moved a distance s then the work done by this force is Fs This can be expanded to BIls

If the current I has been flowing for t seconds then the charge transferred is: Q = It If we consider EMF  (Voltage) (remember potential difference is the work done per unit charge) ε = W/Q = BIls / It = Bls /t The conductor of length l has moved through a distance s, this gives us an area A :  = BA /t

The product (BA) of the magnetic flux density (B) and the area (A) is called the magnetic flux () Magnetic flux has the derived units of (Tm2) and is given the special unit Weber (Wb) The induced EMF in our example is equal to the magnetic flux cut through per second  =  /t

Lenz’s law Lenz’s law states that : The direction of the induced current is always such as to oppose the change which has caused the current to be induced

We can then simply use the “Solenoid rule” to establish the direction of the induced current North Anticlockwise South clockwise Looking into the coil Looking into the coil

Practical applications When current is being established, the induced emf will be setup in a way to prevent the current from increasing. When the current is being switched off, the induced emf will try and keep the current going. This shows that the induced emf opposes any forward emf – back emf.