PHY 5.3 S2

• Faraday Law states that the magnitude of the induced emf is proportional to the rate of change of the magnetic flux.
• The negative sign indicates that the direction of induced emf always oppose the change of magnetic flux producing it (Lenz's Law)

  d∅ : change of magnetic flux

  dt : change of time

  E : induced emf

  
• For a coil of N turns is placed in a uniform magnetic field & but changing in the coil's area 4, the induced emf is given by.

  E =-NB(cos∅)(dA/dt)

  
• For a coil is connected in series to a resistor of resistance R and the induced emf & exist in the coil

  IR = -N(d∅/dt)

  
• To calculate the magnitude of induced emf, the negative sign can be ignored.
• Lenz Law states that an induced electric current always flows in such a direction that it opposes the change producing it. This law is essentially a form of the law of conservation of energy.
• • In Figure 5.6 the magnitude of the magnetic field at the solenoid increases as the bar magnet is moved towards it. • An emf is induced in the solenoid and the galvanometer indicates that a current is flowing.

  
• To determine the direction of the current through the galvanometer which corresponds to a deflection in a particular sense, then the current through the solenoid seen is in the direction that make the solenoid upper end becomes a north pole.

  • This opposes the motion of the bar magnet and obey the Lenz's law.
• Consider a straight conductor PQ is placed perpendicular to the magnetic field and move the conductor to the left with constant velocity as shown In Figure 5.7.

  • When the conductor move to the left thus the Induce current needs to flow in such a way to oppose the change which has Induced it based on Lenz's law.

  • Hence galvanometer shows a deflection.

  
• To determine the direction of the induced current (induced emf) flows in the conductor PQ, the Fleming's right hand (Dynamo) rule is used

  
• Therefore the induced current flows from Q to P as shown in Figure 5.7. • Since the induced current flows in the conductor PQ and is placed in the magnetic field then this conductor will experience magnetic force. • Its direction is in the opposite direction of the motion

  
• • At the moment when the switch S is closed, current/ begins to flow • The magnetic flux through the solenoid Q increases with time. According to Faraday's law, an induced current due to induced emf will exist in solenoid Q.

  • The induced current flows in solenoid Q must produce a magnetic field that oppose the change producing it (increase in flux). • Hence based on Lenz's law, the induced current flows in circuit consists of solenoid Q is anticlockwise (Figure 5.9a) and the galvanometer shows a deflection.

  
• • At the moment when the switch S is opened, the current / starts to decrease in the solenoid P and magnetic flux through the solenoid Q decreases with time. • According to Faraday's law an induced current due to induced emf will exist in solenoid Q. • The induced current flows in solenoid Q must produce a magnetic field that oppose the change producing it (decrease in flux). • Hence based on Lenz's law, the induced current flows in circuit consists of solenoid Q is clockwise (Figure 5.9b) and the galvanometer seen to deflect in the opposite direction of Figure 5.9a.

  
• For a coil of N turns is placed in the changing magnetic field B, the induced emf is given by E = -NA(cos∅)(dB/dt)