Electromagnetic Induction Class 12 Concepts in Detail

Michael Faraday and Joseph Henry found that electricity could be produced without a battery just by changing the magnetic environment of a conductor. As per Faraday, whenever a bar magnet is pushed within a coil of wire that is connected to a galvanometer, the needle will deflect to show the flow of current. When the magnet is pulled out, it reverses the direction of current. No movement of magnet indicates that there is no flow of current.

Joseph Henry conducted the coil-to-coil test in which he placed two coils close together. He, then, changed the current in first coil and observed that the current appeared momentarily in the second coil. This determined that the induced current exists only till the time magnetic field is changing. If the magnetic fields are steady, there will be no current flow.

Magnetic flux (Φ) determines the amount of magnetic field that passes through a given surface. Mathematically, magnetic flux is expressed as: 

 Φ = B × A × cos θ

Here:

• B is the magnetic field strength measured in tesla (T)
• A is the surface area measured in m²
• θ is the angle between field lines and the normal which is perpendicular to the surface
• In case, the field is perpendicular to the surface i.e. θ = 0°, then, cos θ = 1 and Φ = B × A which is the maximum flux.
• If the field is parallel to the surface i.e. θ = 90°, then, cos θ = 0 and the flux will be zero.

Do note that any change in the magnetic flux through a loop produces an induced emf as per the Faraday’s law. Questions based on its application will be asked in the JEE Main entrance exam.

Faraday’s law builds a mathematical relation between the changing magnetic flux and the induced voltage. As per the formula (relevant for NEET exam aspirants),

 ε = − dΦ/dt

Here:

• ε is the Induced emf measured in volts
• dΦ/dt indicates the rate of change of magnetic flux
• The negative sign comes from the Lenz’s law which indicates that the induced emf opposes any change in flux.

Do note that faster changes in flux will result in larger induced emf.

According to Lenz’s law, the induced current will always flow in a direction that is opposite to the cause of its production. This flow direction will ensure that energy is conserved. We cannot get electrical energy without doing work against induced magnetic effects.

Suppose, we are moving a magnet towards the coil. This coil will generate a magnetic field which pushes back against magnet’s approach. You will be required to do extra work in order to overcome this opposition. This work will get converted into the electrical energy in the coil.

If Lenz’s law were not valid, then it was possible to create energy from nothing, which is impossible in real life.

In case a conductor moves through the magnetic field, its free electric charges will experience a magnetic force or Lorentz force which will separate charges and create an emf. Mathematically, this force is expressed as:

 ε = B × l × v

Here:

• B is the Magnetic field (T)
• L is the length of the conductor in the field (m)
• v is the velocity which is perpendicular to the field (m/s)
  

This principle is used in electric generators and moving conductors in magnetic fields for generating electricity. Once learnt, practice some IIT JAM exam-level questions based on this concept. 

Inductance is a property to measure a coil’s resistance to changes in current due to the magnetic field it produces. There are two types of inductances including self inductance and mutual inductance. 

1. Self-inductance occurs when the coil induces emf within itself whenever its own current changes. It is mathematically represented as ε = − L (dI/dt)
2. Mutual inductance (M) occurs when change in the current of one coil induces emf in the nearby coil. Mathematically, it is expressed as ε₂ = − M (dI₁/dt)

Inductance is dependent on various factors including coil turns, area, length, and magnetic material inside the coil.

An AC generator is a machine that converts mechanical rotations into alternating current electricity using the process of electromagnetic induction. The following steps explain how it happens: 

• Say there is a rectangular coil that rotates inside a uniform magnetic field between poles of a magnet.
  
• As this rectangular coil rotates, magnetic flux through this coil will keep on changing continuously.
  
• As per the Faraday’s law, this will lead to the generation of an alternating emf in the rectangular coil.
  
• The output will be sinusoidal with maximum emf whenever the coil is perpendicular to the magnetic field lines.