➔ Electromotive force (emf)
➔ Magnetic flux and magnetic flux linkage
➔ Faraday’s law of induction
➔ Lenz’s law
Applications and skills
➔ Describing the production of an induced emf by a changing magnetic flux and within a uniform magnetic field
➔ Solving problems involving magnetic flux, magnetic flux linkage, and Faraday’s law
➔ Explaining Lenz’s law through the conservation of energy
➔ Flux: Φ = BA cosθ
➔ Faraday’s / Neumann’s equation: ε = –N (dΦ/dt)
➔ emf induced in moving rod: ε = Bvl
➔ in side of coil with N turns: ε = BvlN
11.1 Electromagnetic Induction
Electromagnetic induction: When an electric charge moves in a magnetic field, then a force acts on it. In a reverse sense, a movement or change in magnetic field relative to stationary charge gives raise to an electric current.
Induced emf (ε)
Definition: Potential difference generated by electromagnetic induction.
For a rod of length L moved with velocity v in a region of magnetic field B:
If the rod moves from left to right, and thus, its electrons move perpendicular to the magnetic field, they experience a downward force along the rod and an electric field is established.
Flow of electrons quickly stops due to electrostatic repulsion at the bottom, and thus, the current exists for a short period of time.
Without movement, emf is not induced.
Formula if the rod is moved connected to wires (the work done to separate electrons leads to an induced emf): ε = BvL.
Magnetic flux (Ф)
Definition: “Product of the magnitude of the normal component of magnetic field strength and area through which it passes.”
Intuitive picture: Number of magnetic field lines crossing a certain area.
Formula: Ф = BAcosθ, where A is the area and θ is the angle between the magnetic field strength direction and the direction normal to the loop area.
Units: weber (Wb)
Definition for a rod: “Product of magnitude and the rate at which the area swept out by the rod is changing” = ∆Ф/∆t.
Magnetic flux linkage: Magnetic flux multiplied by the N turns in a loop. Ф = NBAcosθ.
Magnetic flux density: numerically equivalent to magnetic field strength.
Induced emf = magnetic flux density x rate of change of area = B∆A/∆t.
Definition: “Induced emf is equal to the negative rate of change of magnetic flux linkage.”
Negative sign exists due to Lenz’s law (see below).
Formula: ε = -N∆Ф/∆t.
Magnetic field away from viewer
Coil of area A with N turns
Example of Faraday’s Law:
A coiled wire is moving into a magnetic field.
The induced current should create a magnetic field in a direction opposed to the existing field (in this case, opposing “away from”, therefore, towards the viewer)
Rod (perpendicular to field): in time ∆t, a rod of length L will move a distance s = v∆t, cutting magnetic field lines as it moves in the magnetic field. A = Ls
Formula: ∆Ф = ∆BAcos0º = ∆BA = ∆BLs = BLv∆t, and hence, ε = BvL.
Definition: “The induced emf will be in such a direction to oppose the change in the magnetic flux that crea
ted the current”. It is equivalent to energy conservation.
Work done by magnetic forces that arises due to current is dissipated as thermal energy.
Rod: Force in the rod must oppose
Use left-hand rule twice: Firstly to find the direction of the current in the loop. Secondly, to find the force induced on the rod due to the current the motion. Hence, if it moves towards the right, a leftwards force will appear indicating a counter-clockwise induced current.
Loop wire and a wire with increasing current: Magnetic flux is increasing into the page. Hence, to oppose the increase in magnetic flux (inside the loop), a magnetic field out of the page must exist, and thus, a counter-clockwise current is induced.
Bar magnet through a loop of wire:
When approaching the loop, magnetic flux is increasing, and thus, magnetic field must oppose the increase, with a counter-clockwise current.
When leaving the loop, the magnetic flux is decreasing, and the current is now clockwise.
The opposite magnet (south pole first) would have the exact opposite effect.
THE EXPERIMENTS OF FARADAY AND HENRY
FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION
METHODS OF INDUCING E.M.F.
- By changing B
- By changing A
- By changing θ (orientation of the coil)
INDUCED E.M.F. AND ITS DIRECTION
ω = angular velocity, l = length of conducting rod.
ω = angular velocity of disc.
- if key K is closed then current in P will flow in clockwise direction and consequently induced current in Q will flow in anticlockwise direction. (see fig.a)
- when key K is opened then current in P falls from maximum to zero and consequently induced current in Q will flow in clockwise direction. (see fig.b)
- the direction of induced current in the loop will be clockwise so that it may oppose the increase of magnetic flux in the loop in downward direction.
- the direction of induced current in the loop will be anti-clockwise so that it may oppose the increase of magnetic flux in the loop in upward direction.
- Induced electric field lines form closed loops (different from the electric field lines used to depict electric field produced due to charges)
- Induced electric field is non-conservative in nature (again a difference from the electric field produced by electric charges)
- An emf is induced in a circuit where the magnetic flux is changing even if the circuit is open. But obviously no current will flow. If we close the circuit, the current will start flowing.
- In a loop moving in a uniform magnetic field, when the loop remains in the field, the net emf induced is zero.
APPLICATIONS OF EDDY CURRENT
- Dead beat galvanometer
- Energy meter
- Electric brakes
- Single phase AC motor
- Induction furnace
SELF INDUCTANCE AND MUTUAL INDUCTANCE
- Energy stored in a coil (inductor) =
- The self inductance is a measure of the coil to oppose the flow of current through it. The role of self-inductance in an electrical circuit is the same as that of the inertia in mechanics. Therefore it is called electrical inertia.
- The magnetic energy density (energy stored per unit volume) in a solenoid
N2φ2 = MI1
- M is numerically equal to the flux-linkage in one circuit, when unit current flows through the other. (we use this definition to calculate M)
- M is numerically equal to the e.m.f. induced in one circuit, when the current changes in the other at the rate of one ampere in each second. (it is used to describe the mutual behavior of two circuits).
- Coefficient of self inductance of two coils in series
- Coefficient of self inductance of two coils in parallel
- The coefficient of coupling between two coils having self inductance L1 & L2 and coefficient of mutual inductance M is
- Generally the value of K is less than 1.
- If K is 1, then the coupling of two coils is tight while if K < 1, then coupling is loose.
- Inductance is pure geometrical factor, and is independent of current or applied e.m.f.
- If the angle between the axis of two closely placed coil is θ then