**Faraday's law** states that the absolute value or magnitude
of the circulation of the
electric field **E** around a closed loop is equal to the **rate of change** of
the magnetic flux through the area enclosed by the loop. The
equation below expresses Faraday's law in mathematical form.

ΔΦ_{B}/∆t (through a
fixed area)
= -Σ_{around loop} **E∙**∆**r** (at a
fixed time)

The minus sign in this equation tells us about the direction of
the circulation. (See below.)

When the magnetic flux through the area enclosed
by the loop changes, Σ_{around loop} **E∙**∆**r** is not zero, the
electric field **E** circulates.**E∙**∆

If the loop is an actual wire loop, then there is actual work done by the induced field on free charges.

Σ

This is an

The induced emf causes a current to flow without a potential difference due to separated charges.

ΔΦ_{B}/∆t (through a
fixed area) = induced emf

The induced electric field is **NOT**
a conservative field. When you move a charge against the induced
field once around the loop, you have to do work. But your work is
**NOT** stored as
potential energy. You cannot let the electric field do work to recover
the energy you expended in moving the charge. The induced electric field disappears as
soon as the magnetic flux is no longer changing. The work you do
on a charge against the induced field is not locally
stored. The energy may be transported away in the form of an
electromagnetic wave.
Electromagnetic waves carry energy through free space.

The minus sign in the equation expressing Faraday's law tells us about the
direction of the induced field.

There is an easy way to remember this direction.
The circulation of the induced field is equal to an emf.

Any current
flowing as the result of that emf produces a magnetic field that opposes the
flux changes that produce it.

This is called
Lenz's law.

An induced emf acts as to oppose the change in flux that produce it.

A magnet is moved quickly towards a wire loop as shown.

The flux through the wire loop is increasing in the downward direction.

A current starts flowing
in the loop in the direction indicated by the arrow.

The magnetic field produced by this current
points upward, it opposes the flux
changes that produce it.

The magnetic force due to the loop on the magnet acts to slow down the
approaching magnet.

The beauty of **Lenz's law** is that you do not have to look at the details. If
the magnetic flux through a conductor changes, currents will flow to oppose
whatever caused the change. If some relative motion causes a flux change, the
current will try to stop that relative motion. If a change in a current in
a circuit is responsible for the flux change, then the induced emf will try to
prevent the change in the current of that circuit.

Link: Electromagnetic Induction and Faraday's Law (Youtube)

Consider a flat square coil with N = 5 loops.

The coil is 20 cm on each side, and has a magnetic field
of 0.3 T passing through it.

The plane of the coil is perpendicular to the
magnetic field: the field points out of the page.

(a) If nothing is changed, what is the induced emf?

(b) The magnetic field increases uniformly from 0.3 T to 0.8 T in 1 s.
While the change is taking place, what is the induced emf in the coil?

(c) While the magnetic field is changing, the emf induced in the coil causes a
current to flow. Does the current flow clockwise or counter-clockwise
around the coil?

Solution:

- Reasoning:

If the magnitude of the magnetic field B changes, then the flux Φ = BA changes, and an emf is induced. - Details of the calculation:

(a) An emf is induced by a changing magnetic flux. If nothing changes, the induced emf is zero.

(b) The coil has 5 turns. Each turn has area A = (0.2 m)^{2}. The initial magnetic flux through each turn of the the coil is Φ_{0 }= B_{0}A = 0.3*(0.2)^{2 }Tm^{2 }= 0.012 Tm^{2}.

The final magnetic flux through each turn of the coil is Φ_{f }= B_{f}A = 0.8*(0.2)^{2 }Tm^{2 }= 0.032 Tm^{2}.

The total change of the flux through the coil N(Φ_{f }- Φ_{0}), with N = 5. The induced emf is

emf = -N∆Φ/∆t = -N(Φ_{f }- Φ_{0})/∆t = [-5*(0.032 -0.012)/1.0] V = -0.1 V.

(c) While the magnetic field is changing, the magnetic flux increased out of the page. According to Lenz's law, the emf induced in the loop by this changing flux produces a current that sets up a field opposing the change. The field set up by the current in the coil, points into the page, opposite to the direction of the increase in flux. To produce a field into the page, the current must flow clockwise around the loop according to the right hand rule.

If a long coil of wire of cross sectional area A and length ℓ with N turns is connected or disconnected from a battery, the changing magnetic flux through the coil produces an induced emf. The induced current produces a magnetic field, which opposes the change in the magnetic flux. The magnitude of the induced emf can be calculated using Faraday's law.

- The magnetic field inside the long coil is B = μ
_{0}(N/ℓ)I. - The flux through the coil is NBA = μ
_{0}(N^{2}/ℓ)IA. - The change in flux per unit time is μ
_{0}(N^{2}/ℓ)A ∆I/∆t = L*∆I/∆t, since I is the only quantity changing with time.

L = μ_{0}(N^{2}/ℓ)A is called the**self inductance**of the coil. The units of inductance are**Henry (H)**. 1 H = 1 Vs/A. - The induced emf is emf = -L*∆I/∆t, where the minus sign is a consequence of Lenz's law.

The induced emf is proportional to the rate of change of the current in the coil. It can be several times the power supply voltage. When a switch in a circuit carrying a large current is opened, reducing the current to zero in a very short time interval, this can result in a spark. All circuits have self inductance, and we always have emf = -L*∆I/∆t. The self inductance L depends only on the geometry of the circuit.

A coil has an self inductance of 3 mH, and a current through it changes from 0.2 A to 1.5 A in a time of 0.2 s. Find the magnitude of the average induced emf in the coil during this time.

Solution:

- Reasoning:

The emf due to self induction is emf = -L*∆I/∆t. - Details of the calculation:L = 3 mH, ∆I/∆t = (1.5 A - 0.2 A)/0.2 s = 6.5 A/s.

emf = -L*∆I/∆t = -(0.003 Vs/A)(6.5 A/s) = -0.0195 V.

The minus sign indicates that the induced emf opposes the flux changes that produced it.

A 25 turn circular coil of wire has a diameter of 1 m. It is placed with its
axis along the direction of the Earth's magnetic field (magnitude 50 microT),
and then, in 0.2 s, it is flipped 180^{o}. What is the average emf
generated

Solution:

- Reasoning:

Φ_{B}=**B**∙**A**is the flux of B through the area A. Initially B and A are aligned, finally they are anti-aligned. The dot product changes sign. - Details of the calculation:

emf = -∆Φ_{B}/∆t. Φ_{B}(initial) = NAB = 25*π*(0.5 m)^{2}50*10^{-6 }T = 9.82*10^{-4}Tm^{2}.

Φ_{B}(final) = -Φ_{B}(initial) since the coil is flipped.

|∆Φ_{B}| = 2Φ_{B}(initial).

|∆Φ_{B}/∆t| = 2*(9.82*10^{-4 }Tm^{2})/(0.2 s) = 9.82*10^{-3 }V.

A 0.5 m radius, 500-turn coil is rotated one-fourth of a revolution in 4.17 ms, originally having its plane perpendicular to a uniform magnetic field. Find the magnetic field strength needed to induce an average emf of 10,000 V.

Solution:

- Reasoning:

emf = -∆Φ_{B}/∆t. Φ_{B}= NABcosθ changes from NAB to 0 in 4.17 ms, since θ changes from 0 to 90^{o}in 4.17 ms. - Details of the calculation:

|∆Φ_{B}| = NAB = 500*π*(0.5 m)^{2}*B = (393 m^{2}) * B.

We want

|emf| = |∆Φ_{B}/∆t| = (393 m^{2})/(4.17*10^{-3}s) * B = (94174 m^{2}/s)*B = 10000^{ }V.

B = 0.1 Vs/m^{2}= 0.1 T.

If you miss having regular lectures, consider this video lecture.

Lecture 16: Electromagnetic Induction