- Faraday’s Law Definition: Faraday’s law of electromagnetic induction is defined as the principle that a changing magnetic field within an electric circuit produces an electromotive force.
- First Law: Faraday’s first law states that any change in the magnetic environment of a coil induces an EMF, known as induced EMF, and, if the circuit is closed, induces current as well.
- Second Law: Faraday’s second law clarifies that the induced EMF’s magnitude equals the rate at which the magnetic flux linkage through the coil changes.
- Enhancing EMF: Increasing the coil’s number of turns, the magnetic field strength, or the relative motion speed between the coil and the magnet can amplify the induced EMF.
- Applications and Impact: The faraday law of electromagnetic induction underpins the functionality of transformers, generators, and even musical instruments, demonstrating its broad influence in both technology and culture.
What is Faraday’s Law
Faraday’s law of electromagnetic induction, or Faraday’s law, relates induced electromotive force to the time rate of change of magnetic flux in electromagnetism. Flux depends on the magnetic field, the bounded surface and their orientation. A changing flux can induce EMF around an electric circuit even when the circuit is open and no current flows. This phenomenon is electromagnetic induction.
Faraday’s law states that changing magnetic flux induces EMF; a closed conducting path allows that EMF to drive current. Lenz’s law of electromagnetic induction gives the sign: any resulting current creates a field that opposes the change in flux, rather than always opposing the original field. For motional generator action, Fleming’s right-hand rule can give conventional-current direction.
Faraday’s law explains induced voltage in transformers, rotating motors, generators, and inductors. Michael Faraday demonstrated electromagnetic induction in 1831 using changing current in linked coils and relative motion between magnets and conductors.
Faraday’s Experiment
In the illustrated experiment, a galvanometer closes the coil circuit and indicates current. With the magnet stationary relative to the coil and its field unchanged, flux is constant and the needle stays at zero. Moving the magnet towards the coil changes flux, so the needle deflects in one direction.
When relative motion stops, flux becomes constant and the needle returns to zero. Moving the magnet away reverses the flux change and the deflection. Moving the coil instead gives the same result because relative motion controls this example. Faster flux change produces a larger induced EMF or voltage.
| Position of magnet | Deflection in galvanometer |
| Magnet at rest | No deflection in the galvanometer |
| Magnet moves towards the coil | Deflection in galvanometer in one direction |
| Magnet is held stationary at same position (near the coil) | No deflection in the galvanometer |
| Magnet moves away from the coil | Deflection in galvanometer but in the opposite direction |
| Magnet is held stationary at the same position (away from the coil) | No deflection in the galvanometer |
Conclusion: Relative motion induces voltage only when it changes flux linkage. Motion is one method; changing field magnitude, loop area or orientation can also change flux without relative translation.
The traditional first and second statements below summarise the qualitative condition and quantitative relationship known as Faraday’s laws of electromagnetic induction.
Faraday’s First Law
A change in magnetic flux through a conducting loop induces EMF around the loop. If the conductor forms a closed circuit, the EMF can drive current. Flux may change through field magnitude, bounded area or orientation.
Common ways to change flux include:
- Moving a magnet towards or away from the coil so that linked flux changes
- Moving the coil into or out of a non-uniform magnetic-field region
- Changing the area enclosed by a coil in the field
- Rotating the coil to change the angle between its area normal and the field
Faraday’s Second Law
The induced EMF magnitude equals the time rate of change of flux linkage. If N stationary turns each link the same flux Φ and N is constant, the signed relationship is ε = -N dΦ/dt. The negative sign gives the Lenz-law direction under the chosen loop and surface convention.
Faraday Law Formula
Consider a magnet approaching a stationary coil. Compare flux linkage at times T1 and T2, using one positive surface-normal direction.
The first image gives flux linkage at time T1:
The second image gives flux linkage at time T2:
Subtract the initial linkage from the final linkage:
Write this change as Δ(NΦ):
For constant N with equal flux per turn, the linkage change is NΔΦ:
Divide by the time interval to obtain the average rate of linkage change:
For an instantaneous value, take the time derivative:
The following expression gives that rate for constant turns:
Faraday’s law makes induced EMF equal to the negative time rate of change of flux linkage. The magnitude equals the absolute value of that rate.
The negative sign applies Lenz’s law using the chosen positive directions.
Where:
- Flux Φ in Wb = B·A for a uniform field normal to a flat area; in general, Φ is the surface integral of B·dA
- B = magnetic flux density in teslas
- A = bounded surface area in square metres
How To Increase EMF Induced in a Coil
- Increase the number of turns N while keeping the same changing flux through each turn. The total flux linkage then changes faster in direct proportion to N.
- Increase the rate at which magnetic field strength changes through the coil. A stronger but constant field produces no sustained transformer EMF in a stationary coil; the rate of flux change is the deciding quantity.
- Increase relative speed when that motion causes a faster flux change, such as moving a magnet through the coil’s non-uniform field region. Motion parallel to a uniform field or along an unchanged-flux path need not increase EMF.
Applications of Faraday’s Law
Faraday’s law applies wherever changing magnetic flux produces EMF. The examples below span electrical machines, heating, measurement and signal pickup.
- Power transformers use changing core flux to induce secondary-winding EMF.
- An electrical generator uses mechanical motion to change winding flux and induce EMF. Transformers are the clearer example of mutual induction between separate windings.
- An induction cooker drives alternating current through a coil beneath suitable cookware. Changing flux induces circulating currents in the pan, and electrical resistance converts their energy into heat.
- An electromagnetic flow meter measures electrically conductive fluid moving across a magnetic field. Electrodes sense an induced voltage proportional to the relevant fluid-velocity component after instrument calibration.
- The Maxwell-Faraday equation states that a time-varying magnetic flux density produces a circulating electric field. Its integral around a closed path is the induced EMF.
- Magnetic pickups in electric guitars and some electric violins produce a signal when vibrating strings or other moving magnetic material change flux through a pickup coil.
