Faraday's law
Electromagnetism - OCR A-Level Physics
Key Definition
Faraday's law
The induced e.m.f. is equal to the rate of change of magnetic flux linkage.
The induced e.m.f. is equal to the rate of change of magnetic flux linkage.
$$\varepsilon = -\frac{d(N\Phi)}{dt}$$
Key Definition
Lenz's law
The direction of the induced e.m.f. (and hence the induced current) is such that it opposes the change producing it.
The direction of the induced e.m.f. (and hence the induced current) is such that it opposes the change producing it.
- The minus sign in Faraday's lawThe magnitude of the induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is proportional to the rate of change of magnetic flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns).. represents Lenz's lawThe direction of an induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is such that it opposes the change in magnetic flux that produced it. This is a consequence of conservation of energyThe capacity to do work. Measured in joules (J).. mathematically.
- Lenz's lawThe direction of an induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is such that it opposes the change in magnetic flux that produced it. This is a consequence of conservation of energyThe capacity to do work. Measured in joules (J).. is a consequence of conservation of energyThe capacity to do work. Measured in joules (J).Energy cannot be created or destroyed, only transferred from one form to another. The total energy of a closed system remains constant.: if the induced currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). aided the change, it would increase the flux further, inducing more currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). - creating energy from nothing.
- An e.m.f. is induced whenever the flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns). through a circuit changes, whether by changing B, A, θ, or N.
- For a coil rotating at angular velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.The rate of change of angular displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).. The angle swept per unit time for an object moving in a circle. Measured in rad s⁻¹. ω: NΦ = $BAN\cos$($\omega$t), so ε = $BAN\omega\sin$($\omega$t). The e.m.f. is sinusoidal.
Examiner Tips and Tricks
- When explaining Lenz's lawThe direction of an induced EMF is such that it opposes the change in magnetic flux that produced it. This is a consequence of conservation of energy., always link to energy conservation.
- A common 6-mark question asks: 'Explain why a dropped magnet falls more slowly through a copper tube.' The changing flux induces currents that create a field opposing the magnet's motion, converting kinetic energyThe energy an object possesses due to its motion. into thermal energy in the tube.