What is Faraday's law?
What is electromagnetic induction?
How is this described?
- Faraday's law, due to 19ᵗʰ century physicist Michael Faraday. This relates the rate of change of magnetic flux through a loop to the magnitude of the electro-motive force induced in the loop. The relationship isThe electromotive force or EMF refers to the potential difference across the unloaded loop (i.e. when the resistance in the circuit is high). In practice it is often sufficient to think of EMF as voltage since both voltage and EMF are measured using the same unit, the volt.
- Lenz's law is a consequence of conservation of energy applied to electromagnetic induction. It was formulated by Heinrich Lenz in 1833. While Faraday's law tells us the magnitude of the EMF produced, Lenz's law tells us the direction that current will flow. It states that the direction is always such that it will oppose the change in flux which produced it. This means that any magnetic field produced by an induced current will be in the opposite direction to the change in the original field.Lenz's law is typically incorporated into Faraday's law with a minus sign, the inclusion of which allows the same coordinate system to be used for both the flux and EMF. The result is sometimes called the Faraday-Lenz law,In practice we often deal with magnetic induction in multiple coils of wire each of which contribute the same EMF. For this reason an additional term representing the number of turns is often included, i.e.
What is the connection between Faraday's law of induction and the magnetic force?
Faraday's experiment : Induction from a magnet moving through a coil
- Magnet at rest in or near the coil: No voltage observed.
- Magnet moving toward the coil: Some voltage measured, rising to a peak as the magnet nears the center of the coil.
- Magnet passes through the middle of the coil: Measured voltage rapidly changes sign.
- Magnet passes out and away from the coil: Voltage measured in the opposite direction to the earlier case of the magnet moving into the coil.
A small 10 mm diameter permanent magnet produces a field of 100 mT. The field drops away rapidly with distance and is negligible more than 1 mm from the surface. If this magnet moves at a speed of 1 m/s through a 100-turn coil of length 1 mm and diameter just larger than the magnet, what is the EMF induced?
If the magnet is dropped north-pole first, what direction (clockwise or counterclockwise) will the current first flow in the coil?
Suppose the ends of the coil are electrically connected to each other, ensuring that any current generated is dissipated as heat in the resistance of the wires. What effect would you expect this to have on the falling magnet? Hint: consider conservation of energy.
Induction in parallel wires
Figure 5 shows a pair of parallel wires. One is connected to a battery via a switch and current meter while its neighbor forms a loop with just a current meter in series. Suppose the switch is briefly switched on then off. Qualitatively speaking, what will happen to the current measured in the neighbor?
Figure 6: Current pulses due to induction between parallel wires.