Lenz’s law describes the direction of induced current. Lenz’s law uses the magnetic effect of electric current to explain direction of the induced current.
Consider the setup illustrated below:
When the magnet is moved towards the coil, the galvanometer deflects. When moved away, the deflection occurs in the opposite direction. This shows that the direction of change of the magnetic flux linkage determines direction of the induced current.
Galvanometer deflection according to direction of current
We need to investigate how the current flows. To figure out how current will flow in a circuit, consider the circuit below.
- Close the switch and note the deflection on the galvanometer when current flows from A to B.
Observations about current flow through the galvanometer
The current enters the galvanometer through A then leaves through B. The deflection on the galvanometer is to the right. From the diagram, the current leaves the cell towards A.
If the cell is reversed such that the current enters the galvanometer through B, the deflection happens to the left.
Determining direction of the induced current
Consider a coil of wire and a galvanometer connected as shown with a magnet being moved towards the coil.
observations on experiments to verify Lenz’s law
When the north pole of the magnet is moved towards the coil, the galvanometer deflects to the left. This indicates that the direction of the induced current is clockwise. It follows the direction DCBA.
When the north pole of the magnet is moved away from the coil, the pointer deflects to the right. This indicates that current flow is in the anticlockwise direction or direction DABC.
Explanations on lenz’s law
As the north pole of the magnet approaches the coil, the induced current flows in the coil. It attempts to form an electromagnet with a north pole at the end near the incoming magnetic pole. This is meant to resist movement of the incoming north pole.
We study the magnetic effect of electric current. From this study, we can use the clock rule to determine the direction of the induced current. The current flow in the conductor is as illustrated below. The end near the north pole forms an opposing north pole.
Remember the basic law of magnetism states that:Like poles repel, unlike poles attract
Laboratory experiments on direction of induced current
click the image below to experiment about the direction of current:
The North pole of the magnet is moved away from the coil. The induced current flows in the coil. It forms a south pole at the coil’s end near the leaving magnet. This formation opposes the movement of the receding magnet. Using the clock rule, the direction of induced current flow is as illustrated in the diagram below.
From the above experiments and observations, the direction of the induced current in a coil can be determined in Lenz’s law which states that:
The direction of the induced e.m.f is such that the induced current which it causes to flow produces a magnetic effect that opposes the change producing it.
Lenz’s law applies the principle the principle of conservation of energy. Mechanical energy moves the magnet to the coil. This energy is converted to electrical energy. This conversion occurs in the form of an induced current. Hard work is needed to push the magnet towards the coil against a repulsive force that must be overcomed. This repulsive force comes from the induced pole of the coil which causes current to flow in an attempt to oppose the incoming pole.