Physics: Lenz's Law

Understanding Physics: Lenz's Law

A reliable way to solve direction problems is to work in stages. First decide whether the amount of magnetic field passing through the loop is increasing or decreasing. Then decide what effect the loop must produce to fight that change.

Only after that should you use the right hand grip rule to find the current direction. Point your right thumb in the direction of the field made by the induced current. Your curled fingers show the current around the loop.

Students often skip the first stage and choose a direction from the magnet pole alone. That causes mistakes when a magnet moves away, when a coil rotates, or when the external field changes strength.

The size of the induced effect depends on how quickly conditions change. Moving a magnet slowly through a coil gives a smaller voltage than moving it quickly. A coil with more turns gives each turn a chance to contribute, so the total voltage is larger.

A stronger magnetic field usually produces a larger effect too. The angle matters because a flat loop catches the most field when the field passes straight through its surface. When the field runs along the plane of the loop, very little passes through.

This is why rotating coils are useful in generators. Their changing orientation produces a changing electrical output.

Lenz's law is closely connected to conservation of energy. Imagine that an approaching magnet made a coil attract it in the same direction of motion. The magnet would speed up while the coil produced electrical energy, with no extra work needed.

That would create energy from nowhere. Instead, you must push against a magnetic force as the induced current appears. The work done by your hand becomes electrical energy in the circuit.

If the circuit has resistance, much of that electrical energy becomes thermal energy. The same idea explains why a spinning generator becomes harder to turn when it powers a device.

A solid piece of metal can act like many tiny loops. When it moves through a changing magnetic field, circulating currents can form inside it. These currents can heat the metal or create forces that slow its motion.

This effect is useful in induction cookers, where a changing field heats a suitable pan. It is useful in some train and roller coaster brakes because braking can happen without rubbing surfaces together. Engineers sometimes cut slots into metal parts or build them from thin insulated layers.

Those changes break up the current paths and reduce unwanted heating. When studying examples, keep track of what is changing, the direction of the external field, and the location from which you view the loop. A clockwise current from one side looks counterclockwise from the other.