Practice Lenz's Law in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

The direction of an induced current is always such that it opposes the change in magnetic flux that produced it.

Nature resists change — when you push a magnet into a coil, the coil creates its own magnetic field that pushes back.

Showing a random 20 of 50 problems.

Example 1

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Two coaxial coils sit side-by-side. The current in coil 1 is steadily increasing in the clockwise direction (viewed from coil 2). What direction does the induced current in coil 2 flow (viewed from coil 2)?

Example 2

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A loop has flux Φ(t)=(0.4 Wb)cos⁡(10t)\Phi(t) = (0.4\text{ Wb})\cos(10t). At t=0t = 0, is the induced EMF zero, maximum positive, or maximum negative?

Example 3

challenge
A coil sits in an oscillating field B(t)=B0sin⁡(ωt)B(t) = B_0\sin(\omega t). Describe how the induced current direction behaves over one cycle.

Example 4

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A south pole of a magnet is pushed toward a coil. Which pole does the near face of the coil become?

Example 5

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A conducting ring is dropped through a region of magnetic field. As it enters the field, the induced current creates an upward force on the ring. Explain this using Lenz's law.

Example 6

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A circular loop is in a magnetic field pointing into the page. The field strength decreases. According to Lenz's law, in which direction does the induced current flow?

Example 7

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A bar magnet is dropped through a coil connected to a resistor. Describe the kinetic energy outcome.

Example 8

challenge
A circular loop lies flat; a magnet's north pole approaches from above. State the induced current direction (viewed from above) and the resulting force on the magnet.

Example 9

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A loop with flux into the page is increasing. Induced current direction?

Example 10

easy
Lenz's law tells us the ____ of the induced current.

Example 11

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What does Lenz's law tell you about an induced current?

Example 12

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A magnet falls through a vertical copper pipe. Does it fall faster or slower than free fall?

Example 13

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A horizontal circular loop sits with its plane perpendicular to a uniform vertical magnetic field BB pointing up. BB is steadily decreased. From above, in which direction does the induced current flow?

Example 14

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A copper ring is dropped over a vertical bar magnet (north up). As the ring slides down past the magnet, does it speed up, slow down, or fall at gg?

Example 15

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If the flux through a loop is constant, what is the induced current?

Example 16

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Push a magnet's north pole toward a coil. What pole does the coil's near face become?

Example 17

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A loop's area increases while the perpendicular field BB (out of the page) stays constant. State the direction of the induced current.

Example 18

challenge
A superconducting ring (zero resistance) initially has flux Φ0\Phi_0 through it. The external field is then changed. What happens to the total flux through the ring?

Example 19

easy
A north pole is moved away from a coil. The induced current's resulting magnetic effect on the magnet is to attract or repel?

Example 20

easy
Lenz's law gives the direction; which law gives the magnitude of the EMF?
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