Electromagnetic Induction and usage of appropriate hand rules

A square coil of conducting wire is placed in a uniform magnetic field which is directed into the page. The current in the coil flows anti-clockwise as seen from above. which force acting on the coil is correct at its specific location?

• A

• B

• C

• D

Answer explanation

Fleming’s Left-Hand Rule is used to determine the direction of force acting on a current-carrying wire in a magnetic field.

In this case, the current in the top segment flows left to right, and the magnetic field is into the page.

Applying FLR:

• First finger (field) = into the page

• Second finger (current) = right

• Thumb (force) = upward

  Therefore, the force on the top segment is directed upward, which matches option A.

Rules hack:

• If you are given a wire that already has current and is in a magnetic field, and you’re asked for force → Use Left-Hand Rule

  • 👉 Thumb = Force (which may cause motion)

• If the wire is moving through a magnetic field and you’re asked for induced current → Use Right-Hand Rule

  • 👉 Thumb = Motion

A conducting ring encloses an area of 2.0 cm² and is perpendicular to a magnetic field of strength 5.0 mT. The direction of the magnetic field is reversed in a time 4.0 s. What is the average emf induced in the ring?

• 0

• 0.25 μV

• 0.40μV

• 0.50μV

Answer explanation

Faraday's law of induction

A uniform magnetic field directed into the page occupies a region of width L.  A conducting coil of width L moves at constant speed v, from left to right, through the field. From the instant that the coil enters the field until the instant that the coil leaves the field, which of the following best describes the direction of the current induced in the coil?

• Anticlockwise

• clockwise

• Anticlockwise then clockwise

• clockwise them anticlockwise

Answer explanation

As the square is entering the field the flux linkage is increasing, while as it is leaving the linkage is decreasing. As per  Lenz’s Law , direction of the current is such as to try to reduce the change in flux linkage.

Hence it should set up a magnetic field out of the page as the loop enters the field. and into the page when loop is leaving the magnetic field

Hence answer is C

• A

• B

• C

• D

Answer explanation

Step 1: Use Right-Hand Grip Rule for Loop A

Loop A is carrying a clockwise current (increasing).

🖐️ Right-hand grip rule (also called the corkscrew rule):

• Thumb = direction of magnetic field

• Fingers curl = direction of current

So:

• If current in loop A is clockwise, the magnetic field inside loop A is into the page (or into the screen ⬇️).

📌 Direction of magnetic field by loop A = into the page (⬇️)

And since current is increasing, the magnetic field is increasing into the page.

Step 2: Apply Lenz’s Law for Loop B

Lenz’s Law says:

“The induced current in a loop always acts to oppose the change in magnetic flux.”

• In loop B, the flux into the page is increasing.

• Loop B doesn’t like that!

• So it tries to create a magnetic field out of the page (⬆️) to oppose it.

Now…

🖐️ Use the Right-hand grip rule again:

• Thumb = out of the page

• Fingers curl = anti-clockwise

✅ Induced current in loop B = anti-clockwise

Step 3: Now Analyze Loop C

Loop C is outside loop A. What’s happening there?

• For points outside a current-carrying loop, a clockwise current in loop A creates magnetic field out of the page (⬆️) — this is the opposite of what happens at the center.

• But again — since loop A’s current is increasing, the outward magnetic field is increasing at loop C’s location.

• Loop C wants to oppose that increase.

• So it tries to create a magnetic field into the page (⬇️).

🖐️ Use the right-hand grip rule again:

• Thumb = into the page

• Fingers curl = clockwise

✅ Induced current in loop C = clockwise

Faraday’s law of electromagnetic induction states that the electromotive force (emf) induced in a conductor is proportional to

• the change of magnetic flux density.

• the change of magnetic flux linkage.

• the rate of change of magnetic flux density.

• the rate of change of magnetic flux linkage.

Answer explanation

FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION

Whenever the number of magnetic  lines of force (flux) linked with any closed circuit change, an induced current flows through the circuit which lasts only so long as the change lasts. An increase in the number of lines of force produces an inverse current, while a decrease of such lines produces a direct current.

The induced emf is equal to the negative rate of change of magnetic flux.

i.e.

The -ve sign shows that the induced emf opposes the change in magnetic flux (Lenz’s law).

The diagram shows a loop L of wire in a uniform magnetic field B.

The loop encloses an area A and the field is directed at an angle θ to the normal to the plane of the loop. The strength of B is increasing at a uniform rate R. What is the emf induced in L?

• RA/Cosθ

• RACosθ

• RA/Sinθ

• RASinθ

Answer explanation