Physics · Electromagnetic Induction · NEET
No. The minus sign is not about a positive or negative value. It is a direction rule. It tells you that the induced EMF and its current act to OPPOSE the flux change. When you calculate the size (magnitude) of the EMF for NEET numericals, you drop the sign and use ε = N dΦ/dt to get a positive value, then decide direction separately using Lenz's law.
Yes, exactly. Lenz's law is a statement in words: the induced current opposes the change in flux. The minus sign is that same statement written in the equation ε = -N dΦ/dt. They are two forms of one idea. NEET can ask it either way, so treat the minus sign and Lenz's law as identical.
For finding the numerical VALUE of EMF, use only the magnitude: |ε| = N (dΦ/dt). The minus sign never changes the number, only the direction. Use Lenz's law separately to state which way the current flows. Adding the sign to your arithmetic is a common mistake that wastes time.
A negative dΦ/dt means the flux is decreasing with time. When you feed that into ε = -N dΦ/dt, the two minus signs give a positive EMF, meaning the induced current now tries to KEEP the flux up (to replace what is being lost). If dΦ/dt is positive (flux rising), the induced current tries to push the flux back down. Either way, it opposes the change.
The minus sign is deeply linked to energy conservation. Because the induced current always opposes the change, you must do work against it to keep changing the flux (for example to keep pushing a magnet in). That work is exactly what becomes the electrical energy. If the sign were plus, the current would help the change and energy would appear from nothing, which is impossible.
A strong bar magnet (N pole facing right) is moving from solenoid-1 toward solenoid-2. The directions of the induced current in solenoid-1 and solenoid-2 respectively are along:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
To build in Lenz's law. Faraday's experiments gave the size of the EMF (rate of change of flux). Lenz added the direction: the EMF always opposes the change. Writing ε = -N dΦ/dt packs both facts into one equation, which is why NEET uses this exact form.
Yes. Back EMF in an inductor is ε = -L dI/dt. The same minus sign means the inductor opposes any change in its own current, which is why current in a coil cannot jump instantly.
Not for the value. Examiners want the magnitude of EMF plus, where asked, the correct direction. Report a positive number for the EMF and state the direction using Lenz's law. Do not write a negative EMF value.
You would predict that the induced current helps the flux change. That would mean the current grows on its own, releasing energy from nothing and breaking conservation of energy. The minus sign is what keeps the physics honest.
