Physics · Electromagnetic Induction · NEET
Only the change. This is the single most tested point. An inductor does NOT fight a steady current - once the current is constant, di/dt = 0, so back EMF = -L(di/dt) = 0 and the inductor behaves like a plain wire. It only pushes back while the current is rising or falling. Steady current: inductor is invisible. Changing current: inductor fights it.
Back EMF depends on the RATE of change of current, not the current value. Formula: ε = -L(di/dt). In steady state the current stopped changing, so di/dt = 0 and ε = 0. That is why in a DC circuit long after the switch is closed, the inductor drops no voltage - it acts like a short (a wire). It only 'wakes up' the instant you switch on or off.
The minus sign is Lenz's law written for self-induction. It says the induced EMF acts in the direction that OPPOSES the change that caused it. If current is increasing (di/dt > 0), ε is negative, meaning it opposes the increase. If current is decreasing (di/dt < 0), ε becomes positive, meaning it tries to maintain the current. It is not a real negative number to plug blindly - it tells you the direction.
No. L (self-inductance, unit henry) is a fixed property of the coil - it depends on its geometry and number of turns and does not change with current. Back EMF is the voltage that appears, and it changes moment to moment depending on how fast the current is changing. Relationship: back EMF = L times (rate of change of current). L is the 'stubbornness constant'; back EMF is how hard it pushes right now.
When you open a switch, you force the current to drop to zero almost instantly, so di/dt is huge and negative. Back EMF = -L(di/dt) becomes a very large positive voltage that tries to keep the current flowing. This large voltage can jump the air gap as a spark. This is why inductive circuits (motors, relays) need protection diodes - the inductor 'refuses' to let its current stop suddenly.
AB is a part of an electrical circuit. The branch contains an inductor of 1 H, a 5 V battery, and a 2 ohm resistor in series between A and B. The potential difference V_A - V_B, at the instant when current i = 2 A and is increasing at a rate of 1 amp/second, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the self-induced voltage ε = -L(di/dt) that an inductor produces to oppose any change in the current through it.
ε = -L(di/dt). L is in henry (H), di/dt in ampere per second (A/s), and ε in volt (V). So 1 volt = 1 henry x 1 A/s.
Because it resists a change in current the way mass resists a change in velocity. L plays the role of mass, current plays the role of velocity. This analogy is a common NEET memory tool.
When the current changes fastest - for example the instant you switch a circuit on or off. That is when di/dt is largest, so ε = -L(di/dt) is largest.
No, it depends on the rate of change of current (di/dt), not on i itself. A large steady current gives zero back EMF; a small but rapidly changing current can give a large back EMF.
