Back emf is the voltage induced in a motor or inductor that opposes the change in current through it. In Principles of Physics II, it shows up in Lenz's law, motors, and inductors.
Back emf is the induced voltage that pushes back against a change in current in an inductor or a running motor. In Principles of Physics II, you usually meet it when a coil or motor is reacting to a changing magnetic field, not when it is just sitting there unchanged.
The name can be a little misleading. It is not a separate battery-like source added to the circuit, and it is not a force in the Newtonian sense. It is an induced electromotive force, meaning it is a potential difference created by electromagnetic induction, and its direction is such that it opposes the change that produced it.
For an inductor, back emf appears any time the current is trying to rise or fall. If current through a coil increases, the magnetic field around the coil changes, and the coil induces an emf that resists that increase. If the current drops, the induced emf tries to keep the current going. This is why inductors do not let current change instantly.
In a motor, the same idea shows up as the spinning coil cuts through magnetic field lines and generates an induced emf. As the motor speeds up, that induced voltage grows. The result is a smaller net voltage across the motor windings, which reduces current draw. That is why a motor that is already spinning usually draws less current than a stalled motor.
You can think of back emf as a built-in feedback effect. The faster the motor turns, or the more quickly current changes in a coil, the stronger the opposing emf becomes. That self-limiting behavior is not a bug, it is part of why real motors and inductive circuits behave smoothly instead of taking unlimited current.
A common misconception is that back emf means the circuit is somehow wasting voltage. What is really happening is energy conversion and conservation. The induced emf is tied to changes in magnetic flux, and the opposing direction comes straight from Lenz's law. If you know the sign of the change, you can predict which way the back emf points and whether it is resisting an increase or trying to sustain a decrease.
Back emf shows up any time you analyze how real electric devices behave instead of idealized ones. In Principles of Physics II, it is one of the clearest examples of Lenz's law turning into something you can measure in a circuit or motor.
It matters because it explains why motors do not draw the same current in every situation. A motor at startup or when stalled has very little back emf, so the current can be large. Once the shaft starts turning, back emf rises and the current drops. That difference is a big reason real motors need protection and why stalled motors can overheat.
It also gives you a way to connect magnetic flux, induced emf, and energy conservation in one story. When you see a coil, a changing field, or a spinning motor, back emf tells you how the circuit responds to the change instead of just describing the change itself.
In lab or problem sets, you may be asked to compare a stationary coil with a rotating one, explain why current changes over time, or predict the direction of the induced emf. Back emf gives you the language for those answers, especially when you need to explain why the circuit resists change instead of simply reporting that it does.
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Visual cheatsheet
view galleryLenz's Law
Back emf is the circuit-level result of Lenz's law. Lenz's law gives you the direction of the induced emf, namely that it opposes the change in magnetic flux that caused it. When you explain back emf, you are usually applying Lenz's law to a coil or motor and describing how the induced voltage resists a rise or fall in current.
Motor
A motor is one of the most common places you see back emf in action. As the motor spins, the rotating coil cuts through magnetic field lines and induces a voltage that opposes the supply voltage. That means the motor's current depends on speed, which is why a running motor draws less current than one that is just starting or stuck.
Inductance
Inductance measures how strongly a coil resists changes in current. Back emf is the visible effect of that resistance to change. A larger inductance usually means a larger induced emf for the same rate of current change, so the current builds up or drops off more slowly in an inductor-rich circuit.
induced electromotive force
Back emf is a specific kind of induced electromotive force. Induced emf is the broader term for any emf created by changing magnetic flux, while back emf refers to the induced emf that opposes the current change in the device itself. That distinction matters when you compare generators, motors, and inductors.
A quiz question might give you a motor, a coil, or a graph of current versus time and ask why the current changes after the circuit is switched on. Your job is to recognize that back emf grows as the magnetic situation changes, so the net voltage across the device drops and the current is limited. If a motor is described as stalled, you should connect that to a very small back emf and a potentially large current draw. In problem sets, you may also need to explain the direction of the induced emf using Lenz's law or compare the current in a motor at startup versus steady rotation. The safest move is to state the cause, the opposing induced voltage, and the effect on current or speed.
Back emf is a subtype of induced electromotive force, but they are not always interchangeable. Induced emf is the broad term for any emf created by a changing magnetic flux. Back emf is the induced emf that specifically opposes the change in current or motion in the same device, such as a motor coil or an inductor.
Back emf is the induced voltage that opposes a change in current in a motor or inductor.
It comes from changing magnetic flux, so it is a direct application of Lenz's law.
In a motor, back emf increases as speed increases, which lowers the net current.
A stalled or just-started motor has little back emf, so it can draw a much larger current.
Back emf is part of why real inductive devices resist sudden changes instead of responding instantly.
Back emf is the induced voltage that opposes the change in current in a coil or motor. It appears because the magnetic flux through the device is changing, and the induced emf points in the direction that resists that change. In a motor, it grows as the motor speeds up.
Lenz's law tells you the direction of the induced emf: it opposes the change that created it. Back emf is what that looks like in a motor or inductor. If current is rising, the back emf resists the rise. If current is falling, it tries to keep current flowing.
As the motor spins faster, it generates a larger back emf. That induced voltage subtracts from the supply voltage, so the net voltage across the motor windings gets smaller. With less net voltage, the current drops.
Not exactly. Induced emf is the general term for any emf caused by changing magnetic flux. Back emf is the induced emf that opposes the current change in the same device, especially in inductors and motors. So back emf is a special case of induced emf.