EMF is an acronym for electromotive force. Scientists tend not to use the spelled-out version of this term, in part because it can be misleading: EMF is not, in fact, a force in the way physicists use the term. Rather, it’s the energy produced by the interaction between a current and a magnetic field when one (or both) is changing. It’s measured in volts, and is sometimes equated with voltage, or potential difference.
The tutorial below describes both EMF and a related phenomenon, back EMF (or counter EMF). EMF explains the sudden flashing of the light bulb in the illustrated circuit both when it is connected and disconnected.
The tutorial contains a simple circuit of a battery, knife switch and light bulb (acting as a resistor, resisting the flow of current). It also contains an inductor, in the form of a wire coil. Inductors store energy in the form of the magnetic fields that are generated around them by the current passing through the wire.
See how this works by clicking on the blue Turn On button to throw the knife switch and power the circuit, depicted by yellow glow in the circuit. Note the blue magnetic field lines (a manifestation of EMF) that form around the coil of the inductor, explained by Faraday’s Law of Induction. Note also that the light bulb flashes for a moment, then dims. This effect is explained by back EMF.
When electricity travels through the circuit, its initial preference is to avoid the light bulb and travel down the path of least resistance, through the coiled wire. But for a few moments, at least, the electricity does surge through the light bulb, prompting a brief flash. It does this when the coil briefly puts up its own resistance to the current in the form of back EMF. This back EMF is produced as a result of Lenz’s Law, which says that in a circuit with an induced EMF caused by a change in a magnetic field, the induced EMF causes a current to flow in the direction that opposes the change in flux. In other words, if an increasing magnetic field induces an EMF, the resulting current will oppose a further increase.
So as the magnetic field in the inductor grows, it induces a current that works to counter the battery-generated current. As a result, it is easier for the battery’s current to flow through the light bulb – at least until the inductor’s magnetic field reaches a steady state (stops changing), putting an end to the back EMF.
The effects of back EMF can also be seen when you click the red Turn Off button to interrupt the circuit. Note that the magnetic field lines begin to collapse as the flow of electricity slows. In this manifestation of Lenz’s Law, the decreasing magnetic field induces an EMF, and the resulting current opposes a further decrease. The resulting current flows through the circuit to the light bulb, which flashes with that surge, then dims as the electricity completely drains from the circuit.
Thanks to our scientific advisor on this page, Mr. James Andy Powell, Electronics Engineer in the MagLab's Instrumentation & Operations division.