What term is used to refer to back emf or counter emf?

Back electromotive force (emf), often referred to as counter emf, is the voltage that opposes the change in current that created it within an electrical circuit. It arises in inductive components, such as motors and transformers, when the magnetic field created by the current flow induces a voltage that opposes the initial current flow.

The term "induced voltage" is appropriate in this context because back emf is a type of induced voltage generated by the magnetic field change in the inductive element. It effectively acts to limit the amount of current that flows through the circuit, thereby influencing the operation and efficiency of electrical devices, particularly in the context of DC motors where it plays a crucial role in controlling speed.

The other terms do not accurately describe back emf. Residual voltage typically refers to a small voltage that persists after a component has been disconnected from the power source. Electromotive force is a broader concept that refers to the potential difference generated by a source, not necessarily opposing current. Input voltage is simply the voltage supplied to a system, which does not encompass the opposing nature of back emf.