Q.State the difference between the EMF and the terminal potential difference of a cell. Under what condition are the two equal in magnitude?

An EMF DEVICE (a cell, battery, solar cell, fuel cell, or generator) maintains a potential difference across its own terminals by doing WORK on the charge carriers passing through it, driving them from low to high potential energy internally using some non-electrostatic energy source (chemical energy in an ordinary cell, absorbed photon energy in a solar cell). Its emf, $\varepsilon=dw/dq$, is the work done per unit charge, with SI unit joule/coulomb (equivalently the volt). …