All voltage sources have two fundamental parts: a source of electrical energy that has a characteristic electromotive force (emf), and an internal resistance r . The emf is the work done per charge to keep the potential difference of a source constant.

The emf is equal to the potential difference across the terminals when no current is flowing. The voltage output of a device is called its terminal voltage V terminal and is given by V terminal = ε - I r , where I is the electric current and is positive when flowing away from the positive terminal of the…

voltage output of a device — called its terminal voltage V terminal and is given by V terminal = ε - I r , where I is the electric current and is positive when flowing away from the positive terminal. terminal voltage — potential difference measured across the terminals of a source when there is no load attached. equivalent resistance — resistance of a combination of resistors; it can be thought of as the resistance of a single resistor that can replace a combination of resistors in a series and/or parallel…. series — the sum of the individual resistances: R s = R 1 + R 2 + R 3 + ⋯ = ∑ i = 1 N R i R s = R 1 + R 2 + R 3 + ⋯ = ∑ i = 1 N R i.