Electric current flows in a closed circuit, from high potential to low potential. When an electric current moves through a component of electrical resistance, there is a reduction in electrical potential energy because electrical energy is used on this resistance. In order for the electric current to continue to flow from high potential to low potential, there must be a device to add electrical potential energy, the tool is an electromotive force (emf) or more accurately called an electric voltage source. Emf or a voltage source is a component that converts a type of energy into electrical energy, such as batteries, solar cells, or electricity generators.
Source of electromotive force (emf)
The figure beside is a simple illustration of a closed circuit that has emf (ε) and resistance (R). Long vertical lines at the emf source represent high potential while short vertical lines represent low potential. Electric charges move from high potential to low potential. An electric charge that moves during a certain time interval is called an electric current, where the direction of the electric current is indicated by an arrow.
When it is at a high potential, the charge has electrical potential energy. Electric potential energy decreases when the electric charge moves past the resistor. When an electric charge moves in the source of emf from a low potential to a high potential, the electric potential energy increases.
When the electric charge moves inside the source of the emf, the charges are experiencing resistance. The actual electrical source in life usually has an internal resistance, which is expressed by the symbol r. When an electric charge flows through an internal resistance (r), there is electrical potential energy reduced.
Terminal voltage (V) is the net voltage produced by an electric voltage source or source of emf. Mathematically, expressed through equations:
V = ε – I r
V = Terminal voltage, ε = emf, I = electric current, r = internal resistance.
If a 12 Volt battery has a resistance in 0.2 Ω then when 2 A flows from the battery, the terminal voltage is 12 – (0.2) (2) = 12 – 0.4 = 11.6 Volt.
When there is no electric current flowing from the battery, the terminal voltage value = emf. Suppose that emf = 12 Volt then the terminal voltage = 12 Volt. If there is an electric current flowing from the source of emf, the terminal voltage is smaller than the emf.
Emf is the electrical voltage listed for example on the battery, while the terminal voltage is the actual electrical voltage or actual potential difference, where the value is affected by the electric current flowing in the battery.