The key difference between terminal voltage and EMF is that terminal voltage is a closed-circuit voltage, whereas EMF is an open-circuit voltage.
Often, the terms EMF (electromotive force) and voltage are mistaken as the same, but these are two different terms. Some people also mistake these terms for electromagnetic fields as well, which is a completely different term. However, electromotive force and voltage are related to each other directly when considering the generation of electromagnetic fields. A combination of terminal voltages is given an EMF.
CONTENTS
1. Overview and Key Difference
2. What is Terminal Voltage
3. What is EMF
4. Terminal Voltage vs EMF in Tabular Form
5. Summary – Terminal Voltage vs EMF
What is Terminal Voltage?
Terminal voltage can be defined as the potential difference across the terminals of a load when the circuit is turned on. Generally, the terminal voltage is measured using a voltmeter. The voltage output of a device can be measured across the terminals, where it is called terminal voltage, V. The equation for this term is as follows:
V = emf – Ir
V is the terminal voltage; emf is the maximum potential difference delivered by a cell or a generator when there is no current flowing through it, r is the internal resistance, whereas I is the current that flows at the time of the measurement.
If the current is flowing away from the positive terminal, I is positive. The terminal voltage becomes smaller when the current that is flowing through the circuit is larger. Similarly, the largest internal resistance is the largest upon the minimal terminal voltage.
What is EMF?
EMF can be described as the maximum potential difference delivered by a cell or a generator upon zero current flow through it. The EMF value is usually measured using a potentiometer. The term EMF stands for electromotive force.
Typically, all the voltage sources tend to create a potential difference, and they can also supply current as long as they are connected to a resistance. When considering a smaller scale, the potential difference can create an electric field that can exert force on charges. This causes the flow of a current.
Although the name electromotive force uses the word force, it is not a force at all. We can define it as a special type of potential difference of a source when there is no current flowing through the circuit. The units of EMF are volts.
Moreover, EMF is directly related to the source of potential differences in a particular combination of chemicals in a battery. EMF differs from the output of the voltage of the particular device upon the flow of current. For example, the current flowing between the terminals of a battery is lower than the EMF value when the battery is supplying current. It can also decline further upon the depletion of the battery or loaded down.
What is the Difference Between Terminal Voltage and EMF?
Terminal voltage is the potential difference across the terminals of a load when a circuit is switched on, while EMF is the maximum potential difference a cell or a generator can deliver when zero current is flowing through it. The key difference between terminal voltage and EMF is that terminal voltage is a closed-circuit voltage, whereas EMF is an open-circuit voltage.
The below infographic presents the differences between terminal voltage and EMF in tabular form for side by side comparison.
Summary – Terminal Voltage vs EMF
Terminal voltage is the potential difference across the terminals of a load when the circuit is switched on, while EMF or electromotive force is the maximum potential difference a cell or a generator can deliver when zero current is flowing through it. The key difference between terminal voltage and EMF is that terminal voltage is a closed-circuit voltage, whereas EMF is an open-circuit voltage.
Reference:
1. “Terminal Voltage.” Kolibri Teacher In a Box.
Image Courtesy:
1. “Faraday emf experiment” By Eviatar Bach – Own work (CC0) via Commons Wikimedia
2. “Three-phase three-wire network, whose total instantaneous power is measured using instantaneous line voltages and currents, taking the phase B as common, connected to two two-terminal elements” By Alej27 – Own work (CC BY-SA 4.0) via Commons Wikimedia