27.1
Electromotive Force
Electromotive force (emf) is the force that causes current to flow from a higher to a lower potential. The term "electromotive force"is used for histo…
Alessandro Volta, inventor of the battery, coined the term electromotive force or emf.
Emf is not a force, but rather a potential difference between the two terminals of a battery.
Emf represented by the symbol epsilon, is the maximum voltage a battery can deliver in an open circuit. It is expressed in units of volt.
When a wire is connected between the two terminals, the emf pushes the electrons from the negative terminal to the positive terminal.
Emf is defined as the amount of work done to move a unit charge across the terminals.
In a closed circuit, the voltage measured across the battery's terminals is defined as its terminal voltage, which is always less than the emf value. This is because the battery has an internal resistance r.
The terminal voltage is thus, the difference between its emf and the voltage drop across the internal resistance.
View the full transcript and gain access to JoVE Core videos
Q1: Why is electromotive force called a force when it is not actually a force?
The term electromotive force is used for historical reasons, coined by Alessandro Volta, the inventor of the battery. Despite its name, emf is not a force but rather a potential difference between the two terminals of a battery. It represents the maximum voltage a battery can deliver in an open circuit, measured in volts.
Q2: What is the relationship between emf and terminal voltage in a battery?
Terminal voltage is the voltage measured across a battery's terminals when current flows through it, and it is always less than the emf. This difference occurs because the battery has internal resistance. Terminal voltage equals the emf minus the voltage drop across the internal resistance, which increases as the battery depletes.
Q3: How does a battery function as a charge pump in a circuit?
An emf source functions as a charge pump, moving negative charges from the positive terminal to the negative terminal to maintain the potential difference. Work must be done on these charges against the electric field, requiring energy from chemical reactions in the battery. This process raises the charges' potential energy and maintains the high potential at the positive terminal.
Q4: What are examples of devices that produce electromotive force?
Examples of emf sources include batteries, electric generators, solar cells, thermocouples, and fuel cells. All these sources transform energy of some kind—mechanical, chemical, thermal, or other forms—into electric potential energy and send it to the circuit. This energy transformation is essential for maintaining current flow in any circuit.
Q5: How is electromotive force defined in terms of work and charge?
Electromotive force is defined as the amount of work done to move a unit charge across the terminals of a battery. When there is no current flowing, the emf equals the work done on the charge per unit charge. Its unit is the volt, derived from joules of work divided by coulombs of charge.
Q6: What causes the internal resistance of a battery to increase over time?
Internal resistance generally increases as a battery is depleted due to oxidation of the plates or reduction of the acidity of the electrolyte. This increase in internal resistance causes a greater voltage drop across the battery's internal resistance, further reducing the terminal voltage available to the external circuit.
Q7: What happens to electron flow when a wire connects the terminals of a battery?
When a wire is connected between the two terminals, the emf pushes electrons from the negative terminal to the positive terminal through the external circuit. Simultaneously, the battery's internal charge pump moves negative charges from the positive terminal to the negative terminal internally, maintaining the potential difference needed for continuous current flow.