Introduction - What is Electromotive Force?
Electromotive Force is the amount of energy delivered per unit electric charge by a power source such as a generator or a battery (abbreviated E or EMF). As the generator or battery works on the electric charge being transported within itself, energy is changed from one form to another. The device's positive terminal becomes positively charged, while the negative terminal becomes negatively charged. The Electromotive Force is the work done on a unit of electric charge, or the energy gained per unit of electric charge. It is abbreviated E in the international metric system, but it is also known as EMF.
Electromotive Force has force in its name but technically it is not a force. It is generally measured in volts, which is equal to one joule per coulomb of electric charge in the meter–kilogram–second system. In the centimeter–gram–second system, the statvolt, or one erg per electrostatic unit of charge, is the electrostatic unit of Electromotive Force.
Electromotive Force, measured in volts, is the electrical activity produced by a non-electrical source in electromagnetism and electronics. Devices, such as batteries (which convert chemical energy) or generators, create an Electromotive Force by converting various sources of energy into electrical energy (which convert mechanical energy). Electromotive Force is sometimes described using a water pressure analogy. (In this example, "force" does not refer to forces of interaction between bodies.)
The electromagnetic work that would be done on an electric charge (an electron in this case) if it traveled once around a closed loop of a conductor is described as EMF in electromagnetic induction. The electric potential's scalar field is not specified for a time-varying magnetic flux joining a loop due to a circulating electric vector field, but an EMF accomplishes work that is abbreviated E in the international metric system, but it is also known as EMF.
Overview
The Electromotive Force is abbreviated as EMF. The Electromotive Force is the voltage at the terminals of the source in the absence of an electric current. The concept of Electromotive Force refers to the amount of work required to separate the charge carriers in the source current, such that the force acting on the charges at the terminals of the source is not a direct consequence of the field. EMF is developed as a result of internal resistance.
What is meant by Electromotive Force? The Electromotive Force (EMF) is defined as- The amount of work done in the energy transformation (or conversion) and the amount of electricity that passes through the electrical source or the generator. The Electromotive Force (EMF) is measured in Volts and denoted by the symbol ε (or E). In this article, we will discuss mainly what is Electromotive Force, what is EMF in Physics, etc.
What is EMF in Physics?
Now we will understand what EMF is in Physics and what is meant by EMF in Physics. So, the Electromotive Force is the maximum potential difference between two electrodes of the cell when no current is drawn from the cell. The Electromotive Force is denoted by E or sometimes it is also denoted by the symbol ε.
We know that the charges move in the electric circuit, for the motion of the charges in a given electric circuit we need to apply an external force to it. We say that the battery or an external electric source such as a battery applies such force which will give acceleration to the charges and it is known as the Electromotive Force. Despite its name, it’s not a form of force but a potential difference.
If the Electromotive Force is not a form of force at all, then why is it termed as the Electromotive Force, what are EMF and potential difference, and what is the source of EMF? To answer these doubts, consider a simple circuit of a lamp connected to a battery, as shown in the Figure below.
(Image will be updated soon)
The battery (any electro-voltaic cell) can be represented as a two-terminal device that keeps one terminal at a higher potential than the second terminal. The higher electric potential is sometimes known as the positive terminal and it is generally labeled with a plus sign. The lower-potential terminal is known as the negative terminal and is labeled with a minus sign. This is known as the source of the EMF.
When the source of the Electromotive Force is disconnected from the lamp, there is no net flow of charges within the EMF source. Once the battery is reconnected to the lamp, charges will flow from one terminal of the battery, through the lamp (causing the lamp to glow), and back to the other terminal of the battery. If we consider positive current flow which is also known as the conventional current flow, positive charges leave the positive terminal, travel through the lamp, and enter the negative terminal of the EMF source. This is how an EMF source is configured. Now, what is the Electromotive Force of a cell? The Electromotive Force of a cell is the potential difference developed at both ends of the given battery.
What is the Unit of Electromotive Force?
What is the Electromotive Force measured in? Let us have a look at what is the unit of Electromotive Force, the formula for Electromotive Force is given by,
⇒ ε = V + Ir
Where,
V- The applied potential difference.
I- The current flowing through the circuit.
r- The internal resistance of the circuit.
Therefore, the unit of Electromotive Force is in volts. The Electromotive Force (EMF) is expressed as the number of Joules of energy supplied by the source divided by each Coulomb to enable a unit of electric charge to move across the circuit. Mathematically it is given by:
⇒ ε = Joules/Coulomb
Therefore the dimensions of the Electromotive Force M1L2T-3I-1.The answer to the question what is the si unit of Electromotive Force, from the expression of EMF we can say it is Joules/coulomb.
Example:
Consider an electrical circuit with a potential difference of 7V, a current of 1A, and the internal resistance of the battery is 0.7ohms. Calculate the EMF of the battery.
Given,
Potential difference = V = 7V
Current in the circuit = I = 1A
Internal resistance of the battery = r = 0.7Ω
Now, EMF of the circuit is given by:
⇒ E = I(R + r)
Where,
R - External resistance of the electrical circuit.
r - Internal resistance of the given circuit.
I - Current flowing through the circuit.
On rearranging the above expression,
⇒ E = IR + Ir
We know that the product of current in the circuit and the external resistance is the potential difference across the resistance. Thus,
⇒ E = V + Ir
Substituting given values in the equation,
⇒ E = 7 + (1 × 0.7) = 7.7 volts
Therefore, the EMF of the battery is given by 7.7V.
FAQs on Electromotive Force
1. What is electromotive force (EMF) and how is it different from potential difference in a circuit?
Electromotive force (EMF) is the maximum potential difference provided by a source like a battery or generator when no current is drawn. Potential difference refers to the voltage measured across any two points in a circuit when current is flowing. EMF represents the energy supplied per unit charge, while potential difference represents energy used as charges move through components in the circuit.
2. Explain the significance of internal resistance in calculating the EMF of a cell.
Every real cell or battery has some internal resistance (r). The EMF (E) of a cell is related to the terminal potential difference (V) and the current (I) by E = V + Ir. Internal resistance causes the terminal voltage to drop when current flows, making EMF always equal to or higher than terminal voltage. Considering internal resistance is crucial for accurate circuit calculations.
3. How does Faraday's law relate to the generation of electromotive force?
Faraday's law of electromagnetic induction states that a change in magnetic flux through a coil induces an EMF in the coil. The induced EMF is proportional to the rate of change of flux. This principle is the basis for the working of generators, transformers, and even the operation of many modern renewable energy devices.
4. What is the formula to calculate the EMF of a battery, and what do each of the components represent?
The formula is ε = V + Ir, where:
- V = terminal potential difference of the battery
- I = current flowing through the circuit
- r = internal resistance of the battery
5. Why is EMF referred to as a "force" when it is not an actual physical force?
Despite its name, electromotive force is not a force, but rather a potential difference (measured in volts). The term arises historically because EMF "drives" electric charges around a circuit, similar to how force drives movement in mechanics. Modern physics clarifies that EMF quantifies energy supplied per coulomb of charge, not an actual force.
6. Is it possible for the EMF of a cell to be negative? What does a negative EMF indicate?
Yes, EMF can be negative in situations involving induced EMF from changing magnetic fields. A negative EMF means the induced direction opposes the cause (as per Lenz's law), such as when an inductor generates an EMF to oppose change in current or the applied magnetic field. It reflects the direction, not the existence, of EMF.
7. How does terminal voltage change with increasing current in a circuit containing a battery of fixed EMF?
As current (I) increases, the potential drop across the battery's internal resistance (Ir) also increases. Since terminal voltage (V) = EMF (E) - Ir, the terminal voltage decreases with growing current. This is crucial when designing circuits to prevent excessive voltage drops.
8. What are real-world devices that utilize EMF, and how do they transform energy?
Devices such as batteries, generators, and solar cells utilize EMF by converting different types of energy into electrical energy. For example, batteries convert chemical energy, generators convert mechanical energy, and solar cells convert light energy into an EMF that drives current through external circuits.
9. Can EMF be affected by circuit resistance outside the cell? Explain your reasoning.
EMF is not affected by the external circuit resistance. EMF is defined as the maximum potential difference provided by the source when no current flows (open circuit condition). Only the terminal voltage depends on the resistance in the external circuit as it determines current and internal loss.
10. Compare and contrast EMF and terminal voltage with respect to measurement and circuit conditions.
- EMF: Measured using a potentiometer under open circuit conditions (no current), is the inherent ability of a source to supply energy per charge.
- Terminal Voltage: Measured using a voltmeter across source terminals when current flows, is always less than or equal to EMF due to internal resistance.
