Electromotive Force: Formula, Unit, Dimensional Formula and Real-Life Examples

Electromotive Force is the energy supplied by a battery or generator to move electric charges through an electric circuit. It is the work done per unit charge and is commonly known as EMF. Every time you switch on a torch, use a remote control, or charge your phone, electromotive force helps the electric current flow.

Have you ever observed why a battery can make a bulb glow? It happens because the battery provides the energy needed to push charges through the circuit. This article explains what electromotive force is, its formula, unit, dimensional formula, and its importance in everyday electrical devices.

Table of Contents

• What Is Electromotive Force
• Electromotive Force Formula, Unit, and Dimensional Formula
• Can Electromotive Force Be Negative in a Circuit
• Real-Life Examples of Electromotive Force

What Is Electromotive Force

Electromotive Force (EMF) is the electric potential produced by a battery, cell, or generator that allows electric current to flow in a circuit. In simple words, 

“Electromotive Force (EMF) is the electric potential produced by a battery, cell, or generator that makes electric current flow through a circuit from one point to another.

Although it is called a "force," it is not actually a force. It is a potential difference created between the two terminals of a source.

Do You Know?

The term electromotive force (EMF) was introduced by the Italian physicist and chemist Alessandro Volta. He invented the first electric battery, known as the Voltaic Pile, in 1800, which became the foundation for modern batteries and electrical circuits. 

Any device that produces an electric current has two terminals, a positive terminal and a negative terminal. The work done per unit charge in moving the charge from the negative terminal to the positive terminal inside the source is called the electromotive force of the source.

For better understanding, just imagine a water pump pushing water through a pipe. In the same way, EMF pushes electric charges through an electrical circuit and makes electrical devices work.

Electromotive force (EMF) is represented by the Greek letter ε (epsilon). 

It is equal to the maximum potential difference between the terminals of a battery or cell when no current is flowing in the circuit. 

Electromotive Force Formula, Unit, and Dimensional Formula 

Now that we know what electromotive force is, the next step is to learn how it is measured and calculated. The electromotive force formula, its SI unit, and its dimensional formula help us describe the amount of energy supplied by a battery or cell. Let's take a closer look at each of them one by one. 

Electromotive Force Formula

The electromotive force formula is used to calculate the total potential provided by a battery or cell. It is given by:

ε=V+Ir

Where ε is the electromotive force (EMF), V is the terminal voltage of the battery, I is the current flowing in the circuit, and r is the internal resistance of the battery

This formula shows that the electromotive force of a battery is equal to the terminal voltage plus the voltage lost due to the battery's internal resistance.

Unit of Electromotive Force

The SI unit of electromotive force (EMF) is volt (V). Since EMF is the energy supplied to move a unit charge through a circuit, it is measured in volts.

One volt means that one joule of energy is used to move one coulomb of charge.

1 Volt=1 Joule1 Coulomb

or

1V=1J1C

In simple words, if a battery provides one joule of energy to move one coulomb of charge, its electromotive force is said to be 1 volt.

Electromotive Force Dimensional Formula

The dimensional formula of electromotive force (EMF) is the same as that of potential difference because both are measured in volts.

Since,

EMF=Work DoneCharge

The dimensional formula of work done is,

[M1L2T−2]

and the dimensional formula of charge is,

[IT]

Therefore, we can say

EMF=[M1L2T−2][IT]

=[M1L2T−3I−1]

Hence, the electromotive force dimensional formula is,

  [M1L2T−3I−1]

This dimensional formula is commonly used in physics to verify equations related to electric potential and electromotive force.

Can Electromotive Force Be Negative in a Circuit 

Yes, electromotive force (EMF) can have a negative value in certain situations. A negative EMF means that the produced voltage acts in the direction opposite to the applied voltage or current.

This usually happens in the following cases,

• Charging a rechargeable battery: When a battery is being charged, the current flows opposite to its normal discharge direction. As a result, the EMF acts against the incoming electrical energy and is considered negative.
• Electromagnetic induction: According to Faraday's law, the induced EMF opposes the change in magnetic flux that produces it. 

This is represented by the negative sign in the equation,

E=−dΦBdt

The negative sign followsLenz's law, which states that the induced EMF always opposes the cause that produces it.

Therefore, a negative EMF does not mean the battery has lost energy; it simply shows that the direction of the induced voltage is opposite to the chosen direction of current or applied voltage.

Real-Life Examples of Electromotive Force

From batteries in a torch to huge power stations, electromotive force helps produce and move electric charges in a circuit. Here are some real-life examples of electromotive force that we come across every day.

• Torch Batteries: Batteries in a flashlight generate an EMF that allows current to flow and light the bulb.
• Mobile Phone Batteries: Rechargeable batteries in smartphones produce an EMF that powers the device.
• Solar Panels: Solar panels convert sunlight into electricity and generate an EMF for homes and electronic devices.
• Hydroelectric Power Plants: Flowing water turns turbines connected to generators, producing an EMF and generating electricity.
• Wind Turbines: Wind rotates the blades of the turbine, and the generator creates an EMF that supplies electricity.
• Fuel Cells: Fuel cells generate an EMF through chemical reactions between hydrogen and oxygen.
• Bicycle Dynamo: A bicycle dynamo produces an EMF when the wheel rotates, which powers the bicycle light.
• Electric Generators: Generators used during power cuts convert mechanical energy into electrical energy by producing an EMF.
• Power Stations: Thermal and nuclear power plants use generators to create a large EMF and supply electricity to cities and industries.
• Induction Cooktops: These appliances use changing magnetic fields to induce an EMF in the cooking vessel, producing heat.
• Wireless Charging Pads: Wireless chargers create a changing magnetic field that induces an EMF in the receiving device's coil.
• Microphones: Some microphones generate an EMF when sound vibrations move a coil inside a magnetic field.
• Electric Guitar Pickups: The vibrating strings create a changing magnetic field, inducing an EMF that is converted into sound.
• Portable Power Banks: Power banks store electrical energy in batteries and provide an EMF to charge mobile devices.

These additional examples make the concept of electromotive force more relatable by connecting it with devices that students see and use every day.

Till now, we have learned that electromotive force is an important concept in electricity because it helps charges move in a circuit. From batteries and generators to solar panels and mobile phones, EMF is present in many devices that we use every day.