Concept of E.M.F. and Potential Difference

Illustration

We can better understand the difference between e.m.f. and p.d. by referring to the figure above. Suppose a battery has an e.m.f. of 4 volts—this means it continuously supplies 4 joules of energy per coulomb. As each coulomb leaves the battery’s positive terminal, it delivers most of its energy to the circuit resistors (2 Ω and 2 Ω in this example), with the rest going to the connecting wires. By the time the charge reaches the battery’s negative terminal, it has expended all the energy originally supplied. The battery then replenishes this energy—another 4 joules per coulomb—to repeat the process.

There is a distinct difference between e.m.f. and potential difference.The e.m.f. of a device, say a battery, is a measure of the energy the battery gives to each coulomb of charge. Thus if a battery supplies 4 joules of energy per coulomb, we say that it has an e.m.f. of 4 volts. The energy given to each coulomb in a battery is due to the chemical action.The potential difference between two points, say A and B, is a measure of the energy used by one coulomb in moving from A to B. Thus if potential difference between points A and B is 2 volts, it means that each coulomb will give up an energy of 2 joules in moving from A to B.

The p.d. between any two points in the circuit is the energy used by one coulomb in moving from one point to another. Thus in above Figure between A and B is 2 volts. It means that 1 coulomb will give up an energy of 2 joules in moving from A to B. This energy will be released as heat from the part AB of the circuit.

The following points may be noted carefully :

Above Figure shows a circuit with a cell and a resistor. The cell provides a potential difference of 1.5 V. Since it is an energy source, there is a rise in potential associated with a cell. The cell’s potential difference represents an e.m.f. so that symbol E could be used. The resistor is also associated with a potential difference. Since it is a consumer (converter) of energy, there is a drop in potential across the resistor. We can combine the idea of potential rise or drop with the popular term “voltage”. It is customary to refer to the potential difference across the cell as a voltage rise and to the potential difference across the resistor as a voltage drop.

Note. The term voltage refers to a potential difference across two points.There is no such thing as a voltage at one point. In cases where a single point is specified, some reference must be used as the other point. Unless stated otherwise, the ground or common point in any circuit is the reference when specifying a voltage at some other point.

Applications and Practical Implications

Understanding E.M.F. And potential difference is vital in numerous elements of electrical engineering, electronics, and ordinary existence:

1. Power Sources: Batteries, mills, and sun cells are all examples of gadgets that offer E.M.F. They are used in countless programs, from cell phones to electric powered cars.
2. Circuit Analysis: Engineers and technicians use E.M.F. And capability distinction to layout, troubleshoot, and optimize electric circuits, making sure the green and safe operation of various devices.
3. Home Wiring: In residential wiring, capability difference is vital for handing over electrical electricity to homes and home equipment, making sure they feature properly.
4. Safety: Understanding those ideas is important for safety, as misusing electric gadgets or working with circuits can result in electrical hazards if no longer handled efficaciously.

Electric Potential

When we charge a body, we do work in the process. The body stores this work as potential energy. A charged body can perform work by moving other charges through attraction or repulsion. We call this ability of the charged body to do work its electric potential.

A charged body possesses electric potential, which gives it the capacity to do work.

The greater the capacity of a charged body to do work, the greater is its electric potential. Obviously, the work done to charge a body to 1 coulomb will be a measure of its electric potential i.e.

Electric potential, V =  `frac WQ`

 

We measure work in joules and charge in coulombs.Therefore, the unit of electric potential will be joules/coulomb or volt. If W = 1 joule, Q = 1 coulomb, then V = 1/1= 1 volt.

We say a body has an electric potential of 1 volt when we do 1 joule of work to give it a charge of 1 coulomb.

Thus, when we say a body has an electric potential of 5 volts, we mean that charging the body with 1 coulomb of charge requires 5 joules of work. In other words, each coulomb of charge carries 5 joules of energy. The more energy per coulomb a charged body has, the higher its electric potential.

Potential Difference

We call the difference in the potentials of two charged bodies the potential difference.

If two bodies have different electric potentials, a potential difference exists between the bodies. Consider two bodies A and B having potentials of 5 volts and 3 volts respectively as shown in below Figure Each coulomb of charge on body A has an energy of 5 joules while each coulomb of charge on body B has an energy of 3 joules. Clearly, body A is at higher potential than the body B.

Figure 1

Figure 2

When a conductor connects two bodies (see Fig. 2), electrons flow from body B to body A. The current stops flowing once both bodies reach the same potential. This leads to an important conclusion: current flows in a circuit only when a potential difference exists. If there's no potential difference, no current will flow. People sometimes refer to potential difference as voltage.

Unit. Since the unit of electric potential is the volt, we also define the unit of potential difference as the volt.

➤The potential difference between two points is 1 volt if one joule of work is **done or released in transferring 1 coulomb of charge from one point to the other.

Maintaining Potential Difference

A device that maintains a potential difference between two points develops an electromotive force (e.m.f.). A simple example of this is a cell. The figure below shows a familiar voltaic cell, which includes a copper plate (the anode) and a zinc rod (the cathode) immersed in dilute H₂SO₄.

In a cell, chemical action moves electrons from the copper plate to the zinc rod via the electrolyte (dil. H₂SO₄), charging the copper plate positively (+Q) and the zinc rod negatively (–Q). The work done (W joules) to separate these charges creates a potential difference of W/Q volts. When connected by a wire, electrons flow from zinc to copper, generating an electric current. The cell's chemical process maintains this charge separation—and thus the potential difference—as long as chemical energy is available. Though both potential difference (p.d.) and electromotive force (e.m.f.) use volts as their unit, e.m.f. sustains the p.d., and the two do not represent the same quantity.