Concept of E.M.F. and Potential Difference

Illustration

The difference between e.m.f. and p.d. can be made more illustrative by referring to above Figure Here battery has an e.m.f. of 4 volts.It means battery supplies 4 joules of energy to each coulomb continuously.As each coulomb travels from the positive terminal of the battery,it gives up its most of energy to resistances (2 Ω and 2 Ω in this case) and remaining to connecting wires. When it returns to the negative terminal, it has lost all its energy originally supplied by the battery. The battery now supplies fresh energy to each coulomb (4 joules in the present case) to start the journey once again.

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 a body is charged, work is done in charging it. This work done is stored in the body in the form of potential energy. The charged body has the capacity to do work by moving other charges either by attraction or repulsion. The ability of the charged body to do work is called electric potential.

The capacity of a charged body to do work is called its electric potential.

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`

 

The work done is measured 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.

Hence a body is said to have an electric potential of 1 volt if 1 joule of work is done to give it a charge of 1 coulomb.

Thus, when we say that a body has an electric potential of 5 volts, it means that 5 joules of work has been done to charge the body to 1 coulomb. In other words, every coulomb of charge possesses an energy of 5 joules. The greater the joules/coulomb on a charged body, the greater is its electric potential.

Potential Difference

The difference in the potentials of two charged bodies is called 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

If the two bodies are joined through a conductor [See Fig.2], then electrons will flow from body B to body A. When the two bodies attain the same potential, the flow of current stops. Therefore, we arrive at a very important conclusion that current will flow in a circuit if potential difference exists. No potential difference, no current flow. It may be noted that potential difference is sometimes called voltage.

Unit. Since the unit of electric potential is volt, one can expect that unit of potential difference will also be volt. It is defined as under :

➤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 potential difference between two points is said to develop electromotive force (e.m.f.). A simple example is that of a cell.Below Figure shows the familiar voltaic cell. It consists of a copper plate (called anode) and a zinc rod (called cathode) immersed in dilute `H_2So_4`.

The chemical action taking place in the cell removes electrons from copper plate and transfers them to the zinc rod. This transference of electrons takes place through the agency of dil.`H_2So_4` (called electrolyte). Consequently, the copper plate attains a positive charge of +Q coulombs and zinc rod a charge of –Q coulombs. The chemical action of the cell has done a certain amount of work (say W joules) to do so. Clearly, the potential difference between the two plates will be W/Q volts. If the two plates are joined through a wire, some electrons from zinc rod will be attracted through the wire to copper plate. The chemical action of the cell now transfers an equal amount of electrons from copper plate to zinc rod internally through the cell to maintain original potential difference (i.e. W/Q). This process continues so long as the circuit is complete or so long as there is chemical energy. The flow of electrons through the external wire from zinc rod to copper plate is the electric current. Thus potential difference causes current to flow while an e.m.f. maintains the potential difference. Although both e.m.f. and p.d. are measured in volts, they do not mean exactly the same thing.