Difference Between EMF and Voltage

The most common confusion in students is what’s the difference between EMF and voltage. To understand the difference between EMF and voltage, let us first understand what we mean by the terms EMF and voltage.

Electromotive force and voltage are often mistaken to be the same but there are differences between them. They are sometimes confused with electromagnetic fields as well. One can say that they are all related, but are not the same. Electromotive force and voltage are directly related to the generation of electromagnetic fields (EMF). Electromotive force is an invisible form of energy produced by the interaction of electric and magnetic fields which results in the movement of electrons from one point to another.

Electromotive Force (EMF) 

Electromotive force or also known as EMF is an energy input for charging through a battery cell. In other words, it creates and maintains a voltage in the active cell, by supplying energy in Joules to each unit of Coulomb charge. This is represented by "ε" and the unit of measurement is the same as voltage, which is Volt. 

EMF is the maximum potential difference between two points of the battery when no current flows from the source in the case of an open circuit. That is, it is caused by EMF and is affected by voltage or potential difference. A generator or battery is used to convert one energy into another. In these devices, one terminal is positively charged and the other is negatively charged. Therefore, an electromotive force is work done on a unit electric charge.

Voltage 

Voltage is the force that causes an electric charge to flow. This is the potential difference between two connections where one connection collects more electrons than the other. Voltage is defined as potential energy per charge. 

Voltage is measured in volt (V), which is the derivation unit of potential. A voltage drop is a drop in potential along the current path through a circuit. The higher the resistance of a component, the greater the voltage drop between connections. When electricity encounters resistance,  potential energy is lost because it is converted into another form of energy to do the work. For example,  electric potential energy is converted into thermal energy by a resistor.

Difference between EMF and Voltage 

Points to Remember 

1. EMF or electromotive force is the potential difference generated by one or more cells or a changing magnetic field in a solar cell, and voltage is the potential difference measured at any two points in the magnetic field. 

2. The SI unit and voltage of EMF  are the same (volt). 

3.  The magnitude of the EMF depends on the change in the magnetic field, and the voltage depends on the magnitude and resistance of the current. 

4. Voltage can be thought of as the difference between two electrical states in an electric field, but EMF is the force that causes the difference in electrical states.

What is  EMF?

1. EMF stands for electromotive force. EMF is the voltage at the terminals of the source in the absence of an electric current. 

2. The concept of EMF defines 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.

3. The electromotive force (EMF) is defined as- The amount of work done in the energy transformation and the amount of electricity that passes through the electrical source or the generator.

4. EMF is measured in Volts and denoted by the symbol ε (or E).

What is Voltage?

• The voltage is defined as the amount of energy required to move a unit charge from one end to another end. Voltage is measured in Volts and denoted by the symbol V.

• The voltage is mainly developed between the two poles of the electric circuit i.e. it developed between the anode and cathode of the battery. 

• The positive terminal of the battery is known as the cathode and the negative terminal of the battery is known as the anode. The potential at the cathode of the source will be higher than the potential at the anode.

• When a potential difference or the voltage is developed across the passive elements is known as the voltage drop. (Passive elements-the electrical elements that do not generate power, such as resistors, capacitors, etc. which are used to dissipate, store charges)

• The voltage developed is a result of the electric field.

Difference Between EMF And Terminal Voltage

Now the major emf and voltage difference is, voltage or terminal voltage is too small in comparison with the emf. It implies that the Intensity of Emf developed will be always greater than the voltage as the voltage exists in a loaded circuit. Due to external resistance, there is always voltage drop or energy loss which will lead to varying intensity. But, emf is always constant.

Let us look at other voltage and EMF difference as listed below:

Difference Between Voltage and EMF

These are some major notable differences between the emf and terminal voltages. Though both are measured as potential differences, they are not the same.

Solved Examples

1. Consider an electrical circuit with a potential difference of 5V, a current of 0.9A, and the internal resistance of the battery is 0.7ohms. Calculate the EMF of the battery.

Ans:

Given,

Potential difference = V = 5V

Current in the circuit = I =0.9A

Internal resistance of the battery =r = 0.7

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 = 5 + (0.9 x 0.7) = 5.63 volts

Therefore, the EMF of the battery is given by 5.63V.

2. A battery provides a current of 1A through a 3ohm coil and 0.8A through a 5ohm coil. Calculate emf and the internal resistance of the battery.

Ans:

Given,

Let the emf of the battery be E and the internal resistance of the battery be r.

Now,

Emf of battery 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

For 3 Ω coil: E = 1(3+r)……..(1)

For 5 Ω coil: E = 0.8(5+r)……….(2)

On solving (1) and (2) we get the value of the internal resistance of the battery, r = 5 Ω

Now emf of the battery is, E= 8V

Therefore, the emf and internal resistance of the battery are 8volts and 5ohms respectively.

Did You Know?

• Various types of batteries are available in the market and the emf of batteries will vary from each other. 12V emf batteries are the standard ones used for practical purposes.

• The emf of batteries is also determined by the type of chemical reaction involved. Lead-Acid batteries used in cars and other vehicles are the most common types.

• Though Emf stands for electromotive force, it is still the voltage developed in the circuit. Here force means energy per unit charge.