What is the primary difference between EMF and terminal potential difference?

EMF (Electromotive Force) is the maximum potential difference a cell can provide when no current is drawn from it, representing the total chemical energy converted per unit charge. It's an intrinsic property of the cell. Terminal potential difference (V), on the other hand, is the actual voltage available across the cell's terminals when current is flowing through an external circuit. Due to the voltage drop across the cell's internal resistance ($Ir$), the terminal potential difference is always less than the EMF when current is being supplied by the cell ($V = E - Ir$). They are equal only in an open circuit condition.

Why does a real cell have internal resistance?

A real cell has internal resistance because the materials it's made of—the electrolyte solution and the electrodes—are not perfect conductors. The electrolyte offers resistance to the movement of ions, and the electrodes themselves have some electrical resistance. Additionally, chemical reactions at the electrode surfaces can also contribute to this opposition to current flow. This inherent resistance within the cell itself causes some of the energy generated by the chemical reactions to be dissipated as heat, leading to a voltage drop internally and reducing the voltage available to the external circuit.

How does connecting cells in series affect the overall EMF and internal resistance?

When cells are connected in series, their EMFs add up if they are connected in the same polarity (positive to negative). For 'n' identical cells, the equivalent EMF becomes $nE$. The internal resistances also add up directly, so the equivalent internal resistance becomes $nr$. This arrangement is used to achieve a higher total voltage. If cells are connected with opposing polarities, their EMFs subtract, but their internal resistances still add up.

How does connecting cells in parallel affect the overall EMF and internal resistance?

When identical cells are connected in parallel, the equivalent EMF remains the same as that of a single cell ($E_{eq} = E$). This configuration does not increase the voltage. However, the equivalent internal resistance decreases significantly. For 'n' identical cells, the equivalent internal resistance becomes $r/n$. This reduction in internal resistance allows the combination to deliver a higher total current to an external circuit or to last longer by sharing the load among multiple cells, making it suitable for applications requiring high current or extended operation.

What happens to the terminal voltage when a cell is short-circuited?

When a cell is short-circuited, it means the external resistance ($R$) connected across its terminals is effectively zero. In this scenario, the current drawn from the cell becomes maximum, given by $I_{max} = E/r$, where $E$ is the EMF and $r$ is the internal resistance. According to the terminal voltage equation $V = E - Ir$, if $R=0$, then $V = I imes 0 = 0$. This means all the EMF is dropped across the internal resistance of the cell, and the terminal voltage becomes zero. This condition can be dangerous as it leads to very high currents and significant heat generation within the cell.

Can the terminal voltage ever be greater than the EMF?

Yes, the terminal voltage can be greater than the EMF, but only when the cell is being charged. When a cell is being charged, an external source forces current into the positive terminal of the cell, against its natural EMF. In this case, the current direction is opposite to the discharge current. The terminal voltage equation then becomes $V = E + Ir$. The additional $Ir$ term accounts for the voltage required to overcome the cell's internal resistance and push current into it, making the terminal voltage higher than the cell's intrinsic EMF.

Cells, EMF, Internal Resistance

Physics

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

A cell is a device that converts chemical energy into electrical energy, providing a source of electromotive force (EMF) to drive current in a circuit. The EMF, denoted by $E$ , is the maximum potential difference across the cell's terminals when no current is drawn from it. However, all real cells possess an internal resistance, $r$ , which is an opposition to the flow of current within the cell it…

Quick Summary

A cell is a source of electromotive force (EMF), $E$ , which represents the maximum potential difference it can provide. This EMF is generated by chemical reactions within the cell, converting chemical energy into electrical energy.

All real cells possess an internal resistance, $r$ , due to the materials and processes inside them. When a current $I$ is drawn from the cell, a voltage drop $Ir$ occurs across this internal resistance.

Consequently, the actual voltage available at the cell's terminals, known as the terminal potential difference $V$ , is less than the EMF. This relationship is given by $V = E - Ir$ . If the cell is on an open circuit (no current), $V=E$ .

If the cell is being charged, $V = E + Ir$ . Cells can be combined in series to increase the total EMF ( $E_{eq} = nE$ , $r_{eq} = nr$ ) or in parallel to increase current capacity and reduce equivalent internal resistance ( $E_{eq} = E$ , $r_{eq} = r/n$ for identical cells).

Understanding these concepts is fundamental to analyzing real-world electrical circuits.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

EMF vs. Terminal Voltage Relationship

The core relationship governing a real cell is $V = E - Ir$ . Here, $E$ is the EMF, the ideal voltage source.…

Cells in Series (Aiding)

When $n$ identical cells, each with EMF $E$ and internal resistance $r$ , are connected in series such that…

Cells in Parallel (Identical)

When $n$ identical cells, each with EMF $E$ and internal resistance $r$ , are connected in parallel (all…

EMF ($E$): — Max potential difference (open circuit).

Internal Resistance ($r$): — Resistance within cell.

Terminal Voltage ($V$): — Actual voltage across terminals (closed circuit).

Discharging Cell: —

V = E - Ir

Charging Cell: —

V = E + Ir

Current: —

I = \frac{E}{R+r}

Power to External Load: —

P_{ext} = I^2R = VI

Power Lost Internally: —

P_{int} = I^2r

Cells in Series (Aiding): —

E_{eq} = nE

r_{eq} = nr

Cells in Parallel (Identical): —

E_{eq} = E

r_{eq} = r/n

Cells in Parallel (Non-identical): —

E_{eq} = \frac{E_1/r_1 + E_2/r_2}{1/r_1 + 1/r_2}

rac{1}{r_{eq}} = \frac{1}{r_1} + \frac{1}{r_2}

EMF is 'E' for 'Everything' (total potential). Internal resistance 'r' 'reduces' it. Terminal voltage 'V' is 'Visible' (what you measure). So, 'E' minus 'Ir' equals 'V' (E - Ir = V). For series, 'N' times 'E' and 'N' times 'r'. For parallel, 'E' stays 'E', but 'r' gets 'Reduced' (r/N).