Cells, EMF, Internal Resistance — Explained

Detailed Explanation

The study of cells, electromotive force (EMF), and internal resistance forms a cornerstone of understanding direct current (DC) circuits. While ideal voltage sources are often assumed in introductory problems, real-world power sources, such as batteries and generators, exhibit characteristics that deviate from this ideal, primarily due to their internal resistance.

1. Conceptual Foundation: What is a Cell?

A cell is an electrochemical device that converts chemical energy into electrical energy. This conversion process involves redox reactions occurring at two distinct electrodes immersed in an electrolyte.

One electrode acts as the positive terminal (cathode) and the other as the negative terminal (anode). The chemical reactions drive electrons from the negative terminal, through an external circuit, to the positive terminal.

This continuous flow of electrons constitutes an electric current. A battery is essentially a combination of one or more cells.

2. Electromotive Force (EMF), $E$

Definition: — The EMF of a cell is defined as the maximum potential difference between its terminals when no current is drawn from the cell (i.e., when the external circuit is open). It represents the work done per unit charge by the non-electrical forces (chemical forces) within the cell to move a positive charge from the lower potential terminal to the higher potential terminal. It is the 'driving force' or 'voltage generating capacity' of the cell.
Units: — The unit of EMF is the Volt (V), which is Joules per Coulomb ($J/C$).
Ideal vs. Real Cells: — An ideal cell would have zero internal resistance, meaning its terminal voltage would always be equal to its EMF, regardless of the current drawn. However, such a cell does not exist in reality. The EMF is an intrinsic property of the cell, determined by the chemical nature of its electrodes and electrolyte.
Measurement: — EMF can be measured accurately using a potentiometer, which draws no current from the cell during measurement, or by a high-resistance voltmeter connected across the terminals of an open circuit cell.

3. Internal Resistance, $r$

Origin: — Every real cell possesses an internal resistance, $r$, which is the opposition offered by the electrolyte and electrodes to the flow of current within the cell itself. This resistance arises from the finite conductivity of the electrolyte, the resistance of the electrodes, and the chemical processes occurring at the electrode-electrolyte interfaces.
Factors Affecting Internal Resistance:

* Nature of Electrolyte: Higher concentration of ions generally leads to lower internal resistance. * Nature of Electrodes: The material and surface area of electrodes play a role. * Distance between Electrodes: Greater distance means higher resistance.

* Area of Electrodes Immersed: Larger immersed area leads to lower resistance. * Temperature: Internal resistance generally decreases with an increase in temperature due to increased ion mobility.

* Age of Cell: As a cell ages, its internal resistance tends to increase.

Effect: — When current $I$ flows through the cell, a voltage drop occurs across this internal resistance. This voltage drop is given by $Ir$. This 'lost voltage' means that the potential difference available to the external circuit is less than the cell's EMF.

4. Terminal Potential Difference (Terminal Voltage), $V$

Definition: — The terminal potential difference is the actual voltage available across the external terminals of the cell when current is being drawn from it.
Relationship with EMF and Internal Resistance:

Consider a cell with EMF $E$ and internal resistance $r$ connected to an external resistance $R$. The total resistance in the circuit is $R_{total} = R + r$. According to Ohm's Law, the current flowing through the circuit is $I = \frac{E}{R_{total}} = \frac{E}{R+r}$.

The voltage drop across the external resistance $R$ is the terminal potential difference $V$. So, $V = IR$. Substituting $I = \frac{E}{R+r}$ into $V = IR$, we get $V = \frac{E}{R+r} \times R$. Alternatively, we can express $V$ in terms of $E$ and $Ir$: Since $E = I(R+r) = IR + Ir$, And $V = IR$, Therefore, $V = E - Ir$.

This equation is crucial: it shows that the terminal voltage $V$ is always less than the EMF $E$ when current $I$ is flowing ($I > 0$). If the external circuit is open ($I=0$), then $V=E$. If the cell is being charged (current flows into the positive terminal), then $V = E + Ir$.

Short Circuit: — If the external resistance $R$ is zero (a short circuit), the current drawn will be maximum: $I_{max} = \frac{E}{r}$. In this case, the terminal voltage $V = I \times 0 = 0$. All the EMF is dropped across the internal resistance.

5. Power Delivered by a Cell

Total Power Generated by Cell: — The total power generated by the cell's chemical reactions is $P_{total} = E \times I$.
Power Dissipated Internally: — The power lost as heat within the cell due to its internal resistance is $P_{internal} = I^2 r$.
Power Delivered to External Circuit: — The power delivered to the external resistance $R$ is $P_{external} = V \times I = I^2 R = \frac{V^2}{R}$.
Energy Conservation: — By conservation of energy, $P_{total} = P_{external} + P_{internal}$, which means $EI = I^2 R + I^2 r$. Dividing by $I$ gives $E = IR + Ir$, which is consistent with $E = V + Ir$.

6. Cells in Series Combination

When cells are connected in series, the negative terminal of one cell is connected to the positive terminal of the next.

Aiding Series (Polarities aligned): — If $n$ identical cells, each with EMF $E$ and internal resistance $r$, are connected in series such that their EMFs add up (positive to negative connection), then:

* Equivalent EMF: $E_{eq} = nE$ * Equivalent Internal Resistance: $r_{eq} = nr$ * Current in external circuit $R$: $I = \frac{nE}{R+nr}$

Opposing Series (Polarities reversed): — If one cell is connected in reverse, its EMF subtracts. For two cells $E_1, r_1$ and $E_2, r_2$ in series, but $E_2$ is reversed:

* $E_{eq} = |E_1 - E_2|$ * $r_{eq} = r_1 + r_2$ (Internal resistances always add up, regardless of polarity)

7. Cells in Parallel Combination

When cells are connected in parallel, all positive terminals are connected together, and all negative terminals are connected together. This arrangement is typically used to increase the current capacity or prolong the discharge time, rather than increasing the voltage.

Identical Cells in Parallel: — If $n$ identical cells, each with EMF $E$ and internal resistance $r$, are connected in parallel:

* Equivalent EMF: $E_{eq} = E$ (The voltage remains the same, but the current capacity increases). * Equivalent Internal Resistance: $rac{1}{r_{eq}} = \frac{1}{r} + \frac{1}{r} + ... (n \text{ times}) = \frac{n}{r} implies r_{eq} = \frac{r}{n}$ * Current in external circuit $R$: $I = \frac{E}{R + r/n}$

Non-Identical Cells in Parallel: — For two non-identical cells $E_1, r_1$ and $E_2, r_2$ in parallel:

* Equivalent EMF: $E_{eq} = \frac{E_1/r_1 + E_2/r_2}{1/r_1 + 1/r_2}$ * Equivalent Internal Resistance: $rac{1}{r_{eq}} = \frac{1}{r_1} + \frac{1}{r_2} implies r_{eq} = \frac{r_1 r_2}{r_1 + r_2}$

8. Real-World Applications

Automotive Batteries: — Car batteries are typically 12V lead-acid batteries, consisting of six 2V cells connected in series. Their internal resistance is critical for delivering the high starting current required by the engine.
Portable Electronic Devices: — Batteries in phones, laptops, and other devices are designed with specific internal resistances to optimize power delivery and battery life.
Power Banks: — These devices often use multiple lithium-ion cells in parallel to increase capacity and provide higher current output.
Solar Panels: — Individual photovoltaic cells are connected in series and parallel to achieve desired voltage and current outputs.

9. Common Misconceptions

EMF vs. Terminal Voltage: — Students often confuse EMF with terminal voltage. Remember, EMF is the *source* voltage, while terminal voltage is the *available* voltage to the external circuit when current is flowing. They are equal only when no current is drawn.
Internal Resistance as External Resistance: — Internal resistance is *inside* the cell and cannot be directly accessed or changed by connecting external components. It's an inherent property.
Cells in Parallel always increase voltage: — This is incorrect. Parallel connection of identical cells keeps the voltage the same but reduces the equivalent internal resistance, allowing for higher current delivery or longer discharge times.

10. NEET-Specific Angle

NEET questions often test the understanding of the $V = E - Ir$ relationship, calculations involving cells in series and parallel, and power dissipation. Conceptual questions might focus on factors affecting internal resistance or the difference between EMF and terminal voltage. Numerical problems frequently involve finding current, terminal voltage, or internal resistance given other parameters. Be prepared to apply Ohm's law in conjunction with the cell equations.