Cells Electromotive Force and Internal Resistance

Introduction

To have a deep understanding of the working of electric batteries, you need to have a basic knowledge of the properties of the materials that go into it, i.e., cells and electrolytes. Energy conversion and storage devices are the most powerful and vital of all devices used in electrical engineering and electronics applications. Although their uses can vary greatly, they typically convert one form of energy into another or store energy that can be retrieved later to perform work.

What are Cells?

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All of us have either used or have come across cells on a day-to-day basis. Be it our mobile phones or laptops, we have encountered a cell. A cell is an electrochemical device that converts chemical energy into electrical energy.

To understand it better, let us first explore what in fact is electricity. And how is electricity generated. To answer these questions, one needs to know about atoms which are composed of protons and neutrons in their nucleus surrounded by electrons moving in orbits around them. The number of protons present in an atom determines its type.

These types can be ionic (e.g.: NaCl), metallic (e.g.: Cu) or covalent (e.g.: O₂). These different types of atoms form molecules that makeup solids, liquids, and gases, respectively.

Types of Cells

• Primary Cells: A primary cell is a non-rechargeable battery. When used, these batteries produce energy from an oxidation reaction within them. Primary cells are commonly found in devices that cannot be recharged like flashlights or smoke detectors.

• Secondary Cells: The electrochemical cells in secondary batteries are different from those in primary batteries. In a secondary battery such as lead-acid or nickel-cadmium, energy storage does not take place at an electrode but is achieved within a separate compartment.

• Galvanic Cells: Galvanic cells are two different electrodes in an electrolyte solution that produces electrical energy.

• Photovoilatatie Cells: Photovoltaic (PV) cells are an electrical component that converts light into electricity. PV cells are made up of semiconductor materials that can produce an electric current when exposed to sunlight or other forms of electromagnetic radiation. The amount of current produced is directly proportional to how much light is received by each cell in a given period of time.

• Fuel Cells: In physics, a fuel cell is a device that converts chemical energy directly into electrical energy.

• Solar Cells: Solar cells are devices that converts solar energy into electrical energy. There are two types of solar cells: silicon-based photovoltaic (PV) cell and thin film PV cell.

What is Electromotive Force (EMF)?

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The electromotive force (EMF) is a scalar value of voltage that appears at all points in an electric circuit, due to natural electrical currents. The Electo Motive Force is usually denoted by $\varepsilon$ (E-M-F), although historically it was sometimes denoted with D for the difference. It was thus called in distinction from galvanic or voltaic electricity (V-M-F).

The EMF is measured in volts. A battery has an EMF of 1 volt if its terminals are connected together by a wire that conducts 1 ampere when there is no load on it

Formula

By the following formula, you can easily calculate EMF,

emf = I (R + r)

Where,

I = Current

R = Resistance

r = Internal Resistance

Or,

Emf = E/Q

E = Energy in joules

Q = Charge in coulombs

What is Internal Resistance?

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As stated before, resistance is defined as an opposition to electrical current flow. If you are trying to measure current flow in a circuit with a multimeter and you put your probe on a wire inside of that circuit’s path, it will decrease your measurement. This is because there is an internal resistance present in any device; it's caused by electrons scattering within conductors. The higher a conductor's resistance, the more difficult it is for electrons to move through it.

This can be minimized by using thick wires or using smaller currents, but these options aren't always available or practical. In some cases, especially when dealing with very large currents (such as those from generators), we must deal with high levels of internal resistance to avoid damage to our equipment. The solution for these situations is called Ohm's Law.

Ohm's Law: V = IR.

Using Ohm's Law allows us to calculate voltage based on current flow and vice versa based on voltage/current levels.

Formula

Internal Resistance (r) = (E – V)/I

Where,

E = Electro-Motive Force

V = Potential Difference

I = Current Flow in a device

FAQs

Q1. What is the potential difference across the terminals (VT) of a cell with emf E for the open circuit?

Ans. When a cell is open circuited, there is no path for current to flow between anode and cathode. Hence V0 = 0.

The common dry cell produces a voltage of - 1.5 V (1.5 volts)

Q2. What is the relationship between Faraday's law and Electromotive Force?

Ans. Faraday's law states that when there is a change in magnetic flux through a loop of wire, there is an electromotive force (EMF) induced into that wire. In other words, EMF = n × B where n is the number of turns in a loop of wire wrapped around a magnetic core; B is magnetic flux through that same cross-sectional area; and × represents multiplication.

Q3. What is the difference between Electromotive Force and terminal voltage?

Ans. An electromotive force is a potential that creates an electric current when electrons move from a negatively charged part of a circuit to a positively charged part. Terminal voltage is defined as: The voltage across terminals of an electrochemical cell at equilibrium with its surroundings under specified conditions; it is equal to Ecell / Q where Ecell is electrical potential of electrochemical reaction in V, Q is charge passed during reaction in coulombs.

For example, if we consider two half-cells (electrodes) with different redox reactions on them connected by a salt bridge (which allows ions to pass through), then terminal voltage can be calculated using Nernst equation. In case of galvanic cells (or batteries), terminal voltage is what you will measure using voltmeter connected between positive and negative terminals.