Chapter 11: Electromotive Force

11.1 Introduction to Electromotive Force

Electromotive force (EMF) is a concept in electromagnetism that refers to the potential difference across a source of electrical energy, such as a battery or a generator. EMF is responsible for driving electric current through a circuit. In this chapter, we will explore the concept of EMF and its role in electrical circuits.

11.2 Definition of Electromotive Force

Electromotive force is the potential difference (voltage) generated by a device that converts other forms of energy into electrical energy. It is represented by the symbol (epsilon) and is measured in volts (V). It is important to note that EMF is not a force in the traditional sense; rather, it is a voltage that drives the flow of electric charge in a circuit.

11.3 Sources of Electromotive Force

Various sources can generate electromotive force, including:

1. Chemical energy: Batteries convert chemical energy into electrical energy through redox reactions, producing an EMF between the positive and negative terminals.
2. Mechanical energy: Generators, such as dynamo or alternators, use mechanical energy from a rotating shaft to create a changing magnetic field that induces an EMF in a coil of wire.
3. Thermal energy: Thermocouples generate an EMF by converting temperature differences between two dissimilar metals into electrical energy.
4. Photovoltaic energy: Solar cells convert sunlight (photons) into electrical energy, producing an EMF through the photovoltaic effect.

11.4 Internal Resistance and Terminal Voltage

All real sources of electromotive force have some internal resistance (r), which is due to the resistance of the materials and components within the device. The terminal voltage (V) of a source, such as a battery or a generator, is the potential difference across its terminals when connected to a load (external resistance, R). The relationship between EMF, internal resistance, and terminal voltage can be expressed as:

where

• is the terminal voltage in volts (V),
• is the electromotive force in volts (V),
• is the current in amperes (A),
• is the internal resistance in ohms

11.5 Kirchhoff’s Loop Rule and EMF

Kirchhoff’s loop rule states that the sum of the voltages around any closed loop in a circuit is equal to zero. This rule also applies to electromotive forces within the circuit. In a loop containing an EMF source, the sum of the EMF and the voltage drops across resistors must equal zero.

where

• is the sum of the electromotive forces in the loop,
• is the sum of the voltage drops across resistors in the loop.

Chapter Summary

In this chapter, we discussed the concept of electromotive force, which is responsible for driving electric current through a circuit. We explored various sources of EMF and the relationship between EMF, internal resistance, and terminal voltage. We also examined the role of EMF in Kirchhoff’s loop rule. Understanding EMF is essential for analyzing and designing electrical circuits involving power sources.

Continue to Chapter 12: Resistors in Series and Parallel

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