Ohm’s Law establishes the mathematical relationship between the three primary characteristics of an electric circuit: the driving force, the rate of flow, and the opposition to that flow. This law governs how electric charge moves through a conductor. The formula uses symbols to represent these factors, and the letter ‘E’ represents the electrical pressure or force that initiates the movement of charge.
The Core Concept of ‘E’
The letter ‘E’ in the Ohm’s Law formula traditionally denotes Electromotive Force (EMF). This term refers to the energy-per-unit-charge that a source, such as a battery or a generator, provides to a circuit. Although “force” is in the name, EMF is a measure of energy transfer, not a physical force like gravity.
Electromotive Force converts non-electrical energy (like chemical or mechanical energy) into electrical energy, creating a potential for charge movement. This potential is commonly known as Voltage or Electric Potential Difference. A helpful analogy is a water pump in a plumbing system, which represents the source of EMF creating pressure.
This electrical pressure is analogous to the potential energy that charges possess at a high-potential point, like the positive terminal of a battery. Just as water at the top of a hill has gravitational potential energy, electrical charge has electric potential energy. The potential difference, or voltage, measures the difference in energy between two points in the circuit.
The electrical pressure created by the EMF source makes the flow of charge possible. When a charge flows from a point of higher potential to a point of lower potential, the difference in electrical energy is converted into another form, such as light, heat, or mechanical work.
E’s Relationship to Current and Resistance
The relationship between the driving force ‘E’ and the two other circuit characteristics is defined by Ohm’s Law, written as E = I x R. In this formula, ‘I’ represents the Current (the flow rate of electric charge), and ‘R’ represents the Resistance (the material’s opposition to that flow).
The formula establishes that electrical pressure ‘E’ must be sufficient to overcome the circuit’s opposition, ‘R’, to produce a flow of charge, ‘I’. If resistance remains unchanged, a direct and linear relationship exists between the electrical pressure and the flow rate. Doubling the electromotive force will result in a doubling of the current flowing through the circuit.
The source must continuously supply the potential difference to push the charge carriers past the inherent resistance of the conductor. The material properties of the wire or component determine the resistance. This constant action allows the current to be a steady flow in an ohmic circuit.
Quantifying ‘E’
The standard unit of measurement for Electromotive Force and Voltage is the Volt (V), named after the Italian physicist Alessandro Volta, who invented the first chemical battery. The Volt is defined physically in terms of the fundamental units of energy and charge.
One Volt is equal to one Joule of energy per one Coulomb of electric charge. This definition demonstrates that the potential difference is a measure of the work required to move a certain amount of charge between two points. Therefore, a 9-Volt battery imparts nine Joules of energy to every Coulomb of charge that flows through it.
In practice, electrical pressure is measured using a Voltmeter or a Multimeter. To accurately measure the potential difference, the instrument must be connected in parallel across the two points being analyzed. This parallel connection allows the meter to measure the difference in electrical potential energy without disrupting the flow of current.