Ohm’s Law: The Relationship Between Voltage, Resistance, Power and Current
By: Jason Hall
Tags: Circuit, Ohms Law, Electrical Engineering, Electricity, Electronics

In the early 19th century, German physicist Georg Ohm discovered that the current through a conductor is directly proportional to the voltage across the conductor. This discovery formed the basis for what came to be known as Ohm’s Law, which describes the relationship between voltage (V), current (I) and resistance (R):

Ohm’s law is easily remembered if you memorize the pyramid below. Simply cover the value you wish to find, and the required calculation is shown. Examples: to find voltage, cover V and the answer is I x R. To find current, cover I and the answer is V/R. To find resistance, cover R and the answer is V/I.

The Ohm’s law formula wheel shows the calculations needed to solve for voltage, current, resistance, as well as power. Power being the rate at which electrical energy is transferred by an electric circuit.

To use the formula wheel, you only need to know two variables to solve for a third. As an example, if you know the current and resistance in a circuit, you can calculate the power produced by squaring the current and multiplying that by the resistance: I² x R.

Electromotive Force

Electromotive force, or EMF, can be considered the pressure that forces electrons to flow in an electric circuit. EMF can be referred to as voltage, electric potential difference, electric pressure, or electric tension. In the International System of Units, EMF is measured in volts (V).

Resistance

The electrical resistance (R) of a material is a measure of its opposition to the flow of current. The SI unit of resistance is the ohm. While all materials resist electrical current, some materials have a very low resistance (conductors), while others have very high resistance (insulators).

Factors that affect the resistance of a material are its resistivity, its width, length and, temperature. Each material has a specific resistivity, which is a measure of how strongly it resists or conducts electric current. A wide conductor has less resistance than a narrow conductor, and a long conductor has more resistance than a shorter one (see the water analogy below). A cold conductor has less resistance than a warm one because of the effect that temperature has on the electrons within the conductor: in a cold environment the electrons collide with each other less than they would at a warmer temperature, allowing them to move from atom to atom, carrying their charge more efficiently.

Power

Electrical power (P) is the rate that electrical energy is transferred by an electric circuit. Another way to think of electric power, is that it is the rate of doing work, measured in watts. One watt is the rate at which work is done when one ampere of current flows through a circuit which has an electrical potential (voltage) of one volt. Power is usually measured in kilowatts per hour, or kilowatt hours (kWh).

Current

Electric current (I) is the rate of flow of electric charge. The SI unit of current is the ampere, or amp. An ampere is defined as the flow of electric charge of one coulomb per second, where the coulomb is the SI unit of electric charge.

Electric Charge

As previously stated, electric current is the rate of flow of charge. Charge is a fundamental quality of electricity that is present in electrons and protons. From our physics classes we know that protons are positively charged while electrons are negatively charged. When these particles move an electric current is produced.