How to Calculate Coulombs

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The electrical charge that runs through anything, from an AA battery to a lightning bolt, is measured in coulombs. If you know the current flow in a circuit and how long it flows for, you can calculate the electrical charge in coulombs.

Properties of Coulombs

Electrons are tiny and have a very small charge. In physics, a very large number of electrons is defined as 1 unit of charge called a coulomb. One coulomb is the equivalent of 62 × 1018 electrons. The number of coulombs per second is called the current (i.e., the rate of coulombs flowing in the circuit). The energy of a coulomb is called the voltage and is measured in joules.

How to Calculate Electrical Charge

To determine the amount of electrical charge that flows in a circuit, you need to know the current flow and how long it flows for. The equation is:

charge (coulomb, C) = current (ampere, A) × time (second, s).

For example, if a a current of 20 A flows for 40 s, the calculation is 20 × 40. So the electrical charge is 800 C.

How to Calculate Energy Transferred

If you know the amount of electrical charge in coulombs and the voltage (also known as potential difference), you can work out how much energy is transferred. The equation is:

energy transformed (joule, J) = potential difference (volt, V) × charge (coulomb, C).

For example, if the potential difference is 100 V and the charge is 3 C, the calculation is 100 × 3. So 300 J of energy is transferred.

Using Coulomb's Law

The product of the electrical charges in two bodies (i.e., whether they attract or repel each other) depends on each body's charge in coulombs, as well as the distance between the bodies. If the polarities are the same (both positive or both negative), the coulomb force repels, but if the polarities are opposite (negative/positive or positive/negative) the coulomb force attracts. The electrical charge is also inversely proportional to the square of the separation distance between the two bodies. This is known as Coulomb's law, which is stated as:

F = kq1q2 ÷ r2.

In this equation, F is the force applied to charges (q1) and (q2), k is Coulomb's constant and (r) is the distance between (q1) and (q2). The value of k depends on the medium that the charged objects are immersed in. For example, the value of air is approximately 9.0 × 109 Nm2 ÷ C2. Coulomb's law can be used for many physics problems where you know all of the values but one.


About the Author

Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.