Equation for Current

Current is the amount of charge passing a point per unit time.

For example, if 1 A of current passes a point on a circuit, it means that 1 C of charge (electrons) passes that point every 1 s. If 2 A of current passes a point on a circuit, it means that 2 C of charge (electrons) passes that point every 1 s.

So if 2 A of current passes a point on a circuit for 3 s, then 2 x 3 = 6 C of charge has passed that point. That is, current x time = charge.

flow of charge

It is convenient to write this an equation. Lets represent current by I, time by t and charge by Q. Then charge = current x time can be written as

Q = It.

If we divide both sides by t, we get

Q/t = I, or I = Q/t.

This can be used to calculate the current if we know the charge passing a point and time taken.

Question 1. Suppose a current of 0.03 A passes the light bulb in a circuit for 2 s. How much charge has passed through the bulb?

Answer. Using Q = It, we find Q = 0.03 x 2 = 0.06 C.

Question 2. In 2 s, 0.04 C of charge passes the battery in a circuit. What is the current in the circuit?

Answer. Use I = Q/t. So I = 0.04 / 2 = 0.02 A.

Question 3. A current of 0.03 A passes the light bulb in a circuit for 2 s. How many electrons have passed through the bulb? The charge of one electron is 1.6 x 10^-19 C.

Answer. Using Q = It, we find Q = 0.03 x 2 = 0.06 C. So the number of electrons is 0.06 / (1.6 x 10^-19) = 3.75 x 10¹⁷.