PHYS 2212 Module 5.1

Electrical Current

Learning Objectives

By the end of this section, you will be able to:

• Describe an electrical current
• Define the unit of electrical current
• Explain the direction of current flow

Electric Current

Electric current is defined to be the rate at which charge flows past a point. Imagine looking at one spot on the copper wire in the picture from the video. Now count how many electrons move past that point in one minute. You can take that amount of charge that flowed past (dq) and divide by the time that you made the measurement (dt), and that is the current in the wire.

The units for current are derived from C/s (look at the formula to see where this comes from) and are called Amperes, with symbol: A. This is named after the French physicist André-Marie Ampère. But we usually just say “amps” because we are lazy.

In this example with our parallel plates, we know that the electrons are the particles that move because the protons in the atoms are too tightly bound to the atoms to be removed. We know this because of scientists like J.J. Thompson and Ernest Rutherford, who discovered the structure of atoms in the late 1800s / early 1900s. But when Ben Franklin was researching electricity, he did not even know that atoms existed so he didn’t know that electrons are the particles that move through a conductor. As a result, he defined Conventional Current to be the rate of flow of positive charge.

In this picture, the electrons flow to the left, but with Ben Franklin’s definition we would say that it is equivalent to positive charge moving to the right. Physically it’s not correct but it still works and we still use Ben Franklin’s definition of conventional current today. In summary: the flow of negative charge to the left is equivalent to positive charge moving to the right.