Learning Resources
Lesson
Electric current, and the force that creates a current are critical to electronic circuits. They provide the energy that causes the circuit to work. As you've already seen, current is the flow of electrons through a circuit. But what conditions are needed before electrons will flow as an electrical current?
The conditions are
- a difference in electrical charge between two points in the path.
- a complete path, or closed circuit
Lets look first at the difference in electrical charge by using the example of an electrical cell. A cell provides a chemical reaction between two dissimilar materials called terminals (for example, carbon and zinc), in some form of medium called an electrolyte (for example, an acidic paste) [think of a two solid objects suspended in jelly]. The chemical reaction causes a build-up of electrons on one terminal, and a deficit of electrons on the other. Remember that protons can't move. The difference in the charge from one terminal to the other is described as electromotive force (think electron motion), often abbreviated to EMF.
Batteries are made up of two or more cells joined together. It turns out that each cell can produce a fixed level of electromotive force. Bigger cells can produce the force for a longer period of time than smaller cells.
Figure Electromotive Force Produced by Chemical Reaction in a Cell
The electrical symbol (or schematic symbol) for a cell and a battery are shown in the next figure
Figure Schematic Symbols for Cell and Battery
Electric Current
We are quite familiar with commercial dry cells, although many people mistakenly call them batteries (remember a battery has 2 or more cells). AAA, AA, B, C, and D cells are different physical sizes. D cells, being the largest, have more capacity than AAA cells, being the smallest. More capacity means they can cause a larger number of electrons to flow in a given time, or the same number for a longer time.
Remember the coulomb. It is a measure of capacity of electrical charge. Electrical current is a flow of electrons over time. To create a current with a cell, it needs to be connected in a complete circuit. The two terminals need to be connected externally. If you just connect them with a wire, the wire and/or the cell will likely overheat as the current flow will be at its maximum. Usually a load (for example, a light) is in the circuit preventing this from happening.
Figure Electron Flow Caused by Electromotive Force in a Cell
Using the coulomb (6.25 X 1018 electrons) as a quantity, and counting the electrons over time, we have a way of measuring electric current. Current is measured in Amperes (abbreviated to amps).
1 amp = 1 coulomb of electrons moving past a point in 1 second
or
1 amp = 6.25 X 1018 electrons per second
An amp is a very large electrical current. AAA cells, for example can only produce electrical current measured in thousandths of an amp.
DC and AC
DC, or direct current, is the flow of electrons in one direction only. All battery current is DC. AC, or alternating current, is the flow of electrons in one direction, and then in the other direction, switching back and forth at regular intervals. The electricity in your house wires is AC. It changes direction 120 times per second. A flow in one direction, and then in the other direction, is called a cycle. Household current is 60 cycle (120 / 2).
All portable and battery operated devices use DC. All electronic devices use DC, even those that plug into a wall outlet. All plug-in electronic devices will have a device to convert AC into DC. The 'power brick' that comes with some electronics equipment is used to convert from 120 volts AC to 12, 9, or even 6 volts DC.
For More Information
For more information on how batteries work, check this site
Activity
Assigned activities
Investigate devices that rely on cells and batteries for electrical power. What alternatives exist for powering these devices?
Test Yourself
There is no self test for this lesson.