Learning Resources
Lesson
Electromotive force is the potential difference in electrical charge between two points in a circuit, for example between the two terminals of a battery. EMF is measured in units called Volts. Voltage is a measure of the electromotive force that causes electrons to flow. A single cell produces approximately 1.5 volts. Cells in the AAA to D size all produce approximately 1.5 volts. A 9 volt battery contains 6 cells (6 times 1.5 volts).
It turns out that there is a relationship between electromotive force (volts), electrical current (amps), and electrical resistance (ohms). That relationship is known as Ohm's Law. Ohm's Law essentially says that the more electromotive force you apply to a resistance, the more current will flow through it.
It turns out that one volt is enough electromotive force to cause one amp of electrical current to move through 1 ohm of resistance in one second. Double the voltage and the amps double. This is illustrated below.
Figure Relationship of Voltage, Current and Resistance
Figure Effect of Doubling the Voltage
Ohm's Law is normally expressed by the equation
E = I X R (or E = IR)
where E stands for electromotive force (measured in Volts),
I stands for electrical current (measured in Amps), and
R stands for resistance (measured in Ohms).It is also frequently written as
V = IR (voltage equals current times resistance)
If you know any two values, you can easily find the other one by solving the equation.
This is one of the most fundamental relationships in electricity and it is important to understand it in order to do any work in any field of electricity or electronics. All circuits, even those for simple flashlights, use this relationship, and designing an electrical or electronic device requires good understanding of this relationship.
At home, you can use this information to help you determine if you are plugging too many appliances into a circuit. People tend to make that decision intuitively, occasionally misjudging the electrical current requirements, and blowing a fuse or popping a circuit breaker.
For Additional Information
To see additional information on Ohm's Law, check these sites
- Ohm's Law Explained
- Ohm's Equations and Ohm's Equation Calculators
- Interactive Java Tutorial on Ohm's Law
Activity
Assigned activities
Using Ohm's Law equation, calculate the missing value in each
- E = 12 volts, I = .05 amps
- E = 3 volts, R = 5000 ohm's
- I = .002 amps, E = 120 volts
- I = 15 amps. R = 3000 ohms
- R = 1 ohm, E = 60 volts
- R = .01 ohm, I = .002 amps
Test Yourself
There is no self test for this lesson.