Ohm's law

From Wikiversity
Jump to: navigation, search
Books-aj.svg aj ashton 01b.svg Educational level: this is a secondary education resource.
Sciences humaines.svg Educational level: this is a tertiary (university) resource.
V, I, and R, the parameters of Ohm's law.

Purpose[edit]

Understand Ohm's law in the context of electronics, and the meaning of Voltage, Current, and Resistance (Resistors) in the law.

The Ohm's law[edit]

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points.

I \propto V then:V → ↑I and ↓V → ↓I

In the electrical circuit,there are three factors : current,denoted by I; potential difference, denoted by V; and resistance,denoted by R.

I = \frac V R \;\;\;\;\;\;\; or \;\;\;\;\;\;\;V = I \cdot R

Based on the work of Georg Simon Ohm, Ohm's law is one of three fundamental laws which begin the study of electronics, in partnership with Kirchhoff's Voltage Law and Kirchhoff's Current Law, laws. These three laws form the frame on which the rest of electronics is constructed. It's important to note that these laws don't apply everywhere, but definitely apply with great precision in wires, which are used to connect most electronic parts together in a circuit. Though individual parts may or may not be analysed by Ohm's law, their relationship to the circuit can be. Any student completing a course in electronics should be capable of quoting Ohm's law in his or her sleep. Not because they learn it once, but because it's used repeatedly in conjunction with almost every other task in electronics.

The actual statement of Ohm's Law is: The current flowing through a metallic conductor is proportional to the Electromotive force applied across its ends, provided the temperature and all other conditions remain constant.

Note there is no mention of resistance; that is simply the name given to the constant of proportionality involved.

Something important to take away from this definition is

  • In a passive circuit, the current is a result of the voltage applied

and

  • there are definite thermal effects on the resistance (or effective resistance) of conductors.

Ohm's law is linear and therefore assumes linearity in the electronic part. It's easy to think of the law in terms of the line equation  y = mx considering resistance to be the constant m, current as the independent variable x, and voltage as the dependent variable y. In this way a proportionate relationship between voltage and current is established. Of course, Ohm's law may be rearranged in three valid and easily derivable ways. It's important to be familiar with each of these and comfortable in recognising and switching between them.

I = \frac V R \;\;\;\;\;\;\; or \;\;\;\;\;\;\;V = I \cdot R \;\;\;\;\;\;\ or \;\;\;\;\;\;\; R = \frac V I

Example:

If you have one amp (1A) of current flowing through a 2 ohm resistor, how much voltage will be across it?


External Links[edit]

  • Ohm Law: Electronics for Beginners