The circuit below will be used for exploring Ohms Law.
 
The current direction is shown as the direction that electrons would flow or move in the circuit. This is referred to as non conventional current. Some engineering text books will use a conventional current direction. That is current is assumed to flow from the positive terminal of the battery to the negative. Later when we discuss Kirchhoff's Laws we will explore the different current conventions.  For this lesson and future
lesson's non conventional current flow will
be used.
OHMS LAW
Ohm's law is simply a statement or rule that describes the relationship among the three elements in the circuit above. Normally it is stated as what happens to the current as the voltage or resistance is varied and the other kept constant.
ΓÇó If the resistance is kept constant and the voltage is increased then the current will increase. If the resistance is kept constant and the voltage is decreased then the current will decrease. Or to state it more precisely we can say that
The current (I) in a circuit is directly
proportional to the applied voltage (V)
ΓÇó If the voltage is kept constant and the resistance is increased then the current will decrease. If the voltage is kept constant and the resistance is decreased then the current will increase. Or to state it more precisely we can say that
The current (I) in a circuit is inversely
proportional to the resistance (R) in the
circuit.
OHMS LAW AS AN EQUATION
Ohm's law may be expressed as in an equation as shown below. You will notice that of the three forms each can be derived from the other by algebraic means.
 
 
UNITS USED FOR OHMS LAW
Current Flow
The unit used for current flow is the Ampere or "Amp" and abbreviated by the letter A or a. Current flow is the measure of how much charge is flowing past a given point in 1 second. The basic unit of charge is the Coulomb. One electron contains a charge of -1.6 x 10-19 coulomb's. So to have a charge of 1 Coulomb we would need 6.25 x 1018 electrons. Thus to have one amp of current flowing past a given point we would need 6.25 x 1018 electrons flowing past that point in one second. With todays integrated circuits the currents encountered are on the order of 1/1000 of an amp or abbreviated as milliamp (ma). It is only when you're are dealing with high power that you will ever use currents expressed as Amps. However in some of the problems I may choose to use values for the voltage and resistance that will give large values of current, just for the simplicity of the math. For instance a circuit with a battery of 10 Volts and a resistance of 10 Ω will have a current of 1 Amp
(10 V/10 Ω).
Voltage
When we are dealing with voltage we are really talking about potential energy. To be more precise, the difference in energy between two points in a circuit is referred to as "Potential Difference".
For instance a 12 Volt battery setting on the shelf is not producing anything useful but it has the potential to do something useful due to the difference in energy between the (+) and (-) terminals. If we connect a light to it, then the potential energy is converted to some useful work. We can make a similar analogy to your car. If it is just setting in neutral nothing useful is being produced. It has the potential to do something but nothing happens until you put it in gear then we get useful work from it. Now exactly what is work from the physical definition? Work is defined as a "Force acting through a distance" or
Work = Force x Distance
If you push a box across the floor you have done some work on the box. If you push on the box but not hard enough to move it although you may be tired, from the strict definition no work was done on the box. A battery can be thought of as the force to push electrons through a circuit that is connected across its terminals. The moving electrons can be controlled to produce useful work, such as turning on a light, playing a radio or running your Mac so that you can read this document.
Lets talk about the units of work first. The unit of force in the English system is the pound (lb) and the unit of distance is the foot (ft). The unit of work in the English system is simply "foot pounds" and abbreviated as "ft-lb". In the metric system the unit of force is the Newton and the unit of distance is the meter. The unit of work is the Newton-meter and the Newton-meter is given another name of the Joule. Thus by definition then we have
Joule = Newton-meter
So we see that the unit Joule is a measure of work and it is related to ft-lbs by the following equalities.
1 Joule = .738 ft-lb
1 ft-lb = 1.36 Joule
With this background we can present the formal definition of the Volt.
A volt is the potential difference
necessary to do 1 Joule of work on 1
Coulomb of charge.
In equation form:
 
Next we will discuss resistance and then put all of these definitions into a usable format.
Resistance
Lets talk about pushing the box across the floor again. If the box is heavy and a large frictional force is developed between it and the floor, then you will have to expend more energy to move the box. A very simple definition of resistance is the opposition to work. Now normally we might think that resistance is not a good thing. However when properly used it allows us to control the rate at which work is done. There are many examples that one can think of in everyday life. For instance on most doors there is a mechanism that will close the door behind you. Energy is stored in a spring when you open the door and when you release the door this spring causes the door to close. By using resistance designed into the mechanism you can control how fast the door closes. This prevents damage to the door and to anybody who happens to be following immediately behind you.
In electronics, we use resistance to control how much current is flowing in a circuit. The unit of resistance is the Ohm. The symbol used is the Greek letter Ω. The symbol may typed on your keyboard by pressing "option z" in most fonts.
We define the Ohm as follows
If we apply 1 volt to a circuit and a
current of 1 amp flows in the circuit then
the circuit has one ohm of resistance.
SUMMARY
In summary then we can express Ohms law with units as follows
