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BASIC ELECTRICITY:
OHM’S LAW
Electricity, like everything else in existence, is based on the principle of cause and effect. In other
words, results do not happen by themselves – for every result there must be a reason for that result.
We can think of the relationship between cause, effect and opposition to that effect in the following
way:
With electricity, we can think of the effect as current. The cause is voltage, and opposition to the cause
is called resistance.
What are these quantities?
Voltage can be thought of as an electrical force, or pressure. Just as in mechanics, when you apply a
force, something happens. In the study of electricity, the application of a voltage means you will obtain
a result, or achieve and effect.
This effect is called current. Current is the flow of electrical energy due to the movement of electrons.
The electrons move because of the pressure, or voltage, that is applied.
The opposition to the effect is called resistance. Resistance refers to the inability of the electrons in a
material to transfer energy. There are many reasons why one object or material may have more or less
resistance than another, including such things as size, temperature, molecular structure, etc..
What is Ohm’s Law?
Ohm’s Law is simply the cause-and-effect equation shown above as applied to electric circuits. Keeping
in mind the following relationships:
Current = Effect
Voltage = Cause
Resistance = Opposition
BASIC ELECTRICITY:
OHM’S LAW
we can re-write this equation as:
This equation is Ohm’s Law.
Typically, the symbol I is used to represent current, E represents voltage and R refers to resistance.
When we substitute these symbols for the quantities they represent in the equation above, we have the
three familiar forms of Ohm’s Law:
How can we test this law?
We will use the PASCO electrical kits (white boxes) and an ammeter (a device that measures electrical
current). In the PASCO kits you will find several size D dry cell batteries, connecting wires with alligator
clips and several resistors. The resistors are shown in the following diagram:
BASIC ELECTRICITY:
OHM’S LAW
The values of the resistors are 47 ohms (on the left) and 100 ohms (on the right).
1.) To set up the ammeter, plug the black lead into the jack marked COM and the red lead into the
jack marked mA. Set the pointer to 200m (this is about 8 o’clock on the pointer dial).
2.) Take a jumper lead and connect one end to the end of the red lead.
3.) Connect the other end of the jumper lead to one end of the 47 ohm resistor.
4.) While holding the black ammeter lead on the negative (flat) end of the battery, touch the
unconnected side of the resistor to the positive (with the cap) end of the battery.
5.) Record the ammeter reading:
_______________ mA
6.) Repeat steps 3 and 4 for the 100 ohm resistor.
7.) Record the ammeter reading:
_______________ mA
8.) Now use the equation I = E/R to calculate the current through the 47 ohm resistor. Use 1.5 for E
and 47 for R. Record the result:
_______________ mA
9.) Compare this result with the measurement you took in step 5.
10.) Repeat step 8 for the 100 ohm resistor. Use 1.5 for E and 100 for R. Record the result:
_______________ mA
11.) Compare this result with the measurement you took in step 7.