There is a simple relationship between current,
voltage and resistance. This relationship is called
Ohmís Law. The formula is the following.
Difference in Voltage =
Current * Resistance
or DV = I * R
Form 1 of Ohm's Law.
To find current and
resistance the following forms can be used. They are
the same as the above formula but in a different
Current = Difference in Voltage / Resistance
or I = DV / R
Resistance = Difference in Voltage / Current
or R = DV / I
These formulas are always
used for situations where there are two points with
a resistor between them. DV is the difference in
voltage between the two points and current is what
flows between the two points. These simple
relationships allow us to calculate many things.
Given any two of the three values (Current,
Resistance, and Difference in Voltage) the third can
be found. The most common calculation is for
current. Voltage is easy to measure and the
resistance can be found from the resistor. Once these values are known,
current can be calculated using Form 2 of Ohmís law,
I = DV / R. For example, consider the problem shown
in Figure 1. One side is at 0 volts (ground) and the
other side is at 5 volts (with a multimeter, black
probe on right side, red probe on left side).
The voltage difference
between Point A and Point B is 5 - 0 = 5 volts (DV=5).
There is a resistor between the two points which has
a value of 500 ohms (R=500). We know that current
flows from a point of high voltage to a point of low
voltage so we can draw an arrow from the higher
voltage to the lower voltage.
Now we can find the current
flowing through the resistor using Form 2 of Ohm's
I = DV / R
DV / R = 5 / 500
5 / 500 = 0.01 Amps
0.01 Amps = 10 milliAmps
10 milliamps can be
abbreviated as 10 mA
This means the current is
10 mA. ( I = 10mA )
Now to check our answer we
can use Form 1 and Form 3 of Ohmís law. We have to
use the value of current in Amps for these formulas.
So if we have I = 0.01 Amps and Resistance = 500
ohms then by using Form 1 of Ohmís law we can find:
Difference in Voltage = DV
DV = I * R
I * R = 0.01 * 500
0.01 * 500 = 5 volts
This is the voltage we
started with so the value we found for the current
must be right.
We can also check the
answer with Form 3 by using I = 0.01 Amps and DV = 5
Resistance = R
R = DV / I
DV / I = 5 / 0.01
5 / 0.01 = 500 ohms.
So R = 500 ohms
Now consider the problem
shown in Figure 3. The voltage on one side is 10
volts and the voltage on the other side is 3 volts.
Therefore the voltage difference between the two
points is 10 - 3 = 7 volts (DV = 7 V). The resistor
is 400 ohms (R = 400).
Then the current flowing
from left to right is
I = DV / R
DV / R = 7 / 400
7 / 400 = 0.0175 Amps
0.0175 Amps = 17.5
17.5 milliAmps = 17.5 mA
This means the current
flowing from the left to the right is 17.5 mA.
Now suppose we have two
points with a voltage difference of 5 volts. Point A
is at 5 volts and Point B is at 0 volts (ground).
(Notice that the voltage difference is the important
part. If Point A is at 7 volts and Point B is at 2
volts then the voltage difference is the same, 7 - 2
= 5 volts.) Now suppose we want a current to flow
between Points A and B and we want the current to be
0.02 Amps ( I = 0.02 Amps = 20 mA).
Now we need to find the
value of the resistor so we use Form 3 of Ohmís Law.
Resistance = Difference in
Voltage / Current or R = DV / I
DV / I = 5 / 0.02 = 250
This means that putting a
resistor with a value of 250 ohms between Points A
and B will make a current flow from Point A to Point
B and the current will be 0.02 Amps (20 mA). Now
using the values of voltage and resistance, check
the value of the current using Form 2 of Ohmís law.