OHM’S LAW
The rate of the flow of the current is equal to
electromotive force divided by resistance.
I = Intensity of Current = Amperes
E = Electromotive Force = Volts
R = Resistance = Ohms
P = Power = Watts
The three basic Ohm’s law formulas are:
E
I = –––
R
E
R = –––
I
E=IxR
Below is a chart containing the formulas related to Ohm’s law.
To use the chart, from the center circle, select the value you need to
find, I (Amps), R (Ohms), E (Volts) or P (Watts). Then select the
formula containing the values you know from the corresponding chart
quadrant.
PxR
E
I
M
H
PS
P
E
R
TS
AT
W
P
I
AM
PR
P
I
S
LT
VO E
ExI
RxI
O
2
RxI
S
2
E
R
2
E
R
P
2
I
E
P
Example:
An electric appliance is rated at 1200 Watts, and is connected to 120
Volts. How much current will it draw?
Watts
Amperes = –––––
Volts
P
I = ––
E
1200
I = –––– = 10 A
120
What is the Resistance of the same appliance?
Volts
Ohms = –––––––
Amperes
E
R = ––
I
-1-
120
R = –––– = 12 Ω
10
OHM’S LAW
OHM’S LAW
In the preceding example, we know the following values:
In the preceding example, we know the following values:
I = amps = 10
R = ohms = 12Ω
I = amps = 10
R = ohms = 12Ω
E = volts = 120
P = watts = 1200
E = volts = 120
P = watts = 1200
We can now see how the twelve formulas in the Ohm’s Law chart
We can now see how the twelve formulas in the Ohm’s Law chart
can be applied.
can be applied.
WATTS
AMPS = –––––––
OHMS
I =
P
1200
––– = ––––– =
R
12
WATTS
AMPS = –––––––
VOLTS
I =
P
1200
––– = ––––– = 10A
E
120
VOLTS
AMPS = –––––––
OHMS
I =
E
120
––– = ––––
R
12
VOLTS
WATTS = ––––––
OHMS
P =
E
120
14,400
––– = –––– = –––––– = 1200W
R
12
12
WATTS = VOLTS x AMPS
P = E x I = 120 x 10 = 1200W
2
2
2
100 = 10A
= 10A
2
2
WATTS = AMPS x OHMS
P = I x R = 100 x 12 = 1200W
VOLTS =
WATTS x OHMS
E =
VOLTS =
AMPS x OHMS
E = I x R = 10 x 12 = 120V
WATTS
VOLTS = ––––––
AMPS
2
PxR =
1200 x 12 = 14,400 = 120V
P
1200
E = –– = –––– = 120V
I
10
2
2
VOLTS
OHMS = –––––––
WATTS
E
120
14,400
R = –– = ––––– = –––––– = 12Ω
P
1,200
1,200
WATTS
OHMS = –––––––
AMPS2
P
1200
R = ––2 = ––––– = 12Ω
I
100
VOLTS
OHMS = –––––––
AMPS
E
120
R = –– = ––––– = 12Ω
I
10
-2-