OHM’S LAW and ELECTRICITY
Georg Ohm was a German physics. He was known for inventing the
electrochemical cell (a battery) in the 1800’s. Ohm was able to
define the relationship among voltage, current, and resistance – the
true beginning of electrical circuit analysis. This is now known as
Ohm’s Law.
VOLTS=Voltage (V or E), AMPS=Current (I or A), OHMS=Resistance (R or Ω), WATTS=Power (W)
(“I” may have been used for “intensity”)
Quantity
Symbol
Unit of
Measurement
Unit
Abbreviation
Formula
Current
Voltage or
Potential Difference
Resistance
Watt
Power
Energy
I
Ampere (Amp)
A
I = V/R
V
Volt
V
V = IR
R
W
P or VI
E
Ohm
Watt
Watt
kiloWatt hour
Ω
R = V/I
W
W
kWh
P = VI
E=PxT
V = Voltage
Calculating Power & Energy
I = Current
Power = Voltage x Current:
P = VI
R = Resistance
V = IR
!
V
R
!
V
R=
I
I=
V
I R
!
Examples on next page.....
Energy = Power x Time Used:
E = VI x T
OR
E = PT
Current = Watts / Volts:
C = W/V
Examples
-Voltage or Potential Difference:
What is the voltage if the resistance is 4 ohms and the current is 4 amps?
Formula: (You are looking for voltage.) V = I x R
Work: V = ?
I = (Current/Amps) 4 amps
R = (Resistance/Ohms) 4 ohms
V=IxR
V = 4A x 4Ω = 16
Answer with units: V = 16 Volts
-Current or Ampere:
Calculate the current flowing through a 30 ohm resistor from a 12 volt battery.
Formula: (You are looking for current.) I = V/R
Work: I = ?
V = 12 v
R = 30 ohm resistor
I = V/R
I = 12v
÷ 30 Ω = .4A
Answer with units: I = .4A
-Resistance or Ohms:
! of the heating element of the heating element of a toaster over that carries a current
What is the resistance
of 5 amperes on a 110 volt line?
Formula: (You are looking for resistance.)
R = V/I
Work: R = ?
V = 110v
I = 5A
R = V/I
R = 110v
÷ 5A = 22Ω
Answer with units: R = 22Ω
-Total Resistance/Current:
The total resistance in!a series circuit is equal to the sum of the separate resistors. Two wires are connected
in a series circuit with a 12 volt battery. The resistance of one wire is 2 ohms and the other is 6 ohms.
What is the current flowing through the circuit?
Formulas: (You are looking for current – but you have 2 currents/wires.)
Ω + Ω = Total Resistance (Add together the two resistances.)
I = V/ Total Resistance
Work: 2Ω + 6Ω = 8Ω (Total Resistance)
I = V/R
I = 12v ÷ 8Ω = 1.5A
Answer with units: I = 1.5A
!
-Power Rating:
The voltage of an electric circuit is the standard 120V. A refrigerator has a 4.0A current through it. What is
the power rating of the refrigerator?
Formula: (You are looking for power rating or watts.)
P=VxI
Work: P = ?
V = 120v
I = 4.0A
P = 120v x 4.0A = 480W
Answer with units:
480W
-Current:
A computer has a power rating of 156w and uses the standard voltage of 120v. What is the current through
the computer?
Formula: (You are looking for current.) C = W
Work: C = ?
W = 156w
V = 120v
C = 156w
÷V
!
÷ 120v = 1.3 A
Answer with units: C = 1.3A
-Electrical Energy
!bedroom, you have a lamp that holds a 75W light bulb. During a week, you keep the lamp turned on
In your
for a total of 20 hours. What is the total amount of energy used by that light bulb?
Formulas:
E=PxT
kWh = 1,000
÷W
kWh = kilowatt hours
Work: E = ?
P = (Power or Watts) 75w
T = (Time) 20 hours
!
E = 75 x 20h = 1500 watt hours
NOW – change watts to kilowatts: kWh = total watt hours
kWh = 1500 Wh
Answer with units:
!
÷ 1,000 = 1.5kWh
1.5 kWh
!
÷ 1,000