Ohm's Law

During the early 1800s the German scientist George S. Ohm did considerable experimentation with electrical circuits and particularly with regard to resistances in these circuits. He determined that a relationship exists among the factors in an electrical circuit. This relationship is called Ohm’s law. The following describes this relationship. Letters are used to represent the different electrical factors.

E or V =Voltage (emf), I = Amperage (current), R = Resistance (load)

This relationship may be expressed as follows: Current is directly proportional to voltage and inversely proportional to resistance. This can be simplified to:

1. If V is raised, I will go up.
2. If V is lowered, I will go down.
3. If R is raised, I will go down.
4. If R is lowered, I will go up.

The mathematical formula for this relationship is:

Current in amperes (I) = voltage in volts (V) ÷ resistance in ohms (R).

This equation can be transposed to find any missing value as long as the other two are known.

I = V ÷ R, V = I × R, and R = V ÷ I

Ohm's Law triangle is often used as an aid in remembering these formulas. When the component to be solved for is known, simply cover that letter and the remaining portion of the equation will indicate the proper equation to use. It should be remembered that Ohm's Law does not apply to alternating circuits because any coils in the circuit will produce some effects that will upset the law, but the general ideas presented do apply to alternating circuits.

Example 1: What is the current flowing in the circuit in Figure?

1. Determine which formula to use. I = V ÷ R.
2. Fill in the known components: V = 220, R = 5 Ω.
3. Solve the formula. I = 44 A.

Example 2: What is the resistance of a resistor if the voltage is 220 volts and the current is 2 amperes See in Figure?

1. Determine which formula to use. R = V ÷ I.
2. Fill in the known components. V = 220, I = 2 A.
3. Solve the formula. R = 110 Ω.

Example 3: What is the amount of voltage supplied to the circuit in Figure?

1. Determine which formula to use. V = IR.
2. Fill in the known components. I = 5A, R = 50 Ω.
3. Solve for the formula. V = 250 V.

Electrical Power

Electrical power (P) is measured in watts. A watt (W) is the power used when 1 ampere flows with a potential difference of 1 volt. Therefore, power can be determined by multiplying the voltage times the amperes flowing in a circuit.

Watts = Volts x Amperes or P = V x I

The consumer of electrical power pays the electrical utility company according to the number of kilowatts (kW) used for a certain time span usually billed as kilowatt hours (kWh). A kilowatt is equal to 1000 W. To determine the power being consumed, divide the number of watts by 1000:

P (in kW) = E x I/1000

Example 4: Determine the power used by an electric circuit that is supplied with 220 volts and is using 20 amperes See Figure.

1. Determine which formula to use. P = IV.
2. Fill in the known components. P = 20 × 220.
3. Solve for the formula. P = 240 W.

Example 5: Determine the current draw of an electric heater that is rated for 1500 watts and is to be used on a 240 volt system See Figure.

1. Determine which formula to use. I = P ÷ E.
2. Fill in the known components. I = 1500 ÷ 240.
3. Solve for the formula. I = 6.25 A.

Example 6: Determine the power used in a circuit using 7 amperes current and having 15 ohms resistance See Figure.

1. Determine which formula to use. P = IR.
2. Fill in the known components. P = 7 × 15.
3. Solve for the formula. P = 735 W.

NOTE:

Ohmmeters are to be used only on circuits that are re energized. Also, be sure that the component being evaluated is disconnected from the circuit to avoid inaccurate or false readings.

SAFETY PRECAUTION:

• Do not use any electrical measuring instruments without specific instructions from a qualified person. 
• Follow the manufacturer’s instructions.