Calculating current using Ohm's law

What you should be able to do

Calculate current through a resistive DC load from the voltage across it and its resistance. Label the result in amperes and convert it to milliamperes.

Core Method

For an ohmic load, V = I × R. Rearrange this as I = V ÷ R. Use voltage across the load, not an unrelated voltage elsewhere. Use that load's resistance. Volts divided by ohms gives amperes. Identify the unknown, write the equation, substitute values with units, calculate, then check whether the answer makes sense.

Worked through

A classroom worksheet specifies an ideal 12 V DC source connected across an ideal 240 Ω resistor. Neglect wire resistance for this calculation. Known: V = 12 V; R = 240 Ω. Unknown: I. Equation: I = V ÷ R. Substitute: I = 12 V ÷ 240 Ω. Calculate: I = 0.05 A. Convert: 0.05 A × 1,000 mA/A = 50 mA. The two current values are the same quantity written in different units. Do not report 0.05 mA; that would be 1,000 times too small.

Check Your Result

Multiply calculated current by resistance: 0.05 A × 240 Ω = 12 V. That returns the stated load voltage. Now keep the same 240 Ω resistance but change the worksheet voltage to 24 V: I = 24 ÷ 240 = 0.10 A = 100 mA. With resistance fixed in this model, doubling the voltage doubles current. The example does not authorize applying a higher voltage to real equipment.

Practice With Answers

1. A 24 V ideal source is across a 480 Ω resistor. Calculate current.

Answer: 24 ÷ 480 = 0.05 A = 50 mA.

1. A 12 V ideal source is across a 1.2 kΩ resistor. First convert 1.2 kΩ to 1,200 Ω.

Answer: 12 ÷ 1,200 = 0.01 A = 10 mA.

1. A learner writes 240 ÷ 12 = 20 A for the main example. What went wrong?

Answer: They reversed the division. Current is voltage divided by resistance.

1. A learner writes “50” in the result box. What is missing?

Answer: The current unit; this result is 50 mA, not 50 A.

Apply The Correct Model

These are steady-state resistor calculations. A powered camera, reader, sounder or controller may not behave like a fixed resistor. Use its documented operating and startup current when planning system demand. Also distinguish voltage at a supply from voltage actually across a distant load; cable voltage drop is addressed later. The resistor's power rating and operating temperature are separate checks. A resistance value alone does not establish that a component can dissipate the resulting power. Lesson 024 addresses component loading. This is a calculation lesson, not instructions for inserting a meter into an energized circuit.

Where beginners go wrong

Voice Recap

To calculate current in our resistive DC example, divide volts by ohms. Twelve volts divided by 240 ohms is 0.05 amperes, or 50 milliamperes. Check by multiplying current by resistance. Use the right model, the right voltage, and the right units.

Sources