ASVAB · Electronics Information · 6 min read
Ohm's Law and Electric Power
- Define voltage, current, and resistance, and solve circuit problems using Ohm's Law.
- Calculate electric power and analyze how changes in voltage or current affect power dissipation.
- Determine electrical energy consumption in kilowatt-hours and apply proper unit conversions without a calculator.
The Core Quantities: Voltage, Current, and Resistance
Every electrical circuit relies on three foundational quantities: voltage, current, and resistance. Voltage is the electrical potential difference that drives electric charge through a circuit. It acts as the electrical pressure pushing charges along a conductive pathway. In circuit diagrams and equations, voltage is represented by the letter V or E and is measured in volts (V).
Current is the rate of flow of electric charge through a conductor. When voltage pushes charges, current measures how much charge passes through a point over time. Current is represented by the letter I and is measured in amperes or amps (A). In a simple resistive circuit, no voltage across a resistor means no current through it.
Resistance is the opposition to the flow of electric current. Ordinary materials oppose electric current, although superconductors can have zero electrical resistance under special conditions. Resistance is represented by the letter R and is measured in ohms (Ω). For an ohmic resistor, the voltage across it and its resistance determine the current through it: I = V / R.
| Quantity | Symbol | Unit of Measurement | Unit Symbol |
|---|---|---|---|
| Voltage | V or E | volt | V |
| Current | I | ampere (amp) | A |
| Resistance | R | ohm | Ω |
Common mistakes
- Multiplying current by resistance before squaring when applying the power formula P = I² × R.
- Forgetting to square the voltage before dividing by resistance in the formula P = V² / R.
- Failing to convert milliamperes to amperes (dividing by 1,000) before calculating voltage or resistance with Ohm's Law.
- Multiplying power in watts directly by minutes instead of converting power to kilowatts and time to hours when calculating kilowatt-hours.
Key points
- Voltage (V or E, in volts) drives electric charge, current (I, in amps) is the rate of charge flow, and resistance (R, in ohms) opposes that flow.
- For an ohmic resistor, use V = I × R, I = V / R, or R = V / I to find an unknown quantity when the other two are known.
- For an ohmic resistor, doubling the voltage doubles the current if resistance stays constant; doubling the resistance halves the current if voltage stays constant.
- Electric power is measured in watts (1 W = 1 J/s) and is calculated using P = V × I, P = I² × R, or P = V² / R; doubling current or voltage across a fixed resistor quadruples the power.
- Electrical energy consumption is measured commercially in kilowatt-hours (kWh = kW × hours), where 1 kW equals 1,000 W.
- An open circuit has a broken path with ideally infinite resistance, while a short circuit provides a very low-resistance path that can allow dangerously high current.
- Use consistent units in Ohm’s Law: for example, volts divided by ohms gives amperes, while volts divided by kilohms gives milliamperes.
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Sample lesson: Electricity Basics: Current, Voltage and Resistance25 practice questions on this exam
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