Measuring Electricity: Volts, Amps, Ohms, and Watts

Learning Objectives

Define the four key quantities used to measure electricity: voltage, current, resistance, and power.

Identify the units for each quantity (volts, amps, ohms, watts).

Explain the 'water hose' analogy for understanding voltage, current, and resistance.

Use the formula P = V × I to calculate electrical power.

The Language of Electricity

To work with electricity and understand how circuits function, we need a way to measure and describe it. There are four fundamental quantities we use: voltage, current, resistance, and power.

The Water Hose Analogy

A simple and useful way to visualize the first three quantities is to think about water flowing through a hose.

Voltage (V): Voltage is the pressure that pushes the water through the hose. It's the difference in potential between the start and end of the hose. In a circuit, voltage is provided by the power source (like a battery). The unit of voltage is the volt (V).

Current (I): Current is the flow rate of the water. It's the amount of water that passes a certain point per second. In a circuit, current is the flow of electrons. The unit of current is the ampere (A), often called an 'amp'.

Resistance (R): Resistance is anything that restricts the flow of water. It could be a kink in the hose or a narrow nozzle at the end. In a circuit, resistance is a measure of how much a material opposes the flow of electrons. The unit of resistance is the ohm (Ω).

These three are related by Ohm's Law: V = I × R. This means that for a given voltage, if you increase the resistance, the current will decrease.

Power: The Rate of Energy Use

The fourth key quantity is power. In electricity, power is the rate at which electrical energy is converted into another form, like light, heat, or motion. It tells you how 'powerful' a device is, or how fast it uses energy.

The unit of electrical power is the watt (W). A kilowatt (kW) is 1,000 watts.

Calculating Electrical Power

Electrical power is simply the product of the voltage and the current.

Power = Voltage × Current

P = V × I

Example 1: A light bulb is connected to a 120-volt outlet and draws a current of 0.5 amps. What is its power rating?

Formula: P = V × I

Substitute: P = 120 V × 0.5 A

Calculate: P = 60 W

The light bulb has a power rating of 60 watts.

Electrical Energy vs. Electrical Power

It's important not to confuse power and energy.

Power is the rate of energy use (measured in watts).

Energy is the total amount of power used over a period of time.

Your electricity bill charges you for the total energy you use, not the power. The unit used is the kilowatt-hour (kWh). One kWh is the energy used by a 1,000-watt device running for one hour.

Energy = Power × Time

Running a 100-watt light bulb for 10 hours uses `100 W × 10 h = 1000 Wh = 1 kWh` of energy.

Key Terms

**Voltage (V): The potential difference or 'pressure' in an electric circuit that causes current to flow. Measured in volts.
**Current (I): The rate of flow of electric charge (electrons). Measured in amperes (amps).
**Resistance (R): The opposition to the flow of electric current. Measured in ohms (Ω).
**Power (P): The rate at which electrical energy is transferred or used by an electrical circuit. Measured in watts (W).
**Ohm's Law: A fundamental law of electricity stating that the current through a conductor between two points is directly proportional to the voltage across the two points (V = IR).
**Kilowatt-hour (kWh): A unit of electrical energy equal to the energy consumed by a 1,000-watt device operating for one hour.