Watt's Law For Beginners: Power, Voltage & Current Simplified

Watt's Law states that the electrical power in an circuit is equal to the voltage times the current in that circuit.

In this article, we will be exploring Watt's Law, explained for beginners. Watt's Law is named after James Watt, in whose honor the unit of power and Watt's Law was named after due to his contributions to the development of the steam engine.

What is Watt's Law?

Watt's Law helps us determine how much electrical energy will be consumed in a circuit. The amount of electricity consumed is known as electrical power, measured in Watts. 1 Watt ($W$) is equal to 1 volt ($V$) times 1 ampere ($A$). Mathematically, electrical power is given by the equation $P=VI$.

What Does Electrical Power Mean?

To better understand what a Watt is, let's look at what power really means. Power is how fast energy is being used, or consumed.

A Watt is simply tacking "electrical" to power: electrical power. A Watt measures specifically electrical power.

Think of it like this: the brighter a light is, the faster it's using electricity, and therefore has more power. A dimmer light would have less power, because it's using less electricity per second than the brighter light.

Why do We Need Watt's Law?

Watt's Law relates these 3 units to each other:

• Voltage
• Current
• Power

You can check out the article linked to each of these units to get a deeper understanding, but for a brief overview,

• Voltage is like the pressure in a pipe.
• Current is similar to the flow rate of water past a point in the pipe (note that it is the RATE of flow, not the SPEED of the water).
• Power is the resulting work done from the water pressure and flow rate.

Watt's Law Formula

$P = VI$

Where

• $P$ is electrical power, measured in Watts
• $V$ is voltage, measured in Volts
• $I$ is current, measured in amperes

This law is closely related to Ohm's Law, which tells us that as voltage increases, current must also increase. Additionally, to learn why power creates heat in circuits, you can read about Joule's Law.

As you will learn on your journey in understanding how circuits work, every physics law, whether about kinematics or electricity, is interconnected.

Practice Problems for Watt's Law

In real life, electrical components are commonly given power ratings. This is the voltage that should be supplied times their current consumption. To calculate, use Watt's Law.

Beginner Level (Calculating Power Consumed)

A light bulb is supplied with $20V$ and it draws $5A$ of current. What is the power rating of this light?

Intermediate Level (Calculating Current)

A device consumes $50W$ of power when connected to a $9V$ power supply. How much current is this device drawing?

Advanced Level (Using Watt's Law and Ohm's Law)

A resistor has a resistance of $10\Omega$ and carries a current of $2A$. What is the voltage across the resistor? How much power does it dissipate (or consume)?

How Does Watt's Law Help With Electrical Safety?

Typically, when working with Watt's Law and electricity in the real world, you will know the power rating of a device and how much voltage is applied. The current, because it is a flow rate, largely depends on these two units.

Although it's debated on whether current or voltage kills you, one thing's for sure: you don't want a lot of it running through your body. That's why high-current wires in your house typically have insulation on them. But what if the wire gets so hot it melts the insulation?

This is why things like wire thickness matter. Thicker wires have less resistance, which allows current to flow more easily and reduces heating caused by electrical resistance.

Apply What You've Learned

Two light bulbs both consume $60W$. Bulb A operates at $120V$. Bulb B operates at $12V$. Which bulb draws more current? Which bulb needs thicker wires?

Conclusion

Although formulated over 200 years ago, Watt's Law is one of the most fundamental laws of circuits, as it teaches us the relationship between voltage, current, and power as well as how to stay safe when working with electricity.

After learning about Watt's Law, you can start to understand real world problems that face the future of energy.