2.1 Introduction

Welcome to Lesson 2!

In our last lesson, we explored the forms and sources of energy—from the chemical energy in your breakfast to the radiant energy streaming from the Sun. We learned that energy is the capacity to do work or produce heat, and that it can change forms but is never created or destroyed.

Now, we're adding a crucial piece to that foundation. Have you ever noticed that your phone battery drains faster when you're streaming video versus just texting? Or that a 13-watt LED light bulb uses electricity differently than a 100-watt bulb?

These everyday observations point to a key distinction that scientists, engineers, and even your utility company rely on:

Energy and Power are related, but they are not the same thing.

Think of it this way:

• Energy is the total amount of "fuel" you use—like the total gallons of gas put in your car this month.
• Power is the rate at which you use it—like how fast you burn that gas while accelerating onto the highway.

In physics terms:

Energy (measured in joules or kilowatt-hours) = how much work gets done

Power (measured in watts) = how fast that work happens

In this unit, we will be looking at how to convert energy and power units as well as reading electric bills. We will also learn how to calculate how much energy our household devices use and how much that costs us.

Lesson 2: Learning Objectives

By the end of this lesson, you'll see physics in action every time you flip a switch—and understand exactly what you're paying for when that bill arrives.

• Distinguish work, energy, and power using everyday examples
• Convert between units (joules ↔ kilowatt-hours) to connect physics class to your utility bill
• Calculate energy use: Energy = Power × Time (e.g., a 100 W bulb running 10 hours = 1 kWh)
• Interpret appliance labels to estimate real-world costs
• Decode an actual electricity bill—spotting how many kWh you used and why your cost per kWh isn't just the "supply rate"