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AP Environmental Science: Exam Prep32 chapters | 316 lessons | 30 flashcard sets

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Lesson Transcript

Instructor:
*Sarah Friedl*

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

It's important to understand both how energy is measured and the different units used in those measurements. In this lesson, you'll learn about common energy measurement units, as well as see examples showing how to convert between them.

When you hear the term 'energy' what do you think of? Maybe the electricity that powers the appliances in your home, or the gas that powers your car to get you where you need to go. But, how about a group of kids that just ate a bunch of cookies? Or, the effort it takes you to lift weights at the gym?

All of these are associated with energy because **energy** is simply the ability to do work. But this also means that describing energy can be a bit complicated because you'll need to define what type of energy you're talking about, like thermal, chemical, or electrical, as well as make sure you use the appropriate unit of measure. There are a lot of different units that are used for energy measurement and probably just as many ways to convert between them. Let's go through some of the more common units that you should be familiar with and then work through some examples of how you would go from one unit to another.

It's only fitting that we start with the unit named for the man who is considered the father of modern science, Sir Isaac Newton. The **newton** is the standard unit of force and is represented with the capital letter N. It's equal to 1 kg * m per s^2, which is about the amount of force needed to lift an apple over your head.

The **joule**, which is the amount of energy needed to lift a 1 N object over a 1 meter distance, is also named for a very important scientist, Sir James Prescott Joule. Like a newton, a joule (J) is a fairly small amount of energy. For example, a donut contains about 1-mega joule worth of energy, or 10^6 J. That's one million joules in just that one donut!

Another unit you could use to describe the energy in that donut, and one you're probably more familiar with, is the **calorie** (cal). This is the amount of heat needed to raise the temperature of 1 gram of water by 1°C. This is not the same as the **Calorie** (Cal), which is a kilocalorie, or 1,000 calories. Food calories are always kilocalories, or Calories (with a capital C). So, if that donut you just ate has 700 Calories, that means it actually has 700 kilocalories, or 700,000 calories (with a little c)! A calorie is also the same as 4.186 J, but we'll get into that in just a bit.

If you own an air conditioner, water heater, or furnace, you're probably familiar with the energy unit **Btu**. This stands for British thermal unit and is the amount of heat needed to raise 1 pound of water by 1°F. A single Btu is yet another small amount of energy - similar to the amount of heat one match puts out. Therefore, your household appliances will be rated at large values, like 40,000 or 80,000 Btus. 1 Btu is also equal to 252 cal (with a little c), or 1,055 J.

**Therms**, or thermal units, are larger than Btus. In fact, 1 therm is 100,000 Btu, or about the amount of energy released from burning 100 cubic feet of natural gas. Finally, we have the **kWh**, which stands for kilowatt hour, and is the amount of energy used to produce 1 kilowatt of electricity for an hour. This is what you see on your electric bill, telling you how much energy you've used to power your house each month.

The **watt** is named for James Watt, who invented the steam engine and is the same as 1 J per second. Just like the kilocalorie is 1,000 calories, the kilowatt is 1,000 watts, which means that to get a kWh, we simply multiply 1,000 J per s * 3,600 s (there are 3,600 seconds in an hour), which is the same as 3.6 * 10^6 J (that's over 3 million J!). This is about how much it takes to power ten 100-watt light bulbs for one hour, which shows you just how small 1 J really is.

Of course, this list doesn't include all the units of energy you'll come across, but these are by far the most common ones. So, now that you know what they are, let's look at how you would convert between them. This is an issue you're going to run into all the time in science, so it's important that you know how to go from one unit of energy to another.

Let's say that you have 100 g of water and you want to increase its temperature by 20°C. For this, you would need 2,000 calories because as you now know, 1 calorie is the amount of heat needed to increase 1 g of water by 1°C.

But, what if I asked you how many joules this is? Well, we know that 1 cal = 4.186 J, so we find that 2,000 cal * (4.186 J/1 cal) = 8,372 J. And, how about how many Btus this is? Since 1 Btu = 1,055 J, we simply convert joules to Btus like this: 8,372 J * (1 Btu/1,055 J) = 7.94 Btus.

Easy enough, right? Let's try another one. Say that you cook something in your 1,500-watt microwave for 60 minutes. How many kWh did your microwave use? We use the same conversion process here as before: 1,500 W * (1 kWh/1,000 W) * 1 hour = 1.5 kWh.

What about determining how many therms are needed to power that microwave for an hour? Well, we know that 1 therm = 100,000 Btu, that 1 kWh = 3.6 * 10^6 J, and that 1 Btu = 1,055 J, so this can definitely be done!

First, we determine how many Btus equal 1 kWh: (3.6 * 10^6 J/1 kWh) * (1 Btu/1,055 J) = 3,412 Btu. Knowing this, then, the therms needed for the microwave for 1 hour is found by: 1.5 kWh * (3,412 Btu/1 kWh) * (1 therm/100,000 Btu) = 0.05 therms.

In general, conversions between units of energy are pretty straightforward like these examples as long as you know the conversion factors. And while it may require a little bit of work, as you can see, this is a useful and practical skill if you want to be able to talk with others about energy.

**Energy** is the ability to do work, which sounds simple enough, but in reality can get a little complicated. There are a number of different types of energy, as well as different units that we use to measure it. It's important to not only understand what these different units are and how they measure different types of energy, but also how to convert between them. Being able to convert from one unit of energy to another will not only help you be able to work with other scientists but is also a practical skill for everyday life.

After you've completed this lesson, you'll have the ability to:

- Define energy
- Describe examples of common energy units of measurement
- Explain how to convert between common energy units

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AP Environmental Science: Exam Prep32 chapters | 316 lessons | 30 flashcard sets

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