"Bridge Ices Before Road"

Have you ever seen this sign on a road? Ever wondered why the bridge ices up first? There are actually two reasons.

The first reason is that the underside of the bridge is exposed to the elements. Cold air gets all around the bridge structure, making any water on the surface freeze. The road surface is protected by the soil underneath. Eventually, the road becomes cold enough for the wet surface to freeze.

The second reason is that bridges and roads are made from different materials. Bridges are made mostly of steel and concrete. Both of these materials conduct heat well. If you heat them, they get hot fast. If you cool them, they get cold fast. So, once the air temperature drops, the bridge starts losing heat, leading to relatively quick formation of ice. The road is made of asphalt, which holds heat in. Therefore, it takes a long time for the road to become cold enought to freeze.

Hot! Hot! Hot!

One of the unusual properties of metals is their high heat conduction. This means that, when heated, their temperature goes up faster than than of many other materials. You can show this simply by placing spoons or forks made of metal and other materials into hot water.

What You'll Need:
Microwave-safe mug
Water
Several spoons or forks, made of different materials such as metal, wood, or plastic
Microwave oven
Potholder

Fill the mug with water and microwave it for three minutes. Using the potholder, remove the mug from the oven, and place the spoons or forks in the water. Make sure that they don't touch one another. Carefully touch each spoon or fork over the next few minutes. Which one gets hot fastest? Which one stays cool longest?

You'll find that the metal definitely heats up first. This happens because the atoms in the metal absorb energy from the hot water and begin to move faster. Faster-moving atoms are hotter atoms. Other materials, such as wood or plastic, are insulators, and their atoms resist taking on more energy. So, they stay cooler!

Here's another heat conduction experiment that you've done hundreds of times, probably without realizing that it's an experiment! Touch something made of metal that has been just sitting in the room. Does it feel hot or cold? Metals feel cold to the touch, but does this mean that they are at a lower temperature than your hand? Unless you just took the metal out of the fridge, it's at the same temperature as everything else in the room! So, how can it feel cold? The metal is now absorbing heat from your hand, just like it did from the hot water in the experiment above. Your hand senses that you are losing heat, and your brain then thinks that the metal must be cold - but you're just fooling yourself!

Un-Make A Magnet!

In the last experiment, you made a screwdriver magnetic. But is that change permanent? What if you wanted a non-magnetic screwdriver again. Is there any way to remove the magnetization? In other words, once you've lined up the atomic magnets, is there any way to make them all disorganized again?

Yes, there are a couple of easy ways to do this, but you need to put some energy into the screwdriver blade. You can whack it with a hammer, which disorders the tiny atomic magnets. But this might damage the screwdriver, so it's not the recommended method. You can also heat the screwdriver blade to demagnetize it. Hold the handle with an oven mitt, and touch the blade of a magnetized screwdriver to the burner of an electric stove for about a minute, or put it in the flame of a gas stove for the same amount of time. The blade will get very hot, so don't touch it until it has cooled for about five minutes. Then you should find that it has become an un-magnet, and doesn't pick up paper clips any more!

Hot News!

Here's a heat-related story that has been in the news lately, and you can do an experiment to see if it is true. Dr. Steven Chu, the U. S. Secretary of Energy, said that Americans could save a lot of energy by making the roofs of their houses white and/or shiny, and by paving roads with light-colored concrete instead of black asphalt. Does the color of the roof really matter?

You should do this experiment on a bright sunny day. You'll need a thermometer, a box large enough to hold the thermometer (about as big as a shoe box), and some materials for the "roof", like white paper, black paper, aluminum foil, or anything else you want to try. You can decorate the box with a door and some windows to make it look more like a house, but the experiment will work even if you don't do this.

Put the thermometer in the box, then place the roofing material flat on top of the box. Check the temperature and write it down. Place the box in the sun for one hour, then record the temperature again. Try all the different materials. Keep an eye on the weather, because if the sun isn't equally strong for all the roof tests, your results might not be correct. Which type of roof resulted in the coolest house? What does this tell you about which roof would be best in the summer?

Now, before you go and paint your roof white, ask yourself some more questions - which type of roof would be best in the winter, when you want your house to be warm and toasty? What is the climate like where you live? Is it really cold in the winter? Is it really hot in the summer? How long does the cold weather last, and how long is the hot season? So, the answer to whether a white or shiny roof would help you save energy over a whole year depends a lot on where you live. That's the way it often is with suggestions about saving energy - there's no easy answer that works for everyone!

Have you ever seen a metallic sheep??

What happened to the temperature in the last experiment? And how did the steel wool look at the end? You should have seen that the temperature went up by several degrees, and that the steel wool looked rusty after the experiment was done. What happened?

First of all, what exactly is steel wool? Is it the fleece from metallic sheep? Of course not. Steel wool is just very thin wires made of iron that is sold in clumps that looks something like sheep's wool. It is often used instead of sandpaper for removing paint or smoothing surfaces.

So, where did the heat come from? When iron comes in contact with the oxygen (O₂) and water (H₂O) in air, a chemical reaction called oxidation occurs. Rust is actually a mixture of several compounds of iron, oxygen, and hydrogen, with names like iron oxide and iron hydroxide. Do you remember learning about thermal energy in "Where Does Heat Come From?"? We said that a wood fire feels hot because the thermal energy of the wood was higher than that of the ashes and other material left over from the fire. The same thing applies to the iron. The thermal energy of rust is lower than that of the original iron, and you feel the energy lost as heat.

But why didn't the steel wool rust before you put it into the jar? Normally, steel wool is coated with oil or some other material that protects it from air. The vinegar you rinsed the steel wool with took the coating away, and the bare iron was exposed. So, the vinegar wasn't actually involved in the reaction, but it did allow the reaction to happen.

If you go to the drugstore, you can actually find a product that uses the oxidation of iron to generate heat! ThermaCare heat wraps contain iron (and some other chemicals) that react with air and produce warmth that helps relieve the pain of arthritis and sore muscles.

Chemical Heat

Here's an experiment that shows how a common chemical reaction creates heat. See if you can figure out what the reaction is.

You'll need a good-sized jar with a lid, a small thermometer that will fit in the jar, some steel wool (plain steel wool, not the scrubbing pads that contain soap), and vinegar.

First, take a handful of steel wool, and place it in the jar. Put the thermometer in, making sure that the bulb (temperature-sensing part) in surrounded by steel wool. Record the temperature. Wait a few minutes, then check again. Boring, huh? The temperature shouldn't change very much.

Now for some chemistry! Take the thermometer and steel wool out of the jar. Soak the steel wool in vinegar for a few minutes, then remove it and squeeze the vinegar out (wear a rubber glove when doing this). Quickly put the steel wool and thermometer back in the jar the way it was before. Watch the thermometer for several minutes. Does anything happen? Check back a few times over the next hour or so. What happens to the temperature? Does the steel wool look the same? What do you think happened to it? Here's a hint: don't get distracted by the vinegar - it's not involved in the reaction.

Mechanical Hands

In the last post, you were promised some experiments involving mechanical energy that you can do with your own hands. Wait a minute, you might be saying, my hands aren't mechanical (I assume that none of you are robots!). Here's another case where scientists sometimes use words a little differently than most people do. Mechanical energy is simply the energy of moving objects. Your hands are objects, and when you move them, you are using mechanical energy!

The first experiment is very simple. Place your palms together and rub quickly. What do you feel? Why do your hands get warm? Your muscles create mechanical energy which turns into heat because of friction, which makes it hard to rub two things together. OK, that wasn't much of an experiment. Let's try another one...

Find a simple wire clothes hanger like the ones that come from the dry cleaner. If it is covered in paper, remove the paper. Grab the straight bottom of the hanger with one hand in each corner. Bend the straight part of the wire about eight to ten times. It may be hard at first, but will get easier. Touch the bent part of the hanger. It's hot! Again, your muscles supplied the mechanical energy, and the friction among the atoms in the metal created heat.

Can you think of any other examples of how mechanical energy can be turned into heat?

Where Does Heat Come From?

When we talk about "hot" weather, we are referring to heat that comes from the sun. The sun's heat is created by nuclear fusion, in which hydrogen atoms transform into helium. What other sources of heat can you think of? In the winter, we usually heat our homes by burning a fuel, like oil, natural gas, or wood; this is a chemical reaction called combustion. You've probably also noticed that some electrical appliances, such as lamps and toasters, also get hot. The sun, oil, electricity - these seem to be very different. How can they all produce the same thing - heat?

You might think that they all contain heat, which just escapes when fusion, combustion, or electricity happens. Long ago, scientists believed that matter contained phlogiston (pronounced flow-jis-tun), a substance that had no taste, color, or weight. They thought that phlogiston came out of matter when it burned. There were a lot of problems with this idea, and by around 1800, scientists realized that phlogiston didn't really exist.

So, matter doesn't contain either phlogiston or heat, but it does contain thermal energy. "Heat" and "thermal energy" may sound like they mean the same thing, but to scientists, they don't. This might be easier to understand by a comparison. Think of the aroma of cookies baking in the oven. Where does the smell come from? "Smell" isn't something that the cookie dough releases as it cooks. Instead, we smell that wonderful odor when our nose senses certain molecules given off by the cookies. So, smell isn't something in itself, but it is what we experience. Usually, when we say that something is "hot", we mean that it feels warm to the touch. Thermal energy is sort of like the fragrant molecules. When it is released, we feel its effect, and call it heat. Of course, they're not exactly the same, because thermal energy doesn't contain actual molecules, but hopefully you get the idea.

When you burn wood, the fire feels hot because the original wood has more thermal energy that the ashes, water, and other stuff that's left over after the fire. The heat you feel is the thermal energy that is lost during combustion. The same thing happens during the fusion reactions in the sun. You can also feel heat when some other type of energy is transformed into thermal energy. That's why electricity creates heat - the electrical energy is changed into thermal energy. In the next experiment, you'll learn how to turn mechanical energy into thermal energy - using your very own hands!

Hot & Cold

We all know when something feels hot or cold, but did you ever think about what causes it to feel that way? You probably know that everything is made up of molecules, which tiny bits of matter. When something feels hot, it means that the molecules are moving very fast. So, how do the molecules move when something is cold? That's right - they're slow.

Now, you might think that you could never see how fast molecules move. After all, they're really tiny, and your eyes aren't that good! But you can do an experiment that will let you see the effect of their motion. Here's what you need:

2 drinking glasses of the same size and shape
Food coloring
Hot water (from the faucet is OK, but make sure you don't get scalded)
Cold water (also from the faucet)

Fill one glass with hot water. Fill the other glass to the same level with cold water. Add 3 drops of food coloring to each glass. Watch for a few minutes. What do you see? Can you explain your results based on the different speeds of the molecules in hot and cold water?

Summertime Science!

It's been pretty cool in the Northeastern US so far this summer, but the heat and humidity will be here soon! We'll be talking about heat over the next few weeks, and will give you a bunch of hot experiments that you can try at home! As always, be sure to follow the instructions carefully, and make sure that adults are around when you are doing any experiments.