Thursday, January 15, 2009

Heat Energy Movement and Heat Loss

Some of you from the Altoona area had recently read about why it feels colder on a windy day than on a day without wind. A few months ago, we had also discussed how it feels warmer on a cloudy day. So what is the science behind these questions, both of which seem to involve heat. It has a lot to do with the way heat moves from place to place, or as scientists call it – heat transfer. One of the basic facts in nature is that heat (which is a form of energy) always moves from a hotter object towards a colder object.

Heat can move in several ways from one place to another. Let us think about the different ways now. Do you know that when you touch a cold wall or a window, your warm hand is actually losing heat to the window glass? This form of heat movement is called conduction. This is the same manner heat moves from the stovetop to the kettle or to a soup pot. Conduction is heat movement by contact. Here the hot body has to touch the cold body for heat transfer by conduction.

But remember the cold wind story, there the heat is moved away from you by convection. Here there is usually a fluid medium, it can be air or water usually which carries the heat away from the hotter body. This is the same way how heat comes into your room through baseboard heaters when hot air is blown into the room. Convection involves another medium, usually air or water, transferring the heat. When cold air leaks into a house, it is convection which is to blame for our heat loss.

A third form of heat transfer, which does not require any medium or contact to occur, is radiation. Here the heat energy travels in the form of waves which can go through even vacuum. This is how the heat energy comes to earth from the sun, across millions of miles in the space. This is also how we lose heat from a closed car in winter, when it is left parked overnight. On a windy day, the wind can add to the loss of heat by taking heat away from the glass surface of windows and the body of the car. Radiation is also why we like to open our curtains on a sunny day in winter, to let warmth in through the glass windows.

One of the best examples of a man made object that tries to prevent heat loss is the thermos flask. If you ever get a chance to see one, you should examine it closely. One of the big benefits of a thermos flask is that it keeps colds things cold or hot things hot. You can read more about it at: . Also look up the words thermal insulation on the internet and find out what it means.

Little Lion Experiment
We will learn how different forms of heat transfer take place. Caution: We will NOT be using any kind of stove or electric heaters to do these experiments. We will be using hot water from the tap in the house to provide heat to some cold objects. But even with this you need to be extra careful not to spill any on yourself or get scalded. Be very careful and use only small amounts in small mugs. These experiments can all get pretty messy, so do NOT attempt them on carpeted floors at all. Also it is advised to not do it on a wooden floor either as any spill can be slippery and dangerous. Keep plenty of washcloths or paper towels around to take care of spills.

You will need:
1) Cold water
2) Hot water
3) A coffee mug or a cup to pour with
4) A small bucket or a quart saucepan
5) Aluminum foil
6) Plastic wrap
7) A newspaper
8) Two or three Hershey’s kisses kept in a cold place for an hour (yummy chocolate!)
9) A pencil and a small notepad to make notes.

Conduction Experiment Steps:
1) Keep a piece of aluminum foil (10 inch by 10 inch) larger than your hand over a cold glass window and keep your hand on the foil to feel the temperature.
2) Repeat the same step with a newspaper and also with your bare hand (only for a few seconds). Note the case when it felt coldest.

Radiation Experiment Steps:
1) Fill up hot water in a coffee mug almost till the top. Carefully cover the top with plastic wrap till it is snug and tight, tape the overhanging wrap around the cup if possible.
2) Keep the mug inside a bucket/saucepan.
3) Carefully place an unwrapped Hershey kiss on top of the plastic wrap.
4) After 5 minutes, check the condition of the chocolate, has it slightly melted?
5) Try the same experiment, but instead of the plastic wrap, cover the coffee mug with aluminum foil, making sure that the shinier side of the foil faces the hot water. And use a new chocolate.
6) Try the same experiment with newspaper taped to the top of the mug. Observe if any melting occurs.

Think about how the shiny side of the foil acts as a mirror to the radiation heat and prevents it from coming out of the mug. Final tip: you can probably eat the chocolate from the plastic wrap and the foil experiments, but the chocolate from the newspaper experiment may not be clean. Throw it away. Instead of chocolate you can also use small piece of candles/wax.