NSTA Web Seminar: Energy: Stop Faking It! LIVE INTERACTIVE YOUR DESKTOP Tuesday, May 8, :00 p.m. to 8:00 p.m. Eastern time

Bill Robertson May 8, 2007 NSTA Web Seminar Energy: Stop Faking It!

Energy and the behavior of gases

TrueFalse Hot water, just like hot air, rises. Then the cold water rushes in to fill the void. This creates a convection cell.

Put this bottle with the coin on top in the freezer for a half hour. What happens when you take it out and put it on a table at room temperature? Take a guess!

What’s the explanation? When you heat air molecules, they need more room. Therefore, they push up on the coin to get more room. When you heat air molecules, they are farther apart on average, so naturally they push the coin up. Heating air molecules causes the air to expand, so the coin has to lift up. Heating air molecules makes them move faster and push harder, so the coin moves up.

The Kinetic Theory of Gases Gas molecules move in a straight line at a constant speed until they collide with another molecule or with a wall. Molecules bounce off one another or off a wall as if they are billiard balls. When you heat gas molecules, they move faster. When you cool gas molecules, they move slower.

When the people move slowly, how much of the space do they cover? Let’s heat the people up. Now how much of the space do they cover? Some of it All of it

Does this gas of people expand when you heat them? Does this gas of people contract when you cool them? Yes No

Gases do not necessarily expand when you heat them Gases do not necessarily contract when you cool them

Are these people being hot or cold gas molecules?

Gas molecules do not need any more space when you heat them, nor do they need any less space when you cool them.

Put this bottle with the coin on top in the freezer for a half hour. What happens when you take it out and put it on a table at room temperature?

What’s the explanation? When you heat air molecules, they need more room. Therefore, they push up on the coin to get more room. When you heat air molecules, they are farther apart on average, so naturally they push the coin up. Heating air molecules causes the air to expand, so the coin has to lift up. Heating air molecules makes them move faster and push harder, so the coin moves up.

Gases expand when heated if they are allowed to do so by their surroundings and gases contract when cooled if their surroundings are of a nature to contract around them.

Let’s talk about pulleys

How does your pull in the previous slide compare to the force exerted on the washer? The sameTwice as largeHalf as large

Sometimes all a simple machine does is change the direction of the force you exert

How does your pull in the previous slide compare to the force exerted on the washer? The sameTwice as largeHalf as large

The washer experiences approximately twice the force you exert

Work = (net force)(distance object moves in the direction of the force)

Work done on a system adds energy to the system Work done by a system subtracts energy from the system

If we believe in conservation of energy, then the work you do on a system shows up as energy of the components or work done by the system

If we ignore heat losses due to friction, then we can say that Work done on the system = work done by the system or Work in = Work out

F 1 d 1 = F 2 d 2

F 2 is twice as large as F 1, so how does d 1 compare to d 2 ? F 1 d 1 = F 2 d 2

As with all simple machines, it’s a trade-off between force and distance. You get twice the force, but have to input twice the distance.

Mechanical advantage = If there are two strings holding up the weight in a pulley system, then you get two times the force.

If there are three strings holding up the weight in a pulley system, then you get three times the force. To figure out the mechanical advantage of a pulley system, you just count the strings supporting the weight. A simple rule, but meaningless if you don’t know why it works.

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