Heat and Temperature

The heat, as Glen Frey says, is on

First, set up the demo…

• In case I haven’t

Temperature

• Temperature is a measure of how hot or cold something is.

• It is measured on a standard scale in degrees– English unit countries: Fahrenheit– SI countries: Celsius– Scientists: Kelvin

How Thermometers Work

• For those who don’t know, a thermometer is a device used for measuring temperature.

• Nearly all matter expands when it heats

• So you can use liquids (mercury or colored alcohol) that expand at known rates and put them in a tube with a scale on it.

Temperature Scales

• Fahrenheit scale: water freezes at 320F, water boils at 2120F.

• Celsius scale: water freezes at 00C, water boils at 1000C.

• Kelvin scale: same spacing as Celsius, but zero point is at absolute zero (the coldest possible temperature), which is -2730C.

• Fun fact: even with different spacing, -400F and -400C are the same temp.

Quick reference for those raised in the United States

• 00 C = freezing point of water

• 200 C ~ room temperature

• 370 C ~ body temperature

• 400+ C ~ Death Valley

• 1000 C = boiling point of water

A check for understanding

• And now for some volunteers…

But Dr. Mason, what does temperature ACTUALLY measure?

• Think about a cup of coffee.• The molecules in your coffee are constantly moving

around and bouncing off one another.• As the temp of any substance increases, the

molecules of the substance gain energy and move faster.

• The molecules in hot coffee move faster than the molecules in cold coffee.

• This extra motion makes materials expand when hot.

• So, temperature is a measure of the average kinetic energy of the molecules in a substance.

Okay, so what is heat?

• Heat is energy• Heat is energy that flows from one object

to another. • Heat ALWAYS flows from the hotter

object to the colder object.• Heat will only flow if the two objects are in

THERMAL CONTACT.• Heat will flow between two objects until the

temperature becomes equal.

To Review:

• Heat is energy that is transferred between objects, resulting in a change in internal energy. Heat is measured in Joules, just like other kinds of energy (potential and kinetic).

• Temperature is a measure of the kinetic energy of a substance. Temp is measured in degrees (of Celsius, Fahrenheit, etc.).

Hot/medium/cold water demo

• Once again, I need some volunteers…

Thermal Equilibrium

• After two objects have been in thermal contact long enough, they have the same temperature.

• This means they are at thermal equilibrium and no heat flows between them.

Let’s ask some questions…

• Imagine it is the dead of winter and the heater in your house isn’t working.

• You wake up in the morning and have no socks on. When you step onto the floor, would you rather step onto a tile floor, or on a rug?

• Which is hotter? The tile or the rug?

• Why would you rather step on the rug?

Another situation

• Now let’s say it is the middle of the summer. You are outside barefoot at 2pm on a blazing hot day.

• Would you rather stand in the grass, or in the middle of the black asphalt street?

• Which is hotter, the asphalt or the grass?

• Why is it better to stand in the grass?

A check for understanding…

• Once again, the volunteers….

Measurement of Heat

• If something absorbs heat, the temp goes up.

• But the change in temp doesn’t just depend on amount of heat. It also depends on the mass of the something and the kind of the something.

• Calorie: the amount of heat needed to raise 1 gram of water 10 C.

• Kilocalorie: amount of heat needed to raise 1 kg of water 10 C.

Important Note!

• It doesn’t matter what the starting temp of a substance is, as far as raising the temp is concerned.

• The same amount of heat is required to raise 1 gram of water from 30 C to 40 C as it is to raise 1 gram of water from 980 C to 990 C.

• How many calories for each?

Do the (easy) math

• I add the same amount of heat to 2 beakers. One has 1 kg of water and the other has 3 kg of water. Which beaker increases its temp more?

• How many calories of heat are needed to raise 2 grams of water by 10 C?

• How many calories of heat are needed to raise 50 grams of water by 20 C?

• How many calories of heat are needed to raise 400 grams of water by 30 C?

Specific Heat Capacity

• Which one heats up faster: a 1 kg potato or a 1kg metal pot?

• Which one cools down faster: the 2 kg apple pie just out of the oven or a 2 kg aluminum pan just out of the oven?

• The different speeds at which substances raise or lower their temperature is due to their specific heat capacity.

Specific Heat Capacity cont’d

• Specific Heat Capacity: the amount of heat required to raise the temp of one unit mass of the substance by one degree.

• Example 1: gold SHC = 130 J/kg/0C

• Example 2: glass SHC = 837 J/kg/0C

• Example 2: water SHC = 4186 J/kg/0C

SHC concepts

• A substance with high SHC will take more heat to raise its temp, will take longer to heat up and longer to cool down.– Examples: water, baked potato

• A substance with low SHC will take less heat to raise its temp, will take less time to heat up and less time to cool down.– Examples: most metals

Check for understanding

• Once again with the volunteers…

Thermal Expansion

• Remember how we said that all substances expand when you heat them and contract when you cool them? Yeah, that’s still true.

• Gases expand and contract more than liquids which expand and contract more than solids.

• Now would be a good time to set up the demo.

Thermal expansion examples

• Let’s say you have a jar with a stuck lid. How can you make it easier to remove the lid? Why does this work?

• Why are there expansion joints in roadways?

• Read the crazy teaching tip from page 316 of the teacher’s manual.

Bimetallic Strip/Thermostat

• Someone tell me what a thermostat is.• Thermostats use a bimetallic strip. A

bimetallic strip is a strip that has two different metals (say, brass and iron) stuck together.

• These two metals expand and contract at different rates.

• When the metals change temperature, the strip bends.

• See page 317 to understand thermostats.

The crazy expansion of water

• Water behaves very strangely at low temp.

• It contracts as the temp lowers to 40C, at which point it expands again.

• So when water freezes into ice, it actually has a larger volume (and lower density) than water at 40C. See page 320 to see why!

• This is why ice floats.

Check for understaning

• Again with the volunteers….