Physics 102 - Heat




Chapter 15




Does absolute zero really exist?

 

Temperature, heat and expansion

 

 


 

Three temperature scales are in use today. Celsius (or Centigrade) is based on water. The freezing point of water is 0°C and the boiling point is 100°C. The Kelvin scale uses the same size for its degrees but shifts its zero to absolute zero (-273°C). On the Fahrenheit scale, the freezing point of water is 32°F and the boiling point is 212°F

OK, but what is temperature anyway...

In "everydaySpeak" heat and temperature mean roughly the same thing. In physics, these two commodities are quite different. The first thing we need to do is make sense of this.

 

Temperature is related to the average speed at which molecules move. It is an intensive property - it does not depend on the amount of substance present.

If something is hot, it means the molecules are moving fast. Stars have extremely high temperatures. The surface of the sun is about 5000 K, while the core is about 15 million Kelvin.

sun video

sun video link

kinetic motion demo

Something cold has slowly moving molecules. The coldest temperature recorded on Earth is −89.2°C (−129°F; 184 K) at Lake Vostok in Antarctica. Intergalactic space is incredibly cold at about 3 Kelvin but we have achieved temperatures even colder than this in the laboratory.

cold site

liquid nitrogen demo

 

Temperature does not measure the total kinetic energy of a substance. A swimming pool and a goldfish bowl can be the same temperature but the swimming pool has much more kinetic energy. In both cases, the total kinetic energy is the sum of the kinetic energies of all of the molecules.

There are a lot of ways molecules can move. They can change their positions (translation). They can also rotate, vibrate, twist and move in other complicated ways.

Temperature involves translation only. When molecules are moving faster on average, the temperature is higher.

Internal energy involves all kinds of motion of molecules, including rotation, etc. It is an extensive property - it does depend on the amount of the substance involved.

balloon demo

Heat is energy transferred because of temperature differences. It is neither intensive nor extensive - it is the quantity of energy in transit.

 

 

 

 

 

Energy units

Heat energy is traditionally defined in calories.

1 cal = heat needed to raise the temperature of 1 gram of water by 1°C.

Just to make things confusing, if you capitalize calorie, it means something different.

1 Cal = 1000 cal

These are called food Calories. You need to eat about 2000 Calories per day.

Work is defined in physics as energy required to move a massive object or substance. Work is traditionally measured in Joules.

1 J = work done in moving 1 meter against a force of 1 Newton

Heat and work are different forms of energy, and we now prefer to measure both in Joules.

1 cal = 4.2 J

 

When two materials are in contact, they exchange heat until they come into thermal equilibrium. The heat lost by one is gained by the other until they are the same temperature.

 

 

Heat capacity

Specific heat capacity is defined as the amount of heat needed to raise the temperature of a substance by 1 degree Celsius. Different substances have different capacities for storing internal energy.

specificHeatBeakers.jpg

The same applied heat can result in very different temperatures for different substances even if they have the same mass.

specific heat table

 

Water has a very high specific heat - it takes a lot of heat to raise its temperature. This is especially important for life on Earth. Water must lose a lot of heat to lower its temperature. That means oceans are very hard to freeze.

 

 

Thermal expansion

 

Most substances expand with heat. When they expand, they retain their proportions. The amount of expansion for a given change in temperature is a property of the substance - different substances expand different amounts.

 

This explains why heating a jar lid will make it easier to open the jar. The metal lid expands more than the glass jar under the hot water.

ball and ring demo

 

Varying expansion rates are also the reason we use a bimetallic strip how a bimetallic strip in a thermostat.

bimetallic strip demo

Water is different from most other substances in that it expands as it cools. This is also very important for life on Earth. If water contracted when it froze, it would become more dense and sink. The oceans would freeze from the bottom up.

ice bomb demo


Most scientists agree that water is extremely important for life, maybe essential. All life as we know it needs at least some water. Some forms of life can exist in very severe conditions that would be deadly for normal kinds of beings. These are called extremophiles.

blackSmoker.jpg

Sub-oceanic vents called "black smokers" are lush oases for exotic forms of life, including these tube worms.

 

 

 

Scientists have found microbes living in solid ice, and even in solid rock.

 

But the strangest of all are the water bears (tardigrades). They can withstand temperatures from minus 300 degrees to plus 400 degrees Fahrenheit.

Water bears are about .1 mm in length. They live in wet moss and lichen, and water puddles. When the puddle dries up, they curl up and blow away with the dust, only to come to life later when the rains come again. They can survive decades of dehydration. They have also been shown to survive exposure to outer space in a low Earth orbit of around 270 km altitude. This involves being exposed to cosmic radiation and UV radiation from the sun.