Temperature, Heat

and Heat Transfer

M. Rocha

Physics 1 - Chapters 15-16

Temperature

Temperature is the average kinetic energy of atoms and molecules in matter

When something feels hot is because its atoms and molecules are hitting your skin with a lot of kinetic energy

Temperature is the average kinetic energy in a system

The bucket has more total kinetic energy, but the cup has a greater average kinetic energy (energy per molecule is higher) and thus higher temperature

Temperature Scales

The three scales we use to measure temperature are Centigrade (Celsius), Farenheit and Kelvin

F = \frac{9}{5}C + 32

C = \frac{5}{9}(F - 32)

K = C + 273.15

Who the heck uses Celsius anyway?

Everyone but US!

Heat

Heat is the transfer of energy due to differences in temperature. High temperature objects heat (transfer energy to) lower temperature objects.

Thermal Equilibrium

When two objects in contact reach equal temperatures heat transfer stops

Internal energy is the grand total of all the energies inside a substance, i.e. kinetic + rotational + potential energy

Internal Energy

The Internal energy of an object depends on: Temperature, Mass and Composition

Internal energy is the grand total of all the energies inside a substance, i.e. kinetic + rotational + potential energy

Internal Energy

The Internal energy of an object depends on: Temperature, Mass and Composition

Checkpoint

If 1 kg of water stores 400,000 J of internal energy at 100 Kelvin. How much energy would 1 g of water store at the same temperature?

400 Joules

Specific Heat Capacity

When heat is transferred to a material its internal energy rises. How much energy goes into kinetic energy vs. other types of energies depends on the composition of the material, and is called the specific heat capacity of the material

Specific heat capacity is the internal energy required to raise one gram of a material by one degree of temperature

\Delta T = \frac{\mathrm{Heat \ Transferred}}{\mathrm{Heat \ Capacity} \times \mathrm{mass}}

Specific heat capacity is like a thermal inertia since it signifies the resistance of a material to change its temperature

Checkpoint

If the specific heat capacity of a material is 10 Joules/(gram °C), how much heat do you need to raise 1 gram of it by 10 °C?

100 Joules

\Delta T = \frac{\mathrm{Heat \ Transferred}}{\mathrm{Heat \ Capacity} \times \mathrm{mass}}

Specific Heat Capacity

Water has a high specific heat capacity

Water has a high specific heat and is transparent, so it takes more energy to heat up than land does.

The central interior of a large continent usually experiences more extreme temperatures

Checkpoint

Why is it that the climate in the desert is so hot during the day yet so cold at night?

Because sand has a low specific heat capacity

Thermal Expansion

Due to increased molecular motion, most materials expand as temperature increases

Expansion of Water

Water contracts when heated from 0 to 4 °C

This is because below 4 °C, water is full of ice crystals.

Because water expands below and above 4 °C, water's highest density is a 4 °C

Expansion of Water

Heat Transfer

Conduction, Convetion and Radiation

Heat

Heat is the transfer of energy due to differences in temperature. High temperature objects heat (transfer energy) to lower temperature objects.

Heat always flows from high temperature objects to low temperature objects.
Heat flow stops when temperatures equal.
The methods by which heat flows are conduction, convection and radiation.

Heat Flow

Conduction

Conduction is heat flow by direct contact without any overall transfer of matter.

Some materials are good thermal conductors, while others are insulators.

Conduction

Metal is a good conductor

Materials composed of atoms with “loose” outer electrons are good conductors of heat (and electricity also).

Because metals have the “loosest” outer electrons, they are the best conductors of heat and electricity.

Checkpoint

If you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand?

False. Heat flows from your hand to the ice

Conduction

Air is a poor conductor/good insulator

Conduction

Air is a poor conductor, good insulator.

A “warm” blanket does not provide you with heat; it simply slows the transfer of your body heat to the surroundings.

Wool, fur and feathers are good insulators mostly due to the air spaces they contain.

Convection

In convection, heat is transferred by movement of the hotter substance (usually a fluid) from one place to another

Buoyancy causes warm air to rise, which carries thermal energy directly by its motion.

Convection

Convection oven has a fan to enhance the circulation of the air, increasing the transfer of heat

Air is a poor thermal conductor but easily transfers heat by convection

Convection

During the day, the land is warmer than the water, and a sea breeze results

At night, the land is cooler than the water, so the air flows in the other direction

Checkpoint

Water has a heat capacity that is about 4 times greater than that of land. That means that for the same amount of energy transferred by the sun, 1 kg of water will

rise in temperature 1/4x as much as 1 kg of land material

\Delta T = \frac{\mathrm{Heat \ Transferred}}{\mathrm{Heat \ Capacity} \times \mathrm{mass}}