Temperature, Heat

and Heat Transfer

M. Rocha   

Physics 10

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 

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?

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

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?

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?

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 

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 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?

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

Radiation

In radiation, heat is transmitted in the form of radiant energy (electromagnetic waves)

Most of the energy in our planet got here via radiation

Radiation

Radiation

Most of the heat from a fireplace goes up the chimney by convection. The heat that warms us comes to us by infrared radiation

Thermal Radiation

All objects continuously emit radiation. Objects at low temperatures emit long waves.  Higher-temperature objects emit waves of shorter wavelengths

Human body T~300 K, emits infrared radiation

All objects continuously emit radiation. Objects at low temperatures emit long waves.  Higher-temperature objects emit waves of shorter wavelengths

Sun T~6000 K, emits visible radiation

Thermal Radiation

Thermal/Blackbody Radiation

Thermal/Blackbody Radiation

Emission and Absorption of Infrared/Thermal Radiation depends on surface color

Thermos bottle eliminates conduction and convection by having double-walled sides with vacuum.

Silvered interior walls minimize heat transfer by radiation

Controling Heat Transfer

Greenhouse Effect

Glass is transparent to sunlight (short-wavelength).

Glass is opaque to infrared radiation (long-wavelength) produced by objects inside greenhouse, trapping the heat

Greenhouse Effect

Earth's atmosphere transparent to sunlight (short-wavelength).

Earth's atmosphere is opaque to infrared radiation (long-wavelength) due to greenhouse gasses (carbon dioxide and water vapor)

Global Warming

Over past 1000 years temperatures were nearly constant until CO2 emissions increased starting with the industrial revolution