Heat

Heat and Thermodynamics

• The laws of thermodynamics describe what happens to internal energy (mainly heat) as it is transformed into work and to other forms.

• First Law: Energy cannot be created or destroyed, but it can be converted from one form to another.– NO FREE LUNCHES

• Second Law: Impossible to take heat from a source and change all of it into useful work; some heat must be wasted.– CAN’T BREAK EVEN

Laws of Thermodynamics

Work and Heat

• Temperature of the water rises if either:– heat is added– work is done

• The total energy in the water is equal to work done to the a water and the heat added to the water.

The First Law of Thermodynamics

The work done on a system plus the heat added to a system must equal the change

in total energy of the system

Won + Qto = E represents the “change in” something

Q is the symbol for heat

Example

• When a cylinder is compressed, work is done on the cylinder

• No heat added (Q = 0)

Won + 0 = E

• The change in energy is positive and results in an increased temperature (T2 > T1)

Impossibilities

• Impossible Event: It is impossible for heat to spontaneously move a block across a table

• Impossible Machine: It is impossible to convert heat completely into useful energy

• Both do not violate the conservation of energy

(1st law)

The 2nd Law of Thermodynamics

• In order to explain why some events are impossible, we need an additional law besides conservation of energy (1st law)

• The 2nd Law of Thermodynamics: In an isolated system, disorder always increases– “You Can’t Break Even”

• Entropy is a measure of this disorder

Work and Heat

• Two kinds of motion (Energy) that the particles of an object can have.– A coherent motion where they move together.– An incoherent, chaotic motion of individual

particles.

• Work (W) on an object is associated with coherent motion, while heating an object (Q) is associated with its internal incoherent motion (Entropy).

Example of Entropy• Ice has low entropy, liquid water has

more, steam has a lot

IceLiquidWater

WaterVapor

Increasing Entropy

Reason for the 2nd Law

• The number of ways a system can be in an ordered state (low entropy) is much smaller than the number of ways a system can be in a disordered state (high entropy)

• Example: There are a vast number of ways to arrange books randomly on a shelf, but only one way to arrange them alphabetically

Heat and Temperature

• Temperature is a measure of the average internal kinetic energy of the molecules of a substance.

• Heat is a measure of the internal energy that has been absorbed or transferred from one body to another.– Increasing the internal energy is called

heating.– Decreasing the internal energy is called

cooling.

Measuring Heat

• A calorie (cal) is the amount of heat required to raise 1 gram of water 1° Celsius

• A Calorie or Food Calorie is 1000 cal (kilocalorie).

• A Btu (British thermal unit) is the amount of heat required to raise 1 pound of water by 1° Fahrenheit

Measuring Temperature

• Scientists prefer Kelvin to degrees Celsius in measuring temperature

• degrees Celsius = Kelvin - 273• Example: 25° C = 298 K (Kelvin)• Kelvins are useful because no object in

nature can ever have a temperature lower than 0 K (absolute zero)

0 K = -273° C

Some Common Temperatures

Temperature and Heat

• The relationship between temperature and heat is:

Q = mcT– Q = heat in calories– m is the mass in kilograms T is the change in Temperature in degrees

Celsius– c is the specific heat in Joules per gram °

Celsius

Specific Heat and Heat Capacity• The specific heat of a substance is the

number of Joules necessary to raise the temperature of one gram by 1° Celsius

• A material with a high specific heat has a large heat capacity (the ability to store thermal energy).

• An object with a high specific heat/ heat capacity will cool down slower than an object with a low specific heat.

Water has the one of the highest specific heat values and therefore has a high

heat capacity.

Heat also depends on Mass

• If both objects were heated for several hours they will have the same temperature.

• However, the larger array will store nine times more thermal energy than the same one.

Example

• Energy used to take a bath: How much energy is required to heat 200 kg of water from 20°C to 50°C?

• Answer: Q = (200kg)(4,180)(50-20°C) 25,000,000 J

• Note that heat depends upon mass. The more water (mass), the more energy required to heat the water to a particular temperature.

How Hot is the Pizza?

To better illustrate the idea of heat capacity, consider this scenario: Your pizza has just been taken from the oven and you're hungry. The crust is not too hot to handle when you pick it up. You're confirmed in your belief that it's at the perfect temperature when you touch the crust to your tongue. It feels warm, but not uncomfortably hot. So chomp! and Oww! Your mouth is burned by the pizza sauce. How can this be? Obviously, both the crust and the sauce are at the same temperature ... after all, they were heated together in the same oven.

How Hot is the Pizza?

• Even though they were both at the same temperature, the sauce (because it contains more water) contains more thermal energy.

• Because of this, more thermal energy is required to raise the sauce to the same temperature as the crust. When you put the pizza in your mouth, both the sauce and crust lose heat until they reach the same temperature as your mouth. The (water containing) sauce has much more heat to surrender and that's why it burns so much.

Thermal Inertia of the Oceans• A substance with a high thermal inertia both

heats up and cools down at a slow rate. • Due to the large mass and high heat capacity, the

earth’s oceans have considerable thermal inertia.

Thermal Inertia of the Oceans• The good news is that because the oceans are

so large, and take so much time to absorb the thermal energy, we are warming more slowly than would otherwise occur.

• The bad news is that the oceans not only take up heat slowly, the also dissipate heat slowly. So even if we are able to reduce the greenhouse gases in the earth atmosphere to reasonable levels, the thermal inertial of the oceans will still take quite some time to respond and cooling down the earth will take considerable time.

Latent Heat

• Sometimes, adding heat to a system does not result in an increase in temperature

• When a substance changes from one state to another, the transition is called a phase change.

• A phase change always absorbs or releases energy, a quantity of heat that is not associated with a temperature change

Latent Heat

• Latent heat is the hidden energy of a phase change, which is energy that goes in or comes out of internal potential energy

• Recall the three phases of matter– solid– liquid– gas

IceLiquidWater

WaterVapor

Latent Heat of Fusion• The ice warms to the

melting point (0° C), then absorbs heat during the phase change as the temperature remains constant.

• At 0° C, adding heat to ice causes a phase change (to water) rather than a rise in temperature

Latent Heat of Vaporization

• When all the ice has melted, the now liquid water warms to the boiling point (100° C), where the temperature again remains constant as heat is absorbed during the second phase change from liquid to gas.

• After all the liquid has changed to gas, continued warming increases the temperature of the steam.

Latent Heat

Principles of Heat Transfer

• Heat transfer is one way of transferring energy to a body (work is the other)

• Occurs only when there is a temperature difference between the two bodies (heat flows from hot to cold)

• Occurs through three processes: conduction, convection, and radiation

From Hot to Cold

• Heat energy is transferred when there is a difference in temperature

• In an isolated system heat flows from hot to cold until both bodies are at the same temperature

The Three Types of Heat Transfer

• Conduction: Heat is transferred through a material (e.g. insulation or glass)

• Convection: Heat is transferred by air or water currents (e.g. ocean currents)

• Radiation: Heat is transferred when a hot body emits radiation (e.g. infrared radiation given off by a fire)

Conduction• Conduction depends on the

following:– Type of Material: thermal conductivity

(e.g. metal spoons transfer heat better than plastic)

– Temperature Difference– Area (e.g. a thin stirring stick

transfers less heat than a thick spoon)

– Thickness (the distance heat has to travel)

Heat Conduction Equation

• QC/t = heat transferred per unit of time

• k = thermal conductivity

• A = area

• T2 - T1 = temperature difference

= thickness

Examples of Conduction

• Why does crushed ice melt faster than ice cubes?– Answer: Because the exposed

area is larger

• Why do you save money by turning down the thermostat in cold weather?– Answer: Because the

temperature difference (between inside and outside) is smaller

Convection

• Warm air (water) rises and cool air (water) sinks

• Why? Because warm air (water) is less dense and “floats” on cooler air (water)

• The rising of warm air (water) creates circulating convection currents

• Convection can occur in any gas or fluid.

Examples of Convection

• The sea breeze is caused by differences in temperature between the ocean and the shore

• In fact, all weather and ocean currents are caused by convection

• A draft in a cold room is caused by convection currents from air leaking through a window or door

• A “rolling boil” in a pot is the result of convection

Radiation

• Radiation results in heat being emitted “at the speed of light”

• Radiated heat requires no medium (e.g. air) and can propagate through empty space

• Heat is emitted as type of electromagnetic radiation

• Here, radiation does not refer to the emissions of radioactive substances

Types of Electromagnetic Radiation

The Wave Nature of Light• Wavelength is the distance from one crest to the

next• The Frequency (f) of a wave is the number of

complete waves that pass a point in a given time. Hertz is the unit of frequency.

• The Velocity is always the speed of light.

Frequency and Wave Length

• Relationship between frequency and wave length

c = fc = The speed of light = 3.0 X 108 m s-1 = The wavelength of the radiation (m)

f = The frequency of the radiation (Hz or s-1)

**** The shorter the wavelength, the greater the energy.

Radiation from Hot Objects

• Hot objects emit radiation over a wide range of wavelengths

• Room temperature objects emit radiation that is mostly infrared

• Object hotter than ~1000° C begin to emit visible light Light from the Sun

Temperature and Radiation

The Higher the Temperature, the

• Greater amount of radiation being emitted.

• Lower the Wavelength of Radiation being emitted.

• Higher the Frequency of Radiation being emitted.

Temperature and Radiation

• A hot burner on a stove or a fire emits large amounts of infrared and a smaller amount of visible radiation

• Mammals (~40° C) emit mostly infrared radiation

• Our sun (~6000° C) emits a large amount of visible light

• Incandescent lights (regular light bulbs) have heated filaments that emit visible light when the temperature get to 2500 °C

An Example of Heat Transfer

• A radiator works by circulating steam through a series of pipes, where it condenses and releases heat

• Heat is transferred by conduction, convection, and radiation

The Campfire

• If you hold one end of a burning stick (not the burning end!) you will eventually feel it getting hotter. This is heat transfer by conduction.

• If you hold your hand above the fire (but not too close!), it will be warmed by convected air.

• If you are somewhere in the vicinity, you will feel the side toward the fire getting warmer by radiation.