The Cool Physics of Heat

HEAT: A transfer of energy due to a difference in temperature.

By Colin McAllister. colin.mcallister@ymail.com

Students are invited to edit this work according to the Creative Commons license on the References and Copyright slide.

Created at Curtin University of Technology, Sarawak, Malaysia..

Preface

• An introductory outline of the Physics of Heat.• I created this presentation at Curtin Sarawak Malaysia as a basis for

Foundation Physics students and others to edit and expand. • Do you have a story involving heat that you would like to contribute to this

presentation? Students are invited to edit this work according to the Creative Commons license on the References and Copyright slide. You may add photos from your camera phone that illustrate a physical phenomena, or add a description of any physical process that involves heat or temperature. Changes should conform to academic practice; please do not insert images that are protected by copyright, or paraphrase sentences from Physics text books. Add a slide at the end with your name, email, the date and citing any references.

• Except where otherwise noted, this work is licensed under the Creative Commons Attribution-Share Alike 2.5 Malaysia License.

• By: Colin McAllister, teaching Physics in the School of Foundation & Continuing Studies, at Curtin University of Technology, Sarawak, Malaysia.

The Cool Physics of Heat

The combustion of hydrocarbon gas in air is a source of heat.

The Cool Physics of Heat

• Did someone throw cold water on your idea?• Did you get your fingers burned in a business deal? • These two idioms show that heat is a macroscopic

phenomena that we are all familiar with. • Heat exchange occurs in all man-made and natural

systems.• The Perpetual Motion Machine does not exist!• Every activity causes energy to be converted to heat.• Physics explains the macroscopic properties of heat in

terms of the microscopic motion of molecules.• Every science and every branch of engineering uses

measurements of energy and temperature to quantify thermal properties.

Heat as Energy Transfer

• HEAT is the energy transferred from one object to another because of the difference in temperature.

• Heat flows from hot to cold.

• Units of heat: calorie, kilocalorie, Calorie and Joule.

• Physicists use the SI unit: Joule or J

• Temperature is measured in oC, oF or oK

Conservation of Energy

• Heat is equivalent to work done. • Work Done is Force x Distance (parallel)• Friction converts mechanical energy to

heat. How can we reduce unwanted friction?

• The brakes on a vehicle convert the kinetic energy of the vehicle to heat energy in the brake pads, disk or drum.

• Excessive braking wastes petrol. Explain this in terms of the physics involved.

Internal Energy

• The internal energy of a material is due to the motion of molecules.

• The temperature of a material is a measure of its internal energy.

• Molecules in a solid or liquid are restricted, so their motion is in the form of vibration.

• Molecules in a gas or are free to move around, and have Kinetic Energy.

• When internal energy flows from one place to another place, it is known as heat.

Specific Heat

The specific heat capacity of water determines the energy needed to heat it.

Specific Heat

• Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a certain temperature interval. (wikipedia.org)

• In a solid, the rise in temperature is due to the molecules vibrating at larger amplitude. In a gas, the rise in temperature is due to the molecules moving at faster speeds.

Latent Heat

The energy required to convert liquid water into steam is Latent Heat

Latent Heat

Boiling water undergoes a phase change from liquid to gas.

The Strokkur geyser in Iceland ejects a jet of water 60 to 100 feet into the air.

Geothermal heat changes someunderground water fromliquid water to steam, which is a gas. This energy increases the volume andthe pressure,powering theeruption.

Latent Heat

• Latent Heat is the amount of energy in the form of heat released or absorbed by a substance during a change of phase state (i.e. solid, liquid, or gas), – also called a phase transition (wikipedia.org)

• Phase Transitions that release heat: condensation, freezing and deposition (frost).

• Phase Transitions that absorb heat: melting, evaporation, boiling, sublimation.

• (Some materials, e.g. CO2 undergo sublimation: changing directly from a solid to a gas.)

Heat Transfer Mechanisms

• Convection: heat carried by the flow of a fluid, such as air or water.

• Thermal Conduction: flow of thermal energy through a material.

• Thermal Radiation: electromagnetic radiation due to temperature.

• Thermal Radiation is key to explaining how global warming occurs.

Convection

A fan drives transfer of heat by forced convection of air.

Convection

• Convection involves the movement of a fluid between two regions of different temperature. The fluid may be a gas (e.g. air) or a liquid (e.g. water).

• In “free convection” the movement of the fluid is due to the difference in temperature between the regions. The fluid expands in the warmer region, causing its density to be lower. It contracts to its original density in the cooler region. Lower density fluid will rise and higher density fluid will fall under gravity, causing the fluid to flow as a current. E.g. ocean currents.

• In forced convention, the fluid is already moving due to a pressure difference, perhaps driven by a pump or a fan.

• The effectiveness of thermal convection depends on the viscosity and on the specific heat of the fluid.

Thermal Conduction

• Conduction is the transfer of heat within a material, by the random transfer of kinetic energy from molecules with a high amplitude of vibration to neighbouring molecules with a lower amplitude of vibration.

• Metals contain many free electrons, known as conduction-band electrons. These contribute to the conduction of heat in metals.

• The thermal conductivity of a material is a measure of how well it conducts heat.

• Copper and Aluminium have a high thermal conductivity.• Air has a low thermal conductivity. Insulating materials,

like polystyrene foam, have a high air content.

Thermal Radiation• Every object emits thermal radiation from its surface due to its

temperature.• Thermal radiation is electromagnetic radiation that is transmitted by

the electrons in the atoms of the radiating object.• Everyday objects emit thermal radiation that is in the infrared region

of the electromagnetic spectrum.• A hot object, like the filament of a light bulb, emits infrared and

visible thermal radiation.• Thermal radiation is transmitted through air or through the vacuum

of space.• Thermal radiation transports heat from a hotter surface to a cooler

surface, e.g. light from the surface of the Sun to the Earth.• The thermal radiation inside an enclosed oven is at equilibrium,

because all the walls are at the same temperature.

Global Warming

Huge tabular icebergs, calved from the ice shelf in the Southern Ocean's Weddell Sea. (Photo courtesy of Mike Vecchione, NOAA National Marine Fisheries Service),

Global Warming

• Thermal Radiation is key to explaining how Global Warming occurs – The Greenhouse Effect.

• The average temperature of the Earth was constant when the thermal radiation emitted into space, balanced the solar energy absorbed

• Burning of fuel and destruction of forest increases the quantity of greenhouse gases in the Earth’s atmosphere

• Greenhouse gases like carbon dioxide and methane are so called because they behave like the glass that keeps a greenhouse warm.

• Greenhouse gases absorb thermal radiation from the Earth’s surface, causing the Earth to become warmer.

• A small increase in temperature causes many big changes including melting polar ice and rising sea level.

Heating and Cooling

The exhaust pipe of an internal combustion engine.

Heating and Cooling

• Every gadget, from the thinnest mobile phone to the largest electricity generating station, produces heat as a by-product.

• Architects use the physics of heat to design buildings that are comfortable and that make efficient use of energy.

• Computer Engineers add cooling fins and fans to computers; to remove the heat caused by currents inside the silicon chips.

• Materials scientists investigate existing materials and design new materials with special thermal properties.

• When any space vehicle re-enters the Earth’s atmosphere, heat is generated by the friction between the air and the vehicle.

• Tiles on the surface of the Space Shuttle are designed to insulate the spacecraft from the high temperature of re-entry.

Further Study of Heat

• Suggestions for further study include:• The Kinetic Theory of Gases• The Laws of Thermodynamics• Statistical Thermodynamics• Differential Equations of Heat Conduction• Phonons and Lattice Vibrations• Temperature Regulation of the Human Body• How to Overclock a Motherboard• Aerogel, also known as Frozen Smoke• HVAC Employment Opportunities• Cryogenics, the physics and engineering of

temperatures below -150 oC.

References and Copyright

• Slide show and images (cc) 2008 by Colin McAllister, colin.mcallister@ymail.com, teaching at Curtin University of Technology, Sarawak, Malaysia.

• Except where otherwise noted, this work is licensed under the Creative Commons Attribution-Share Alike 2.5 Malaysia License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/2.5/my/deed.en_GB

• Students are invited to edit or contribute to this slide show, in accordance with the above license.

• Ref: D.C. Giancoli, Physics Principles with Applications, published by Prentice Hall.

• Ref: www.wikipedia.org for some definitions.• Iceberg photo: http://publicdomainclip-art.blogspot.com/