why do things feel hot? what is heat? what is temperature?

Why do things feel hot?

What is heat?

What is temperature?

Thermodynamics

Thermodynamics

• Study of the effects of work, heat and energy on a system

• From Greek– Therme heat– Dynamis power

Heat

• Energy in transit

• Heat is the energy transferred between objects because of a temperature difference at the molecular level.

• Symbol Q

Temperature

• Measure of the amount of energy the molecules of a substance has

• Relative measure of hot or cold

• Predicts the direction of heat transfer

Temperature Conversions

• Celcius to Fahrenheit

• Fahrenheit to Celcius

• Celsius to Kelvin

Heat Transfer

• Three mechanisms by which heat (energy) is transferred: – Conduction – Convection – Radiation

Conduction

• Transfer of heat through direct contact

• From warm to cold

• Through solids and liquids

Convection

• Transfer of heat through mass movement of a substance

• the "substance" could be air (a gas) or water (a liquid)

• What does hot air do?– rise

• Why?– Because hot air is less dense than cold

air.

Radiation

• Transfer of energy via electromagnetic waves

i.e. Sun warms your face

• All objects that have a temperature greater than 0 K emit radiation

• Hot object emit more radiation than cold objects.

Concept Question

Will a bullet travel ….?A) Further on a hot dayB) Further on a cold dayC) The temperature doesn’t matter, the

bullet will travel the same distance either way.

Thermal Expansion

∆L =αL0∆T

Change in length = coefficient of linear expansion x original length x change in Temperature

This is why a mercury thermometer works – Thermal expansion

Some Atomic Background

• Atom is from Greek - means indivisible• AMU Atomic mass unit, or unified

atomic mass unit 1u = 1.6605x10-27 kg.• A “u” is based on the 12C atom which

was assigned the value of 12.0000u. See Appendix B

• Einstein, after looking at Brownian motion, calculated the approximate diamter of an atom at 10-10 m.

Zeroth Law of Thermodynamics

• If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

Problem

You need to slide an aluminum ring onto a rod. At room temp (20 ˚C) the ID of the ring is 50.0 mm and the diameter of the rod is 50.1mm.

a) Should you heat or cool the ring to make it fit onto the rod?

b) If the coefficient of linear expansion is 2.5 x10-5 ˚C-1, at what temp will the ring just barely fit onto the rod?

Answer

• You should HEAT the ring. While counter-intuitive, the material expands in all directions thus the hole will expand by the same amount as the surrounding metal.

• ∆L is the amount by which we need the diameter to increase 0.10mm – Convert to meters.

• 0.0001 m = (2.5 x10-5 ˚C-1)(0.05m)∆T

• ∆T = 80˚C Tf = 20 + 80 = 100˚C

Weird Water

• Why does ice float?

• When other materialssolidify they sink.

• In a lake, warm lake water is at the top and the colder water is at the bottom except in winter.

• Water freezes at 0˚ C. • BUT, water is most dense at 4

degrees Celsius (or 39.2 degrees F), which is warmer than freezing. So as water continues to cool from 4 degrees C (39.2 degrees F), it becomes less dense and rises back to the top, leaving the slightly warmer water below.

Sources

• http://apollo.lsc.vsc.edu/classes/met130/notes/chapter2/conduction3.html

• http://www.ajdesigner.com/phptemperature/temperature_equation_convert_f_c.php

• http://images.google.com/imgres?imgurl=http://www.lmvp.org/Waterline/spring2002/images/underice.jpg&imgrefurl=http://www.lmvp.org/Waterline/spring2002/stratification.htm&usg=__MqrrJuuW66fKFwNtEQxpbMdwmok=&h=451&w=312&sz=15&hl=en&start=7&um=1&tbnid=ZpmeKY9yGBxu3M:&tbnh=127&tbnw=88&prev=/images%3Fq%3DWater%2Bdensity%2B4%2Bcelsius%26um%3D1%26hl%3Den%26safe%3Doff%26sa%3DN