Heat – Thermal Energy

ISCI 2002

What is Heat?• Place your finger on the

handle of a ‘hot’ pan. Ouch!

• Heat is energy that is transferred from one ‘system’ to another (two systems have different temperatures).

• Heat transferred to atoms and molecules – increases the KE of each

What is Heat? • Heat – another

form of energy

• Energy transmitted by electromagnetic waves– Sun – Infrared radiation – Energy conversions

• Units of heat:– Calorie (cal) – Joule (J)

Laws of Thermodynamics • Thermodynamics – “movement of heat”

• Thermal energy transfers as heat – no net loss or gain – 1st Law of Thermodynamics

• When heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred.

• Heat spontaneously flows from higher to lower substances, never from lower to higher temperature substances– 2nd Law of Thermodynamics

Energy Conversion

KE and Temperature • KE – Temperature

Relationship

• Temperature is the ‘average’ KE of all atoms and molecules in a system.

• Scales – Fahrenheit – Celsius – Absolute Zero

Heat, Temperature, Mass and the Specific Heat of a Substance

Both beakers contain 1.0 kg of water

Same amount of heat applied

Rise in temperature will be exactly the same.

Heat, Temperature, Mass and the Specific Heat of a Substance

What if you apply the same amount of heat, but double the volume in one beaker?

Left – 1.0 kgRight – 2.0 kg

Heat, Temperature, Mass and the Specific Heat of a Substance

Volumes are 1.0 kg, but on the right is ethyl alcohol

Ethyl alcohol’s temperature will rise twice as fast as the temperature of the water.

Putting it All Together

Phase Changes of Water and Heat A – B : Ice temperature rising to 0 B – C : Ice temp remains at 0 even though heat constantly added; needed to change solid to liquid. Amount of heat need is called heat of fusion

C – D : temperature rises from 0 to 100 C no phase changes occur; only raising temp of water

D – E : At 100 C phase change occurs; amount of heat needed to do this is called heat of vaporization

Thermal Expansion • As heat is transferred through a substance

– Molecules move faster; move far apart

• Liquids expand more than solids

• Engineering Applications – Bridges, Concrete, etc.

• Water expansion – Expands except in 0-4 C range – Occupies greater volume in this range; ice less

dense than water – Freezing increases water’s volume (decreases

density)

Heat Transfer

• Conduction

• Convecti

on • Radiatio

n

Radiation • Radiant Energy – Electromagnetic Waves

• Any substance above absolute zero emit radiant energy– f is proportional to T

• Humans emit ‘infrared’ radiation

• If a substance is real hot – emits light – 500 C (long waves – red light); 1200 C (white hot)

• Sun – emits short wave radiation; Earth emits longer wave radiation (terrestrial radiation) – cool surface

Electromagnetic Spectrum

Greenhouse Effect • How a real

greenhouse works– Glass and visible light – Traps ‘infrared’ sunlight

• Earth’s Greenhouse Effect– Carbon dioxide acts like

glass – Greenhouse gases– Allowed for life to

thrive

• Venus vs. Earth

Absorption of Radiation• Good emitters are good absorbers of

radiation

• A black object filled with hot water will cool faster. Why?