Heat

Chapter 10

Defining Temperature

• Temperature is defined as a measure of the average kinetic energy of the particles in a substance.

Temperature measures energy

• Energy must be either added or removed to change temperature.

Defining Temperature

• “A” has a low kinetic energy so it has a low temperature. Energy is added to “B”, thus it has a higher temperature.

Defining Temperature

• The energies associated with atomic motion are referred to as internal energy.

• Internal energy depends on the size of the sample.

Big sample = more atoms= more internal energy

Thermal Equilibrium• Thermal equilibrium is the state in

which two bodies in physical contact with each other have identical temperatures.

Ex: Hot liquid cocoa + cold milk = lukewarm drink

Converting Temperature Units

Celsius to Kelvin

TK = TC + 273

Kelvin to Celsius

TC = TK - 273

Defining Heat

• Heat – the energy transferred between objects because of a difference in their temperatures

Units of Heat Energy

• SI unit: joule (J)

• Other units are Calorie (cal), Kilocalorie (1000 cal), British thermal unit (Btu) and therm (100,000 Btu)

Conservation of Energy

• Conservation of Energy: if changes in internal energy are taken into account with changes in mechanical energy, the total energy is a universally conserved property.

Conservation of Energy

• Conservation of Energy:

PE + KE + U = 0

• OR

PEi + KEi + Ui = PEf + KEf + Uf

Example

• A vessel contains water. Paddles that are propelled by falling masses turn in the water. This agitation warms the water and increases its internal energy. The temperature of the water is then measured giving an indication of the water’s internal energy increase.

Example

• If a total mass of 11.5 kg falls 1.3 m and all of the mechanical energy is converted to internal energy, by how much will the internal energy of the water increase?

Solution

PE + KE + U = 0

• PEi + KEi + Ui = PEf + KEf + Uf

• PEi = mgh

• Mgh + 0 + Ui = 0 + 0 + Uf

U = Uf - Ui = mghU = (11.5 kg )(9.81 m/s2)(1.3 m)U = 1.5 x 102 J

Specific Heat Capacity

• Specific heat capacity: the quantity of energy needed to raise the temperature of 1 kg of a substance by 1 C at constant pressure

• Air = 1.01 X 103 J/kgC • Water = 4.19 X 103 J/kgC

Ultimately…

• The higher the specific heat capacity, the less change you will see in the temperature of that substance under similar conditions• Temp of Sand vs Water on a Hot Day

• Thermal conductors – transfer energy as heat rapidly

• Thermal insulators – slowly transfer energy as heat

• Conduction – Transfer of energy through matter by colliding particles – solid to solid contact

• Convection - Transfer of energy by the motion of heated particles in a gas or liquid

• Radiation - Transfer of energy in the form of electromagnetic waves (no contact)

Homework

• P 363 #4-5• P 370 #1, 2, 5• P 385 #1-3