Physics Exam 3: Heat

Temperature

A number that corresponds to the warmth or coldness of an object

• Measured by a thermometer

• No upper limit

• Definite limit on lower end

Temperature scales: freezing and boiling points

Celsius

• Freezing point of water - 0 C

• Boiling point of water - 100 C

Fahrenheit

• Freezing point of water - 32 F

• Boiling point of water - 212 F

Kelvin

• Freezing point of water - 273 K

• Boiling point of water - 373 K

• 0 K at absolute zero, no lower temp possible

• Same degrees as celsius scale

Heat + flow of internal energy

Heat

• Internal energy transferred from one thing to another due to a temperature difference

• Internal energy in transit

• Heat is not a thing something has, it is an action

Flow of internal energy

• Energy flows from a high temperature substance to a low temperature substance until equilibrium is reached

Quantity of Heat (Q)

Measured in joules or calories

4.18 J = 1 cal

1000 cal = 1 Cal

Specific heat capacity

• The quantity of heat required to change the temperature of one unit of mass of the substance (g) by 1 degree celsius

  • Water: 4.18 J of heat required to change temp of 1 g of water by 1 C

• Thermal inertia - resistance of a substance to a change in temperature

Heat flow (equation)

Heat flow = mass x specific heat x change in temperature

Q = mcΔT

Q_out = - Q_in

mcΔT (object) = -mcΔT (water)

Heat transfer

Thermal energy can be transferred in 3 ways:

• Conduction

  • Solids touching

• Convection

  • Fluid transfer

• Radiation

  • No medium

Conduction

Conduction - transfer of internal energy by electron and molecular collisions within a substance, especially a solid

Conductors/insulators

• Good conductors conduct heat quickly

  • Substances with loosely held electrons transfer energy to other electrons throughout the solid

  • Ex: silver, copper, other solid metals

• Poor conductors are insulators

  • Molecules with tightly held electrons in a substance, transfer energy slowly

  • Ex: glass, wool, wood, paper, cork, plastic foam, air

• Insulation

  • Doesn’t prevent the flow of energy

  • Slows the rate at which internal energy flows

Convection

Convection - transfer of heat involving only bulk motion of fluids

Ex:

• Visible shimmer of air above asphalt on a hot day

• Visible shimmers in water due to temperature difference

• Hot air rises (cool air falls)

Ex:

• Winds

  • Result of uneven heating of the air near the ground

    • Absorption of sun’s energy occurs more readily on different parts of Earth’s surface

• Sea breeze

  • The ground warms more than water in the day time

  • Warm air close to the ground rises and is replaced by cooler air from above the water

Radiation

• Transfer of energy through empty space using electromagnetic radiation

• All objects above 0 K radiate