Physics 2

Chapter 11Sections 3-4

Latent Heat

• Amount of heat energy per kg that must be added or removed when a substance changes from one phase to another

Q = Lm

Latent Heat

• Lf = latent heat of fusion = 3.33 x 105 J/kg

for ice

• Lv= 2.26 x 106 J/kg for water

• Table of values p 375

Sample Problem

A 7 kg glass bowl (c=840 J/kgC⁰) contains 16 kg of punch at 25 ⁰C. 2.5 kg of ice (c=2000 J/kgC⁰) are added to the punch. The ice has an initial temperature of -20 ⁰C. The punch may be treated as if it were water (c=4186 J/kgC⁰). What is the temperature of the punch, ice, and bowl when they reach thermal equilibrium?

Sample Problem

How much energy is required to change a 42 g ice cube from ice at -11 ⁰C to steam at 111 ⁰C?

Radiation

• Process by which energy is transferred by means of electromagnetic waves

Electromagnetic Wave

• Wave formed by electric and magnetic fields fluctuating together

ex – visible light, radio, microwaves

• Doesn’t require a medium

ex – sunlight travels through vacuum of space

Radiation cont’d.

• All bodies continuously radiate energy in the form of EM waves

• Objects absorb and emit EM waves simultaneously

• If object is same temp as surroundings then amt of radiant energy absorbed = amt of radiant energy emitted

• Material that is a good absorbed is also a good emitter

Stefan’s Law

• Amount of energy emitted by a body depends on time, surface area, temperature

P = ΔQ/Δt = σAeT4

P = rate at which energy is emitted or absorbedσ=Stefan Boltzmann constante=emissivity – number between 0 and 1 that is

characteristic of the material

Convection

• Process in which heat energy is carried from place to place by the bulk movement of fluid

• When part of a fluid is warmed that part expands, decreasing density, causing it to rise -----cooler fluid replaces it ----- repeats

Convection

• Rate of heating can be approximated by

H = hAΔT

H= rate of convection heating

h=convection coefficient

A= surface area of body

ΔT= temperature difference

Conduction

• Process where heat energy is transferred directly through a material

• Bulk motion of the material plays no role in transfer

• Energy is transferred when atoms in hotter part collide and pass on energy to atoms in cooler part

• Metals are good conductors due to free electrons

Conduction

ΔQ/Δt = kAΔT/d

k= thermal conductivity

– depends on type of material