AP Physics B

1. AP Physics B Temperature and Heat

2. A Review of Gas Laws • Boyle’s Law: the volume of a gas is inversely the pressure applied to it (provided the mass and temperature of the gas remain constant). • Charles’ Law: the volume of a gas is directly proportional to the temperature of the gas (provided the mass and pressure of the gas remain constant). • Gay-Lussac’s Law: the pressure of a gas is directly proportional to the temperature of the gas (provided the mass and volume of the gas remain constant).

3. The Ideal Gas Law • If we combine the gas laws from Boyle, Charles and Gay-Lussac, we find that the following is true: P – pressure of gas V – volume of gas T – temperature of gas n – moles of gas R – Universal Gas Constant R = 8.315 J/mol·K NOTE: An ideal gas is not too close to condensing into a liquid and isn’t too dense.

4. Kinetic Theory of Gases • Kinetic Molecular Theory – all matter is made up of smaller particles that are in perpetual, random motion. • This motion is known as Brownian Motion. • The kinetic molecular theory of gases is based upon four postulates: • There are a lot of particles moving randomly (we’ve observed this) • Particles are relatively far from each other (compared to the average size of the particle) • Particles obey Newton’s Laws and only interact with each other in collisions. • All collisions between particles and containers are elastic.

5. Temperature and Heat • The kinetic molecular theory brings us to what we know about temperature. • Temperature – measure of the average kinetic energy of the particles in an object. • Thermal Energy (Internal Energy) – the total energy of all the particles inside a substance. • Heat – amount of thermal energy transferred between objects of different temperatures.

6. Temperature Scales • Celsius (ºC) – metric scale based upon the boiling and freezing points of water under 1 atm of pressure. • Water freezes at 0ºC and boils at 100ºC. • Fahrenheit (ºF) – British scale based upon the melting point of water and the average healthy body temperature. • Water melts at 32ºF and boils at 212ºF. • Kelvin (K) – scale based upon the coldest temperature theoretically possible. • Absolute zero – temperature at which all molecular motion stops.

7. Calculating Heat • Heat depends upon a number of different things: Q – heat (measured in Joules) m – mass of the material c – specific heat of the material DT – change in temperature of the object

8. Specific Heat • Specific Heat – amount of energy needed to raise the temperature of one kilogram of material by one Kelvin (or one degree Celsius). • This is responsible for metal feeling cooler than wood. • If a material has a high specific heat, it will take a lot of energy to heat it up, • BUT a material with a high specific heat will hold its temperature very well. • Typically measured in J/kg·K • Specific heat is unique to each material.

9. Phase Changes • Q = mcDT only works when an object doesn’t melt, freeze, evaporate or condense. • During a phase change, we need to consider latent heat • Heat of Fusion (LF) – energy required to melt or freeze one kilogram of a substance. • Heat of Vaporization (LV) – energy required to evaporate or condense one kilogram of a substance. • Latent heats are measured in J/kg.

10. Temperature vs. Heat for 1 kg of water Temp (ºC) Steam 100 – Water 0 Heat (kcal) -40 – Ice 20 100 200 740

11. Energy Conservation • Heat is a form of energy, so we can still apply the law of conservation of energy. • The amount of energy lost from one substance will be transferred to (gained by) another substance. • NOTE: When an object is getting colder, it is losing energy, so Q will be negative.

12. Methods of Heat Transfer • Conduction – heat transfer via the collision of molecules. • Occurs through contact usually • Convection – heat transfer via the mass movement of molecules. • Occurs in wind currents • Radiation – heat transfer without a medium. • Occurs in the sun