Thermodynamics Ch. 21 to 24

Review Problem If you mix 2 liters of water at 20ºC with 5 liters of water at 70 ºC what will the final temperature of the mixture be?

Think about it… The temperature cannot be less than 20 ºC The temperature cannot be more than 70 ºC The range then has to be between 20&70 You are adding more hot water than cold, therefore the temperature has to be closer to 70 ºC than it is to 20 ºC

Solution T 1 V 1 + T 2 V 2 = T 3 V 3 (20 x 2) + (70 x 5) = ( T 3 x 7) = T 3 x = 7T T = 55.7 ºC

Metals and Heating Different metals expand at different rates. One use for these different expansion rates is the home thermostat.

Bimetallic strip A thermostat uses a bimetallic strip to open and close an electrical circuit. Most bimetallic strips have brass on one side and iron on the other. Brass reacts to heat more readily than iron. It expands and contracts more than iron.

Heat Transfer Q = mcΔT “Q” is the value of heat transferred (in Joules) “m” is the mass of substance (in grams) ΔT is the change in temperature in ºC “c” is the specific heat of the substance ( in joules/gºC)

Specific Heat Measured in joules/gºC For water, specific heat is joules/gºC This means that it takes joules to raise 1 gram of water by 1ºC

Example Calculate the number of joules of heat needed to change 500 grams of water by 50ºC. We are looking for “Q”

Example solved Q = mcΔT Q = (500g)(4.184J/gºC)(50ºC) Q = joules

Specific Heat Q lost = Q gained Heat is conserved.

Example What would the final temperature be if you mixed 1 liter of 20ºC water with 2 liters of 40ºC water?

Example Unknown is final temperature. Constant is c Mass of 1 liter of water is 1kg or 1000g Mass of 2 liters of water is 2 kg or 2000g

Example solved (1000g)(T-20º) = (2000g)(40ºC – T) T = 33.3ºC

Energy Transfer Energy is transferred in 3 ways –Conduction –Convection –Radiation

Conduction Conduction – Applies primarily to solids. It is the direct transfer of kinetic energy when objects collide.

Convection Applies to fluids. Caused by the different densities of fluids at different temperatures. Caused by the movement of the heated substance itself. Example: Hair dryer, wind

Convection Convection currents which stir the atmosphere produce winds. Earth’s surface absorbs heat unevenly. (Some areas are lighter than others.) Uneven absorption causes uneven heating of the air near the surface and creates convection currents

Convection Daytime – land heats up more than the ocean. Nighttime – land cools down more than the ocean.

The Sun The sun does not warm us by conduction – air is a poor conductor. The sun does not warm by convection - it isn’t possible in the empty space between our atmosphere and the sun. The sun warms by radiation in the form of electromagnetic waves.

Radiation Is thermal energy in the form of electromagnetic waves. These waves range from the longest – radio waves to the shortest – gamma waves.

Waves Objects at low temperature emit long waves. Objects at high temperature emit short waves.

Waves Shorter length infrared waves absorbed by our skin produce the sensation of heat. If an object is hot enough, it will also emit visible light. This is called Radiant Energy

Radiant Energy Red light appears at roughly 500ºC White light appears at roughly 1200ºC This why we say an object is “red-hot” or “white hot”.