The First Law

This lesson states th First Law of Thermodynamics, provides the formula, and discusses Joules’ experiment.

Warm Up: {FM} Index cards, derive the conversion from C to F. What is the conversion from C to K?
LO:Students will be able to convert between temperature scales

State Standard:
Heat and Thermodynamics
3. Energy cannot be created or destroyed, although in many processes energy is transferred to the environment as heat. As a basis for understanding this concept:

a. Students know heat flow and work are two forms of energy transfer between systems.

Warm Up: {FM, Index cards} Derive the formula for the temperature at which degrees C = degrees F. — hand in

 

 

LO: Students will understand the relation between work and heat

 

Agenda:
1:practice worksheet on temperature conversions. – 1/2 hour
2: First Law of Thermodynamics (Work is heat)

The First Law of Thermodynamics

The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but cannot be created or destroyed. The first law is often formulated by stating that the change in the internal energy of a closed system is equal to the amount of heat supplied to the system, minus the amount of work done by the system on its surroundings.

One way to write this principle is: Delta U = Q + W
where U is the internal energy of the system, W is positive if work is done on the system and Q is negative if heat flows out of the system.

Joules’ experiment:

JoulesExperiment-schematic

joule_experiment1

This is an extreme example. — Why?

A more practical example, closer to Joules’ actual experiment uses 2 1kg weights dropped through a distance of 1 meter.

As the weights drop, they lose gravitational potential energy.  How much potential energy is lost? (hint: use Delta PE = mgh)

(ans. 2 x 1kg x 9.8m/s^2  x 1m= 17.6 J)

The dropping weights cause the vanes to spin. If it takes 1 calorie to raise the temperature of 1 gram of water 1 degree C (more on this later when we discuss specific heats), and 1 calorie  = 4.18 Joule

By how many degrees C does the water heat up if there was only
1 gram of water?
10 grams?
100grams?
1000grams = 1kg?

Recall: 1 calorie  = 4.18 Joules

In reality, this was too small a temperature change for Joule to measure so he repeated the experiment by turning the crank,to raise the weights, then allowed the weights to drop. He repeated this over and over – let’s say 50 cycles.

How much did the temperature of 1kg of water increase?

Specific Heat

Specific heat or more correctly, specific heat capacity is a constant that describes how much heat it takes, in calories, to raise the temperature of 1 gram of a substance 1 degree C. Specific heat is sometimes defined in terms of kcal per mole.

The formula is:∆Heat = Specific Heat  x  mass  x  ∆Temperature

For water the specific heat “C” = 1 calorie per gram

heat math worksheet