Nov. 17 – Thermal Energy

Today I handed back your labs.  Here are some tips for the next lab:

Lab Tips

- only have ONE independent variable

- only have ONE dependent variable

- Discussions should talk about trends (increase/decreases)

- Discussions should talk about %error

- all graphs and tables should have titles

- a good title is descriptive, NOT ___ vs. __

Use these tips to improve your next lab report.  The next lab is due on Thursday!

Thermal Energy

Today we started a new section on Thermal Energy!

Which one do you think represents the higher temperature?

Definitions

Thermal Energy -  total kinetic and potential energy of all particles in an object.

Temperature - Average kinetic energy of particles in an object.

    Absolute zero - the lowest possible temperature  –273.15 °C   or   0 K.

    Kelvin - Unit of temperature.  Same magnitude as °C, but starts at absolute zero.

Heat - Energy transfer from a hotter object to a cooler object, ∆E

Thermal Equilibrium - When two objects are in contact and the net exchange in energy is zero.  (they are the same temp.)

Lord Kelvin!  The unit of Kelvin was named after William Thomson, 1st Baron Kelvin.

Question: What is the temperature in interstellar space?
Answer: 2.7 K

Stephen Hawking is famous for many things, but one of them is showing that black holes have temperature!

Methods of Thermal Energy Transfer

Conduction: direct contact

    - touching a stove, chocolate melts in your mouth

Convection: particles in a fluid carry energy

    - wind, boiling pot of water

Radiation: electromagnetic radiation to transfer energy

    - sunshine feels warm, spot lights

Heat Capacity

The amount of energy it takes to change the temperature of an object by 1 K.

   - different for every substance

   - depends on mass

   - measured in units of    J / K

Specific Heat Capacity

The heat capacity for 1 kg of a substance.

    - different for every substance

    - measured in units of      J / (K kg)

To find heat capacity, we use this equation

c = Q / m∆T

c = specific heat capacity
Q = heat (energy transferred, ∆E)
m = mass
∆T = temperature change, K

Here are the specific heat capacities for common substances:

Using these values of c for many different substances, we can solve for Q or T.

When one substance loses energy, another one must gain it.  This can be expressed as the following equation:

Q1 = - Q2

Or we can substitute in from the above and get:

Notice the final temperatures are the same on both sides.  The system as reached thermal equilibrium.

The coffee and the cup might start at different temperatures, but eventually will reach the same final temperature.

Homework

You should be able to get started on the homework from the Unit Outline, look for the Temperature and Heat section.