Using the “Clicker”

Using the “Clicker” If you have a clicker now, and did not do this last time, please enter your ID in your clicker. First, turn on your clicker by sliding the power switch, on the left, up. Next, store your student number in the clicker. You only have to do this once. Press the * button to enter the setup menu. Press the up arrow button to get to ID Press the big green arrow key Press the T button, then the up arrow to get a U Enter the rest of your BU ID. Press the big green arrow key.

Temperature What is temperature?

Temperature Temperature is a measure of the average internal energy of an object or a system. Internal energy is energy associated with motion of atoms and/or molecules. Temperature is thus a measure of the average kinetic energy of the atoms and molecules making up an object or a system. (More on this next time)

Temperature scales On the worksheet, see how much you know about the various temperature scales and how to convert between them.

Temperature scales A change by 1°C is the same as a change by 1K. The Celsius and Kelvin scales are just offset by about 273. A change by 1°C is the same as a change by 1.8°F. To convert between Celsius and Fahrenheit we use:

Equations involving temperature If the equation involves T, use an absolute temperature (we generally use a Kelvin temperature). If the equation involves ΔT, we can use Celsius or Kelvin.

Measuring temperature A device used to measure temperature is called a thermometer, and all thermometers exploit the fact that properties of a material depend on temperature. Examples of temperature-dependent properties include: the pressure in a sealed container of gas the volume occupied by a liquid the voltage generated across a junction of two different metals All these effects, and plenty of others, can be used in thermometers.

Thermal expansion Linear expansion Most materials expand when heated. As long as the temperature change isn't too large, each dimension of an object experiences a change in length that is proportional to the change in temperature. or, equivalently, where L0 is the original length, and is the coefficient of linear expansion, which depends on the material.

Thermal expansion Volume expansion For small temperature changes, we can find the new volume using: or, equivalently, where V0 is the original volume.

Bimetallic strip A bimetallic strip is made from two different metals that are bonded together. The strip is straight at room temperature, but it curves when it is heated. How does it work? What is a common application of a bimetallic strip?

Bimetallic strip A bimetallic strip is made from two different metals that are bonded together. The strip is straight at room temperature, but it curves when it is heated. How does it work? The metals have equal lengths at room temperature but different expansion coefficients, so they have different lengths when heated. What is a common application of a bimetallic strip? A bimetallic strip can be used as a switch in a thermostat. When the room is too cool the strip completes a circuit, turning on the furnace. The furnace goes off when the room (and the strip) warms up.

What happens to holes? When an object is heated and expands, what happens to any holes in the object? Do they get larger or smaller? 1. The holes get smaller 2. The holes stay the same size 3. The holes get larger

Holes expand, too Holes expand as if they were filled with the surrounding material. If you draw a circle on a disk and then heat the disk, the whole circle expands. Removing the material inside the circle before heating produces the same result – the hole expands.

Holes expand, too

Thermal Stress If an object is heated or cooled and it is not free to expand or contract, the thermal stresses can be large enough to cause damage. This is why bridges have expansion joints (check this out where the BU bridge meets Comm. Ave.). Even sidewalks are built accounting for thermal expansion. Materials that are subjected to thermal stress can age prematurely. For instance, over the life of a airplane the metal is subjected to thousands of hot/cold cycles that weaken the airplane's structure. Another common example occurs with water, which expands by 10% when it freezes. If the water is in a container when it freezes, the ice can exert a lot of pressure on the container.

Heat What is heat?

Heat Heat is energy transferred between a system and its surroundings because of a temperature difference between them.

Specific heat The specific heat of a material is the amount of heat required to raise the temperature of 1 kg of the material by 1°C. The symbol for specific heat is c. Heat lost or gained by an object is given by:

A change of state Changes of state occur at particular temperatures, so the heat associated with the process is given by: Freezing or melting: where Lf is the latent heat of fusion Boiling or condensing: where Lv is the latent heat of vaporization For water the values are: Lf = 333 kJ/kg Lv = 2256 kJ/kg c = 4.186 kJ/(kg °C)

Which graph? Simulation Heat is being added to a sample of water at a constant rate. The water is initially solid, starts at -10°C, and takes 10 seconds to reach 0°C. You may find the following data helpful when deciding which graph is correct: Specific heats for water: cliquid = 1.0 cal/g °C and cice = csteam = 0.5 cal/g °C Latent heats for water: heat of fusion Lf = 80 cal/g and heat of vaporization Lv = 540 cal/g Which graph shows correctly the temperature as a function of time for the first 120 seconds?

Which graph? Which graph shows correctly the temperature as a function of time for the first 120 seconds? 1. Graph 1 2. Graph 2 3. Graph 3 4. Graph 4 5. Graph 5 6. None of the above

Ice water 100 grams of ice, with a temperature of -10°C, is added to a styrofoam cup of water. The water is initially at +10°C, and has an unknown mass m. If the final temperature of the mixture is 0°C, what is the unknown mass m? Assume that no heat is exchanged with the cup or with the surroundings. Use these approximate values to determine your answer: Specific heat of liquid water is about 4000 J/(kg °C) Specific heat of ice is about 2000 J/(kg °C) Latent heat of fusion of water is about 3 x 105 J/kg

Ice water One possible starting point is to determine what happens if nothing changes phase. How much water at +10°C does it take to bring 100 g of ice at -10°C to 0°C? (The water also ends up at 0°C.) You can do heat lost = heat gained or the equivalent method: Plugging in numbers gives: Lot's of things cancel and we're left with: 100 g = 2m, so m = 50 g. So, that's one possible answer.

Ice water Challenge for next time: find the range of possible answers for m, the mass of the water.

Whiteboard

Using the “Clicker”