Chapter 12

Question 1

Define Thermal energy

Answer 1

is the energy stored in matter, composed of the potential and kinetic energies of the molecules as they move.

Question 2

Define Temperature

Answer 2

is the measure of the average thermal energy per molecule

Question 3

Bathtub example:

Answer 3

= If you come in from a very cold day (i.e., you have frostbite, you will warm up much more by soaking in a cool bathtub than by drinking a cup of hot chocolate. Although the temperature of the water in the bathtub is much lower than the temperature of the hot chocolate, the water has much more thermal energy to transfer to your body than the cup does because there are so many more molecules.

Question 4

Measuring temperature symbol:

Answer 4

T

Question 5

The first standard scale: the Fahrenheit scale.

Answer 5

(1714 → Daniel Fahrenheit, a German Physicist) → Fresh water freezes at 32° & boils at 212°; room temperature ~ 68-70°

Question 6

@Réaumur

Answer 6

(1731 → Rene Antoine Ferchault de Réaumur, French Scientist) → Fresh water freezes at 0° & boils at 80° → This scale is no longer in use, and you don’t need to know about it.

Question 7

® Celsius.

Answer 7

(1741 → Anders Celsius, Swedish Astronomer) → It is also called “centigrade” “, meaning “100 divisions” → Fresh water freezes at 0° & boils at 100°; room temperature ~ 20-21° (Celsius actually set it up in the reverse order, with 0° being the boiling point and 100° being the freezing point. This was reversed after his death in 1744.)

Question 8

Relation between Celsius and Fahrenheit:

Answer 8

F= 9/5C+32° or C=5/9 (F-32°) Also, -40° F = -40° C

Question 9

®Kelvin

Answer 9

(1848 → British Mathematician and Physicist Lord Kelvin (real name William Thomson)- As an object’s temperature drops, the Ek of the molecules decreases. At -273.15° C, all motion of the molecules ceases. It is impossible to have a temperature lower than this. The Kelvin scale calls this point 0. (i.e., this is called absolute zero because it is the lowest possible temp.). - A division in the Kelvin scale, (simply called a “kelvin”, not a “degree Kelvin”) equals a degree division in the Celsius scale, so, 0° C = 273.15 K * Note: no “degree” symbol when measuring in K. → Fresh water freezes at 273 K & boils at 373 K; room temperature ~ 293 K - in 2003, physicists at MIT cooled sodium gas to a temperature of 500 pk. (Half a-billionth of a degree above absolute zero.)

Question 10

Absolute zero

Answer 10

lowest possible temp, all motion of the molecules ceases.

Question 11

© Rankine

Answer 11

(1859 → William John Macquorn Rankine, Scottish Engineer and Physicist) - Same scale as the Fahrenheit scale, but with 0° being absolute zero. (The same as -459.67° F) → Fresh water freezes at 491.67° R & boils at 671.67° R. (You don’t need to know about this one either.)

Question 12

Difference between “heat” and “thermal energy”.

Answer 12

→ Thermal energy is what every object has, whereas “heat” is specifically the energy that transfers from a hotter object to a cooler object.

Question 13

— To raise the temperature of an object, the amount of heat energy needed depends on:

Answer 13

• Mass of the object (i.e., the number of molecules)
• Desired temperature change
• Specific heat capacity of the substance → higher specific heat means it takes more energy to increase the temperature → high specific heat also means it gives off a lot of energy as it cools a small amount

Question 14

Symbol for heat energy:

Answer 14

Q

Question 15

Units for heat energy:

Answer 15

joules (like all energy)

Question 16

How can ΔT be negative?

Answer 16

Note: ΔT can be positive or negative → If energy is removed from the substance, ΔT will be negative, and so Q will also be negative

Question 16

The formula for heat energy:

Answer 16

= mCΔT
Q = heat energy (joules)
m = mass (kg)
ΔT = change in temperature (kelvin or °celsius)
C = specific heat (J/kgK or J/kg°C)

Question 17

What can heat energy also be measured in?:

Answer 17

can also be measured in calories → 1 calorie = 4.18 J (1 Calorie, also known as the “large calorie” , is actually 1 kilocalorie = 4180 J

Question 18

Note about the high specific heat of water

Answer 18

→ it is an exceptional substance. To raise the temperature of water requires a large transfer of heat energy. Similarly, it gives off a lot of energy as it cools slightly. That is why large bodies of water moderate a climate so much!

Question 19

The conversion of energy from hydro-dam to hot water in a kettle :

Answer 19

→ Initially water has Ep due to its height. → Water flows through the dam (Ep converted to Ek), causing turbines to turn (transfer of Ek). → Turbines turn generators in which Ek is converted to electromagnetic energy. → Electromagnetic energy flows through power lines to the house, where it enters the electric kettle. → In the kettle, an element converts electrical energy into heat which is transferred to the water as it heats up.

Question 19

Notes about how the specific heat capacity of water is 4180

Answer 19

→ this is where the definition of the large calorie (Calorie) came from. It is the heat energy needed to raise the temperature of 1 kg of water by 1° C.

Question 20

Measuring energy in kW*h:

Answer 20

• Recall that “watts” are a measure of power. Your “power meter” on your house does not measure power (although it sort of indicates power by how fast the wheel is turning), it actually measures the amount of electrical energy you use.
  Remember P = W/t or P- energy transferred or transformed / t
  Therefore, Energy = Pt
  → So it can be measured in Ws, (i.e. J), or in larger units, kW.h Larger amounts of energy, particularly electrical energy, are often measured in kW-h, as your “power meter” and your bill from BC Hydro does.

Question 21

Efficiency:

Answer 21

Although energy is always conserved, when it is transformed, usually only part of it ends up in the desired form. To determine how efficient a device is at converting energy, we use the formula: (Useful energy Output/ Energy input) × 100%

Question 22

The mathematical relationship between joules and kW*h:

Answer 22

(1 K W x h) x (1000W) x (3600s) = 3. 6 x 10^6 J