Unit 2

Matter & Energy

Chapters 3 & 10

Energy

Recall that Energy is the capacity to do work.

Energy can be given off or absorbed. Different terms are used to describe

processes that give off or absorb energy.

Exothermic

Describes a process which releases energy into the environment.

It gives off heat and/or light. That energy can be used to do work.

Endothermic

Describes a process which absorbs energy from the environment.

It absorbs heat and/or light. The absorbed energy is used to

do work.

Law of Conservation of Energy

Energy can neither be created nor destroyed, but changed from one form to another.

We can change Heat into Light and Light into Heat.

We cannot permanently remove light or heat from the universe.

Heat

Energy that is transferred between objects. Heat always travels from hotter objects to

cooler objects. Energy will be transferred until the objects

are at equal temperatures.

The 3 Methods of Energy Transfer

Radiation– Transfer via light emission.

Conduction– Transfer from one material to another by direct

contact. Convection

– Transfer via circulating molecules.

The Usual Manner

Most of the time, at least two methods of energy transfer occur at the same time.

For example, when you light a match, energy is emitted in the form of light.

The molecules closest to the flame gain energy via conduction.

And the room is warmed via convection.

Types of Energy

There are two basic types of energy that matter can have:

Kinetic Energy (KE) is the energy of an object due to its motion.– Can be calculated using KE = ½ mv2

– KE increases linearly with mass.– KE increases exponentially with velocity (speed).

Potential Energy

Energy of an object due to its location or construction.– How far can it fall?– How much energy is stored in its bonds?

Since there is always more PE, we always talk about a change in PE.

There are 3 Modes of KE

Translational KE is motion through space. Vibrational KE is how strongly bonds

vibrate. Rotational KE is how molecules rotate in

space and the rotation of Atoms within a molecule.

The faster Translation, Vibration, or Rotation, the more KE.

Temperature

A measure of the average kinetic energy of the particles in an object or system.

Higher Temp = higher KE– This means more motion (translational,

vibrational, rotational). Lower Temp = lower KE

– This means less motion.

Kinetic Theory of Matter

All matter is constantly moving (except at Absolute Zero, 0 K)

The greater the temperature, the more motion there is!

Evaporative Cooling

Since Temp is an average, when molecules w/ higher KE evaporate, the remaining group has less overall KE.

The temperature drops. This is why we sweat:

– As sweat evaporates, avg KE decreases.

Sweating it Out

When our body temperature rises, our sweat glands secrete a salty mixture that is mostly water.

Our capillaries near our skin flush and heat is transferred from our blood to the sweat.

Water molecules gain enough KE to evaporate, cooling our bodies.

Pot-In-Pot Refrigerator

Invented in 1995 by Mohammed Bah Abba

Won Rolex Laureate award in 2000

Two clay pots nestled with a layer of sand

Sand is soaked in water Evaporation cools interior.

Needs no electricity or fuel – just water from

time to time!

Why would that be useful?

Temperature Conversions

ºC = 5/9 (º F – 32) ºF = 9/5 ºC + 32 K = ºC + 273 ºC = K – 273 The coldest possible temperature is 0 Kelvin Absolute Zero – where no motion occurs

Complete the Table:# Temperature Fahrenheit Celsius Kelvin

1. Water Freezes 32°F

2. Water Boils 373 K

3. Room Temp 20°C

4. Warm Day 86°F

5. Medium Oven 449.67 K

6. Iron Melts 1,809 K

7. Liquid Nitrogen -320.17°F

8. Common Temp 233 K

9.Space

Background Temperature

-451.3ºF

Delta T

T = change in temperature T = Tf – Ti

All T must be in Kelvin Since Kelvin and Celsius are actually the

same scale (but with a different zero point), T in K = T in ºC

It doesn’t matter if you use Kelvin or Celsius,as long as both Tf and Ti are in the same unit!

Don’t use ºF!!!

Units of Energy

SI Unit: Joule (J) = kg·m2/s2

Common Unit: calorie (cal) = the amount of energy (heat) required to raise 1.00 grams of H2O by 1oC (or 1 K)

Food Unit: Calorie = 1 kcal (1,000 calories) 1 calorie (cal) = 4.184 Joules (J) 1 Calorie (Cal) = 4.184 kiloJoules (kJ)

Convert the following:

60.1 cal to J

450 J to cal

190 Cal (in a yogurt cup) to J

60.1 cal * 4.184 J = 251.4584

1 cal

450 J * 1 cal = 107.5525813 4.184 J

190 Cal * 1000 cal * 4.184 J = 794,960

1 Cal 1 cal

251 J

110 cal

790,000 J

Turn off dah lights!

A 100.0 Watt light bulb gives off 100.0 J per second. If the bulb is on for 1.00 hours, how many Calories of energy are used?

1.00 hr * 3600 s * 100.0 J = 360,000 J 1 hr 1 s

360,000 J * 1 cal * 1 Cal J = 86.042065 4.184 J 1000 cal

86 Cal

Eat Your Wheaties…

One small bag of trail mix contains 1880 kJ of energy. What is the Calorie value listed on the bag?

449 Cal

All that and…

A bag of chips has 260 Calories per serving. How many joules are in 1 serving?

If the bag contains 6 servings, how many kilojoules in the entire bag?

62,000 J/serving 372

kJ/bag