Lecture: Heat

What is heat? What is the unit for heat? Does the amount of energy of a heated steel nail equal the energy of a heated aluminum nail? How does heat move? We all know what cool, warm and hot are: they are description of heat. So, what is heat?

Heat is: the movement of thermal energy thermal = temperature, so what is a energy? energy = ability to do work what is work? w = force x distance, so what’s moving? Because heat flows/moves, what direction does it ALWAYS flow?

Energy (heat) always flows from hot to cold: when you go outside on a cold day, you lose your heat to the air, the cold air does not seep into you; pick up a hot pan: heat moves into your hand!! Temperature is a measure of kinetic energy of molecules’ motion Heat is measured by temperature, thus related to each other, but the terms are not the same The amount of heat an object has does depend on the object’s temperature AND the amount of matter the object has: two gold bars at the same temp: a kilogram of gold will have twice as much heat than 1/2 a kg of gold: more molecules moving, thus more energy and heat!

More matter = more energy (more molecules moving) = more heat:

If work = force x distance, what is the unit for work? Because energy is motion (distance), if you were to measure the amount of energy, what is the unit for measuring energy? That’s right, the joule thus it is the metric unit for thermal energy (heat) too! Another way to measure energy is by counting calories (c): 1calorie = 1g of water increases 1 degree Celsius Now, this is different than a food Calorie (C - capital): which is the same as a kilocalorie (what is kilo? 1,000 calories) Calorie is a very old term which should be done away with

A third way to measure energy, non-metric, is by BTU: British thermal unit = increase 1lb of water 1 degree Fahrenheit So what are the three units for measuring heat energy?

If you have two objects at the same temperature, say 25 degrees C, do they both have the same amount of energy? You should be asking yourself two questions: -how much of each substance do you have? -what type of material are each substance? These two questions relate to specific heat (Cp) of a substance: how much heat energy needed to increase one kilogram one degree Celsius Cp Unit = J kg ◦ C

Compare the specific heats of these two substances (note: mass, temp, energy): Different energy Same mass and temp Different Materials

What is inertia? resistance to change of motion remember particle motion is heat: thermal energy Specific heat = thermal inertia = resistance to change in temperature: increase inertia = increase specific heat Specific heat is a characteristic property:(finger print) Inertia is due to: 1. movement of molecules of substance (changes the temperature and changes E K 2. internal movement within molecules (does not change temp and does not change E K

So, a large specific heat means more energy is needed to increase the temperature of a substance, (hummer vs prius), it will change its temperature slowly or a lot of energy must be lost in order for the temperature to change much More Cp = more molecules = more inertia = more energy needed to move molecules: thus retain more energy

Water is one of the highest Cp substances on Earth! water= 4,184 J/Kg C, gold= 129 J/Kg C thus, Cp is not density It is for this very reason that people who live near the ocean do not complain of their climate: the ocean will hold more energy (high Cp) than the land (low Cp) and at night releases its energy to the cooler surrounding land, so they stay warmer at night: Los Angeles, London-Gulf stream-latitude. cool, I mean warm, I mean specific heat

If we look at different substances, yes they have different specific heats and in general, -light atoms will have a higher s.p.heat -More atoms (more inertia) higher sp.heat and specific heat measures the energy per particle (its motion) per temperature change More particles per kg More inertia More specific heat

If we heat a pot of water on a stove, it might take 15 minutes to bring it to a boil If we put an equal amount of iron on the same flame, it would take only 2 minutes to raise it to the same temperature For silver, it would take less than a minute Why? They have different specific heats, why? Due to their molecular arrangement and number of particles per given mass

This is the formula correlating all the following aspects: energy, specific heat (type of substance), mass, and temperature change: is called the heat equation: E = m C P (T 2 – T 1 ) where: E = energy (J) m = mass (in kg) C P = specific heat (Joule/kg C) (T 2 – T 1 ) = temperature change (delta)

Solve using the energy formula, show work: E = m C P (T 2 – T 1 ) kg x J/kg C ◦ x C ◦ (note unit remaining = ?) Required: know, ?know, formula, show work, circle ans 1.Calculate the heat needed to raise the temperature of 13.5 grams of glass from 24.0 °C to 32.5 °C. The specific heat of glass is 0.800J/g °C. (heat energy = 91.8J) 2. What quantity of heat is absorbed by 54.8 grams of copper as it is heated from 32.0 °C to 40.0 °C? The specific heat of copper is 0.385J/g °C. (heat energy = 168J) 3. What is the specific heat of a material if a 46.5-gram sample absorbed 26.5 joules as it was heated from 18.5 °C to 25.0 °C? (Cp = J/kg °C)

Energy transfer: If you put your hand on your desk, you will feel it is cool. What you have done is felt the movement of heat: thermal energy in motion: called thermal transfer Now, how do you know there was motion? Any time there is a difference in temperatures (energy), heat moves The direction heat (energy) moves is ALWAYS from hot to cold There are three ways heat can be transferred:

1.Thermal radiation did you know that the sun is the source for nearly all energy on Earth? (exception?) when you stay out in the sun too long, you can get burned by the sun’s rays: which ones? the sun emits many waves of energy (electromagnetic spectrum waves). Visible light (roygbiv) is only a small part of it the EMS. u.v. rays (next to violet) cannot be seen, but that is what gives you a sun burn. If we get our energy from the sun, then those energy rays traveling through space have to be able to travel through a vacuum: space!

thermal radiation is energy transferred by electromagnetic waves because of their temperature: including light, uv rays, infrared rays. this not only includes the sun, but any atoms with thermal energy emit thermal radiation: your desk, you; any thing with temperatures above absolute zero thermal radiation is not only emitted, it is also absorbed, the degree of absorption depends on the material: a black parking lot absorbs more than the white lines in the lot

Thermal radiation:

2. Heat conduction when an object touches another object, it can transfer its energy by touch: touch your pen, it will heat up particle to particle transfer of heat is heat conduction heat energy only flows if: -atoms are touching -there is a difference in temperature, when thermal equilibrium (?) is reached, heat stops flowing

Conduction:

because heat travels through atoms, the more atoms touch one another, the better the thermal conductor of heat: wood vs metal glass vs foam cup Insulators are the opposite: do not conduct thermal energy well: foam, feathers and yes a vacuum: why? Dewar 

conductors vs. insulators:

3. Convection we know heat can be moved/transferred, that is why you can get burned from lifting a hot pan. This is by conduction Thermal energy can also move by the hot substance moving What is the definition of a fluid? So, it flows and if it is less dense than its surroundings then it will flow/move: up this explains why it is hotter near your ceiling than your floor this is why firefighters always crouch or crawl into burning buildings this is why hot air balloons rise

convection currents:

Convection: global wind patterns: Doldrums? Horse latitudes ?