Term
| Why does an ice cube feel cold? |
|
Definition
Heat from your hand enters the cube by conduction.
The hotter object (in this case your hand) has more energy per particle. Energy therefore leaves your hand and goes into the ice. Important: Heat always flows from hot to cold! Also note that ice requires a lot of heat to change phase.... and it might even be below the freezing point. because your hand is just changing temperature, not phase, the heat loss you experience definitely cools the temperature of your hand. |
|
|
Term
| Why are good thermal conductors generally good electric conductors also? |
|
Definition
Electron motion is responsible for determining both properties.
Heat flow requires motion of particles, so that they can interact with each other, or even move long distances, carrying heat energy with them. Because the electrons within a metal are "free" (not bonded to individual nuclei), they are more free to pick up heat and move along until they collide with another particle. Because electrons are also charge carriers (of negative charge a we will later learn), the free electrons are better able to have charge flow too (which is electric current). |
|
|
Term
| You are watching a science fiction movie where people on a space ship are traveling around the universe observing stars with different temperatures. Which of the following stars has the lowest surface temperature? |
|
Definition
A reddish star
Stars emit heat energy through radiation. The color of the radiation, and total amount of radiation, depends on the temperature of the object emitting radiation. Stars that look red are really about the same temperature as a red-hot stove. (We can see those far away stars because they are still BIG... the total radiation also relates to surface area). White or yellow stars are emitting across the spectrum pretty evenly. Blue stars (like a blue flame) are hottest.
Note: even people and animal emit some heat as radiation. Our body temperature produces heat that is in the infra-red region of the spectrum... lower energy, but able to be seen by the infrared cameras or viewing glasses like those used in police shows.
Because the radiation from "black-body radiation" is broad-band (a spread of colors, not a single color), our star also emits IR and UV light. The highest energy light is in the UV region, and it's that light that our sunscreen and glasses should protect against the most. |
|
|
Term
| A shiny, white transparent surface prevents heat transfer because |
|
Definition
it has emissivity of zero and reflects all light that strikes it.
White surfaces have low emissivity... which means they neither emit OR absorb light. In the desert, you may want to wear white... because it won't absorb the sun's radiation. But the sand is also light, and doesn't absorb the sun's radiation much. Therefore the temperature drops at night (especially with convective cooling). Since your body will be making the heat to keep you warm, you'll want to be using a white blanket at night because it will not emit the heat you have, and you may want to layer, underneath that white blanket) a black blanket you put out while the sun was still hot. The blankets also will provide an insulating layer to protect you from convective losses.
Note: one reason to wear dark clothes (emissivity ~ 1) during the day in the desert is of you plan to be in the shade. That way any heat you do gain (through absorption during brief stints in the sun, your own body's chemical conversions, or through convection in hot desert winds) will be better emitted heat through radiation. |
|
|
Term
| The main reason housing and natural animal insulation works because: |
|
Definition
the insulating materials trap air and prevent convection.
Insulating layers are most important because they protect against convective losses. You warm the layer next to your skin, and it's not blown away. Also, the material in these layers, often air) air does not have good thermal conductivity, but that's a secondary factor. |
|
|
Term
| You are talking with your significant other and looking up into a beautiful night sky. He/she comments “You know, it is sad that we couldn’t tell if there were any black objects out there no matter how hot they were because a black object would never emit light at any temperature.” You would comment nothing because: |
|
Definition
even though you know it is a misconception and that black objects absorb and emit radiation perfectly, you don’t want to ruin the evening.
Black objects actually radiate BEST. A black iron ball that is VERY hot will actually glow red -- but cool off quickly. When it's cooler, its radiation will be at lower energy frequencies, like IR or radio -- which are not visible to our eyes. Only object at 0 Kelvin will not radiate (but remember that theoretically we can't even reach 0K, due to quantum mechanics still having an energy/temp even at it's lowest possible state). |
|
|
Term
| Soil heats up much faster than water when the two are exposed to sunlight. Use that fact and your understanding of heat transfer to predict which way the wind will blow near the surface of the earth as the sun sets near the seashore. |
|
Definition
The surface wind will blow from the land toward the water.
Heat flow is from hot to cold. The water will reflect the radiation back into air. The soil will absorb it, then transfer it through some conduction/convection into the surface layer above the soil. The surface layer of air is warmed above the oil... and the warm air moves via wind convection into the cold area above the water. |
|
|
Term
| When cooking a potato in a conventional oven it is useful to stick a nail in it, which makes it cook faster. This is because: |
|
Definition
the nail is a good conductor of heat.
The nail's top absorbs heat from the local air in the oven (or even from the radiation of the burner in the oven, depending on how the oven and burner are located). If the oven is a convection oven, the currents will even blow hot air to the nail to increase its heating as it takes heat from the local air). The iron nail transports this heat down its length, and then transfers some of that heat into the inside of the potato (via conduction). This is quicker cooking than transferring all the heat from the surface layer of air around the potato into the potato just via surface conduction. |
|
|
Term
| A common glass thermometer contains a red or silver liquid that rises upward in a glass tube as the temperature of the thermometer increases. The liquid moves up the tube because: |
|
Definition
while both the glass, air and liquid expand as their temperatures increase, the liquid expands more rapidly than the glass does and the air can be compressed.
Air has the highest expansion coefficient... but it is compressible. The expansion coefficients of solids compared to liquids tends to be small, so the tube doesn't get wider and longer even as the liquid expands. Therefeor the liquid is forced to move up the tube and compress the air. |
|
|
Term
| Wind is one of nature’s ways of transferring energy through: |
|
Definition
convection.
Since air is moving long distances and carrying the particles and their temperature with it, the heat flow is through convection.
Conduction is when heat is transferred from one set of locally restrained molecules to the next, and radiation is when heat is moved through EM waves. |
|
|
Term
| In making ice cream, the flavored cream is in a container in contact with ice. In order to freeze the ice cream why is it essential to make the ice melt at a very low temperature? |
|
Definition
The low temperature of the ice as well as its melting removes energy from the flavored cream, causing it to freeze.
The ice probably starts even below 0 degrees C. It will warm, then begin to melt at 0 degrees C. When it is melting, the temperature will be constant, at a cold temperature that will hopefully freeze your ice-cream (which contains some fats that will solidify better). Adding the salt lowers the temperature of the melting transition (the salt prevents any ice that has turned to water from re-freezing because it gets in-between the molecules). With a lower temperature (still constant during melting as long as the solution is still "saturated" (with as much dissolved salt as possible), the ice cream will solidify even faster as the heat is drawn from it and is used to melt the remaining ice in the ice-salt mixture. The actual freezing temperature of your ice-cream depends on the fats, as well as any sugars (which decrease the temperature of the transition -- if your ice-cream is too sugary and not fatty, and if your ice-salt solution doesn't contain much salt, you might not be able to freeze your ice cream). |
|
|
Term
| Which temperature scale has 373 as the boiling point of water? |
|
Definition
Kelvin
The most important temperature scales to us are Celsius, Kelvin, and Fahrenheit.
Water freezes at 0 degrees C and boils at 100 degrees C.
Water freezes at 273 Kelvin and boils at 373 Kelvin. All motion of molecules stops at 0 Kelvin (this is theoretically not possible).
Fahrenheit is only important to us regionally (with weather temperatures and baking temperatures). Water freezes at 32 degrees F and boils at 212 degrees F. |
|
|
Term
| You are hired as an engineer at a thermometer factory. Suppose you built a water thermometer by replacing the mercury in a mercury thermometer with water. You want to measure temperatures above freezing with it. How would it behave as the temperature drops from 5o C to 1o C? |
|
Definition
| The reading will decrease and then begin increasing. |
|
|
Term
| In making ice cream, the flavored cream is in a container in contact with melting ice. When the ice cream freezes it give energy up to the ice / water mixture. In theory, what should you observe happening to the temperature of the ice / water mixture? |
|
Definition
| It will not change because the energy is going into its melting the ice. |
|
|
Term
| A popular physics trick/demonstration involves placing a paper cup with water in it on a burner, and boiling the water in the cup. Although part of the cup may burn, the part containing the water does not because |
|
Definition
the water absorbs most of the heat from the flame.
Wet paper, in a thin layer, is a good conductor of heat (don't try to pick up a hot pan with a wet towel!) But water in bulk, like in a cup, requires a lot of heat to change temperature. You can be holding the cup in the falme a long time before the water boils... and even longer before the water boils off and the dry cup catches fire. (The combustion energy required is going to be higher then the temperature it can reach with the water inside). |
|
|
Term
| Some scientists assert that there would be no life on earth if ice did not have a molecular structure with a lot of open space because: |
|
Definition
the density of ice would be greater than water and lakes would freeze solid - from the bottom up.
This is discussed well in your text. The ice layer forms on the top of a pond or in even larger bodies of water, like the oceans, as there a is an exchange of heat, via convective exchanges as water and airs swirl at the top (where there may be waves and winds that help convective exchange). Because of the air pockets, the ice floats, and can form a layer that prevents further convective losses. The densest water is actually around 4 degrees C... above freezing) and it settles at the bottom of a pond. Life can be at the bottom and not be frozen. Because the food chain starts with algae or single-celled organisms, then works it way to top, this can help the larger animals (who live near the bottom) survive. If the water gets too cold for these organisms, there a chance that some of them are at the bottom to regrow the populations (and evolve into other life-forms) when temperatures rise again. |
|
|
Term
| You get a job as a dish washer to help with your college bills. The first thing the management tells you is to be very careful of the steam – it is exceedingly dangerous. Why? |
|
Definition
Steam at 100 degrees C can burn you much more than water at 100 degrees C, because it needs to lose heat and condense before it begins to cool.
Warning: Hot water is also dangerous, because water has a high heat capacity and will need a lot of heat loss to decrease in temperature. But it's worse if it starts as steam! Steam must lose lose heat to condense, before it can lose heat to cool off. You will burn more in steam than in hot water. |
|
|
Term
| Why do you feel cold when you step out of a swimming pool and are wet? |
|
Definition
The process of the water evaporating off your body removes more heat from your body than air convection with the dry surface.
The molecules of water on your surface have a spread of energies. In evaporation, the ones with the highest energies leave, taking their contribution to heat away with them. The lower energy (low temperature) ones are left. You cool off pretty efficiently. This is why it is useful that our bodies sweat when we start to overheat. |
|
|
Term
| What temperature is frozen water (assume pure water)? |
|
Definition
| 0 degrees Celsius or less. |
|
|
Term
| Why do you add salt to water before making spaghetti (or another pasta)? |
|
Definition
Salt raises the boiling point of the water to a higher temperature, so your pasta cooks faster.
Adding something that dissolves lowers the melting point AND increases the boiling point. The liquid range is expanded on BOTH ends. |
|
|
Term
| Which of the following is the measure of time it takes for an oscillating object to complete a full cycle back and forth to its original location? |
|
Definition
|
|
Term
| The SI unit of frequency is: |
|
Definition
inverse seconds or Hertz (Hz)
In oscillations, Hertz (Hz) is the SI unit of frequency. It could also be expressed as cycles per second or inverse seconds (1/s or s-1).
In oscillations, seconds is the SI unit of period. Period (in s) is 1 divided by the frequency (in H). T = 1/f.
Newtons is the unit of force.
Newtons/meter is the unit of spring constant. |
|
|
Term
| You are a superb proposal writer and convince NASA to let you tag along to Mars. You bring a pendulum clock in order to study gravity (note: the local gravitational acceleration is 38% that of Earth). Compared to how it would be on Earth, the time it takes for the pendulum to compete one cycle will be: |
|
Definition
more (it oscillates slower than on Earth).
The period (time per cycle) of a pendulum depends on length and "g." As the restorative force (relating to g) weakens, it doesn't accelerate back to equilibrium as quickly when it is offset, so the period gets bigger (the individual oscillations are slower). |
|
|
Term
| On a warm, balmy summer day you decide to relax by watching the latest building renovation in your neighborhood. You notice that a very heavy wrecking ball is attached to a much lighter chain and is dangling from a crane and swinging back and forth. As long as it does not swing too high the time it takes the wrecking ball to complete one full oscillation will be independent of: |
|
Definition
both the amplitude of the motion and the weight of the ball.
The period of a pendulum only depends on L and g. L effects the rotational mass (and inertial factors) with a pendulum. A long L takes a long time to turn around because it has a high inertial factor. Mass effects the rotational mass too... but m is also in the restorative factor, the weight of the wrecking ball). Interestingly, mass cancels out.
Or maybe this might not surprise you: pendulum motion is just "modified falling" -- and the downward motion of two different masses is still side by side if they are released from rest. Mass doesn't have an effect. |
|
|
Term
| You are hanging a tire swing for your younger cousin. When you pull back the empty swing and let it go for practice, you measure the time it takes to swing back and forth, and decide it is too short for your cousin's comfort. Should you make changes, and if so, what should you do? |
|
Definition
You should make the rope that holds the tire swing longer (increasing the height of the support if needed.
If you want to LENGTHEN the period, LENGTHEN the rope. it increases the rotational mass (the inertial factor of the pendulum's motion). |
|
|
Term
| You just bought an old car which you want to restore. It still drives, though, so you decide to take some friends for a ride in it and you notice that, at stop lights the car bounces up and down because the shocks are gone. They’re not bad; they’re gone. Anyway, later that day you take many more friends in your car. At stop lights now, you notice that the bouncing frequency will: |
|
Definition
be lower than when it contained less friends.
Increasing the mass on a simple up and down spring-mass system increases the inertial factor of the system and therefore lengthens the period. It's harder for a spring to turn back systems with inertia. |
|
|
Term
| An astronaut leaves Earth and goes to the Moon. While preparing to leave, he notices that he bounces up and down a bit in his space boots when he jumps off a bench and lands on the ground. The space boots have springs in them. When he goes to the Moon and jumps off the spaceship onto the ground, what will he observe? Assume the characteristics of the ground are the same. |
|
Definition
The oscillations will be the same frequency
The local g has an effect on the resting position of the astronaut on his boots (the springs won't compress as much on the moon when he stands still), but g has no effect on the frequency of oscillation -- the oscillation is effected only by his mass and the spring constant (which is a function of the spring). |
|
|
Term
| Which of the following will change when an astronaut in space boots (with springs in the soles), goes to the Moon (where g is smaller)? |
|
Definition
the weight of the astronaut.
The spring constant (in N/m) is a function of what material the spring is made of, as well as it's dimensions (coil spacing, diameter, etc.).
The mass is just how many protons, electrons, and neutrons that are in the astronaut. Unless the astronaut ate a lot of food or fasted, this won't change measurably.
The frequency of the oscillating motion is only dependent on the above two factors... which don't change.
The astronaut's weight is the gravitational force on the astronaut (mg... in Newtons!), this will change when g changes. In this case, the astronaut's weight is smaller. |
|
|
Term
| You have transported to the future (when humans can vacation on the Moon). You have a pendulum clock and clock that is made from a mass on a spring. Which should you give to your cousin, who is moving to the Moon to open a hotel? Your cousin does not want to recalibrate the clock. |
|
Definition
You should give him the clock made from a mass on a spring.
The pendulum clock will run slow (to fix it, you will need to shorten it's length appropriately. The spring-mass clock is not effected. |
|
|
Term
| What does frequency measure? |
|
Definition
| the number of cycles in a unit of time (ex units like cycles per second or revolutions per minute). |
|
|
Term
| The pitch of a note (whether it is a high note like a flute or a low pitch like a sousaphone) is determined by which of the following properties of a sound wave? |
|
Definition
|
|
Term
| The loudness of a sound is determined by a wave's: |
|
Definition
amplitude.
In a sound wave, the amplitude variations (in pressure) determine the loudness. When a sound wave reaches your ear (or a microphone) the molecules of the wave distort the surface (or your eardrum or the microphone sensor) and are interpreted as "loudness." |
|
|
Term
| Sound is a longitudinal wave. This means that the motion of the particles in a traveling sound wave (air particles or particles of another medium carrying the wave, such as water) is best characterized as: |
|
Definition
| back and forth oscillation along the direction of the wave's propogation. |
|
|
Term
| In a cartoon, a bunny is surfing on a 10 foot wave crest that is rapidly approaching the shore. The wave crest travels onto the shore intact, with the bunny still on top of it, and continues to carry him across level land for the next twenty miles. This sort of thing can't happen in real life because: |
|
Definition
| wave crests are made from local water and there is no water on land from which to build the crest. |
|
|
Term
| Which of the following best characterizes the motion of an object bobbing on a water wave? |
|
Definition
| Circular motion. Because the water at the surface is free to oscillate up and down as well as horizontally, water waves at the surface have a circular motion in their oscillation. The "Rayleigh waves" (surface waves) of an earthquake also have this structure and can therefore cause a lot of damage. The "L-waves" (Love waves) are transverse horizontally, the "S-waves at transverse vertically and the "P-waves" are longitudinal. These generally cause less damage. They also travel at different speeds in the ground (and their speed depends on local ground structures). |
|
|
Term
| A wave is traveling in a medium (material)? Which properties determine the speed of the wave? |
|
Definition
Only the properties of the material.
For our purposes: the speed of a wave in the material depends only on the characteristics of the material in which the wave is traveling. (There are some "dispersive materials" that respond differently to different incoming frequencies... but we won't be discussing that until we get to light. Even then, it is the MATERIAL that is determining the speed. |
|
|
Term
| You send wave pulses down a long rope with a given frequency (back and forth oscillation of your hand). How can you increase the speed of the wave pulses? |
|
Definition
Switch to a rope with a smaller mass per unit length.
Because the speed of the wave pulses depends on the material in which the wave travels, you need to change the nature of the material. you an speed up a wave on a string by either increasing the tension of the string (increasing its "restorative factor") or by decreasing its mass per unit length (its "inertial factor"). |
|
|
Term
|
Definition
fastest in solids.
Solids have the strongest interactions between molecules (because of their proximity and bonding) so sound travels quickest in solids. |
|
|
Term
| Two waves are traveling towards each other, and two of their crests meet each other. At this particular moment in space and time, they are said to be "in phase" and the crests momentarily add, creating a larger crest. This is best described as: |
|
Definition
| constructive interference. |
|
|
Term
| The term "timbre" is best described as: |
|
Definition
superposition of harmonic overtones that produces a characteristic sound.
Timbre changes the quality of a tone, without changing its overall pitch. You can sing the words "we" and "you" at the same pitch (or main frequency)...put the sound is different and you can distinguish the words because of the different overriding frequencies that have different amplitudes relating to the slight differences in the way you adjust your mouth and throat. |
|
|
Term
| An instrument's simplest vibrational standing wave is also known as its: |
|
Definition
|
|
Term
| The sound wave that you hear from the instrument is a: |
|
Definition
| longitudinal traveling wave. |
|
|
Term
| The string on a guitar is vibrating in its fundamental mode. The wave on the guitar string is best described as a: |
|
Definition
| transverse standing wave. |
|
|
Term
| True or false: The speed of a wave on a vibrating guitar string is the same as the speed of sound in air. |
|
Definition
|
|
Term
| Which guitar string will have the lowest pitch? Assume the length is the same. |
|
Definition
| a heavy string under low tension. |
|
|
Term
| When the first overtone (second harmonic) of a string is excited, there is a spot in the middle where the string is not moving up and down, but is instead stationary. This spot is known as: |
|
Definition
|
|
Term
| In order to break a wine glass, an opera singer produces a note at the same frequency as one of the natural vibrational modes of the glass. The wineglass starts to vibrate because of: |
|
Definition
|
|
Term
| One instrument is well-tuned, and the other is slightly off. When they are played at the same time, the sound doesn't make you immediately cover your ears... but you hear a wavering in loudness, as the two waves sometimes interfere constructively, and sometimes destructively. This is known as: |
|
Definition
|
|
Term
| A musician plays a flute. When you hear the note, the energy transfers: |
|
Definition
| from a standing wave in the flute to a traveling wave outside, both of which have no long distance motion of particles. |
|
|
Term
| Waves are efficient carriers of energy because: |
|
Definition
Waves are efficient carriers of energy because:
Important: Particles do not more long distances in waves. They interact with neighboring particles and transfer energy efficiently. Since these are molecular interactions, energy transfer is often very efficient. |
|
|
