Month: January 1997

What is heat?

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What is heat? — PM, Princeton, NJ

Heat is thermal energy that’s flowing from one object to another because of a temperature difference between those two objects. Whenever an object contains thermal energy—which it always does—the atoms and molecules in that object are jittering about microscopically. Each atom or molecule isn’t completely stationary; instead it is vibrating back and forth, and pushing or pulling on its neighbors. The object’s thermal energy is the sum of the tiny kinetic and potential energies of those atoms and molecules as they move back and forth (kinetic energy), and push or pull on one another (potential energy). The hotter an object is, the more thermal energy each of its atoms has, on average, so this thermal energy tends to flow to a colder object when you touch the two objects together. When that thermal energy is flowing from the hotter object to the colder object, we call it “heat.”

In today’s lecture, you stated that a person accelerating downward OR UPWARD doe…

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In today’s lecture, you stated that a person accelerating downward OR UPWARD does not feel the effects of gravity. How do you explain the g-forces felt by astronauts at escape velocity? – TH

In the lecture, I said that a person who is falling does not feel the effects of gravity, even when they are traveling upward. But when they are falling, they are accelerating downward at a very specific rate—the acceleration due to gravity, which is 9.8 meters/second2 at the earth’s surface. When an astronaut is accelerating upward during a launch, they are not falling and they do feel weight. In fact, because they are accelerating upward, they feel particularly heavy.

You said that from the moment the ball leaves your hand (after you threw it upwa…

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You said that from the moment the ball leaves your hand (after you threw it upward), it accelerates downward even though you threw it upward. However you then said that the ground (gravity) pushed on your foot to make you accelerate, so why would you also not be accelerating in the opposite direction, like the ball? Why would you not accelerate in the direction in which you were pushed?

I got ahead of myself by using forces I had not yet introduced. I was using friction to push me horizontally across the floor! Here is the complete story:

When I tossed the ball upward and it was rising, gravity was pulling downward on it and it was accelerating downward. But when I obtained a force from the ground, it was not gravity that exerted that force on me; it was friction! As we will discuss in a few days, whenever you try to slide your foot across the floor toward the left, friction pushes your foot toward the right. In class, I traveled toward the right because I was being pushed by friction toward the right. I was actually accelerating in the direction I was pushed, just as you expect.

When accelerating, can you decelerate by going in a direction that is not opposi…

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When accelerating, can you decelerate by going in a direction that is not opposite (your velocity)? For example, going north can you decelerate by going east?

Decelerating is a very specific acceleration—always in the direction opposite your velocity. If you were heading north and accelerated toward the east, your velocity would soon point toward the northeast. It would have some northward aspect because you were initially heading north and hadn’t yet accelerated toward the south. It would have some eastward aspect because you had initially been heading neither eastward nor westward and had since accelerated toward the east.

On the other hand, if you were heading north and then turned toward the east, you would have lost your northward velocity and obtained an eastward velocity. This “turning” would have involved a southward acceleration (to get rid of the northward velocity) and an eastward acceleration (to acquire an eastward velocity).

Warner Brothers has been misleading children! The coyote and the anvil hit the g…

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Warner Brothers has been misleading children! The coyote and the anvil hit the ground at the same time!

You’re exactly right. Occasionally one of those cartoons shows the coyote falling with the anvil directly above his head and the distance between them remaining constant, which is what should happen (ignoring air resistance). But more often, the coyote falls much faster than the anvil, hits the ground first, and is then pounded by the anvil. It sure would be neat to live in a cartoon—the laws of physics just wouldn’t apply.

How do rotary telephones work?

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How do rotary telephones work? — JG, DeSoto, Kansas

As your finger turns the dial of the telephone, you wind a spring and store energy in that spring. When you remove your finger, the spring unwinds and its stored energy drives the dialing mechanism. This mechanism consists of a cogged wheel and a switch, as well as a centrifugal governor. As the dial unwinds, the cogged wheel turns and it’s cogs close and open a switch one time for each number on the dial. For example, if you dial a “6”, the switch closes briefly 6 times. For a “0”, the switch closes 10 times. Each time the switch closes during this action, it “hangs up” the telephone briefly. The switching system at the telephone company recognizes these brief hang-ups as signals for establishing the connection. The centrifugal governor controls the rate at which the dial unwinds and makes sure that the pulses coming from the telephone occur at a uniform rate.

How are the paints made that artists (like Rembrandt and Monet) used in the past…

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How are the paints made that artists (like Rembrandt and Monet) used in the past? — SB, Oedenrode, The Nederlands

These paints consisted principally of a pigment and a drying oil binder. The pigment was usually a colored powder that didn’t dissolve in the oil. Historically, these pigments were materials collected from nature. The drying oil binder was usually linseed oil, obtained from the seed of the flax plant and a byproduct of the linen industry. Like most organic oils, linseed oil is a triglyceride—it consists of a glycerin molecule with three fatty acid chains attached to it. But while in typical animal or tropical plant oils the carbon atom chains of the fatty acids are completely decorated with hydrogen atoms (saturated fats) or almost completely decorated (monounsaturated fats), the carbon atom chains in linseed oil are missing a significant number of hydrogen atoms (polyunsaturated fats). The polyunsaturated character of linseed oil makes it vulnerable to a chemical reaction in which the chains stick permanently to one another—a reaction call polymerization. With time and exposure to air, the molecules in linseed oil bind together forever to form a real plastic! This “drying” process takes weeks, months, or years, depending on the chemicals present in the paint. It can be accelerated by the addition of catalysts—chemicals that assist the polymerization process but that don’t become part of the final molecular structure of the plastic.

What is an electric field and how does it affect us?

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What is an electric field and how does it affect us? — MT, Brampton, Ontario

Electrically charged particles exert forces on one another. For example, a negatively charged particle attracts a nearby positively charged particle and repels another negatively charged one. These attractions and repulsions are mediated by electric fields that are created by those charges. By this statement, I mean that the negatively charged particle creates an electric field around itself and this electric field is what ultimately exerts forces on the other two charges—attracting the nearby positively charged particle and repelling the negatively charged one. Whenever an electrically charged particle finds itself in an electric field, it experiences a force. The direction of that force depends on its electric charge (either positive or negative) and on the direction of the electric field (which may have somewhat different directions at different points in space). The strength of that force depends on the amount of electric charge on the particle and on the strength of the electric field (which can vary from nothing at all to extremely strong).

But while electric fields always exist around charged objects and exert forces on any other charged objects that enter them, electric fields can also exist far away from charges. Electromagnetic waves contain electric and magnetic fields (the magnetic equivalents of electric fields) and these two fields sustain one another as the wave travels. Although electromagnetic waves are created and destroyed with the help of charged particles, they can travel alone and without any nearby charged particles to assist them.

While electric fields exert forces on the charged particles in our bodies, the response of those charges isn’t likely to injure us. When you are exposed to an electric field, there is a subtle rearrangement of electric charges on the surface of your body that then creates its own electric field. The result is that there is essentially no electric field inside you. Only when you are exposed to extremely strong electric fields, and spark and currents begin to flow through you, is there any significant effect to you.

Where does the wax from a burning candle go? Also, why do beeswax candles burn v…

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Where does the wax from a burning candle go? Also, why do beeswax candles burn virtually completely, leaving no wax behind at all? — SC, Rhode Island

The wax molecules in the candle react with oxygen in the candle flame and are converted into water molecules and carbon dioxide molecules. That reaction is associated with combustion and it releases energy so that the candle produces light and heat. The molecules formed by this combustion drift off into the air.

Normal candle wax (paraffin wax) consists of relatively large hydrocarbon molecules. Each molecule in paraffin is a chain of between 30 and 50 carbon atoms that are surrounded by hydrogen atoms. Because its molecules are fairly long and they stick together reasonably well, paraffin is a firm, crystalline solid. If the chains were shorter, say 20 to 30 carbon atoms long, the material would be softer—it would be a liquid-like wax known as petroleum jelly. If the chains were much longer, say 2000 to 3000 carbon atoms long, the material would be firmer—it would be a solid known as polyethylene. Still shorter chains are used in machine oil, diesel fuel, unrefined gasoline, and finally petroleum gases such as propane and methane. The shorter the chain, the softer, thinner, and more volatile the hydrocarbon is at any given temperature. All of these hydrocarbon molecules can burn completely, leaving only water molecules and carbon dioxide. In a candle, the heat of the flame vaporizes the wax molecules—they become a gas—and they then burn completely in the flame itself. As long as the wax doesn’t drip away from the flame, the flame will consume it all completely and leave no ash or wax. Although the structure of the molecules in beeswax is slightly different from that in paraffin, beeswax also vaporizes from the heat of the flame and then burns completely.

When friction is made by two atoms rubbing

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When friction is made by two atoms rubbing — it makes heat. But how and why? — GN, Marine City, MI

When two surfaces slide across one another, some of the mechanical energy in those surfaces is converted to thermal energy (or heat). That’s because the surfaces are microscopically rough and their atoms collide as the surfaces slide pass one another. Each time a collision occurs, the atoms that collide begin to vibrate more vigorously than before. In this process, the surfaces lose some of their overall mechanical energy but the atoms gain some randomly distributed local vibrational energy—more thermal energy. Those surface atoms become hotter. As the sliding continues, large regions of the surfaces become hotter and the surfaces lose much of their energy. If you don’t push them to keep them sliding across one another, they’ll come to a stop as all their mechanical energy is converted into thermal energy.