THERMO Spoken Here! ~ J. Pohl © TOC     NEXT    ~   156

Hot, Heat, Cold, Work, etc

Ideas, terminology, events and conditons of some special systems are discussed in regard to their "being hot, hotter, heat, work, etc." Considerations include mechanical and thermal aspects.

  1. Nose Themometer: Can your nose tell what's hot and what's not?
  2. Locke's Parlor Game: A game about hot water and cold water.
  3. Hot Potato, LN2 and Molten Lead: How to touch these without a glove.
  4. Put Out the Candle: Snuff of blow?
  5. Hammer Heats a Penny Simple test of heat.
  6. Homecoming Car Bash: Pay $1 to hammer the old car.
  7. Winch Hauls Anchor: Steel on steel sliding causes smoke.
  8. Car Door Jimmy: Spring steel is more than iron.
  9. Impact Melts Musket Ball: Do bullets melt on impact?

 1)  Nose Thermometer: Take your friend into the kitchen. From a drawer select a metal spoon and a wooden spoon. Ask your friend to tell you which spoon is colder. Now place the metal spoon against your friend's nose then place the wooden spoon agains the nose. By the sensations felt, your friend will say the metal spoon is cold and the wooden spoon is warmer. But what does physics say?

The friend is mistaken. The metal and wood are the same temperature. Before the "nose measurements" both spoons had been in the kitchen hours and would have attained the same temperature, that of the kitchen space. The sensaton, "cold" or "hot," of nose-to-material-contact is governed by "heat" as well as temperature difference. With the wood and metal spoons temperature difference is the same but heat was greater with the metal spoon because the conductivity (a property of materials) of metal is greater than that of wood.

 2)  Locke's Parlor Game:  To amuse guests, John Locke (English philosopher ~ 1632-1704) conducted a parlor game regarding hot and cold water. Two waist-high tables were set parallel with space for a person to walk between them. Onto the tables four tanks of water were placed in pairs. Two tanks in progression on the left table and likewise two placed on the right table. The first pair of tanks contained water decidedly hot (left tank) and decidedly cold (right tank). Next came the trick. The next pair of tanks were filled with water at room temperature.

A volunteer guest (blind-folded for more excitement) would step between the first pair of tanks and immerse the left hand into the left tank and right hand into the right tank. Whereupon the guest exclaimed, "left is hot" and "right is cold."

Hands dried promptly, and guided a step or so forward, the guest's left hand and right were immersed in the next set of tanks containing water at room temperature whereupon the guest exclaimed, "left is cold" and "right is hot." Removing the blind-fold, the guest realized the trick, all laughed, and drank champagne.


3)  How to Handle a Hot Potato, Liquid Nitrogen, or Molten Lead:  In your kitchen you can remove a potato baking with a temperature of 350°F from your oven bare-handed. You just need to bounce the tuber from hand to hand. On YouTube a video will show a happy physics prof plunge his hand into a pot of molten lead (622°F 450°C) or into a dewer of liquid nitrogen (-196°F) snatch his hand out, smile and say "Ahaaa!"

Your fingers (or hand ) boils at 212°F so contact with the potato and molten lead could hurt. Also your hand freezes at 32°F so liquid nitrogen could make the fingers solid. Heat happens when there is a temperature difference. The greater the temperature difference the greater likely heat. However, heat takes time. Just get your hand outa there. Okay! A hand that moves fast can avoid finger-destruction" by heat of finger boil or finger freeze.


4)  To Put Out the Candle: One way is to "blow it out." Another is to "snuff it."

  • A blow of your breath onto a lit candle pushes the combusting gases away from the wick and also cools the wick below its burning temperature. Thereby the flame is extinguished.
  • Quick fingers can put out a candle provided. The candle wick pinched rapidly is deprived of oxygen whereupon the flame is extinguished.


5)  Hammer Heats a Penny: Next time you are in a machine shop run this experiment. Put a penny against your nose as "measurement" of its temperature. The penny will seem cool. Next get a heavy hammer, place the penny on a shop anvil and with the hammer bludgeon the penny forcefully. Promptly retrieve the penny and touch it to your nose again. It will be noticeably hotter. So did hammer heat the coin?

Prior to the event, the penny, hammer and anvil had equal temperatures (were in thermal equilibrium). Since temperature difference is required for heat there was none prior to impact. Upon impact the temperature of the penny increases not by heat: there was too little time. Also had heat happened the penny would be less hot (why?).

The cause of the temperature increase was "destructive" compression work on the copper. The blow increased the internal energy of the penny. Internal energy is a function of the measurable properties temperature and specific volume, that is U = U(T,v). The blow did not change the specific volume (v), it changed the temperature (T).

Two other considerations while we are here:
(i)  Suppose you repeated the actions with another coin but did not pick it up after the impact. Returning later you would find the coin visibly distorted but "by your nose" the same temperature as before impact. Your delay in picking up the bludgeoned coin permitted heat whereupon its temperature returned to ambient.
(ii) Our shop event involved impact of a penny (made of copper), a hammer and an anvil (both made of steel). The penny temperature increased noticibly. It was deformed and remained deformed. The penny experienced inelastic deformatiom. Also the anvil and hammer were deformed but not permanently. They, were experienced temperature increase also but only momentarily. The steels of the hammer and anvil (mixtures of iron and carbon) are resilient, resistent to deformation.


6) Homecoming Car Bash: Suppose you were a witness to the following scenario.

For homecoming rally our frat painted an old car "colors-of-our-opponent" and parked it on our lawn to be sledge-hammered for $1 a shot.

A crowd gathered to see Zack, our 320-pound tackle demolish a fender. Ham­mer raised to the trees, Zack landed earth-shaking blows. The fender, loose but not off, was bent substantially. In fact, the fender was enough out of the way that the inflated tire below was fully exposed. A bit winded, Zack intended to deliver another blow. If his hammer next hit a bit low of the fender, it would land full on the tire?

What should you do and why?

You should ask Zack not to swing. If Zack's hammer blow lands on the inflated tire the hammer will recoil from the grip of his hands into the crowd. Whe the sledge-hammer blow lands on metal of the fender the energy of the blow is dissapated into the car material causeing further bending of the car, (with some increase of temperature of the metal) in a harmless way. However, if he hits the tire someone might get hurt.


7)  Winch-Hauls-Anchor Spectacle: To get under way the winch of a tanker must haul the anchor chain and massive anchor shipboard. Initially the winch purrs taking up slack as links (~8"x18") slide smoothly through the closed chock (see image). Soon the load increases and the winch starts to grind, pause, seem to stutter. Then it creaks, thumps, noises like "plucked strings" are heard. Straing full-power, the winch almost growls to create great tension in the chain. Chain movement is sporadic.

Quite a spectacle is seen where the chain wraps to pass through the closed chock. For moments the chain is stationary - as tension increases. Then boom a foot chain might blast through the chock. As that happens there is smoke, pops, crackles. Bits of flaming steel flash like fireworks. The noise, smoke and flash are more than I can describe.

The great tension in the chain causes severe abrasion at the sliding contact of a steel link over the steel chock. A component of the contact force will be tangential or frictional. The normal component of link/chock contact might get large enough to liquify the metal thereby causing the pyrotechnics.


8) Jeep-Door-Jimmy:  Teenage twins locked themselves out of their jeep; keys were visible on the seat. They just needed a "Jimmy," a wire bent to have a hook to insert past the door then pull the door handle from inside. One twin found a length of wire made of spring steel, the other found coat-hanger wire.

"I'll do it," said the twin holding spring-steel wire. To form a hook he bent the wire but it straighten - no hook formed. He bent it again; it again straightened - no hook. Bent a third time, the wire was still the same temperature but still no hook was made.

"Outta the way," shoved his brother. He bent a hook in the coat-hanger wire but it was too large. So he straightened the wire, bent it again - this hook was too small. Straightened again he noticed the wire had become hot. On the third bend the coat-hanger snapped.

Of course the wire materials were different. Spring steel is a combination of iron and carbon. When the spring steel wire was bent (deformed) by work, its internal energy increases in an "organized" way with negligible friction (temperature increase). Some call this type event "reversible." When the spring was released nearly all of the energy left the spring as work returning to the bending fingers. This spring also will break - eventually. (After many, many attempts to bend.)

When coat-hanger wire is bent, its metal receives the energy change of work in an unorganized, inelastic way. The equivalent of friction happens inside; wire temperature increases. The bent event proceeds promptly; too soon for heat. Bent again, bent, then once more; the wire snaps. Had there been heat, the wire might have lasted a bend or so more before failure.

3.16 About Heat and Temperature

Science students are physically aware of temperature and the associated event, heat. When heat and temperature are being considered, the model BODY or point mass is inadequate. The model required is pure substance or material (metal, solid and so). In addition a new energy form is needed: internal energy.