heat and energy
Outline
- heat expands most matter
- cold makes water expand
- cold is the absence of heat
- conduction can feel cool
- heat flow can be slowed
- liquids can reduce friction
- insulation reduces heat flow
- conduction is heat by touch
- compression warms; expansion cools
heat expands most matter
Lightning Strikes, Wheel Washers, and Cup Cracks
Yard work can be sweaty work, even in autumn. Ajay takes a short break and sits under an apple tree. When warm, we relax. We stretch out. Matter does too. Heat makes matter stretch out as its bits move apart.
Ajay sits in the shade, with an eye on clouds gathering over the horizon. Storm clouds could bring lightning. Rapid heat makes matter expand quickly, and a lightning stroke is both rapid and hot. Hotter than the surface of the sun. A bolt can blow your clothes off—or worse.
Lightning sends a shockwave of thunder as heat expands air around the bolt. Rapidly heated air spreads out as a shockwave that rumbles, echoing off the ground and off clouds.
Lightning striking a tree can travel through the sap, heating and expanding so fast it blows off the bark. People don’t have sap, but we sweat. Doing yard work, Ajay has a layer of moisture over his skin. Lightning can heat and expand sweat like sap, turning it to steam so fast it blows off clothes.
Let’s leave Ajay under the tree and step inside. Loosening a lid is like a lightning strike, but a lot safer. To loosen a stubborn metal lid, run the jar under hot water. Heat in the water works like lightning, at least in concept. It expands metal in the lid, creating a gap between the jar and the lid stuck on by friction. Increasing the gap decreases friction, so the lid is easier to remove.
Pioneers attached wagon wheels this way, by heating a washer (a metal disc, like a coin). Unlike a coin, a washer has a hole in the centre. When heated, the metal expands and so does the hole.
Picture it. First put the wagon wheel on its axle. Heat the washer to expand the hole. When the hole is wide enough, slide the washer onto the axle. As the washer cools, it contracts snugly onto the axle. No need to run to a hardware store, which were few and far between for pioneers.
Apart from blowing off Ajay’s clothes, rapid heat expansion can crack cups. Tea cups and coffee cups are made of ceramic. Tea cups are thin; coffee mugs, thick. Let’s borrow Ajay’s thick ceramic mug, the one that says “Hot Stuff.” Heat moves slowing through ceramic. That’s another way of saying ceramic is a poor conductor of heat.
Pouring hot water into “Hot Stuff” is again like lightning. The inside expands rapidly, but the mug conducts poorly and so is slow to warm. That creates stress between the hot interior and still-cool exterior. Under stress, the mug will crack. Under stress, many crack.
Rather than cracking Ajay’s favourite mug, we could use a tea cup. Thinner cups reduce the temperature difference, inside to outside. There is less ceramic in the cup wall, so less difference. Another option is to put a metal spoon in “Hot Stuff” before the hot water. The metal absorbs some of the heat, so there is less to cause stress.
Under the tree, Ajay might wonder whether there is inverse lightning. Would quickly cooling the inside of “Hot Stuff” crack it from the outside-in? Dry ice is almost a hundred degrees below zero. A cool idea to think about on a hot summer’s day.
cold makes water expand
Earth Tilt, Pop cans, and 7th Inning Breeze
Winters are cold in the Northern Hemisphere. Jaya, next-door, says that’s because Earth is farther from the sun.
JAYA
Earth wanders out even farther some years, so those years it is colder than usual.
Ajay doesn’t see it that way. Sure, the Earth orbits old Sol. Not in a circle; more of an ellipse, like a watermelon. But Earth does not stand up straight to face the sun. It tilts a bit. It leans back from the sun in winter. Tilting away makes the sun lower in the sky.
AJAY
Lower sun makes for shorter days, less heat from the sun. Less heat from the sun is the cold of winter.
Cold is the absence of heat, much as a shadow is the absence of light. Cold does not exist on its own; it’s a measure of what is missing. Less heat, more cold. So, Earth tilt makes winter cold.
As they cool, most objects contract. Metal cans, for instance. Water and rubber are exceptions. They expand when chilled. Sometimes explosively.
AJAY
Grape windows, yes. I remember. Last winter, I put a couple cans of grape pop on the ledge outside to cool. I forgot about them and, in the morning, found a spray of frozen grape soda all over the window.
Pop is water. Water expands. It was expanding as the can was contracting. Boom! Grape windows. That’s bad news for Ajay, who had to clean it up, but good news for fish. Good because ponds and lakes freeze from the surface first. If they froze bottom up, ice build-up would leave little room for fish.
As water cools, it expands, spreads out. More volume, less density, like a chunk of styrofoam. Big, but light. Cooler, less dense water rises to the surface. Warmer, more dense water sinks.
As cooler, less dense water rises to the surface, the surface water reaches the freezing point first. So, the pond or lake freezes from the surface down. At the same time, the relatively warmer water pushed down keeps the fish from freezing.
Cold on top gave Ajay an idea during a summer fishing trip. He put cans of soda pop in a tub that has no lid.
AJAY
I bought a block of ice at the bait and tackle shoppe. To keep the cans cool, I put the ice block on top.
Warm air around the cans rises. Rising up, it bumps into the ice, cools, shrinks, then sinks due to gravity and the cycle repeats. This is a convection current, circulating warmer and cooler fluids.
The fishing trip was successful. After dinner, Ajay built a fire in the cabin fireplace and tuned in a baseball game on his radio.
AJAY
The windsock showed no breeze outside. Even so, around the 7th inning stretch, I could feel cool air enter by the open window.
Ajay, the fireplace, and even the radio give off heat. Warm air in the cabin rises and exits out vents in the roof. Cooler evening air from outside moves in to fill the gap. As evening draws on, it becomes cooler outside than inside.
Something similar occurs outside by the lake, often called sea breeze or lake effect. Land warms faster than water, so warm air above the land rises as cooler air above the lake moves in.
AJAY
On shore, I feel cool air from the lake. Lake effect. In my cabin, I feel cool air from outside. I call this the 7th Inning Effect.
cold is the absence of heat
Fridge Doors, Root Cellars, and Heat Leaks
Arriving at his cabin on the outset of winter, Ajay hears a hum and finds it warmer inside than expected. His nephew was here last weekend and must have left the refrigerator door open. The motor is straining.
A refrigerator cools, but it doesn’t work like an air conditioner. The motor gives off more heat than is absorbed by refrigerated air. If left unplugged and the door open, that would cool off the cabin, but only for a while.
Ajay restocks the ’fridge and heads to the cellar, where he keeps a stock of potatoes, onions, garlic, beets, carrots. Most, he grew in the autumn. Freezing temperatures are expected overnight.
AJAY
I keep a big pot of water near my vegetable bin to protect the produce from freezing. I borrowed this bit of wisdom from the shed, where I keep my car ready to start even in winter. The shed doesn’t have a block heater for the car.
A block heater plugs in and a trickle current keeps the radiator warm enough to prevent freezing. Instead, Ajay keeps a tub of water near the radiator. Water has a certain amount of heat, keeping it liquid. The water might not feel warm, but hidden (latent) heat is there. Water releases latent heat as it turns to ice, so the tub of water is a heat reservoir.
Before leaving, Ajay’s nephew turned down the heat in the cabin. Not off, but he did set the thermostat low. Jaya, next door, says it takes more energy to reheat the house when you return, but that’s not how Ajay sees it.
AJAY
The cabin is like a leaky bucket that you have to keep filling so the water stays a certain level. Due to leakage, less water is needed to refill an empty bucket than to constantly keep it at the same level.
Likewise, less heat is needed to reheat a house than to keep it warmer than the out-of-doors. Fancy analogy, says Jaya. Simple idea, Ajay replies. Any house loses heat when it is colder outside than inside. A house continually loses heat, so it has to be continually reheated.
JAYA
Maybe so, but turn down the thermostat and you let in the cold. It comes in like a wave just opening the door. I feel a chilly draft now.
AJAY
It isn’t a wave. You don’t let it in. Cold is an absence, like a shadow or donut hole.
JAYA
My fridge isn’t cold, just not warm? Then what keeps outer space hundreds of degrees below zero?
Heat is a measure of the motion of molecules. In a total vacuum there are no molecules to vibrate, so temperature becomes meaningless (and so does time, which is a measure of relative motion).
Where there are molecules like radiation left over from the Big Bang, cold is still the absence of heat. At absolute zero, there is no heat, no motion.
AJAY
Some winters, it feels like absolute zero in my cabin.
conduction can feel cool
Double Blankets, Chilly Tubs, and Baking Tins
Hot air rises. That’s how a hot-air balloon works. As hot air rises in a chimney, it creates a draft (pulls in air) at the base. That sucks air out of the room. This can cool a building even on hot days with no wind.
During the day, the sun heats the chimney, especially if the chimney is painted black to absorb heat. Warm air inside rises up the chimney, creating an updraft at the base that pulls cooler air into the room or building.
Ajay’s cabin has a loft, but Alan and Lana like to sleep in front of the fireplace. When the fire goes out, so goes the heat radiated by the fire. However, the draft continues to draw warm air out of the room.
With warm air pulled out of the cabin, blankets are needed. The cabin has both thick and thin blankets. A thick blanket is an efficient heat insulator, which is to say it resists heat passing through. A thin blanket is a poor heat insulator; it easily lets heat through. Ajay has a couple blankets for each person: one thick, one thin, and in various materials.
Alan’s blankets are both cotton and he put the thick blanket on top, figuring it will block the heat from rising. He slept warmly. Since his blankets are made of the same material, it does not matter which is on top. The blankets are heat insulators in series. That is, the heat must go through both before it escapes.
Lana has a thin satin blanket and a thick cotton blanket. She put the satin blanket next to her. Lana felt chilly, even though the temperature under the covers was the same regardless which blanket is on top. She felt cooler with skin touching the satin than if touching the cotton. Satin conducts heat away from her, more than the cotton would have. Same temperature as cotton, but satin is a better heat conductor.
In the morning, Lana went for a bath. Leaning on the tub to turn the tap made her feel chilly. The wood door, bath mat, and towels did not feel cold, but the tub did even though all are in the same room. The tub is cast iron coated with porcelain enamel. Metal conducts heat away, so the tub feels cooler, even at the same room temperature. The tub felt like it was lower temperature—like the satin blanket.
Alex is baking muffins and tests whether they are done by inserting a toothpick. If the toothpick comes out clean, the muffins are done. He reaches into the hot oven barehanded and presses in a toothpick. The oven is several hundred degrees, but he isn’t burned when reaching in, testing the muffins.
ALEX
With the last batch, I touched a thumb to the muffin tin. A painful blister popped up almost immediately.
The air in the oven is the same temperature as the muffin tin, but air has very little mass. The atoms are more spread out, not many atoms bumping against one another, so air is a poor conductor of heat.
The metal tray has a lot of atoms packed together, bumping into their neighbours. As a result, it is thousands of times better than air at conducting heat. Air is an insulator; metal is a conductor.
After the blanket and tub experiences, Lana was happy to have something warm in hand. She held the fresh muffin, letting its heat conduct into chilly fingers, inhaling sweet convection currents.
heat flow can be slowed
Cast-Iron Cakes, Hot Coffee, and Fresh Snow
Lana found a cast-iron skillet in the cabin cupboard and slid it over a stove element, preparing to make pancakes. After a while, she plopped a drop of water into the skillet to test its temperature.
The drop sizzled and hopped around the hot pan. In a hot skillet, the bottom of the drop turns to a vapour cushion. The drop rides above the pan, much as a puck hovers over an air hockey table. When the water drop dances, Lana knows the skillet is ready for pancakes.
LANA
This test won’t work with a drop of vegetable oil. Oil can’t form for a vapour cushion and would flop onto the skillet, however hot.
Lana likes thick, cast-iron skillets. They have a uniform temperature across its bottom. Food seldom sticks. Many pans are thin steel. While lighter, a thin pan can have hot spots over the burner, making food stick to the pan.
Alex pours his uncle a cup of coffee.
AJAY
Thanks. I will be back in a moment. I need to fetch firewood, but go ahead and add the cream.
ALEX
I’ll wait. The cream is cool. If I add it later, the coffee will stay hotter.
Not so. To keep the coffee hotter, add the cream now. That which has more heat, has more to lose and at a faster rate. The coffee and cream combination loses heat at a slower rate than hot coffee alone.
In any case, Ajay won’t have to wait. Alan and Lana offer to fetch kindling and an armful of logs from the shed. The weather has been dry and cold for days, resulting in static sparks putting on clothing.
Alan and Lana notice how quiet it is outside. Last night it snowed. Fresh snow is not packed. The fluffy stuff has many small spaces between flakes. Air spaces trap sound waves like acoustic tile. Even so, their feet make a crunch-crunch sound across the snow. If a bit warmer, pressure from their weight would partially melt flakes under foot and lessen vibrations, so there would be no crunch.
Below -15 °C is too cold for snow crystals to melt under pressure. Instead, snow crystals slide past one another, compressing with a crunch. When it is too cold to melt with weight, it can be difficult to ice skate. Lana mentions the snow on return.
LEXA
That’s good news. We have one last crop to bring in. In this cold, rain would freeze on contact.
Freezing cell fluids burst their walls, ruining the crop. Fresh snow is protective, like an igloo. Trapped air pockets are a poor conductor of heat, so a snow blanket insulates the ground’s heat, protecting the crop from much colder air above the snow.
liquids can reduce friction
Tire Traction, Stone Cold, and Sand Sliding
Ajay and Alex took the truck into town to pick up supplies. Before heading out, they noticed the tires looked a little flat.
Most matter expands when heated, pushing out against a container. That is why basketballs bounce higher on hot days and are flatter in the cold.
Most matter shrinks when cooled, which why the snow tires on the Alex’s truck appear a bit flatter today. Flatter, they have more contact with the road, better traction.
Good tires grip by creating friction. Liquids, less so. That which flows has less friction than solids. Oil, for instance, can make a surface slick. So can rain. That’s important since ice can’t take much pressure. It melts and the melted layer is slick.
Snow melts unless it is what Ajay calls stone-cold: too cold to pack a snowball. Packing requires pressure melting, at least on the surface, to bond the snowball together. That’s not possible when stone-cold.
LANA
When it is stone-cold outside, ice is like pavement, making it is too cold to skate.
It would be like trying to skate on dry ice (solid carbon dioxide). Lots of grip, but little glide. Skates need to glide over a thin liquid layer, but dry ice does not melt into a liquid. It sublimates from solid to gas. When not so cold, standing on ice pressure melts to a thin layer of liquid, slick as oil.
So walking on ice is walking on water, without the sinking part. That’s why ice can turn a sidewalk into a slide-walk. It’s slippery when wet. Walking on bumpy ice is even more difficult with little grip by friction.
Bumpy ice has with fewer points of contact; mostly at the top of the bumps. Less contact can be helpful with ball bearings or over-filled bicycle tires. Less contact means less resistance due to friction.
Walking on the moon is more difficult than walking on Earth because of less friction between shoes and the surface. Weight on the moon is one-sixth of the weight on Earth. That means one-sixth of the grip.
Walking in a backyard pool is more difficult than walking on the deck. Water in the pool buoys up a person. Being pushed up causing less traction between a person’s feet and the bottom of the pool.
Walking on beach sand is more difficult than walking on a boardwalk. On a beach is more difficult because sand grains are not connected and act like a fluid. They provide little push-back (traction).
Little traction or not, in the stone-cold weather of mid-winter, Alex and Lexa wish they were on a warm beach somewhere. Alan and Lena, however, want only a few degrees warmer to glide across the pond.
insulation reduces heat flow
Silver Warmth, Ceiling Fans, and Walking on Fire
Lana switched to cotton after her experience with the satin blanket. She also puts a hot water bottle under the covers on chilly nights. The bottle will stay warm for hours since water resists heat changes.
That water resists heat changes is also why the water drop danced in Lexa’s skillet. A drop of oil would pass heat through itself quickly compared to water. Instead, the water heated and evaporated on one side.
Cool water likewise resists warming. Food that has a lot of water, such as melon, will stay cooler for longer than other items in a picnic basket. Foods with oils, such as potato salad or coleslaw, spoil faster.
Compared to metal, water needs a lot of energy to warm up. Water also gives off a lot of heat as it cools. Ten times as much as iron. Twenty times as much as silver.
Some things we want to warm quickly. Silverware, for instance. Silver is desirable for cutlery since it warms quickly in a person’s mouth. Gold warms even faster, twice as fast as silver.
Alex and Lexa had wine with supper, served in long-stem wine glasses. They keep wine refrigerated and say it tastes better cooler. A wine glass has a long stem to prevent a person’s hand warming the wine.
Ajay filled his “Hot Stuff” mug half with water, half ice cubes. When the cubes melt, there won’t be a spill. The cubes push aside an amount of water equal to their weight. As they melt, the level of liquid stays the same.
If the ice cubes contain a lot of air bubbles, as often happens with tap water, there will still be no difference on melting. Heavier or lighter, they push aside (displace) as much water as is equal to their weight.
The great room of the cabin has a large fireplace and an equally large ceiling fan at the apex of the pitched ceiling. To set the air draw for the fan, up or down, Ajay pulls a cord dangling from the fan.
Ceiling fans do not change the temperature in a room. They circulate air. To feel cooler, set the ceiling fan so it will pull air upward, drawing warm air up away and so make a person feel cooler.
Warm air from the fireplace rises, bringing cooler air in and onto those in the cabin. To feel warmer, Ajay pulls the cord on the ceiling fan so it pushes the rising warm air back downward.
Warm air rising is one reason the insulation is typically thicker in a roof or attic than in the walls and why there is little or no insulation under the flooring.
Insulation, whether it is a foam or a blanket of fibre, contains many tiny air pockets. Air is a good insulator because its molecules are spread out and so do not conduct heat.
A goose down blanket or down coat has air pockets and so retains warmth. Even crumpled paper stuffed into coat sleeves will hold in a person’s body warmth better than the sleeves alone.
A blanket or insulation or igloo don’t produce heat. They reduce heat loss. A blanket around a thermometer will not raise the temperature since neither blanket nor thermometer produce heat.
AJAY
Insulation is the secret to walking over hot coals. Wait until they burn down to have a layer of ash. Ash, like fresh fallen snow, has pockets of air which resist passing heat along. But walk quickly.
Walking over hot coals is easier if the coals are from wood. Wood is a poor conductor of heat. Nervous sweat may also help as water resists heat change. Lana figures she will stick with the hot water bottle.
conduction is heating by touching
Efficient Engines, Icy Bridges, and Popsicle Sticks
The winter fair has an auto show. One company at the show has on display what they claim is a 100% efficient motor.
ALAN
Can’t be. Just feel it. A completely efficient motor would lose no heat.
ALEX
Noisy too. An engine that is 100% efficient would lose no energy to heat or through vibration.
Ajay, catching up with them, watched as a technician added fuel to the engine.
AJAY
An all-efficient engine would not need fuel to run or oil to reduce friction, nor would it waste any fuel as exhaust. It would also be a perpetual motion machine.
They step outside into the freezing night air, each wearing a different kind of coat. Ajay’s puffy coat traps heat. Air is a poor conductor, so Ajay’s body warmth isn’t lost to the night air.
Alan’s leather jacket looks cool, but makes him feel cold. It offers little protection: a single layer, no lining or padding, and large conductive metal zipper. A flannel shirt under keeps him from freezing.
Alex’s coat has a thin layer of insulation to reduce heat loss. It also has a reflective material sewn inside to radiate body heat back to Alex. The reflective material is similar to the shiny side of aluminum foil.
Out in the night air, Ajay, Alan, and Alex can see vehicles for the winter fair in various states of preparation. One car has no wheels and the chassis sits flat on the ground. This one has no frost on it.
Heat from the ground rises into vehicles touching it, such as the one with no wheels. But rubber tires prevent heat from being conducted from the ground and so the cars on tires are frosty.
ALEX
Driving back to the cabin, let’s avoid the bridge. Roads can be icy over bridges.
Earth radiates heat, but there is no ground below a bridge to conduct heat up to the pavement and prevent ice. It’s like mittens. Mittens keep fingers warmer than gloves on cold days since, in mittens, fingers touch one another and so share heat. Or like snuggling two in a sleeping bag or more in a tent on a frosty night.
On a frosty night, frost forms on the tips of grass. It doesn’t form on dirt, stone, or other heat conductors. Heat from the ground conducts into stones and trees, so the frost is less likely to form around them.
ALEX
Heat can be conducted quickly, having learned the hard way as the kid who licked a flagpole in freezing weather.
ALAN
Lick a popsicle and your tongue might also become stuck.
AJAY
Heat is conducted from tongue to the popsicle or flagpole. Stick with the popsicle stick. It won’t stick since wood is a poor conductor of heat. Kind of like walking on ash over hot coals.
compression warms; expansion cools
Hot Springs, Sliding Doors, and Boiling Spaghetti
Lana and Lexa went to the mountain-top spa, part of the winter fair. Warm air rises, yet it is colder on the top of a mountain than at the bottom. High up, air loses heat to outer space faster than it can be warmed by the sun.
But that’s not the whole story. As air rises, there is less air pressing down from above, so it expands. With particles moving further apart, they have fewer collisions causing heat and so the gas cools.
Ascending the mountain is like releasing air from the tire. Pumping up a bicycle tire pressure heats the air inside the tire. Releasing air spews cool air out the value. Cool because it is rapidly expanding.
Lana figured out which way the breeze is coming from by holding a wet finger in the air. Evaporation is greater on the windy side, which will feel cool. The cool side of Lana’s chilly digit is toward the breeze.
Stepping into the warm spa, Lexa’s glasses fog up. Heat passes into the lens, leaving moisture to condense on the surface of her lenses. Fogging from warm breath happens when she wears a surgical mask.
It is hot in the cedar spa and both of them sweat. Sweat evaporates and carries heat away so a person feels cooler. This sulphur spring is 37 °C, the same as a person’s normal internal temperature.
People perspire, but there is no evaporation in the water, no cooling. To avoid having customers become ill from overheating, the spa limits how long patrons can be in the sulphur spring.
LANA
Walking to the restaurant, I saw a cloudy sliding glass door. It must have a crack.
Double pane windows and doors have a layer of nitrogen gas between the panes to prevent heat from passing through. A crack lets in air. Air contains moisture, which condenses and causes the panes to become cloudy. The process is similar to Lexa’s foggy glasses. Warmth passes through glass, leaving behind moisture.
If there is warm inside and cold outside, the warmth passes inside to outside, leaving the moisture indoors. If there is warm outside and cold inside, the warmth passes from outside to inside, leaving the moisture outdoors.
Lana has soup and salad for lunch. The soup is too hot. Blowing across the top of the soup increases evaporation and removes the warm vapour layer that works like a lid to hold heat in the soup.
Lexa ordered spaghetti. Spaghetti is often cooked in vigorously boiling water so the spaghetti strands don’t stick to the pot or to one another. A gentle boil is better for cooking macaroni or soup, or potatoes.
Whether gentle or vigorous, water boils at the same temperature: 100 °C. Anything more than that is carried away with the steam. Vigorous boiling does not cook spaghetti better or faster.
LEXA
At home, I boil spaghetti gently, stirring from time to time. Once the water starts to boil, I will turn down the temperature to a gentle boil.
Stirring takes the place of a vigorous boil, but a pressure cooker is even more efficient. It cooks at 120 °C by preventing steam from escaping. The boiling point of water goes up as pressure increases, so it cooks faster and uses less energy in the process than conventional boiling.
