Heat and Thermal Effects

General Science teaches how heat moves. This lesson is about what heat does to machinery, and it is the practical half.

Thermal expansion

Materials expand when heated because their particles move further apart.

Different materials expand by different amounts, which is the fact the whole topic hangs on.

Designs must allow for it, or the expansion produces enormous forces. A constrained material that cannot expand pushes on whatever is stopping it, and that force buckles rails, cracks concrete and shears fasteners.

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Three solutions, all in the figure:

An expansion gap, as between rail sections, so a hot rail has room to grow.

An expansion loop or joint, as in a pipe run, which absorbs the length change by flexing.

A bimetallic strip, which does not prevent expansion but uses it.

The bimetallic strip

Two metals with different expansion rates are bonded together. Heat them and one grows more than the other, and since they cannot slide apart, the strip curls.

It always curls toward the metal that expands less, which is the side on the inside of the curve.

That curl is a switch, which is why bimetallic strips are used in thermostats, circuit breakers, and older turn-signal flashers. Heat produces motion, and the motion opens or closes a contact.

Cool it and it straightens again, so the switch resets on its own.

Other expansion consequences

A loose lid comes off after hot water is run over it, because the metal lid expands more than the glass jar.

A gap is left between concrete slabs for the same reason as the rail gap.

A bridge sits on rollers or expansion bearings at one end so it can change length.

A tight metal ring fits over a shaft when heated and grips when it cools, which is how some bearings and gears are mounted.

Power lines sag more in summer than in winter.

A hole in a heated plate gets bigger, not smaller. That surprises people: the metal around the hole expands outward along with everything else, so the hole grows. It is asked.

Moving heat

Conduction, convection and radiation, as the General Science lesson covers. What matters here is which one a machine uses.

A cooling system is a heat transport system. The coolant conducts heat out of the engine, convects it to the radiator, and the radiator gives it to the air.

Nothing is destroyed. The heat is moved from where it would do damage to where it can be released, which is the point a question about what a radiator does is usually testing.

Radiation depends on the surface. A dark, dull surface absorbs radiant energy and heats up faster in sunlight than a light one; a shiny surface reflects it. That is why the inside of a vacuum flask is silvered - to reflect radiant heat back in - and why insulation works by trapping pockets of air, which conduct heat poorly.

Measuring heat: the BTU

A British thermal unit is the heat needed to raise one pound of water one degree Fahrenheit. So heat in BTUs is simply pounds times degrees for water:

How much heat raises 10 pounds of water 20 degrees Fahrenheit?

10 x 20 = 200 BTUs. Fifty pounds raised 40 degrees takes 2,000.

Heat as a loss

Friction converts mechanical energy to heat, which is why no machine is fully efficient and why bearings, brakes and gearboxes get hot.

Excess heat is destructive: it thins lubricating oil, weakens metal, and expands parts until clearances close up and they seize.

That is why machinery has cooling at all. It is not comfort, it is preventing the failure that heat would otherwise cause.

What you can skip

Across the 64 questions on this topic in our bank:

• Metric heat units. Calories and kelvins never appear here, and Celsius once. The heat quantity questions are in BTUs and degrees Fahrenheit.
• Latent heat calculations. Latent heat comes up twice, as an idea.
• Expansion coefficients. No question asks you to compute an expansion from a coefficient; the rail and beam questions give the expansion and ask what it means.

Where people lose points

Thinking a hole shrinks when a plate is heated. It grows.

Saying a bimetallic strip curls toward the metal that expands more. It curls toward the one that expands less.

Thinking a radiator destroys heat. It transfers it to the air.

Forgetting that constrained expansion produces force.

Saying convection works in a solid. It needs a fluid.

Treating thermal expansion as negligible. Over a long span it is large enough to buckle steel.

Work one in under a minute

Why is a gap left between sections of railway track, and what would happen without it?

Steel expands when heated, so on a hot day each rail section grows longer.

The gap gives that growth somewhere to go.

Without it the rails would press against each other, and because they cannot lengthen, the force would build until the track buckled sideways out of line.

Expansion that is prevented becomes force, and that sentence answers most questions in this topic.

Where this leads

Thermal expansion and cooling are the mechanical side of the heat that friction produces, and the engine cooling system is the applied case.