Heat and Temperature

The distinction in the first bullet is the whole topic. A bathtub of warm water contains far more heat than a spark from a grinder, and the spark is at a far higher temperature. Quantity against intensity.

Heat against temperature

Temperature measures the average kinetic energy of the particles - how fast they are moving, on average. It does not depend on how much of the substance there is.

Heat is energy transferred because of a temperature difference. It depends on how much substance there is as well as how hot it is.

Heat always moves from hotter to colder. There is no such thing as cold moving into something. A cold drink does not give cold to your hand; your hand gives heat to the drink. That reframing answers a whole family of questions.

"Cold" is the absence of heat, not a substance. Insulation does not keep cold in; it slows heat coming in.

The three transfer methods

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Radiation is the only one that crosses a vacuum, which is why it is how the Sun's energy reaches Earth. That fact is the most asked point in the topic, and a question mentioning space is asking about radiation.

Convection requires a fluid, so it cannot happen in a solid. Heated fluid becomes less dense and rises; cooler fluid sinks to replace it, and the loop is a convection current. That is what drives weather, ocean currents and the mantle.

Metals conduct well; wood, plastic, air and water conduct poorly. A metal handle feels hotter than a wooden one at the same temperature because it moves heat into your hand faster - the sensation is about the rate of transfer, not about the temperature, which is a good question.

Good insulators work by trapping air. Still air conducts heat very poorly, so materials full of tiny pockets of it - styrofoam, fiberglass, down, a wool sweater - are the insulators the questions offer. A double-pane window insulates for the same reason: the layer of air between the panes.

Temperature scales

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A Kelvin degree is the same size as a Celsius degree, just counted from a different zero. So K = C + 273, which is the easiest conversion on the test.

Absolute zero is 0 K, the temperature at which particle motion is at its minimum. There are no negative Kelvin temperatures, which is a question.

Fahrenheit to Celsius: subtract 32, then multiply by 5/9. Celsius to Fahrenheit: multiply by 9/5, then add 32.

The order matters both times, and doing the addition first is the standard error.

There are 180 Fahrenheit degrees between freezing and boiling against 100 Celsius degrees, which is where the 9/5 comes from: 180 over 100.

Specific heat

Specific heat is the energy needed to raise one unit of mass by one degree.

Water has an unusually high specific heat, which has consequences the test asks about:

• Water takes a lot of energy to heat up and releases a lot as it cools.
• Coastal areas have milder climates than inland ones at the same latitude, because the ocean warms and cools slowly.
• Water is a good coolant, which is why engines use it.

Metals have low specific heat, so they heat and cool quickly. A metal pan on a stove is hot in seconds; the water in it takes minutes.

Given equal masses and equal heating, the substance with the lower specific heat gets hotter faster.

What you can skip

Across all 2,104 General Science questions in our bank:

• The laws of thermodynamics by number. The word "thermodynamics" never appears, and entropy comes up once. Know that heat flows on its own from hot to cold, which this lesson already covers.
• Calorimetry setups. The specific heat questions are the simple multiplication in this lesson - mass times specific heat times temperature change - not lab procedures.

Where people lose points

Treating heat and temperature as the same quantity.

Saying cold flows into something. Heat flows out.

Saying convection happens in solids. It needs a fluid.

Forgetting that radiation needs no medium.

Reversing the order of operations in a temperature conversion.

Accepting a negative Kelvin temperature.

Work one in under a minute

Why does a metal railing feel colder than a wooden bench on the same winter morning?

Both are at the same temperature - the air temperature.

Metal conducts heat away from your hand much faster than wood does, so more heat leaves your skin per second and the sensation is of cold.

The feeling reports the rate of heat transfer, not the temperature. Same reasoning explains why a metal handle feels hotter than a wooden one on a hot pan.

Where this leads

Convection is what drives weather and ocean currents, and thermal expansion reappears in Mechanical Comprehension and in Auto and Shop.