Understanding Heat Exchange

I had a really excellent science teacher for 9th grade “Integrated Science” class. Her name is Kathy and the thing that’s stuck with me most from her class (besides the Photosynthesis Song) is our unit on heat.

Heat transfer is shockingly simple; there are a few equations and constants which govern the exchange of all thermal energy. It’s incredibly useful to have a good understanding of heat transfer in your daily life. Here are some key terms:

calorie The amount of energy required to raise one gram of water one degree Celsius. Calorie (with capital C) means Kilocalorie, or 1000 calories. Kilocalories are the units used to measure the energy contained in food. One calorie is 4.185 Joules.
phase change When matter transitions to a different form due to addition or subtraction of heat energy. The forms of phase change are boiling, freezing, melting, evaporation, and sublimation.
convection A form of heat transfer in which heat moves through a medium (like air). For example, an oven or forced air heating system rely on convection.
conduction A form of heat transfer where heat moves through an object. When your teaspoon heats up after being immersed in a hot beverage, this is an example of conduction.
radiation A form of heat transfer in which waves of infrared light transmit heat to the surface of an object. When the sun heats up the surface of your car, this is radiation.
conservation of energy In a closed system under normal circumstances, energy is never created or destroyed. This is a fundamental law of physics. This means that heat never disappears, it simply moves around. From this, we may infer that heat lost must equal heat gained. For example: As your cup of tea cools down, the mug, saucer, and air in the room become warmer.
specific heat capacity The amount of energy a particular material can store. Water has a specific heat capacity of 1 calorie per gram; Aluminum is more like 0.2 calories per gram.
temperature The average heat of the particles which make up a substance. Measured in degrees.
heat The total amount of thermal energy contained in a substance. If you have a bucket and a cup of water, both at 20°C, they have the same temperature but the bucket has much more heat because its mass is greater.
Q=MCΔT  That weird triangle thing is the Greek letter delta, which is scientific shorthand for change or difference. This is the fundamental heat transfer equation. Heat (Q) is equal to the mass of the sample (m) multiplied by the specific heat capacity of the sample (C) multiplied by the change in temperature (ΔT).

Say you want to know how much energy it takes for your freezer to make an ice cube. Q, the amount of energy, is our unknown. Assume the mass of our water is 100mL, and its current temperature is  23°C (we want to bring it down to 0). Since water’s specific heat capacity is 1 calorie per gram, this makes the math really easy. Q = (100gr)X(1cal/gr)(23°) = 230cal.

I’ve noticed that many people have trouble grasping the fact that liquid water can never (under standard atmospheric conditions) get hotter than its boiling point of 100°C. Assume you have a pot of water simmering on the stove at 100°C. If you turn up the heat all the way, the water will not get hotter, it will simply evaporate more quickly.

A real life example: Some friends of mine live in an apartment with old-fashioned radiators for heat. They were concerned that paper or other flammable items placed near the radiator might combust due to its heat— this seems like a sensible fire safety precaution. But since the radiators use heated water and/or steam to distribute heat, it can never get hot enough to cause a fire. Feel free to store your books on the radiator— but don’t try this with any other form of heating device! Counterintuitively, radiators do not employ radiation to transfer heat. Instead, heat is transferred from the hot water to the metal pipes via conduction which in turn heats the air through convection.

Imagine you have a container full of hot steam and a thermometer. You put the container into the freezer, and check the temperature every minute or so. If you were to plot the time and temperature on a graph, you’d get something like this:

 

When the line is flat, the system is undergoing a phase change. While the phase change is in progress, the temperature does not change because all the energy is being used in the phase change. Once the phase change is complete, the temperature will continue to change until the next phase is reached. The system reaches equilibrium when the stuff inside the container is the same temperature as the environment outside the container.

Ever wonder how scientists measure the amount of Calories in your food? Remember that the process by which your body converts food into energy is not so different than how a fire converts fuel to heat. Scientists place a sample of food, for example 100grams of butter, into a machine called a bomb calorimeter. Under ideal, high-oxygen conditions, the sample is burned rapidly. It heats a known mass of water, and the change in temperature is measured. Just solve for Q and you’ve got your nutrition facts! Of course, once you know the heat energy contained in known masses of common ingredients like oil or flour, you can just add them together to find how many calories are in your tortilla chip. Probably easier than setting it on fire.

I think understanding the basic physical laws which govern energy exchange is important in daily life. If you understand heat transfer and phase changes, you can better understand the energy consumption of tasks like cooking or making coffee. Remember that calories or joules (units of energy) can be converted to watts per second/minute/hour (units of work). I imagine that it will become increasingly important as melting ice (a phase change if there ever was one) continues to affect our planet’s habitability. Take some time to brush up on your 9th grade science!

In case this wall of text was too much for you, here are some videos to help you understand.

The inimitable Bill Nye (30min episode):

Chemistry Lesson: Heat and Specific Heat Capacity (not quite so fun, but simple and informative). 12 minute presentation.

Doc Physics- Latent Heat of Fusion and Vaporization (9min)

5 thoughts on “Understanding Heat Exchange

  1. Nicely done Caleb. Having a knack to dissect the complicated into its various simple parts is a key skill. It would be great to see the proper terms added to some of your work, such as sensible heat, latent heat of vaporization, heat of solidification, saturated temperature, saturated pressure, Zeroth Law of Thermodynamics, and 1st Law of Thermodynamics. Perhaps these would be great additions for a Part-2? Kind Regards, Terry

  2. Nicely done, Caleb (glad you re-considered photosynthesis and the song!).

    I would ask you to celebrate what you know of science and include that in your work; do not be afraid of over-complicating, over-layering anything. This is the time for pushing your limits and going as far as you can — if not further. You can dumb down for the real world if you have to later on in life. This is your time to do just the opposite.

    Good work. Now do more!