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# Heat Transfer with Hot Coffee

by Ron Kurtus

A good application of heat transfer can be seen when you buy a cup of hot coffee at your favorite coffee shop.

The hot coffee heats the paper cup by conduction. The cup provides little insulation, so many coffee shops provide a cardboard insulator to protect your fingers from getting burned. A plastic top not only prevents spilling when you take out the coffee cup, but it also helps to reduce convection cooling of the coffee.

Questions you may have include:

• Why is the coffee cup made of paper?
• How does the holder prevent burns?
• What is the purpose of the coffee cup lid?

This lesson will answer those questions. Useful tool: Units Conversion

## Paper cup

The typical take-out coffee cup is usually made of a heavy paper material. Although paper is a fair insulator, the cup material is thin enough that the hot coffee heats it by conduction, making the outside surface hot to the touch.

Paper coffee cup

(Note: You can see that I drank some of the coffee before taking the picture. Mmm, good!)

The lip of the cup is rolled over for strength and so you won't cut you lips.

### Styrofoam coffee cups

Styrofoam coffee cups do a better job of insulating, because the material does not conduct heat well. Also, A Styrofoam cup is several times thicker than a paper coffee cup, adding to its insulating properties.

Most businesses no longer use Styrofoam cups because they are not biodegradable and are thus not good for the environment. There also is concern about chemicals from the Styrofoam leaching into the hot coffee.

## Cup holder

Since the paper coffee cup gets so hot, many coffee shops provide a thin cardboard holder so that you won't burn your fingers when holding the cup.

Coffee cup holder prevents burned fingers

The holder is corrugated, such that are there air gaps and only the ridges touch the cup.

Coffee cup holder has ridges inside

The air gaps mean the heat transfer must be done by convection, which is less effective than conduction. With only the ridges of the holder touching the hot cup, much less heat transfer by conduction occurs.

## Lid

Usually, a plastic lid is included with the take-out coffee cup.

Coffee cup with lid

This is primarily to prevent spilling the coffee when you walk or drive. But also, the lid traps in the hot air surrounding the coffee, preventing heat loss from convection.

## Summary

Heat transfer can be seen when you buy a cup of hot coffee, which heats the paper cup by conduction. The cup provides little insulation, so a cardboard insulator is provided to protect your fingers from getting burned. A plastic top prevents spilling and helps to reduce convection cooling of the coffee.

Savor the aroma of life

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