How do light bulbs work, and why do onions give you bad breath?

We explore the curious questions that science can answer

Let there be light: the combination of a filament and a vacuum is illuminating (Getty/iStock)

How do light bulbs work?

A light bulb contains a thin wire filament. When a current flows through this filament, the metal heats up and then glows, giving out light. As the inside of the light bulb is a vacuum, there’s no oxygen to oxidise the filament, so it lasts a long time.

How does a radiator work?

Radiators heat the air around them, which then rises because hot air expands and is, therefore, less dense than the air around it. This leaves a gap into which the cold air in the room can flow.

Cold air near the radiator then heats up and again moves out of the way. This cycle keeps going until (theoretically) the air in the room is all at the radiator’s temperature.

 How does a tyre pressure gauge work?

A tyre pressure gauge is like a syringe with a valve on the end where the needle would be. There’s also a spring holding the inside and outside of the “syringe” together. The air in the tyre is under such pressure that it forces air inside the syringe through the one-way valve. The air can’t get out again until the gauge is removed from the tyre. This air forces the inside of the gauge to move out.

But the spring tries to stop the inside from moving. Eventually, the pull from the spring is sufficient to balance the push from the air and the inside stops. This is the point at which the tyre pressure is shown on a scale along the side of the gauge.

Does a hot drink give you more energy than a cold drink (of the same liquid)?

In theory, yes. One kilogram of water absorbs about 4,200 Joules (equivalent to 1 kilocalorie) for every increase of 1C in temperature. Nevertheless, if you raise your temperature above its normal level, the body uses energy in an attempt to return to normal.

However, if your body is below its normal temperature (for example, you may be suffering from hypothermia), then a hot drink will go some way to warming you up. Since the drink partly does the work of warming you up, less energy has to leave the body, so, in effect, the drink has given you more energy.

Why does a hot chip pan erupt when cold water is poured on top?

Hot oil in a pan has a temperature well above 100C, the boiling point of water. If you pour water into the pan, it tries to sink under the oil and gets mixed in with it. Besides this mixing, the water is heated rapidly and starts to turn to steam. Being a hot gas, the steam tends to rise and expand: any oil on top of it will get pushed up in the eruption.

This produces a cloud of oily droplets that bursts into flames when it comes into contact with the cooker’s gas flame or electric element. That is why it is always wise to have a non-water fire extinguisher in your kitchen.

Why does sherbet fizz?

The “fizzy” feeling on your tongue is a mild pain response. Sherbet and fizzy drinks cause fizzing in similar ways. In fizzy drinks, the carbon dioxide in the bubbles is converted by an enzyme in saliva to carbonic acid, a weak acid that irritates the tongue. Sherbet contains citric acid and tartaric acid, which irritate the tongue in the same way.

Why do onions give you bad breath?

Because of the chemicals inside the cells of the onion. These are released when we chop the onion (we smell them) and when we chew it. Once it is in the stomach, our bodies break the onion into even smaller pieces. All of these processes release the chemicals inside the onion cells. Some are gases, so they are expelled back into the mouth from the stomach, often in the form of a belch. The bad breath is simply the presence of onion gases in the mouth and throat; it goes once the onion has been properly digested.