How can sweeteners contain no calories, and how do metal detectors work?

We explore the curious questions that science can answer

Sugarcoated science: sweetness and calories are not measured in the same way (Getty/iStock)

How can artificial sweeteners taste like sugar yet contain no calories?

Sugar is very sweet and has a high energy content, or calorific value. But sweetness and calorie content aren’t necessarily linked.

Sweetness – which is a difficult property to measure, unlike calories – appears to be caused by a different chemistry to calorie content. In fact, we can record sweetness only through taste tests.

But chemical structure and sweetness do appear to be linked. Most sweet compounds seem to have a chemical structure known as “AH.B”, where A and B are negative atoms that are thought to form bonds with a protein in our sweet-sensitive taste buds. To do this effectively, A and B must be around 0.3 nanometres (billionths of a metre) apart. This is a property seen in sucrose, glucose, saccharin and aspartame.

When a person is floating (eg, in a swimming bath) and tenses their muscles, the body begins to sink. Why?

The only factor that will affect buoyancy is the weight (or amount) of air in the person under water – inhalation makes you go up and exhalation makes you go down. This is standard practice in diving. Tensing your muscles without breathing will not affect your buoyancy per se, though it might perhaps change your centre of buoyancy (ie, by moving your diaphragm), resulting in a small displacement. Or you might unconsciously tense one set of muscles slightly more than another – which would affect your orientation in the water, and so possibly your position. Or by tensing your muscles, you might actually move your arms and legs, albeit slightly and without your knowledge. This would change the relative positions of your centre of gravity and centre of buoyancy, causing a small change in position.

However, all the movements described above would be slight, and they would certainly not cause you to sink. Experts at the National Sports Medical Institute and several diving schools agree: it simply doesn’t happen. So they wonder – are you sure you aren’t breathing out when you tense your muscles?

Why does newspaper go yellow faster than other paper?

The main components of wood are cellulose and lignin. Cellulose is long-fibred and strong – so paper remains supple over a long time. Lignin is a polymer that makes wood hard, and is acidic. When making high-quality paper, the pulp is cooked, which removes the lignin. However, the newspaper publishers’ main concern is to get the news to the reader as quickly and as cheaply as possible. Thus they use the cheapest papers and the cheapest inks. As newspapers have a very short shelf life, there isn’t much point in making them out of posh paper. Newspaper is made from wood that hasn’t been cooked, so most of the lignin remains. This is what turns yellow on exposure to sunlight.

Why doesn’t dew form on a car parked in a car port, even though it is open on all sides?

This is because of heat loss. The car port is insulated by the roof both from excessive heat build up during the day, and from sudden heat loss at night. So during the day, moisture-heavy air doesn’t collect under the port because the air there is cooler than that around it (and so can’t absorb as much moisture as warm air). At night, when the Earth starts to lose heat, the car port stays at relatively the same temperature as before; so there isn’t such a sharp drop in temperature, nor so much moisture in the air there. The result: dew doesn’t form.

How do metal detectors work?

To sniff out metallic objects, they all rely on a metallic “search” coil (sometimes called a search loop) with an alternating current running through it. The AC causes a uniform alternating electromagnetic field around the coil. But when that field passes over a metallic object, it induces tiny currents – called eddy currents – in the object. That weakens and distorts the detector’s field (because the eddy current comes from the field’s energy). When the field passes over a magnetic material (say, iron or magnetite) it strengthens the field. An audible note linked to the energy in the coil then alerts you when the search loop passes over a conducting metallic object (less energy) or a magnetic material (more energy). Thus you can also distinguish between metallic and magnetic objects.