What’s actually happening when we use a straw, and can stainless steel rust?

We explore the curious questions that science can answer

It’s all to do with air pressure... (Getty/iStockphoto)

When we use a straw to suck water from a glass, what is actually happening?

This is all to do with air pressure. We have to think about the pressure in two places, first the pressure of air inside the straw, and also the air pressure outside, which is pressing down on the surface of the water in the glass. When you suck with your mouth, you remove some of the air from the inside of the straw. This means that the air pressure inside the straw is less than the air pressure outside. This difference in pressure means that air from outside tries to get inside the straw to even out the difference. As the air tries to get in, it pushes the water ahead of it, and hence up the straw.

It sounds odd, but when you suck water through a straw, it is actually being pushed from underneath, rather than being sucked from above.

Can a household lightbulb be run off normal batteries? If so, how many?

A desklamp may have a 60W bulb that runs off 240V mains. (W = watts, the unit of power; V = volts, the unit of voltage.) The current would be 0.25 A (amps). This is calculated by dividing the power by the voltage.

Batteries have different voltage ratings, so the number needed to try to light a desklamp would be affected by the voltage of each battery (or strictly speaking, cell).

The lower the voltage of each cell, the more would be needed. If we assume each cell is rated 1.5V, you would need 60 of them connected in series (all in a line) to supply the required 240V.

There is a catch to this simple calculation. The current supplied may not be sufficient to light the bulb to normal brightness. This is because the more cells you use, the more resistance there is in the circuit, and this reduces the current that can be drawn.

For practical reasons as well, the more connections there are in the circuit, the more chance there is of a connection not being sound, and so either making a gap in the circuit that current can’t flow past, or at the least, will introduce a large resistance that will make it difficult for current to flow. Finally, using batteries is massively more expensive than using mains electricity.

What happens to the candle wax when a candle burns? Where does it go?

Candle wax is the fuel which is burnt to keep the candle going. When you light a candle, the flame melts the wax at the top of the candle into liquid form. This liquid travels up the wick via capillary action and is transformed into a gas by the heat of the flame. This gas combines with oxygen in the air, giving out the heat and light that you see as a flame. Candle wax is a mixture of chemicals, which contain carbon, nitrogen, oxygen and hydrogen. When combined with the oxygen in the air, new chemicals are created. These are gases – carbon dioxide, nitrogen, and water vapour – which float into the atmosphere, so you can’t see where the wax has gone.

How does polish, such as Mr Sheen, prevent dust from sticking to objects?

There are many different types of antistatic spray on the market. They form a protective layer so that things that were previously insulators can now carry a small amount of charge. Dust, which will either have an extra electron or be missing some electrons, will be charged. Charged things are attracted to other charged things – like the screen of a television or computer. The dust then is likely to stick to it. These sprays allow electrons to either flow into the dust or away from the dust and so the dust returns to a neutral state. This dust then won’t stick to charged objects like the screen and will fall to the floor under the pull of gravity.

Can stainless steel rust?

Some stainless steels will rust, especially cheaper grades. Sometimes makers try to pass off chromium alloy steels as stainless steels (technically, steel has to have at least 11 per cent chromium to be “stainless”). Essentially, new stainless steel forms a thin surface layer of oxide, which is transparent and prevents further decay. If this layer is eroded or compromised, corrosion will occur. No grade is totally immune to all chemicals, although the more expensive grades remain shiny while becoming pitted over time.