New Scientist - USA (2019-11-16)

54 | New Scientist | 16 November 2019

Cold feat

Why is a freezer door difficult to

open just after you shut it?

Frank Mulholland

Woodbury Salterton, Devon, UK

When a freezer is opened and

then closed, some of the cold

air is replaced with air at room

temperature. This warm air then

cools. Gay-Lussac’s law states that

the pressure of a given mass of gas

varies directly with the absolute

temperature of the gas, when the

volume is kept constant.

The cooling of the gas causes

a reduction in pressure inside

the freezer, resulting in a partial

vacuum that makes the door

more difficult to open.

However, a domestic freezer

isn’t perfectly sealed and air will

slowly leak in from outside and

equalise the pressure. So, after a

few minutes, the door becomes

easier to open.

Christina Hart

Melbourne, Australia

Air is a mixture of fast-moving

molecules, mostly nitrogen and

oxygen. The spaces between the

molecules are huge compared

with the size of the molecules,

so they move freely in mostly

empty space and exert pressure

on anything they strike.

In the freezer, the molecules

move slightly slower than those

in the warmer air outside, so

each molecule hits the door with

less force than the molecules on

the outside do. But the slower

molecules are also slightly closer

together than the faster ones, so

relatively more molecules strike

the door, which helps balance the

air pressure from the outside.

When you open the freezer

door, you replace some of the slow

molecules inside the appliance

with faster moving and more

spread out molecules from the

warmer air outside. These slow

down after you shut the door as

they cool. Now, there are relatively

fewer molecules in the freezer

compared with before, so they

exert less pressure than before.

This, for a short while, makes it

a bit harder to open the door.

Raymond Poland

Farnborough, Hampshire, UK

Consider air outside the freezer at

15°C and standard atmospheric

pressure. This fills the freezer

when the door is opened. Let the

area of the freezer door be 1 square

metre. On closing the door, the

temperature of the air inside drops

by1°C before you try to reopen it.

The ideal gas equation says that,

under these conditions, the net

force holding the freezer door shut

due to pressure differences will

be about 350 newtons, or about

36 kilograms. That is quite a lot.

Martin J. Greenwood

Stirling, Western Australia

My freezer has drawers. These trap

cold air, so relatively little escapes

when the door is opened. The door

effect is more noticeable on my

fridge, which has shelves instead.

That air exchange costs you

electricity to cool the warm air

that gets in, which is why you

should minimise the number

of times you open the fridge or

freezer door. Better still, get a top-

loading freezer, located away from

draughts. The cold air remains

trapped, reducing the “loss of

cold” when the door is opened.

John Woodgate

Rayleigh, Essex, UK

If the door is hard to open,

gently prise the seal back with

a fingernail. The pressures inside

and out quickly equalise and you

can open the door easily.

Simon Goodman

Griesheim, Germany

When I worked in a lab, some

biochemicals were stored at -80°C.

If you forgot to get something

out of that freezer the first time

around, you could have a long wait

before the pressure equilibrated

through the door seals, letting you

get in again. The pressure inside

reduced by about 35 per cent on

opening and reclosing.

Sitting pretty

We are frequently told we need

plenty of exercise and that sitting

is bad for us. Is the problem with

sitting merely that it stops you

exercising, or is sitting bad in itself?

Chris Daniel

Glan Conwy, Clwyd, UK

There are many detrimental

effects of sitting in addition to the

physical inactivity itself, which has

links to obesity, diabetes and so on.

For example, when standing,

the spine has a characteristic ‘s’

shape whereas sitting curves the

spine into a ‘c’ shape, which

compresses the discs between the

vertebrae and can increase the risk

of back injury. Hip and knee flexor

muscles may shorten with too

much sitting, which reduces the

range of motion of those joints.

Muscle inactivity can result in

pooling of blood in the legs, which

can lead to varicose veins and

increases the risk of deep-vein

thrombosis. Reduced lymphatic

drainage leads to swelling at the

extremities, puffiness in the eyes

and face, and fatigue headaches.

In prolonged sitting, internal

organ function is impaired.

Bowel mobility is reduced and

your digestion isn’t as efficient,

while reduced diaphragm

movement can cut lung capacity

after as little as an hour.

A simple way of reducing the

risks from sitting is to stand up

and move at least every half an

hour and to take daily exercise. ❚

This week’s new questions

Kicking the habit My 72-year-old mother gave up smoking

at the beginning of the year. Are there any health benefits of

quitting so late in life? Jo Dunn, Cape Town, South Africa

Camera precaution In light of the recent tragic deaths of

39 people whose bodies were found in a refrigerated lorry

in Essex, UK, is there any technology that could help stop

this happening? Could truck companies install cameras

to make sure there is no one inside, for instance?

John Hastings, Whittlesey, Cambridgeshire, UK

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Is there any benefit

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