Cooling
So why do things cool down?
It’s very simple.
If the room was heated until it was the same temperature as the coffee the coffee would not cool down. It would be gaining as much heat energy from the room as it was loosing and so it would stay at the same temperature.
Chest freezers and upright freezers
Leaving the door of an upright freezer open is likely to have more effect on the contents than leaving the lid of a chest freezer open. The cold air will ‘fall’ out of the upright freezer while in the chest freezer it will lie in the chest – so keeping the contents cool for longer.
The hotter things are compared with their surroundings the faster they cool
The bigger the difference in temperature between and object and its surrounding the faster it will cool down. If you take a dish of hot food out of the oven and put it on a kitchen work surface it will start to cool – rapidly at first because it is much hotter than the room but more slowly as time goes by. Eventually if you leave it long enough it will reach the same temperature as the room and so stop cooling. Its temperature will then remain the same.
You can see this shown in the graph. The temperature is falling quickly at the start where the cooling is rapid but slows off as the temperature of the object is lower.
Hot plates in a restaurant
It is always a good guide to the quality of a restaurant if the plates are warm or even hot. Putting hot food on a cool plate will mean that the food will loose heat to the plate and so cool down. When you get your meal it may well be cold.
Do you add the milk straight away?
People often ask whether it is better to add the milk to a hot cup of coffee straight away or wait for a while to let it cool down and then add the milk.
Some years ago two scientists did some research into the cooling of a cup coffee. They found that, under laboratory conditions, a cup of coffee took 425 seconds to cool enough to drink if the milk was put in at once. However if you waited 310 seconds to add the milk the coffee cooled in just 30 additional seconds, a total 340 seconds - 85 seconds faster than adding the milk straight away.
Why not try to repeat their experiment and test their results!
Cooling and planets
The temperature of the Earth is only stable because the Earth is constantly receiving heat energy from the Sun but also radiating heat energy to space. If the Earth was unable to radiate heat into space because of a thick atmosphere or a build up of ‘greenhouse gases’ its temperature would slowly rise – the so called Greenhouse Effect. On the other hand if the Sun was to ‘go out’ the Earth would cool to the temperature of deep space (around –270 oC) and life as we know it would come to an end.
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