- In convection the atoms themselves move.
- Whilst moving the atoms carry the heat energy.
- The atoms loose the heat energy the further away from the heat source they travel.
If, for example, water in a pan is heated from below, the liquid closest to the bottom expands and its density decreases. Therefore the hot water as a result rises to the top and some of the cooler water descends toward the bottom, this causes a circulatory motion called convection current. A convection current can not travel through a vacuum, as there are no particles to move and carry the heat energy.
Radiation
This process is different from both conduction and convection in that the substances exchanging heat need not be in contact with each other. A vacuum can separate them. Heat energy in radiation is carried by infrared waves, which are given off by the heat source itself.
Surfaces can absorb or reflect radiation. Generally, dull, rough surfaces absorb more heat than bright, polished surfaces. That is why cooking utensils generally have dull bottoms for good absorption and polished sides for minimum emission to maximise the heat transfer into the contents of the pot.
Why the advice has been produced?
Always wear a hat: This advice has been given, as the hat will trap a layer of air between the top of the head though the hair and then the hat. As air is a poor conductor, conduction can not occur in a strong enough for the heat energy to be able to pass the through the air through to hat. Heat loss through convection can also slowed as the atoms in the blood move the heat energy up into the head. This movement will be slowed, as the material that the hat is made of will be a poor conductor of heat. Radiation may also have an effect with heat loss through the head because of the colour of the hair i.e. blond hair will reflect infrared radiation back into the head.
Wear many layers of clothing: The clothes will trap layers of air between them and the next layer or the skin. This will slow heat loss through conduction, as air is a poor conductor. If the clothes are lightly coloured it will reflect all the infrared radiation back into the body. The more layers worn the longer it will take for a convection current to pass through.
Avoid clothes from becoming wet: Damp clothes will loose heat more easily as when the body heats up the heat energy produced will be carried away through evaporation. Water is a poor conductor of heat so will cool the body down quicker. The body itself uses water in sweat glands. These glands secrete moisture, which evaporates and cools the body surface down.
Shelter from the wind: The atoms in the wind help convection currents carry the heat energy away from the body. This same process is used in air-conditioning systems in homes and offices to cool down the buildings by blowing cool air currents through the top of the floor; this removes any convection currents and help slow down conduction.
For this experiment we are going to reconstruct these pieces of advice that have been given to walkers. Personally I predict that the hat experiment will turn out with the best results. This is because of the conventional air and water currents transporting the heat energy away from the beaker.
Preparation Work
To help and carry out this experiment as successfully as possible a preparation experiment was carried out. This was to check which material out of a selection we were going to use to represent clothes for the proper experiment.
The three materials that were tested to try and produce the best results for insulation were: Corrugated brown card, Bubble wrap and green school paper towels. 100mls of hot water were poured into three beakers and left for ten minutes. The temperature of the water was taken at the start and also at the end and compared with the other materials. The results were as follows:
To make this a fair test the same amount of water was used, the beakers were only wrapped with one layer of the material and the temperatures for the beakers were all taken at the same time with digital clocks. As the results show paper towel was the best insulator so this meant that this was going to be used in the investigation to represent clothing. To represent a hat in the experiments polystyrene and corrugated card were cut out into the shape of the top of the beaker. A hole was placed in with a pencil to place through the thermomiter whilst recording the temperatures, this was then coated in tin-foil to help keep in infrared radiation heat inside.
Method
By using the simple laboratory equiptment we had to investigate the best piece of advice that had been given. To try and keep these experiments fair we used the same amount of water (100mls) meausered form a measuring cylinder, the same colour paper towels (so not to interfere with the radiation given off and then reflected back by different colours). The same lid for each of the required and three out of the four experiments were conducted in the same position in the same lab so try and not alter room temperature. To try and not cause any anomalies we would just use the items required that were set (eg there were no paper towels wrapped around the beakers for the hat experiment).
For the hat experiment the lid was placed on top of the beaker with the thermometer in the hole. 100mls of water from the kettle was measured and poured into the beaker the first reading was took and the clock was started. Once every minute for ten minutes a reading was taken and recorded for ten minutes. This was then repeated with out the lid on and the results were recorded.
To investigate the clothing advice 100mls of water from the kettle was poured into the beaker. One paper towel was wrapped around the beaker and held into place by tape. Results were again taken once a minute for ten minutes. This experiment was carried out a second time with two layers of paper towel wrapped the beaker and results recorded.
For the experiment to investigate the damp clothes advice 100mls of hot water from the kettle was poured into the beaker. This was wrapped with one paper towel that had been soaked in water from the tap. This was stuck to the side of the beaker with tape and the hot water was added. Results were taken ten times in ten minutes and recorded.
Placing a fan on full power aimed at a beaker standing on a tripod produced the wind experiment. 100mls of water was added to the beaker and results were taken and recorded once a minute.
Apparatus
The following pieces of apparatus were used to conduct these experiments: a Pyrex beaker, thermometer, clock, kettle, tap water, tape, paper towels, hand made lid, electric fan, tripod and a measuring cylinder. These all fit under the category of simple laboratory equipment as specified in the task.
Results
The hat experiment
The experiment was carried out in science 3 on a cold day
With lid placed on beaker
The same results were taken without the lid being placed on the beaker
Wear many layers of clothes
This experiment was conducted in science four on a cold day
With one layer
With two layers
The keep all clothes dry
The experiment was conducted on science four on a cold day
One layer of damp paper towels
One layer of dry paper towels
Wind experiment
Experiment conducted on a cold day in science four
Fan on full power
No wind
Evaluation
All four of these experiments were designed to model a human body and the circumstances that cause hypothermia. From my hypothesis I predicted that the experiment with the hat would turn out with the best result, but this was proved wrong by the experiment with the fan. The reason for this is that the stronger airflow will have produced a faster convectional current taking the heat energy away from the beaker faster than normal. This proves that sheltering from the wind is the main way of stopping the symptoms of hypothermia from setting in.
The sources of error
As these experiments were only carried out once to fit in with the amount of work in the space of time there could have been a number of sources of error to cause anomolys these are:
- The difference in room temperature in the two rooms used.
- The amount of tape on the beakers, as this was not measured
- The time taken to get the water from the kettle, let the thermometer reach the right height and then record and start the clock.
- The amount of water may have always not have been the same e.g. just less/more than the required 100 mls.
- The temperature may have not been taken exactaly on one minute from the previous time.
- The temperature on the thermometer may have not been exactly at a certain degree and may have been in between two and rounded.
- The paper towels when wrapped around the beaker may have been too slack or taught.
- The paper towels may have covered the bottom of the beaker in some of the experiments.
- The hat might have not been placed on top of the beaker properly leaving room for heat to be carried away from the beaker.
