Thursday, 23 May 2013

Sweat content and cooling

This is an experiment to investigate how the electrolytes that make up part of sweat affect the cooling rate due to evaporation.

Equipment

Thermometer
Cotton wool
Elastic band
Clamp and stand
Stopclock
Salt solutions of five differing concentrations (10g/litre - 50g/litre in 10g/litre steps)

Method

Wrap a wad of cotton wool around the thermometer and secure in place with an elastic band.
Dip the wool in the lowest concentration salt solution, stir to it is thoroughly soaked.
Gently shake off any excess, clamp in place, note the initial temperature and start the stopclock.
After 10 minutes note the final temperature.
Repeat using fresh cotton wool for each of the other salt solutions.

Care should be taken to try and ensure the same amount of cotton wool is used each time.
If you have access to 5 thermometers all experiments can be run at the same time.
A digital temperature probe and datalogger will allow you to capture the cooling curve for each experiment if you are able to use one.





Modelling sweat - cooling effect

Sweating is a way of losing thermal energy to maintain body temperature and prevent overheating.

This experiment will allow you to model how the cooling due to sweating is affected by wind speed and wind temperature.

Equipment

Clamp & stand x 2
Boiling tube
Thin cotton material
Thermometer
Water bath
Stopclock
Hairdrier with cool, warm and hot settings
Small sponge
Water trough
Measuring cylinder

Procedure

Set the water bath to heat water to 37 degrees Celsius.
Attach a layer of the cotton around the boiling tube, either with elastic bands at the top and bottom, or by gluing a sleeve with a small overlap.
Clamp the boiling tube into the stand and add the thermometer.
Clamp the hairdrier on the other stand so that it points at the boiling tube at a distance of 5cm.
With water from the water bath use the sponge to wet the cotton and add 50ml to the boiling tube.
Switch on the hairdrier on it's cool setting and start the stopclock.
Record the temperature on the thermometer after 5 minutes. If the cotton appears to be drying out during this time reapply water with the sponge.

Repeat the experiment increasing the distance between the hairdrier and the boiling tube.
Each time the hairdrier is moved be sure to start with fresh water from the water bath.

To simulate the effect of increased wind temperature repeat the whole experiment using the hairdrier on medium and hot settings.

Safety notes

Take care using electrical appliances around water.
Mop any spills up straight away.


Monday, 20 May 2013

Electrolysis - electrode mass vs current

Equipment

Power supply
Rheostat
Ammeter
Electrode holder
Carbon electrodes
Copper sulphate solution
Beaker
Stopclock
Top pan balance


Method

Add 150 ml of the copper sulphate solution to the beaker.
Connect the electrodes to the power supply via the rheostat and ammeter in a series circuit.
Weigh the negative electrode on the balance.
Add the electrodes to the solution and set the rheostat to minimise the current.
Start the stopclock and switch on the power, noting the current.
After 5 minutes switch off the power, remove the negative electrode and rinse using the water.
Dry the electrode and weigh it again.
Empty and resupply the beaker with the same volume of fresh solution.
Adjust the rheostat, replace the electrode and repeat the process.

Aim to get at least five different readings of mass gained for different currents.
If you have time repeat the entire experiment 3 more times

Plot a graph of the mass gained vs current.

Note - Copper Sulphate is toxic, care should be taken when handling it and hands should be washed after the experiment.




Resistance characteristic of a component

Equipment

Power supply
Connecting leads
Crocodile clips
Ammeter
Voltmeter
Variable resistor

Method

Set up a circuit as shown in the diagram below.
Add the component to be tested in the space between the circular terminals.
Record the current and voltage.
Slowly alter the variable resistor to increase the current and read off the voltage.
Aim for a minimum of ten readings over the full range of the variable resistor.
Repeat the whole process twice more and calculate an average.






Analysis

Plot a graph of the current and voltage ensuring that the current in plotted on the X-axis.
The gradient of your graph is equal to the resistance of the component.

Notes

You can turn your component around to test the effect of changing the current passing through it in the opposite direction.
Temperature also has an affect on the resistance of a component and some components will heat significantly when in use.




Resistance - factors affecting resistance

The following practical set up will allow you to test a number of factors affecting resistance.

When finding the resistance of a wire you will need to measure the current passing through the wire and the voltage across it. Ammeters are connected in series and voltmeters are connected in parallel across the test wire.

A circuit should be set up as shown below, with the wire being tested connected between the circular terminals on the diagram, which should be crocodile clips or points on a breadboard.



To fully investigate the factors affecting the resistance of a wire you should first choose a fixed length and material for the wire. You may then investigate how the area of the cross-section of the wire affects the resistance.

Once you have recorded values of current and resistance for different thickness wires select one thickness and alter the length placed between the crocodile clips.

To find the resistance you will need to use the equation V / I = R with the readings on the Ammeter and Voltmeter.

You may need to do a little preliminary testing as, depending on your power supply,  a wire which is too short or too thin may melt.

Once you have collected your data you should plot graphs of area vs resistance and length vs resistance.