Wednesday, 27 November 2013

Transformers - voltage & coil ratios.

Aim

To investigate how the ratio of number of turns on the primary and secondary coils affects the potential difference across them.

Equipment

laminated iron C cores and holder
Insulated wire for winding
Power supply (12V a.c.)
Digital Multimeter or a.c. voltmeter
Connecting leads & crocodile clips

Method

Wind each side of the C core with 50 turns of wire.
Connect one side to the a.c. power supply set to 6V - this is the primary coil.
Connect the other side to the multimeter (set to measure a.c. voltage) or the a.c. voltmeter - this is the secondary coil.
Join the C cores using the holder.
Switch on the power supply and record the reading on the meter, then turn off the power supply again. Repeat the reading twice more.

On the side connected to the multimeter UNWIND 5 turns.
Take another three readings for the voltage.
Continue unwinding and taking readings until you have only 5 coils on the secondary side of the transformer.

Risk assessment

Wires may become hot if current is allowed to flow for too long due to low resistance so the power supply should not be left on. If it is left on too long wires should be allowed to cool after the power is shut off before moving the equipment.

There is a risk of electric shock with this experiment.
It is very important to start with the maximum number of turns on the secondary and then unwind it. This prevents any stepping up of the voltage which could cause sparking.
The current will increase as the voltage is stepped down so all changes should be made to the secondary coil without the power being turned on.
The equipment should not be touched when turned on.
Anyone with a heart condition or who has a pacemaker should be especially careful.

Tansformer power measurements

This experiment will allow you to perform a basic investigation of the power transfer & efficiency of a simple transformer.

Equipment

Power supply (12V a.c )
12V bulb
Variable resistor
Two a.c Voltmeters
Tw a.c Ammeters (multimeters could be used)
Connecting leads with croc clips
Wire for winding (laminated or insulated)
Iron rod 15 - 20 cm long

Method

From the power supply attach the variable resistor and one ammeter in series. Create a primary coil of fifty turns around one end of the iron rod using the winding wire and use croc clips to connect this pimary coil in series with the ammeter and variable resistor back to the power supply. Connect one voltmeter in parallel across the primary coil.

Create a secondary coil at the other end of the iron rod, again using fifty turns. Using crocodile clips connect the bulb and the second ammeter in series with the secondary coil and connect the voltmeter in parallel to teh secondary coil.

With the variable resistor turned completely one way not the readings on all of the meters.

Turn the variable resistor and take readings in five to ten different positions until the variable resistor reaches the other limit of rotation.

Repeat twice more, using the variable resistor to give your the same values for current and voltage on the primary coil each time.

Analysis

Find the mean readings for each step and then use the equation P = I V to find the power on the primary and the power on the secondary.

Plot a graph of the input and output power and describe any relationships you notice. Why is there a discrepancy in the power between each side?

Sunday, 2 June 2013

Reflection from a curved mirror

This experiment uses a laser to determine the focal point of a curved mirror on its concave surface.

On a piece of paper draw a line which will be your principal axis. Place the mirror at one end of this line so that the centre of the mirror bisects the line.

Use the laser to shine a ray of light parallel to the principal axis so that it reflects off the mirror. Using a sharp pencil mark the path of the ray on the paper.

Repeat this for a number of different starting positions - the point where all the lines cross the axis is the focal point of the lens.

It should be noted that there are risks using lasers. Care should be taken to choose a laser which is eyesafe at the aperture and to be careful with the laser alignment so that stray reflections are minimised.

Law of reflection

Equipment

Protractor
Plain paper
Plane mirror
Laser
Sharp pencil
Ruler

Method

Determine a line on the paper to place the mirror on, mark this line and mark the mid-point.
Draw a line from the midpoint perpendicular to the base line - this is your normal and all angles should be measured from this line.
Shine the laser so that it hits the mirror at the midpoint - mark two dots in the centre of the laser beam incident to the mirror and two more on the reflected ray.
Use the ruler to mark in the path of light and measure the angles of each beam from the normal.
Repeat this process another 5 times for different angles of incidence.

Analysis

You should find the angles on incidence and reflection are the same, however there may be some variance. Sources of error include how well the points in the rays were marked, how well the lines were then drawn with the ruler and finally your proficiency in using a protractor.

Electrolysis - purification of a sample


Impure samples of a metal can be purified by using the method described below. This method is for purifying a copper sample, but by substituting a different metal and a solution of one of its salts you will achieve the same results.

Place the sample of copper to be purified onto the positive terminal and a piece of the pure copper metal onto the negative terminal.

Both electrodes should be placed into a solution of a salt of the copper, copper sulphate is ideal for this purpose, though care should be taken as it is toxic. It is important that the two electrodes do not touch.

The copper from the impure electrode will be taken into solution and copper from the solution will be deposited on top of the pure copper electrode.

You may wish to weigh the electrodes (dry) before and after this experiment and compare the mass lost by one to the mass gained by the other.

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.



Monday, 25 February 2013

Carbonate content 2 - reaction with hydrochloric acid.

This method describes how to ascertain the relative proportion of carbonate in an ore sample based on the duration of a reaction with HCl.

Equipment

Conical flask
Balance with .01g resolution
Measuring trays
HCl 2mol/litre
Ore samples
Pestle and mortar
Spatula
Measuring cylinder
Stopclock

Method

Grind your first sample in the pestle and mortar until it is finely divided into a powder.
Measure out 20ml of acid and add to the measuring cylinder and pour into the conical flask.
Measure out 1g of the ground sample into a measuring tray.
Add the ground sample to the conical flask and start the stopclock, giving the flask a quick swirl to ensure the two mix.
Stop the stopclock when the reaction finishes and no more gas is evolved.

Record the time and move onto your next sample.

Goggles should be worn whenever working with acids.

Results

The longer the reaction continues for, the greater the proportion of carbonate in your sample. Depending on the proportions you may find the reaction completes too quickly to be able to time, or continues for too long. If this is the case you could either adjust the amount of sample used, or change the concentration of the acid.

Carbonate content 1 - Thermal measurement

This method will allow you to measure the amount of carbonate present in a sample of a material. It uses thermal decomposition to remove carbon dioxide from the carbonates and uses mass differences to assess the amount of carbonate in your sample.

This method assumes that carbonates will be the only gases evolved in the thermal decomposition and that you are working with carbonates of less reactive metals.

Equipment

Carbonate containing samples
Pestle and mortar
Crucible
Bunsen
Pipe clay triangle
Tripod
Tongs
Balance with .01g resolution

Method

Set up bunsen under a tripod with a pipe clay triangle on it.
Grind up some of your first sample (around 2-5 grams) in the pestle and mortar until it is finely divided.
Record the mass of your crucible.
Add your ground sample to the crucible and record the mass of both.
Strongly heat the sample for sufficient time to thermally decompose the carbonates. This can be a lengthy process with the end point associated with no further colour change of the material. You may need to agitate the crucible with the tongs to ensure that all the carbonates have been decomposed.
Record the mass of sample and crucible again and calculate the mass difference.
Repeat for each of your other samples.

Analysis

Divide the mass lost by each sample by the initial mass of each sample to find the a relative proportion of carbonates within each sample. This process could be calibrated against a known mass of a sample of calcium carbonate.

Reflection 2 - Mirrors and angles

This set of experiments demonstrates some of the properties of reflections in a pair of plane mirrors.

When we consider a reflection in a plane mirror we have an object and an image which is produced 'in' the mirror (as it is a virtual image).

Experiment 1 - number of images

If we begin to use two mirrors which we hinge together along one side and change the angle between them the path of the light and the number of images can be altered.

Equipment

Two plane mirrors
Sellotape to hinge them
A protractor
A small object such as a pencil sharpener

Method

Place the object on your page and draw a line a few centimetres away from it to place one of the mirrors along - this one will be fixed. Adjust the angle between the mirrors to 180 degrees (so flat) and note that including the object you can see two of whatever it is you have chosen.

Keeping the one mirror still slowly reduce the angle between the mirrors stopping when you have three versions of your object - i.e. the object and two reflections. Note the angle between the mirrors.

Repeat this for additional whole numbers of object + images, nothing the angle each time.

Results

You should see a clear pattern between the angles and the number of objects - if not then consider dividing the angle in a complete circle by the number of object & images you saw at each step.

Experiment 2 - the corner reflector

This experiment demonstrates how a combination of mirrors can be used to reflect light back parallel to its initial path - an arrangement which is used in the reflectors on bicycles.

Equipment

Two plane mirrors
Sellotape to hinge them
A protractor
A ruler
A single slit raybox and powerpack
A sharp pencil

Method

Fix the mirrors such that the angle between them is 90 degrees.
Place them on a page and shine a ray from the raybox so that it bounces of both mirrors.
Mark the path of the ray with dots and then remove the mirror so you can join them with the ruler. Add arrows to show the direction the ray travelled.

Try this for a different incident angle, and again mark the rays.

Results

You should notice the the reflection off the second mirror is parallel to the incident ray. Use the law of refelction to see if you can explain why this should be. (Hint - think about the angle the normal is to the mirror surface)















Reflection 1 - Plane Mirrors

This experiment seeks to demonstrate the law of reflection - the angle of incidence is equal to the angle of reflection when measured from the normal.

Equipment

Single slit raybox with power supply
Paper
Ruler
Protractor
Sharp Pencil
Plane mirror
Support for mirror (e.g. wooden block with a groove in, or plasticine)

Method

Draw a line on the paper. Place the mirror on the line and support it so it does not move.
Shine the beam from the raybox towards the mirror. Use the pencil to carefully mark two dots in the centre of the incident and reflected rays.
Move the mirror to one side and use the ruler to join the dots to show the complete path of the ray. Add arrows so you know which direction the ray travelled.
At the point where the ray reflects from the mirror add a line perpendicular to the mirrors surface - this is the normal line.
Use the protractor measure the angle between the normal and the incident ray, and the normal and the reflected ray.

Note these angles in a table and then repeat the experiment for at least three more different angles.

Care should be taken when moving the raybox as those which use an incandescent bulb can get hot to the touch.

Results

You should find that the results show that the incident angle and reflected angle are equal. Your results may be a little out, due to errors introduced with how carefully you marked the path, the normal and measured the angle.