Thursday, 29 May 2014

Centripetal force vs speed of rotation

Equipment

Glass tube 15cm length with ends flamed so that they are smooth.
1.5m length of string
Hooked mass hanger for 100g slotted masses
Large rubber bung with home through centre
Marker pen
Stopclock

Method

Tie the bung to one end of the string. then measure 60cm from the centre of the bung along the string and make a mark with the pen.
Run the string through the tube and tie a loop at the end to hang the masses from.
Hang the mass hanger and spin the bung at the correct speed to keep the pen mark at the top of the tube.
Start the stopclock and time how long it takes for 10 swings of the bung. Take two more readings of the time for 10 swings to calculate an average and check for anomalies.
Add a mass to the hanger and repeat the swinging and timing being careful to ensure that the pen mark stays at the top of the tube. It is important that the radius of the swing does not change as this is another factor that affects the centripetal force needed and so will affect the speed you measure.
Continue adding masses and timing 10 swings until you have 500g on the hanger.

Calculate the centripetal force applied by multiplying the mass used by the gravitational field strength.
Calculate the average speed by taking the circumference of the circle the bung travels in and dividing by the average time for one swing.

Plot a graph of centripetal force vs speed of motion.

Risk assessment

Ensure you are working in a clear area before starting to swing. After each repeat check the string to make sure it is not beginning to fray. There is still a risk of the string breaking and the bung flying free. As such only rubber bungs should be used. Do not attach a metal mass to the swinging end. Goggles should be work to protect eyes from any possible flying bungs. As you are using a glass tube care should be taken not to grip it too tightly as this increases the chance of the tube breaking. The tube should also be well supported at the rotating string end to prevent any extra stress on the tube increasing the risk of it breaking.

Friday, 23 May 2014

Factors affecting circular motion & centripetal force

This is a series of three short experiments which examine how a different factors affect the size of the centripetal force needed to maintain motion.

The three factors you will investigate are; Mass of object, Radius of circle, Speed of object.

Equipment

Plastic ballpoint pen tube sanded to smooth off the ends
1.5 m length of string marked every 10 cm
Slotted masses on a hanger 100g/division and 10g/division
Large and small rubber bung with a hole through the centre
Stopclock

Diagram



Method 1 - Effect of mass.

Set up the equipment as shown using the small bung with a 50cm length between tube and bung centre. Maintain this distance throughout
Add 100g mass to the end of the string and swing the bung so that the 50cm mark stays in position.
Time 10 swings and record the time. Stop and start again and record the time for 10 swings. Repeat this once more and take an average of the three readings.
Replace the small bung with a large one.
Start to swing the bung at the same rate as before (same time for 10 swings).
If the bung moves outwards and will not maintain the 50cm radius when swung at this rate add more masses to the hanger. If it slides towards the ballpoint tube remove masses from the hanger.
Keep adding/subtracting masses until the larger bung will swing at the same distance and at the same rate as the smaller bung. Note down the new mass needed and check three times as before that it is swinging at the same rate as for the small bung.

Does a larger bung need a larger or smaller centripetal force to make it swing at the same rate and distance?

Method 2 - Effect of radius

Using the small bung start with 100g mass and a 10cm radius. Sing the bung to maintain the 10cm radius and time 10 swings. Repeat as above twice more and calculate the average time at this radius.
Increase the radius by 10cm.
Start to swing the bung at the same rate as before (same time for 10 swings).
If the bung moves outwards and will not maintain the 20cm radius when swung at this rate add more masses to the hanger. If it slides towards the ballpoint tube remove masses from the hanger.Keep adding/subtracting masses until the  bung will swing at the correct distance and at the same rate as for 10cm. Note down the new mass needed and check three times as before that it is swinging at the same rate as for 10cm.
Repeat the process at 10cm intervals up to 50cm radius.

How does the radius of the swing affect the centripetal force required to make it swing at the same rate for the same mass object?

Method 3 - Effect of speed

Using the small bung start with 100g mass and a 50cm radius. Sing the bung to maintain the 50cm radius and time 10 swings. Repeat as in method 1 twice more and calculate the average time at this radius.
Add a new mass to the hanger and repeat the above, timing 10 swings.
Repeat for one more addition of mass.

How does the speed affect the centripetal force required to keep a fixed mass moving at a constant radius?

In each experiment it is important you only change one variable and control the others so that you can see the effect it has on the centripetal force. Do not for example change the bung, the radius and the speed of swing all at once as you will not be able to tell which factor is affecting the mass required to provide teh centripetal force.

Risk assessment.

Ensure you are working in a clear area before starting to swing. After each repeat check the string to make sure it is not beginning to fray. There is still a risk of the string breaking and the bung flying free. As such only rubber bungs should be used. Do not attach a metal mass to the swinging end. Goggles should be work to protect eyes from any possible flying bungs.



Wednesday, 14 May 2014

Thermistor resistance dependent on temperature

Two methods of measurement are presented here depending on the equipment you have available.

Equipment

NTC Thermistor
Hot water
Jug of cold water
Beaker
Thermometer
Digital Multimeter (option 1)
Ammeter and Voltmeter (digital or analogue - option 2)
12V d.c. Power supply (option 2)
Connecting leads

Method - Option 1

Connect the thermistor to the multimeter and set to read Ohms.
Immerse the thermistor in hot water in the beaker. Use the thermometer to measure the temperature and record this and the resistance.
Add a small amount of cold water to change the temperature by at least 5 degrees C and repeat the readings.
If you have time repeat the experiment twice more.
See image below for setup.

Method - Option 2

Connect the thermistor and ammeter in series to the power supply (set to 10V). Connect the voltmeter in parallel to the thermistor.
Immerse the thermistor in hot water in the beaker. Use the thermometer to measure the temperature and record this, the current and the voltage..
Add a small amount of cold water to change the temperature by at least 5 degrees C and repeat the readings.
Divide the voltage by the current to find the resistance.
If you have time repeat the experiment twice more.
See image below for setup.

Safety

Care should be taken with the hot water and a stack of paper towels should be kept handy to mop spills up straight away. You should also be aware that hot water can cause scalds so should work standing up to allow you move away quickly if there is a spill. leads should be prevented from wrapping around the beaker causing extra chance of a snag and spill occurring. If performing option 2 extra care should be taken to ensure that the water and power supply are kept as far away from each other as possible.

Option 1 Setup

Option 2 setup

Tuesday, 13 May 2014

Diffusion, surface area and volume.

This experiment will allow you to find how surface area/volume ratio affects the rate of diffusion.

Equipment

Hydrochloric Acid solution (1 molar)
Beaker
Stopclock
Agar cubes of different sizes (three each with sides of 0.5 to 3 cm in 0.5cm intervals) made using alkali and Phenolphthalein indicator
Tweezers
White paper
Deionised water
Goggles


Method

Using the tweezers add one of the smallest cubes to the beaker. Place the beaker on the white paper so as to me able to monitor the colour change easily.
Pour the enough acid into the beaker to just cover the cube and start the stopclock. Stop the stopclock when the cube has become clear. Record the time taken.
Empty the beaker and rinse out using the deionised water.
Repeat with the remaining two 0.5cm side cubes.
Repeat the entire process using the remaining cubes.

Calculate the surface area and volume of each cube. Divide the surface area by the volume and plot a graph of this value against the time taken for the diffusion to complete.


Safety

Goggles must be worn when working with acids. Tweezers should be used when moving the cubes as the indicator used acts as a laxative. Care should also be taken to wash hands after completing the experiment.



Resistance of a Thermistor

This experiment gives a way to find the resistance of a thermistor at a fixed temperature.

Equipment

Water bath held at 20 degrees C
Jug of cold water
Thermometer
Thermistor
12V d.c. power supply
Variable resistor
Connecting leads
Ammeter
Voltmeter

Method

Place the thermistor in the water bath with the thermometer to allow you to monitor the temperature throughout.

Connect the power supply, thermistor and variable resistor and ammeter in series. Then connect the voltmeter in parallel across the thermistor.

Adjust the variable resistor so it is allowing the least current to flow through the circuit and note the current. Then adjust the variable resistor to find the maximum value for the current flow. This is the range of your independent variable. Use these values to decide on a suitable interval for your current values to ensure that you get at least 5 different values of the current

Starting at the lowest current value record the current and voltage. Increase the current using the variable resistor and again record the current and voltage. Repeat this increase and measurement until you reach the maximum current.

Monitor the temperature of the water to ensure that the thermistor is not heating the water during operation.

Take another two sets of readings for the same values of current you used on the first run though. Then calculate an average of these values.

Safety

Be careful that the water from the water bath does not splash onto any of the electrical equipment being used. Also be aware that they thermistor may be warm after use - the temperature should be 20 degrees however if you are not carefully maintaining this the thermistor may get warm.

Analysis

Plot a graph of current on the x axis and voltage on the y axis. The gradient of this graph will be the resistance of the thermistor