Physics Core Practicals

Investigate the relationship between force and extension for a spring

Sample Method

1. Set up the equipment as shown

2. Add 100g (1N) to the mass holder

3. Measure the extension of the spring and record the result 

4. Repeat steps 2 to 3 for a range of masses from 1N to 10N

Variables 

• The independant variable is the one deliberately changed - in this case, the force on the spring
• The dependant variable is the one that is measured - the extension

Investigate the relationship between force and extension for a spring

Considerations, Mistakes and Errors

• The extension is the total increase in length from the orignal unloaded length. It is not the total length or the increase each time
• Adding too many masses can stretch the spring too far, which means repeat measurements cannot be made

Hazards and Risks

• The biggest hazard in this experiment is masses falling onto the experimenter's feet. To minimise This risk, keep masses to the minimum for a good range of results

Investigate the effect of varying the force and / or the mass on the acceleration of an object

Sample Method

1. Set up the equipment as shown 

2. Release the trolley and use lightgates to take the measurements needed to calculate acceleration

3. Move 100g (1N) from the trolley onto the mass holder

4. Repeat steps 2 and 3 until all the masses have been moved form the trolley to the mass holder

If investigating the mass, keep the force constant by removing a mass from the trolley but not adding it to the holder

Variables

• The independant variable is the force or the mass
• The control variable is kept the same. In this case, the force if the mass is changed or the mass if the force is changed

Investigate the effect of varying the force and / or the mass on the acceleration of an object

Considerations, Mistakes and Errors

• When changing the force it is important to keep the mass of the system constant. Masses are taken from the trolley to the holder no extra masses are added
• Fast events often result in timing errors. Repeating results and finding the mean can help reduce the effect of these errors
• If the accelerating force is too low or the mass is too high, Then frictional effects will cause the results to be inaccurate 

Hazards and Risks

• The biggest hazard in this experiment is masses falling onto the experimenter's feet. To minimise this risk, masses should be ket to the minimum needed for a good range of results

Identify the suitability of apparatus to measure the frequency, wavelength and speed of waves in a ripple tank

Sample Method 

1. Time how long it takes one wave to travel the length of the tank. Use this to calculate wave speed using                          speed = distance ÷ time

2. To find the frequency, count the number of waves passing a fixed point in a second

3. Estimate the wavelength by using a ruler to measure the peak-to-peak distance as well as the waves travel

4. Use a stroboscope to make the same measurements and compare the result

Variables

• The key control variable is water depth. Its important to ensure that the depth of the water is kept constant across the tank as, for a given frequency, the depth will affect the speed and wavelength 
  

Identify the suitability of apparatus to measure the frequency, wavelength and speed of waves in a ripple tank

Considerations, Mistakes and Errors

• Using  a stroboscope can improve the accuracy of measurements
• By projecting a shadow of the waves onto a screen below the stroboscope, flash speed can be adjusted to make the waves appear stationary. This makes wavelength measurements much more accurate
• For high frequencies that are difficult to count, this can be used with the wave speed measurement to calculate the frequency using wave speed = frequency x wavelength 

Hazards and Risks

• When using a stroboscope there is a risk to people with photo-sensitive epilepsy. It is important to check that there are no at risk people involved in the experiment or in the area

Investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface

Sample Method

1. Take four glass boiling tubes each painted a different colour:    matt black, gloss black, white and silvered

2. Pour hot water into each boiling tube 

3. Measure and record the start tempurature of each tube 

4. Measure the tempurature of each tube every minute for 10 mins

5. The tube that cools fastest, emits infrared energy quickest

Variables 

• The independant variable is the colour of the boiling tube
• The dependant variable is teh tempurature
• The control variables include volume of water, start tempurature and enviromental conditions

Investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface

Considerations, Mistakes and Errors

• A common error in this experiment is not having the boiling tubes at the same tempurature at the start - a hotter tube will cool quicker initially, which can affect results 
• Evaporation from the surface of water can cause cooling too, which will affect the results. To minimise this, block the top of each tube with a bung or a plug of cotton wool

Hazards and Risks

• The main hazard is being burned when pouring the hot water and when handling the hot tubes. Using a test tube rack to hold the tubes minimises the need to touch the tubes and means hands can be kept clear when pouring the water into them

Investigate the factors that affect the resistance of electrical circuits

Sample Method

This example looks at how lengths affect the resistance of a wire:

1. Set up the standard test circuit as shown

2. Pre-test the circuit and adjust the supply voltage to ensure that there is a measurable difference in readings taken at the shortest and longest lengths

3. Record the voltage and current at a range of lengths, using crocodile clips to grip the wire at different points

4. Use the variable resistor to keep the current through the wire the same at each length

5. Use the volatge and current measurements to calculate the resistance

Investigate the factors that affect the resistance of electrical circuits

Variables 

• The independant variable is the length of wire 
• The dependant variable is the voltage
• the control variable is that current (which is kept the same because if it was too high it would cause the wire to get hot and change its resistance)

Considerations, Mistakes and Errors

• Adjusting the supply voltage to ensure as widea range of results as possible is important, as measurements culd be limited by the resolution of the measuring equipment
• The range of measurements to be tested should always include at leats five measurements at reasonable intervals. This allows for patterns to be seen without missing what happens in between, but also without taking large numbers of unnecessary measurements

Investigate the factors that affect the resistance of electrical circuits

Hazards and Risks

• Current flowing through the wire can get very hot
• To avoid being burned by the wire a low supply voltage should be used, such as the cell of the diagram, and adjust the variable resistor to keep the current low

Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature

Sample Method

1. Set up the standard test circuit as shown

2. Use the variable resistor to adjust the potential difference across the the test component

3. Measure the voltage and current for a range of voltage values

4. Repeat the experiment at least three times to be able to calculate a mean 

5. Repeat for the other components to be tested

Variables

• The independant variable is the potential difference across the component ( set by the variable resistor)
• The dependant variable is the current through the component, measured by the ammeter

Investigate the V-I characteristics of a filament lamp, a diode and a resistor at constant temperature

Considerations, Mistakes and Errors

• Before taking measurements, check the voltage and current with the supply turned off. This will allow zero errors to be identified
• A common error is simply reading the supply voltage as the voltage across the component. At low component resistances, the wires will take a sizable share of this voltage, resulting in a lower voltage across the component. This is why a voltmeter is used to measure the voltage across the component

Hazards and Risks

• The main risk is that the filament lamp with get hotter as the current increases and could cause burns. If it overheats, the bulb will 'blow' and must be allowed to cool down before attempting to unscrew and replace it

Investigate the density of regular and irregular solids and liquids

Sample Method 

1. Set up the equipment as shown 

2. Record the height of the water in the measuring cylinder and the mass of the solid / liquid being tested 

3. Add the solid / liquid being tested to the measuring cylinder

4. Record the new height in the measuring cylinder

5. Subtracting the original height from the new height gives the volume of the solid / liquid being tested

6. Now the density can be calculated

Variables

• The independant variable is the material being tested
• The dependant variables are the volume and mass
• The control variable is the temperature

Investigate the density of regular and irregular solids and liquids

Considerations, Mistakes and Errors

• If a solid that is less dense than water is tested, the volume measurement will be incorrrect because the solid will not be fully submerged
• When reading from the measuring cylinder, the reading must be taken from the bottom of the meniscus
• The temperature of the water must be exactly the same through all tests, as an increase in temperature could cause the material or water to change volume slightly through expansion

Hazards and Risks

• There are very few hazards, unless the materials being tested are hazardous or react with water
• The main hazards could be a slip if water is spilt