The Required Practical Assessment
Investigating reaction time
Aim: To measure and investigate factors affecting human reaction time.
Variables:
Independent Variable: This is the factor you change in the investigation. Examples from the sources include:
Whether or not a stimulant like caffeine has been consumed.
Potentially, the person being tested (to compare different individuals).
Dependent Variable: This is what you measure. In this practical, the dependent variable is reaction time. This is often determined by the distance the ruler falls before it is caught.
Control Variables: These are factors that you need to keep the same to ensure a fair test and that only the independent variable affects the dependent variable. Examples include:
The same person catching the ruler (if investigating other factors).
Using the weaker hand of the person catching the ruler.
The position of the forearm and fingers over the edge of the desk.
The same ruler being used.
The starting height (zero mark) of the ruler above the catcher's fingers.
The instruction for the catcher to look at a point across the room and use their peripheral vision.
Taking a short rest between repeats to avoid the participant getting used to the stimulus.
Method:
Sit at a table with a partner.
Place your weaker forearm across the desk with your thumb and forefinger slightly over the edge, with a small gap between them.
Your partner holds a metre ruler vertically with the zero mark just above your thumb and forefinger.
You must look at a point across the room and use the corner of your eye to detect the ruler's movement.
Your partner will randomly drop the ruler without telling you when.
As soon as you see the ruler start to fall, close your thumb and forefinger to catch it.
Record the distance the ruler has fallen (the mark on the ruler between your fingers) in a table. Ensure measurements are taken in centimetres initially.
Repeat this process several times (e.g., 10 times) to get multiple readings.
Calculate the average distance the ruler fell.
You can then swap places with your partner to gather their results.
How to Find Out Things (Data Analysis and Interpretation):
Converting Distance to Reaction Time: The distance the ruler falls is related to the time it takes to react due to gravity. You can use a conversion table to convert the average distance into an average reaction time in seconds. The exact relationship comes from the equation of motion: (s = ut + \frac{1}{2}at^2), where (s) is the distance, (u) is the initial velocity (0 in this case), (a) is the acceleration due to gravity (approximately 9.8 m/s²), and (t) is the time (reaction time). Therefore, (s = \frac{1}{2}gt^2), which can be rearranged to (t = \sqrt{\frac{2s}{g}}).
Calculating a Mean: To find the average reaction time, you sum all the reaction time measurements and divide by the number of measurements taken. This helps to reduce the impact of random errors.
Identifying Anomalies: When you have your set of results, look for any measurements that are significantly different from the others. These could be anomalies and might need to be excluded when calculating the mean if there's a clear reason for them (e.g., distraction).
Investigating the Effect of a Stimulant (e.g., Caffeine): To investigate the effect of caffeine, you would measure a person's reaction time before and after they consume a drink containing caffeine. Ensure control variables (like the time of day and amount of rest) are kept consistent. You would then compare the average reaction times in both conditions. A better reaction time is a shorter time.
Drawing Conclusions: Based on your results, you can describe what effect the independent variable had on the dependent variable (reaction time). For example, if investigating caffeine, you would state whether caffeine increased or decreased reaction time and by how much, based on your average results.
Sources of Error and How to Prevent Them:
Anticipation: The person catching the ruler might anticipate when it's going to be dropped, leading to a faster reaction time than genuine. To prevent this, the person dropping the ruler should do so randomly and without giving any cues.
Parallax Error: If the person reading the ruler is not looking directly at eye level with the markings, they might misread the distance. Ensure the reading is taken at eye level.
Movement of the Arm: If the catcher moves their arm downwards before catching the ruler, this could affect the measured distance. Ensure only the fingers close to catch the ruler, and the forearm remains still.
Distractions: External noises or movements can affect the concentration of the person being tested, leading to variations in reaction time. Conduct the experiment in a quiet environment with minimal distractions.
Getting Used to the Test: Taking short rests between trials can help prevent the person from becoming too familiar with the dropping of the ruler, which could artificially improve their reaction time.
Resolution of Equipment:
Ruler: The resolution of a standard metre ruler is typically 1 millimetre (or 0.1 centimetres). This means you can only read the distance to the nearest millimetre.
By following this method and carefully controlling variables, you can investigate factors affecting reaction time and draw conclusions based on your collected data.
Investigating motion
Force = mass x acceleration
Aim: To investigate the relationship between force, mass, and acceleration, and to verify Newton's second law (F=ma). This can be done by varying either the force applied to a constant mass or by varying the mass while keeping the applied force constant.
Variables (when investigating the effect of force on acceleration):
Independent Variable: Force accelerating the trolley. This is typically changed by varying the weight of the hanging masses.
Dependent Variable: Acceleration of the trolley.
Control Variables:
Total mass of the system (trolley + hanging masses). When changing the hanging mass to alter the force, masses should be transferred from the hanger to the trolley to keep the total mass constant.
The track or surface should be kept consistent to ensure friction remains as constant as possible.
The position of the light gates (if used) should be consistent between readings for a given setup.
Variables (when investigating the effect of mass on acceleration):
Independent Variable: Mass of the trolley (this can be increased by adding weights to the trolley).
Dependent Variable: Acceleration of the trolley.
Control Variables:
Force accelerating the trolley. This should be kept constant throughout the experiment (e.g., by using a fixed hanging mass).
The track or surface should be kept consistent to ensure friction remains as constant as possible.
The position of the light gates (if used) should be consistent between readings.
Method (Investigating the effect of force on acceleration - Constant Mass):
Set up a track with a trolley. Attach a string to the trolley, running it over a pulley at the end of the track.
Hang masses from the other end of the string. The weight of these hanging masses provides the accelerating force on the system (trolley + hanging masses). Remember that the force (weight) in Newtons is calculated using (W = mg), where (m) is the mass in kilograms and (g) is the acceleration due to gravity (approximately 9.8 N/kg, though sometimes approximated as 10 N/kg for simplicity in calculations).
Measure the total mass of the system (trolley + all masses that will be used, including those initially on the trolley) using a balance. This total mass should remain constant throughout this part of the investigation.
Use a method to measure the acceleration of the trolley. The source describes three possible methods:
Data Logger with two light gates: Place two light gates at a measured distance along the track. Attach a card of known width (a "flag") to the trolley. The data logger measures the time the flag takes to pass through each gate, calculates the velocity at each gate, and then calculates the acceleration between the gates. The flag width needs to be entered into the data logger. Ensure the string is long enough so the trolley passes through the second light gate before the hanging masses hit the floor.
One light gate: Measure the speed ((v)) of the trolley as it passes through the light gate (using the flag width and time). If the trolley starts from rest, and you know the distance ((s)) travelled to the light gate, you can calculate acceleration using the formula (a = v^2 / (2s)). Ensure you measure the distance from the starting point (middle of the flag) to the light gate consistently.
Stop clock and measured distance: Measure the time ((t)) it takes for the trolley to travel a known distance ((s)) starting from rest. The acceleration can be calculated using the formula (a = (2s) / t^2). Ensure measurements are in metres to get acceleration in m/s².
Start with a small hanging mass (e.g., 10 g, which exerts a force of approximately 0.1 N). Record the hanging mass (and calculate the force) and measure the acceleration.
Change the force by increasing the hanging mass. For each new hanging mass, transfer an equal mass from the hanger to the trolley to keep the total mass of the system constant. Record the new force and measure the new acceleration. Take multiple readings for each force and calculate a mean acceleration.
Repeat for several different forces.
Method (Investigating the effect of mass on acceleration - Constant Force):
Set up the track, trolley, pulley, and hanging mass as before. Keep the hanging mass (and therefore the accelerating force) constant throughout this part of the investigation.
Measure the acceleration of the trolley with its initial mass using one of the methods described above. Take repeat readings and calculate a mean.
Increase the mass of the trolley by adding known weights to it. Measure the new total mass of the trolley.
Measure the acceleration of the trolley with the increased mass, keeping the hanging mass (force) the same. Take repeat readings and calculate a mean.
Repeat for several different masses of the trolley.
How to Find Out Things (Data Analysis and Interpretation):
Calculating Force: The accelerating force is the weight of the hanging masses: (F = m_{hanging} \times g). Ensure you convert the mass to kilograms before calculating the weight in Newtons.
Calculating Acceleration: Use one of the methods described above to obtain the acceleration in m/s². Remember to convert all measurements to standard SI units (metres, kilograms, seconds).
Graphing Results (Effect of Force): Plot a graph of force (on the y-axis) against acceleration (on the x-axis). According to Newton's second law (F=ma), this should produce a linear relationship passing through the origin. The gradient of this graph will be equal to the mass of the system (trolley + any added masses, which should be constant in this part of the experiment). You can calculate the gradient by drawing a large right-angled triangle on your line of best fit and dividing the change in force (height of triangle) by the change in acceleration (width of triangle). Compare this gradient to the measured mass of the trolley and any added masses; they should be similar, though the gradient might be slightly higher due to friction.
Graphing Results (Effect of Mass): To investigate the inverse relationship between mass and acceleration (when force is constant), you can plot a graph of acceleration (on the y-axis) against mass (on the x-axis). This should show a curve. To get a linear relationship, you can plot acceleration (on the y-axis) against 1/mass (on the x-axis). The gradient of this linear graph should be equal to the constant force applied.
Verifying Newton's Second Law: By observing the linear relationship between force and acceleration (at constant mass) and the inverse relationship between acceleration and mass (at constant force), you can verify Newton's second law, (F = ma), or its rearranged forms, (a = F/m) and (m = F/a).
Sources of Error and How to Prevent Them:
Friction: Friction between the trolley and the track, and in the pulley, will oppose the motion and reduce the acceleration. This means the measured acceleration will be less than expected for a given force and mass. Using a frictionless track (like an air track if available) can minimise this. For a regular track, ensure the wheels of the trolley move freely. The calculated mass from the graph of force against acceleration might be slightly higher than the actual mass due to friction.
Air Resistance: Air resistance acting on the trolley (especially at higher speeds) can also affect the acceleration. Using smaller, more streamlined trolleys can help reduce this.
String Angle: Ensure the string is horizontal between the trolley and the pulley to maximise the horizontal force on the trolley.
Mass of the String and Hanger: The mass of the string and the hanger itself are usually small but can introduce a small error, as the accelerating force is technically acting on the trolley, the string, and the hanging masses. Using low-mass strings and hangers minimises this. The source mentions the hanger having a mass (10g).
Reaction Time (if using a stop clock): If using a stop clock, human reaction time in starting and stopping the clock can introduce errors in the time measurement, which will affect the calculated acceleration. Using light gates and a data logger provides a more precise measurement of time and speed.
Measurement Errors: Ensure accurate measurements of distances (between light gates, or the total distance travelled) and the width of the flag on the trolley. Read measuring instruments at eye level to avoid parallax errors.
Ensuring Constant Total Mass (when varying force): When transferring masses between the hanger and the trolley, ensure the correct amount of mass is transferred to keep the total mass of the system constant.
By carefully controlling variables and taking repeat readings, you can obtain reliable results and investigate the relationship between force, mass, and acceleration effectively.
Hooke’s Law
The practical investigation for Hooke's Law aims to explore the relationship between the force applied to a spring and its extension.
Variables:
Independent Variable: The force applied to the spring. This is varied by adding known masses to the spring. The force is calculated from the mass using the equation (F = mg), where (m) is the mass in kilograms and (g) is the acceleration due to gravity (approximated as 10 N/kg in this experiment).
Dependent Variable: The extension of the spring. This is the change in length of the spring from its original, unloaded length. It is measured in metres.
Control Variables:
The spring itself should remain the same throughout the experiment to ensure its properties (like stiffness) don't change.
The way the ruler is clamped and the initial measurement of the spring's length should be consistent.
Method:
Hang the spring from a support.
Clamp a ruler vertically next to the spring. Ideally, the ruler should start its measurement at zero at the bottom of the spring when no load is applied. This avoids the need for adjustments later. Ensure the ruler is clamped securely and is vertical.
Record the initial length of the spring (when no masses are attached). This is the starting point for measuring extension. With zero force applied, the extension is zero.
Add masses to the spring one at a time. The sources use 100 g masses, each exerting a force of approximately 1 Newton (using (g = 10 , N/kg)).
For each mass added, allow the spring to settle and stop oscillating before taking a measurement. Get down to eye level with the bottom of the spring and the ruler to take an accurate reading, avoiding parallax error. Be careful to read the correct scale on the ruler if it has more than one.
Record the new length of the spring for each added mass.
Calculate the extension of the spring by subtracting the initial length from the new length. Remember to convert measurements from centimetres or millimetres on the ruler to metres for calculations and plotting graphs. To convert from centimetres to metres, divide by 100.
Repeat the process for several different masses (and therefore forces). The example in the source uses forces of 1 N, 2 N, 3 N, 4 N, and 5 N.
After taking the measurements, remove the weights from the spring and check if it returns to its original length. If it doesn't, it indicates that the spring has been stretched beyond its elastic limit, and Hooke's Law may no longer apply to the higher forces used.
Data Analysis and Finding Things Out:
Record your results in a table with columns for force (in Newtons) and extension (in metres).
Plot a graph of force (on the y-axis) against extension (on the x-axis). This is done for the purpose of analysis, even though the extension is the dependent variable.
Draw a line of best fit through the plotted points. For a spring obeying Hooke's Law, this graph should be a straight line passing through the origin (or very close to it). If the line doesn't go through the origin, it might suggest a systematic error in measuring the initial length or zero extension.
Calculate the gradient of the line of best fit by drawing a large right-angled triangle on the graph. The gradient is calculated as the change in force (height of the triangle) divided by the change in extension (width of the triangle). The units of the gradient will be Newtons per metre (N/m).
The gradient of the force-extension graph represents the spring constant (k) of the spring. The spring constant is a measure of the stiffness of the spring; a higher spring constant indicates a stiffer spring.
Hooke's Law:
Hooke's Law states that the extension (x or e) of a spring is directly proportional to the force (F) applied to it, provided that the elastic limit is not exceeded. This relationship is expressed by the equation:
[ F = kx ]
where:
(F) is the force applied to the spring (in Newtons).
(k) is the spring constant (in Newtons per metre, N/m).
(x) is the extension of the spring (in metres).
The sources note that the symbol (x) for extension can sometimes be replaced with a lowercase (e), so the equation can also be written as (F = ke).
Sources of Error and How to Prevent Them:
Zero Error: The ruler might not start exactly at zero at the bottom of the spring, leading to a systematic error in length measurements. Using a ruler that starts at zero at the end and carefully aligning it with the bottom of the spring can minimise this.
Parallax Error: Not reading the ruler at eye level can lead to inaccurate measurements of the spring's length and extension.
Oscillations: If measurements are taken while the spring is still oscillating, the readings will be inaccurate. Allow the spring to come to rest after adding each mass. Gentle support might be needed to dampen oscillations.
Exceeding Elastic Limit: Adding too much mass can cause the spring to stretch beyond its elastic limit, meaning it will not return to its original length when the masses are removed, and Hooke's Law will no longer be obeyed. Check if the spring returns to its original length after the experiment.
Spring Catching: Ensure the spring is hanging freely and not catching on the clamp or any other part of the setup, as this would prevent it from extending properly.
Precision of Measurements: Use a ruler with a suitable resolution (e.g., to the nearest millimetre) and record measurements carefully. Convert units correctly to metres for calculations and graphing.
Systematic Error in Initial Length: There might be a small systematic error in determining the initial length of the spring. The intercept of the force-extension graph (if not exactly at zero) can sometimes indicate this.
By carefully following the method, taking accurate measurements, and plotting a graph, you can investigate Hooke's Law and determine the spring constant of a spring.
Ripple Tank
Purpose:
The aim of the experiment is to measure the speed of water waves.
Method:
Here's a breakdown of the method described in the sources:
Set up a ripple tank, which is essentially a pool of water with an oscillator positioned above the surface. The oscillator moves up and down to create waves that travel across the water.
A light source is positioned above the tank to shine light through the water. The waves in the water act as lenses, and their crests and troughs will create patterns of light and shadow on a screen placed below or beside the tank. A mirror might be used to reflect the light towards the camera for better viewing.
Adjust the oscillator so that it is just touching the surface of the water to produce clear and prominent waves.
Turn on the oscillator to generate waves.
A strobe light that flashes at the same frequency as the oscillator can be used to make the waves appear stationary, making it easier to measure their wavelength. If you are sensitive to flashing lights, you can work with lower frequencies where you can count the number of waves passing a point per second to determine the frequency.
To measure the wavelength (λ), it is recommended to measure the length of several wavelengths (e.g., 10) using a ruler on the screen and then divide by the number of waves measured to get a more accurate result. It's important to align the zero of the ruler with the start of the first wave being measured, not counting from one.
It's crucial to account for the magnification of the wave image on the screen compared to the actual waves in the tank. To determine the magnification, an object of known size (like a credit card) can be placed in the ripple tank, and its size can be measured both in the tank and on the screen. The magnification is the ratio of the image size to the actual size. Any wavelength measurements taken from the screen must be divided by this magnification factor to obtain the real wavelength.
The frequency (f) of the waves is determined by the setting on the oscillator (in Hertz, Hz). If a strobe light is used and makes the waves appear stationary, its flashing frequency matches the wave frequency.
To investigate the relationship between frequency and wavelength, repeat the experiment for several different frequencies, measuring the corresponding wavelength each time. It's suggested to have at least five readings across a good range of frequencies.
Calculations:
The speed of the wave (v) can be calculated using the wave equation:
[ v = f \times \lambda ]
where:
(v) is the wave speed (in metres per second, m/s)
(f) is the frequency (in Hertz, Hz)
(λ) is the wavelength (in metres, m)
You can either calculate the wave speed for each frequency and wavelength measurement and then find the mean speed, or you can plot a graph to analyse the data more scientifically.
Graphing the Results:
To obtain a linear graph, instead of plotting frequency against wavelength (which would show an inversely proportional relationship), you can plot frequency on the y-axis and the reciprocal of the wavelength (1/λ) on the x-axis. The graph should be a straight line passing through the origin, and the gradient of this graph will be equal to the speed of the wave in m/s.
Sources of Error and How to Prevent Them:
The "Required practical exam 07.03.24.pdf" mentions "Sources of error/how to prevent:" for the ripple tank experiment but does not list specific examples in the provided excerpts. However, based on the method, potential sources of error could include:
Parallax error when measuring wavelengths on the screen. Ensure your eye is directly above the ruler mark.
Inaccurate determination of magnification. Measure the known object in the tank and on the screen carefully and take multiple measurements.
Difficulty in judging the exact position of wave crests or troughs, especially if the waves are not very clear. Using a strobe light can help with this.
Reflections from the sides of the ripple tank that might interfere with the waves being studied.
Ensuring the frequency of the oscillator is accurately known and remains constant during measurements.
Conclusion:
By analysing the data, you can determine the relationship between the frequency and wavelength of water waves and calculate their speed. As the frequency of the oscillator is decreased, the wavelength of the water waves should increase.
Source indicates that "Ripple tank (measuring wave speed)" is one of the required practicals for an assessment. Source suggests reviewing key details for each practical, including the method, variables, sources of error, and writing conclusions. Source provides headings for the ripple tank practical, including "Method (short bullet points)," "Sources of error/how to prevent," "Resolution of equipment (ruler)," and "Calculation (Wave Speed = Frequency × Wavelength)." It also asks, "Is frequency technically the independent variable?". In this experiment, the frequency is typically the independent variable that is changed by the experimenter to observe its effect on the dependent variable, the wavelength, while the wave speed in the medium ideally remains constant (or changes predictably with factors like water depth).
Waves on a string
Here's some information about investigating waves on a string, drawing on the provided sources:
The practical investigation aims to measure the speed of waves on a string. One method to achieve this involves creating stationary waves (also known as standing waves) on the string.
Method:
Set up a signal generator connected to a vibration generator. The vibration generator has a bit that goes up and down, driven by the electrical current from the signal generator.
Attach one end of a piece of string to the vibration generator.
Run the string over a pulley at the other end of the table and attach a mass (e.g., a 100 g mass) to provide tension in the string. It's important not to use too much mass as it could break the vibration generator.
Turn on the signal generator. The vibration generator will create a wave that travels along the string.
When the wave reaches the end of the string, it will be reflected back. The superposition of the wave travelling one way and the reflected wave travelling the other way can create stationary waves at specific frequencies.
Adjust the frequency of the signal generator until a clear standing wave pattern with one or more loops (antinodes) and nodes (points of little or no movement) is observed.
For each observed standing wave pattern, measure the wavelength of the wave.
For the simplest pattern with one loop, the length of the string is equal to half a wavelength ((\frac{1}{2} \lambda)). Therefore, the wavelength ((\lambda)) is twice the length of the string.
For a pattern with two loops, the length of the string is equal to one whole wavelength ((\lambda)).
For a pattern with three loops, the length of the string is equal to one and a half wavelengths ((\frac{3}{2} \lambda)), and so on. Generally, if there are (n) loops on the string of length (L), then (L = \frac{n}{2} \lambda), so (\lambda = \frac{2L}{n}). Alternatively, you can measure the distance between two adjacent nodes, which is half a wavelength, and then double it.
Record the frequency ((f)) displayed on the signal generator for each standing wave pattern.
Calculate the wave speed ((v)) using the wave equation: (v = f \lambda).
Alternative Method (Implied):
While the detailed method focuses on stationary waves, source also mentions investigating motion with "force = mass x acceleration," and the ripple tank and waves on a string practicals are grouped in P6. This might imply that you could potentially investigate how changing the tension (related to force) or mass per unit length of the string affects the wave speed directly, although the provided transcripts focus on varying frequency and observing standing waves.
Variables:
Independent Variable: In the method described, the frequency of the signal generator is the independent variable, which is adjusted to create different standing wave patterns. Source even poses the question "Is frequency technically the independent variable?" for this practical, suggesting it is often treated as such. However, you could also consider the number of loops in the standing wave pattern as a way to index your measurements at different frequencies.
Dependent Variable: The wavelength of the standing wave that is observed at each frequency.
Control Variables:
The length of the string between the vibration generator and the pulley should remain constant.
The mass (and therefore the tension) at the end of the string should ideally be kept constant throughout the experiment, as tension affects the wave speed. However, this could potentially be an independent variable in a different investigation.
Data Analysis:
Record the frequency and the corresponding wavelength for each observed standing wave pattern in a table.
Calculate the wave speed ((v = f \lambda)) for each frequency and wavelength pair.
You can then find the average wave speed from these calculations.
Alternatively, you can plot a graph. Source suggests plotting frequency (on the y-axis) against the reciprocal of the wavelength ((\frac{1}{\lambda}) on the x-axis). The gradient of this graph will be equal to the wave speed ((v)), since (f = v \times \frac{1}{\lambda}).
Sources of Error and How to Prevent Them:
Difficulty in accurately identifying the nodes and antinodes of the standing wave, which can lead to errors in measuring the wavelength. Ensure you take measurements from the clearest parts of the wave pattern.
Ensuring the frequency reading on the signal generator is accurate. Check the calibration of the signal generator if possible.
The string might not be perfectly horizontal or the tension might not be perfectly constant along the string due to the mass hanging at the end. Use a long enough table to minimize the angle and ensure the mass is hanging freely.
The vibration generator might introduce some movement at the 'nodes' near it, making precise measurement difficult. Try to measure the wavelength over several loops if possible, or focus on the clearer nodes further along the string.
Resolution of Equipment:
Ruler: The resolution of the ruler used to measure the length of the string or the wavelength of the standing wave will affect the precision of your wavelength measurements. Use a ruler with millimetre markings for the best possible resolution.
By following these steps, you can investigate the speed of waves on a string and understand the relationship between frequency, wavelength, and wave speed.
Leslie Cube
The Leslie cube practical is a GCSE physics required practical focused on infrared radiation emission. It can also be used to investigate infrared absorption.
Aim:
The aim of the experiment is to investigate how the type of surface affects the amount of infrared radiation emitted or absorbed.
Method for Investigating Emission:
A Leslie cube, which is a hollow metal cube with different surface finishes on each of its vertical sides (e.g., matte black, shiny metallic, matte white) and sometimes a different top surface, is used.
Hot water from a kettle is poured into the Leslie cube, and a lid is placed on top to reduce heat loss by convection. Using more hot water is better. You might also want to consider heat loss by conduction into the table and use a heat-proof mat.
An infrared thermometer (or an infrared detector connected to a meter) is used to detect the infrared radiation emitted from each surface of the cube.
To ensure a fair test, the thermometer should be held at the same distance and angle from each surface when taking measurements. A distance of 10 cm is suggested as an example.
The temperature (or amount of infrared radiation detected) is recorded for each surface. It's important to allow some time for the surfaces to reach a stable temperature after the hot water is poured in.
Observations and Expected Conclusions (Emission):
The experiment demonstrates that different surfaces emit different amounts of infrared radiation even when they are at the same temperature. You would expect to find that the matte black surface is a much better emitter of infrared radiation compared to shiny surfaces. Shiny surfaces are poor emitters.
Method for Investigating Absorption:
An infrared lamp (a bulb that emits a significant amount of infrared radiation) is used as a source of infrared radiation.
Objects with different surface finishes are exposed to the infrared radiation from the lamp at the same distance. Examples given include:
Copper tubes with bungs, covered in shiny, matte white, and matte black materials, with a thermometer or temperature probe inserted (with cotton wool to prevent contact with the sides) to measure the temperature inside.
Boiling tubes filled with tap water, with different surface finishes applied to them, and a thermometer in a bung with its bulb in the water.
The initial temperatures of the objects should be recorded.
The objects are then exposed to the infrared lamp for a set period of time (e.g., every minute for 10 minutes, or just final temperatures after 5 or 10 minutes can be compared).
The temperatures of the objects are recorded at regular intervals or at the end of the chosen time period.
Observations and Expected Conclusions (Absorption):
This experiment shows that different surfaces absorb different amounts of infrared radiation. You would expect to find that the matte black surface is the best absorber of infrared radiation, resulting in the highest temperature increase. Shiny surfaces are poor absorbers. Matte white surfaces are somewhere in between. This explains why wearing a white T-shirt is preferable to a black T-shirt on a sunny day, as the white one absorbs less infrared radiation.
Variables:
Based on source, for the Leslie cube investigation:
Independent variable: This would be the type of surface of the Leslie cube (for emission) or the type of surface exposed to infrared radiation (for absorption).
Dependent variable: This would be the amount of infrared radiation emitted/absorbed (measured by the infrared thermometer as temperature or a reading on a detector) or the temperature change of the objects.
Control variables: These would include factors that need to be kept the same to ensure a fair test, such as:
The temperature of the hot water in the Leslie cube at the start of the emission experiment.
The distance and angle of the infrared thermometer from the surface.
The time allowed for temperature readings.
The distance and power of the infrared lamp from the surfaces being tested for absorption.
The initial temperature and volume of water in the boiling tubes (if used for absorption).
The type and size of the copper tubes (if used for absorption).
Sources of Error and How to Prevent:
Source indicates the need to consider "Sources of error/how to prevent" for this practical, but the provided excerpts do not explicitly list them. However, based on the method:
Ensuring the infrared thermometer is held at a consistent distance and angle. Using a ruler or a fixed stand can help prevent this.
Heat loss from the Leslie cube by convection or conduction (other than through the surfaces being measured) could affect the surface temperatures. Using a lid and a heat-proof mat can minimise this.
The infrared thermometer itself might have a degree of uncertainty in its readings. Taking multiple readings and calculating an average can help.
For absorption, ensuring all objects are placed at the same distance from the infrared lamp and receive the same intensity of radiation is important.
Relevance to Assessment:
Source explicitly states that the "Leslie cube (Infrared radiation)" is one of the required practicals for an assessment on the 3rd April. Source advises reviewing the method, variables, sources of error, and how to write conclusions for each practical, including the Leslie cube. Source provides a template with key headings to consider for this practical in preparation for an exam on the 7th March, suggesting that understanding these aspects is crucial for assessment.
In conclusion, the Leslie cube practical is used to investigate the relationship between the type of surface and the emission and absorption of infrared radiation. Matte black surfaces are found to be the best emitters and absorbers, while shiny surfaces are the poorest. Understanding the method, variables, potential errors, and expected conclusions is important for assessment.