Required Practicals

AQA A-Level Physics has 12 required practicals that you must have completed during the course. These practicals form the basis of questions on Paper 3 Section A, and practical techniques are also tested throughout Papers 1 and 2. You need to know the method, variables, safety precautions, expected results, data analysis, and common exam questions for each one.

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Required Practical 1: Stationary Waves on a String

Investigation into the variation of the frequency of stationary waves on a string with length, tension, and mass per unit length.

Method

1. Set up a vibration generator connected to a signal generator at one end of a string
2. Pass the string over a pulley and attach hanging masses to provide tension (T)
3. Adjust the frequency on the signal generator until a clear stationary wave pattern is observed (fundamental mode: one antinode)
4. Measure the length (L) of the vibrating string between the vibration generator and the pulley
5. Record the frequency (f) at which the fundamental mode occurs
6. Vary one variable at a time (length, tension, or mass per unit length) and repeat

Variables

Variable	Role	How to change it
Frequency (f)	Dependent	Read from signal generator
Length (L)	Independent	Move the pulley or vibration generator
Tension (T)	Independent	Change the hanging mass (T = mg)
Mass per unit length (mu)	Independent	Use different strings

Key Equation

f = (1/2L) sqrt(T/mu)

Expected Results

• f is inversely proportional to L (plot f vs 1/L: straight line through origin)
• f is proportional to sqrt(T) (plot f vs sqrt(T): straight line through origin)
• f is inversely proportional to sqrt(mu)

Data Analysis

• Plot f against 1/L (keeping T and mu constant) — gradient = (1/2) sqrt(T/mu)
• Or plot f² against T (keeping L and mu constant) — gradient = 1/(4L²mu)

Safety

• Wear safety goggles in case the string snaps
• Do not use excessive masses that could break the string or topple the stand
• Keep the area clear below the hanging masses

Common Exam Questions

• Explain how to identify the fundamental mode (one antinode, two nodes at fixed ends)
• Calculate the speed of waves on the string using v = f lambda (lambda = 2L for fundamental)
• Discuss sources of uncertainty (difficulty identifying the exact resonant frequency, parallax in length measurement)

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Required Practical 2: Interference and Diffraction

Investigation of interference effects to include two-slit interference and diffraction grating.

Method — Young's Double Slit

1. Shine a monochromatic light source (e.g. laser) through a double-slit arrangement
2. Observe the interference pattern on a screen placed several metres away
3. Measure the fringe spacing (w) using a ruler or travelling microscope
4. Measure the slit separation (s) and the distance from slits to screen (D)
5. Use the equation: lambda = ws/D

Method — Diffraction Grating

1. Direct a monochromatic light source (e.g. laser) at a diffraction grating
2. Observe the diffraction pattern — bright maxima at specific angles
3. Measure the angle (theta) to each order maximum using a protractor or by measuring the distance on a screen
4. Use the equation: d sin theta = n lambda, where d = 1/(number of slits per metre) and n is the order number

Variables

Variable	Role
Fringe spacing (w) or angle (theta)	Dependent
Slit separation (s) or grating spacing (d)	Independent
Wavelength (lambda)	What you calculate
Distance to screen (D)	Controlled (or measured)

Expected Results

• Double slit: evenly spaced bright and dark fringes (equal spacing confirms coherent interference)
• Diffraction grating: sharp, bright maxima at specific angles; higher orders at larger angles
• Calculated wavelength should match the known wavelength of the light source

Data Analysis

• Double slit: lambda = ws/D — calculate lambda and compare with the accepted value
• Diffraction grating: plot sin theta against n — gradient = lambda/d

Safety

• Never look directly into a laser beam — Class 2 or higher lasers can cause eye damage
• Display a warning sign when the laser is in use
• Avoid specular reflections from shiny surfaces

Common Exam Questions

• Explain why the fringes in a double-slit experiment are equally spaced
• Explain why a diffraction grating gives sharper maxima than a double slit
• Calculate the maximum number of orders visible: n_max = d/lambda (rounded down)
• Describe how to improve the accuracy of the wavelength measurement

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Required Practical 3: Determination of g by Free-Fall

Determination of g by a free-fall method.

Method

1. Set up an electromagnet to hold a steel ball bearing at a measured height (h) above a trapdoor switch
2. When the electromagnet is switched off, a timer starts automatically
3. The ball falls and hits the trapdoor, which stops the timer
4. Record the time (t) for the fall
5. Repeat for different heights (h) and take repeat readings at each height

Variables

Variable	Role
Time (t)	Dependent
Height (h)	Independent
Mass of ball, air resistance	Controlled

Key Equation

h = ½gt² (since u = 0 for free fall from rest)

Data Analysis

• Plot h against t² — gradient = ½g, so g = 2 x gradient
• Alternatively, plot 2h/t² for each data point and take the average
• Compare the experimental value of g with the accepted value (9.81 m s⁻²)

Sources of Error

Error	Type	Effect
Reaction time of electromagnet	Systematic	t is slightly too large (g appears smaller)
Air resistance	Systematic	t is slightly too large
Measurement of height	Random	Uncertainty in h
Ball not releasing cleanly	Random	Variation in t

Safety

• Place a cushion or sand tray below the drop zone to catch the ball
• Keep feet clear of the drop area

Common Exam Questions

• Explain why the graph of h against t² should pass through the origin
• Calculate percentage uncertainty in g
• Suggest improvements (use a longer drop distance to reduce the percentage uncertainty in t)

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Required Practical 4: Young Modulus

Determination of the Young modulus of a material.

Method

1. Clamp a long, thin wire (e.g. copper) horizontally between two fixed supports, or vertically from a clamp
2. Measure the original length (L) using a metre rule and the diameter (d) using a micrometer screw gauge (take readings at several points and average)
3. Apply increasing loads (masses) and measure the extension (delta L) using a marker and ruler, or a travelling microscope
4. Record load (F = mg) and extension for each mass added
5. Continue loading until the elastic limit is approached (do not exceed it if you want elastic behaviour)

Key Equations

Stress = F/A, where A = pi(d/2)² Strain = delta L / L Young modulus E = stress/strain

Data Analysis

• Plot stress against strain — gradient = Young modulus (E)
• Or plot force against extension — gradient = EA/L, so E = (gradient x L)/A
• A straight line through the origin confirms Hooke's law is obeyed (elastic region)

Sources of Error

Error	How to reduce it
Uncertainty in diameter (small value)	Use a micrometer; take multiple readings at different positions; calculate the mean
Difficulty measuring small extensions	Use a long wire (2-3 m) to produce a larger extension; use a travelling microscope or vernier scale
Wire may not be perfectly straight	Apply a small initial load to remove kinks before starting measurements
Thermal expansion	Work in a temperature-controlled room

Safety

• Wear safety goggles (wire may snap under tension)
• Do not stand under the loading masses
• Use a cushion or sand tray below the masses

Common Exam Questions

• Explain why a long, thin wire is used (larger extension for a given stress, reducing percentage uncertainty)
• Calculate the Young modulus from given data
• Explain the significance of the elastic limit on a stress-strain graph

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Required Practical 5: Resistivity of a Wire

Determination of resistivity of a wire using a micrometer, ammeter, voltmeter, and metre rule.

Method

1. Measure the diameter of the wire using a micrometer screw gauge at several points; calculate the mean and then the cross-sectional area A = pi(d/2)²
2. Set up a circuit with the wire connected to a power supply, ammeter (in series), and voltmeter (in parallel across the wire)
3. Measure the length (L) of wire in the circuit using a metre rule
4. Record the current (I) and voltage (V); calculate resistance R = V/I
5. Change the length of wire in the circuit (by moving the crocodile clip) and repeat
6. Plot R against L

Key Equation

R = rho L / A, so rho = RA/L

Data Analysis

• Plot R against L — gradient = rho/A, so rho = gradient x A
• The graph should be a straight line through the origin (if the temperature remains constant)

Sources of Error

• Wire heating up (changes resistivity) — use low currents and switch off between readings
• Uncertainty in diameter measurement — take multiple readings
• Contact resistance at crocodile clips — ensure firm connections

Safety

• Do not allow the wire to overheat (risk of burns)
• Switch off the power supply between readings

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Required Practical 6: EMF and Internal Resistance

Investigation of the EMF and internal resistance of electric cells.

Method

1. Connect a cell to a variable resistor, ammeter (series), and voltmeter (across the cell terminals)
2. Vary the resistance using the variable resistor
3. For each resistance value, record the terminal pd (V) and the current (I)
4. Plot V against I

Key Equation

V = epsilon - Ir (epsilon = EMF, r = internal resistance)

Data Analysis