You are viewing a free preview of this lesson.
Subscribe to unlock all 4 lessons in this course and every other course on LearningBro.
Success in A-Level Physics exams comes not only from understanding the physics but from knowing how marks are awarded. This lesson covers the structured approach to calculations, the equations you must know versus those provided, unit analysis, uncertainty and error propagation, graph skills, common mistakes, and the patterns examiners use in mark schemes.
Every calculation in A-Level Physics should follow this disciplined seven-step process:
flowchart TD
A["1. IDENTIFY the physics<br/>What principle or law applies?"] --> B["2. SELECT the equation<br/>From memory or data sheet"]
B --> C["3. LIST known values<br/>with correct units"]
C --> D["4. CONVERT units if needed<br/>(mm to m, mA to A, etc.)"]
D --> E["5. SUBSTITUTE and REARRANGE<br/>Show all working"]
E --> F["6. SOLVE and state the answer<br/>with correct units"]
F --> G["7. CHECK sig figs and<br/>reasonableness of answer"]
Question: A wire of length 1.80 m and diameter 0.56 mm has a resistance of 3.2 ohm. Calculate the resistivity of the wire.
Exam Tip: Examiners award marks at each stage. Even if your final answer is wrong, you can still earn marks for selecting the correct equation, converting units, and substituting correctly. ALWAYS show your working.
One of the most important revision tasks is knowing which equations are provided and which must be memorised. Here is the comprehensive breakdown:
Mechanics:
| Equation | Description |
|---|---|
| v = u + at | First SUVAT equation |
| s = ½(u + v)t | Second SUVAT equation |
| W = Fs cos theta | Work done by a force at angle theta |
| Ek = ½mv² | Kinetic energy |
| Ep = mgh | Gravitational potential energy (near surface) |
| P = W/t | Power |
| P = Fv | Power (force x velocity) |
| p = mv | Momentum |
| F = delta p / delta t | Force as rate of change of momentum |
| F = kx | Hooke's law |
| E_elastic = ½Fx = ½kx² | Elastic strain energy |
Electricity:
| Equation | Description |
|---|---|
| Q = It | Charge = current x time |
| V = IR | Ohm's law |
| P = IV = I²R = V²/R | Electrical power |
| V = W/Q | Potential difference |
| epsilon = I(R + r) | EMF equation |
| V_out = R_2/(R_1 + R_2) x V_in | Potential divider |
| R_series = R_1 + R_2 + ... | Resistors in series |
| 1/R_parallel = 1/R_1 + 1/R_2 + ... | Resistors in parallel |
Waves:
| Equation | Description |
|---|---|
| v = f lambda | Wave speed equation |
| n = c/v | Refractive index |
| n_1 sin theta_1 = n_2 sin theta_2 | Snell's law |
| lambda = ws/D | Double slit equation |
Particles and Radiation:
| Equation | Description |
|---|---|
| E = hf | Photon energy |
| hf = phi + Ek_max | Photoelectric equation |
| lambda = h/p = h/(mv) | de Broglie wavelength |
| E = mc² | Mass-energy equivalence |
Further Physics:
| Equation | Description |
|---|---|
| v = omega r | Linear speed from angular velocity |
| omega = 2pi/T = 2pi f | Angular frequency |
| a = -omega² x | SHM defining equation |
| E_total = ½m omega² A² | Total energy in SHM |
| C = Q/V | Capacitance |
| E = ½QV = ½CV² = ½Q²/C | Energy stored in a capacitor |
| pV = nRT | Ideal gas equation (amount in moles) |
These are given — but you must still know when and how to use them:
Exam Tip: Even though equations are given on the data sheet, you must practise using them. In the exam, you do not have time to figure out how to apply an unfamiliar equation. Know what every symbol means and how to rearrange.
Unit analysis is a powerful tool for checking answers and is explicitly tested in the AQA specification.
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
To find the units of a quantity, substitute the units of the quantities on the right-hand side of its defining equation.
Example 1: Units of resistance
R = V/I. The volt is defined as J/C = kg m² s⁻³ A⁻¹. The ampere is A.
So R has units: (kg m² s⁻³ A⁻¹)/A = kg m² s⁻³ A⁻² = ohm
Example 2: Units of the spring constant k
F = kx, so k = F/x. Units: N/m = kg m s⁻² / m = kg s⁻²
Example 3: Show that the units of epsilon_0 are C² N⁻¹ m⁻² (or equivalently F m⁻¹)
From Coulomb's law: F = Q_1 Q_2 / (4pi epsilon_0 r²)
Rearranging: epsilon_0 = Q_1 Q_2 / (4pi F r²)
Units: C² / (N m²) = C² N⁻¹ m⁻²
Exam Tip: In unit analysis questions, always start from a defining equation and substitute units systematically. Show every step clearly.
Percentage uncertainty = (absolute uncertainty / measured value) x 100%
| Operation | Rule | Example |
|---|---|---|
| Addition or subtraction (a + b or a - b) | Add absolute uncertainties | If delta_a = 0.1 and delta_b = 0.2, then delta_(a+b) = 0.3 |
| Multiplication or division (a x b or a/b) | Add percentage uncertainties | If %a = 2% and %b = 3%, then %(a x b) = 5% |
| Power (a^n) | Multiply percentage uncertainty by n | If %a = 2% and you calculate a², then %uncertainty = 2 x 2% = 4% |
Given: R = 3.2 ± 0.1 ohm, L = 1.80 ± 0.01 m, d = 0.56 ± 0.02 mm
% uncertainty in R = (0.1/3.2) x 100 = 3.1% % uncertainty in L = (0.01/1.80) x 100 = 0.56% % uncertainty in d = (0.02/0.56) x 100 = 3.6%
Since A = pi(d/2)², and we use d²: % uncertainty in A = 2 x 3.6% = 7.1%
rho = RA/L, so: % uncertainty in rho = 3.1% + 7.1% + 0.56% = 10.8%
The absolute uncertainty in rho = 10.8% x 4.4 x 10⁻⁷ = 4.8 x 10⁻⁸ ohm m
So rho = (4.4 ± 0.5) x 10⁻⁷ ohm m
Subscribe to continue reading
Get full access to this lesson and all 4 lessons in this course.