Cambridge A Level Physics: Equations

An extended equation sheet for AS and A2 revision. Not all equations are provided in the booklet during exams.

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[AS Chapters][A2 Chapters]

AS Ch1 Physical Units and Quantities

All 2D vectors can be broken down into two components, right angles to each other.
Horizontal vector component, \(F_x = F \cos \theta \)
Vertical vector component, \(F_y = F \sin \theta \)
To recombine vector components, \(F = \sqrt{F_x^2 + F_y^2} \)
Fractional uncertainty = \( \frac{\textrm{absolute uncertainty}}{\textrm{measurement}} = \dfrac{\Delta x}{x} \)
Percentage uncertainty = \( \dfrac{\Delta x}{x} \times 100\) %
If \( p=\dfrac{x^{3} y^\frac{1}{2}}{7z^4} \), then percentage uncertainty p is \( \pm \; (3 \Delta x) + (\frac{1}{2} \Delta y) + (4 \Delta z) \)%
... and fractional uncertainty \(\frac{\Delta p}{p} = 3\frac{\Delta x}{x} + \frac{1}{2}\frac{\Delta y}{y} +4 \frac{\Delta z}{z} \)

AS Ch2 Kinematics

Displacement, s = area under v-t graph
Velocity, \(v= \dfrac{\Delta s}{\Delta t}=\) gradient of s-t graph
Acceleration, \(a=\dfrac{\Delta v}{\Delta t}=\) gradient of v-t graph
\(v=u+at\)
\(s=\frac{1}{2}(u+v)t\)
\(s=ut+\frac{1}{2} at^2\)
\(v^2 = u^2 + 2as\)
Projectile components if \(\theta\) is elevation: \(V_x = v \cos\theta\) and \(V_y = v \sin\theta\)
Projectile max vertical height, \(H=\frac{u^2 \sin^2 \theta}{2g}\)
Projectile max horizontal range, \(R=\frac{u^2 \sin 2\theta}{g}\)

AS Ch3 Dynamics

Momentum, \(p=mv\)
Force causes acceleration, \(\Sigma F= \dfrac{\Delta p}{\Delta t}=ma\) (2nd law)
Impulse, \(F \Delta t = \Delta(mv)\)
Total momentum, \(m_1 u_1 + m_2 u_2 = m_1 v_1 + m_2 v_2\)
Elastic collisions \(u_1 - u_2 = v_2 - v_1 \) (leftward is negative, rightward is positive)
Weight, \(W=mg\)

AS Ch4 Forces, Density, Pressure

Gravitational force, \(F_{g}=mg\)
Electrostatic force, \(F_{e}=qE\)
Friction, \(F_{f}= \mu R\)
Drag or viscous force, \(F_D \propto v^2\)
Moment of force/torque \(= F \, \times d_{\perp} = F_{\perp} \times d\)
Density, \( \rho = \frac{\textrm{mass}}{\textrm{volume}} = \dfrac{m}{V} \)
Pressure (Pa), \( P=\frac{\textrm{Force}}{\textrm{Area}}=\dfrac{F}{A}\)
Pressure in fluids, \(\Delta P = \rho g \Delta h \)
Upthrust force, \(F_{up}= \rho V g\)
Equilibrium in fluid \( \rho V g = mg\)

AS Ch5 Work, Energy, Power

Work Done (J), \(W = force \times distance = Fs \cos \theta\)
Gravitational Potential Energy \(E_p = mgh\)
Kinetic Energy \(E_k = \frac{1}{2} mv^2\)
Elastic potential energy \(E_p = \frac{1}{2} Fx = \frac{1}{2} kx^2\)
Electric potential energy in uniform electric field, \(E_p= qEd\)
Work done expanding gas, \( W = P \Delta V \)
Efficiency (%), \( \eta = \frac{\textrm{power or energy output}}{\textrm{power or energy input}} \times 100 \)
Power (W), \(P =\frac{\textrm{work done or energy change}}{\textrm{time taken}}= \dfrac{W}{t} = Fv \)

AS Ch6 Deformation of Solids

Restoring force (Hooke's Law), \(F=k \; \Delta x\)
Parallel springs effective constant \(k = k_1 + k_2 + ... \)
Series springs effective constant \(\frac{1}{k} = \frac{1}{k_1} + \frac{1}{k_2} + ...\)
Elastic potential or strain energy \(E_s = \frac{1}{2} Fx = \frac{1}{2} kx^2\)
Work done on spring \(W = \Delta E_s =\) area under F-x graph
Stress (Pa), \(\sigma = \frac{\textrm{force applied}}{\textrm{cross-section area}} = \dfrac{F}{A}\)
Strain, \(\epsilon = \frac{\textrm{extension}}{\textrm{original length}} = \dfrac{x}{l_0}\)
Young's modulus (Pa), \(E = \frac{\textrm{stress}}{\textrm{strain}} = \dfrac{\sigma}{\epsilon} = \dfrac{Fl_0}{Ax}\) = Gradient of linear stress-strain graph
Strain energy per unit volume = Area under stress-strain graph

AS Ch7 Waves

Frequency (Hz), \(f=\frac{\textrm{one cycle}}{\textrm{period}} = \dfrac{1}{T}\)
Wave speed, \( v=f\lambda \)
Phase difference ratios \(\dfrac{\Delta t}{T} = \dfrac{\Delta x}{\lambda} = \dfrac{\Delta \theta}{2\pi} \)
Wave intensity, \( I \propto A^2 \) and \( I \propto f^2 \) and \( I \propto \frac{1}{r^2} \)
Doppler effect ratios, \( \dfrac{f_0}{f_s} = \dfrac{v}{v \pm v_s} \) (positive away, negative towards)
Malus' Law of Polarized intensity, \(I = I_o \cos^2 \theta\) [2022 syllabus]

AS Ch8 Superposition of Waves

Path difference, \(\Delta L = L_2 - L_1 = a \sin \theta \)
For constructive interference \(\Delta L = 0,\; 1\lambda, \; 2\lambda,\; 3\lambda,... n\lambda \)
For destructive interference \(\Delta L = 0.5\lambda,\; 1.5\lambda,\; 2.5\lambda,... (n+\frac{1}{2})\lambda \)
Young's double-slit fringe separation, \( x=\dfrac{\lambda D}{a} \)
Multi-slit diffraction grating, \( n \lambda = d \sin \theta \) (observing maxima)
Grating slit separation, \( d = \dfrac{1}{N} \) (e.g. N = 30000 lines/metre)
To find greatest number of maxima n, use \( n \lambda < d \sin 90 \)

AS Ch9 Current of Electricity

Total charge (C) of free electrons = \(Q = nALe \)
Current (A), \(I = \frac{total \; charge \;}{time \; taken}\ = \dfrac{Q}{t} = \dfrac{naLq}{t} = nAvq \)
Potential difference aka voltage (V), \( V = \frac{\textrm{energy}}{\textrm{charge}} = \dfrac{W}{Q} \)
Resistance (\( \Omega \)), \( R=\dfrac{V}{I}=\) constant (if obeys Ohms's Law)
Resistance in wire (\(\Omega\) m), \( R=\dfrac{\rho L}{A} \)
Power (W), \( P=IV = I^2R = \dfrac{V^2}{R} \)
Max Power Dissipated, \(P_{max} = \dfrac{E^2 R}{(R+r)^2}\)

AS Ch10 DC Circuits

e.m.f (V) from battery, \(E = IR + Ir =\) terminal p.d. + lost volts
Combined Series Resistance, \( R = R_1 + R_2 + R_3 + ... \)
Combined Parallel Resistance, \( R = \left( \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dfrac{1}{R_3} + ...\right)^{-1} \)
Kirchoff I: At a junction, total current in = total current out
Kirchoff II: For loop in circuit, \( \sum V = 0 \) or sum of e.m.f = sum of potential drops
Potential divider (ratio), \( \dfrac{V_1}{V_2} = \dfrac{R_1}{R_2} \) or \( \dfrac{V_1}{V_{cell}} = \dfrac{R_1}{R_{total}} \)
Potentiometre output p.d. (ratio), \( \frac{V_o}{V_s} = \left(\dfrac{x}{l} \right) \)
Potentiometre to find emf (ratio), \( \frac{E}{V_0}= \left(\dfrac{x}{l} \right) \)
Strain gauge ratio if no change in area \(\dfrac{\Delta R}{R} = \dfrac{\Delta L}{L} \), but if changing area but constant volume then \(\dfrac{\Delta R}{R} = \dfrac{2 \Delta L}{L} \)

AS Ch11 Particle and Nuclear Physics

Alpha decay \( {}_{10}^{100}\text{X} \longrightarrow {}_{8}^{96}\text{Y} + {}_{2}^{4} \alpha \)
Beta-minus decay \( {}_{10}^{100}\text{X} \longrightarrow {}_{11}^{100}\text{Y} + {}_{-1}^{0} \beta + \overline{\nu}_e \)
Beta-plus decay \( {}_{10}^{100}\text{X} \longrightarrow {}_{9}^{100}\text{Y} + {}_{1}^{0} \beta + \nu_e \)
Gamma decay \( {}_{10}^{100}\text{X} \longrightarrow {}_{10}^{100}\text{Y} + \gamma \)

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[AS Chapters][A2 Chapters]

= = = = = = = = = = = = = = = = = = =

A2 Practical Paper 5

General uncertainty, \(\Delta x = \dfrac{1}{2} (x_{max} - x_{min}) \)
Absolute unc. gradient, \(\Delta m = | m_{best} - m_{worst} | \)
Absolute unc. y-intercept \(\Delta C = | C_{best} - C_{worst} | \)
General log uncertainty, \( \Delta log_a (x) = \dfrac{1}{ln(a)} \left(\dfrac{\Delta x}{x} \right) \)
Absolute uncertainty of lg, \( \Delta lg (x) = \dfrac{1}{ln(10)} \left(\dfrac{\Delta x}{x} \right) \)
Absolute uncertainty of ln, \( \Delta ln (x) = \dfrac{\Delta x}{x} \)

A2 Ch12 Circular Motion

Arc length, \(s = r \theta \) (definition of radian)
Angular velocity, \( \omega = \dfrac{\Delta \theta}{\Delta t} = \dfrac{2 \pi}{t} =2 \pi f \)
Average speed or linear/tangential velocity, \(v = r \omega \)
Period of revolution, \(T = \dfrac{2 \pi}{\omega} = \dfrac{2 \pi r}{v} \)
Centripetal acceleration, \(a = vw = r \omega^2 = \dfrac{v^2}{r} \)
Centripetal force, \(F_c = ma = mr \omega^2 = \dfrac{mv^2}{r} \)

A2 Ch13 Gravitation / Gravitational Fields (G-field)

Gravitational force of attraction, \( F = \dfrac{GMm}{r^2} \)
G-field strength, \(g = \dfrac{GM}{r^2} \)
Gravitational potential, \( \phi = - \dfrac{GM}{r} \)
G-potential energy, \(E_g = m \phi = -\dfrac{G Mm}{r} \)
Orbital velocity, \( v = \sqrt{ \frac{GM}{r} } \)
Orbital period, \(T = \dfrac{2 \pi r}{v} = 2 \pi \sqrt{\frac{r^3}{GM}} \)
Escape velocity, \(v_{escape} = \sqrt{\frac{2GM}{R}} \)

A2 Ch14 Temperature & Thermal Properties

Conversion: T/K = T/°C + 273.15
Heat energy to raise liquid temperature, \(Q = mc \Delta \theta + Q_{lost}\)
Energy supplied by heater, \(Q = VIt \)
Internal energy, \(U = \text{sum of random distribution of K.E. and P.E of molecules}\)
Change in internal energy, \(\Delta U = q + W \)
where +q is heat supplied to system, +W is work done on system (V decrease).
Internal energy of ideal gas, \(U = E_k = \frac{3}{2} kT \) (no potential energy)

A2 Ch15 Ideal Gases

State for ideal gas \(PV = nRT = NkT\)
Amount of gas (mol), \(n = \frac{\text{Number of Particles}, N}{\text{Avogadro Constant}, N_{A}} \)
Boltzmann constant, \(k = \frac{\text{Molar gas constant}, R}{\text{Avogadro constant}, N_A}\)
Boyle's Law \( P \propto \dfrac{1}{V} \) (constant T)
Charles' Law \(V \propto T\) (constant P)
Pressure Law \(P \propto T\) (constant V)
Combined laws \(\frac{PV}{T} = constant \)

A2 Ch16 Thermodynamics

Pressure of gas \( P = \dfrac{1}{3} \left(\dfrac{Nm}{V}\right) \langle c^2 \rangle = \dfrac{1}{3} \rho \langle c^2 \rangle \)
NOTE: m is mass of a single particle!
Kinetic energy of gas molecule \(\langle E_k \rangle = \frac{1}{2} m \langle c^2 \rangle = \frac{3}{2} kT \)
Internal energy of ideal gas, \(U = E_k = \frac{3}{2} kT \) (no potential energy)

A2 Ch17 Oscillations / Simple Harmonic Motion (SHM)

Angular frequency, \( \omega = \dfrac{2 \pi}{T} = 2 \pi f \)
Period of a spring, \(T = 2\pi \sqrt{\frac{m}{k} } \)
Period of a pendulum, \(T = 2 \pi \sqrt{\frac{l}{g} } \)
SHM acceleration, \(a = -\omega^2 x \)
Max acc. at max displacement (x=A), \(a = -\omega^2 A \)
SHM velocity, \(v = \pm \omega \sqrt{A^2 - x^2} \)
Max speed at equilibrium (x=0), \(v_{max} = \omega A\)
E.g. if displacement is \(x = A \sin \omega t \) then velocity is \(v = \omega A \cos \omega t \)
Total energy \(E_T = \frac{1}{2} mv^2 + \frac{1}{2} kx^2 \)
Trigonometric identity \( \sin^2 \omega t + \cos^2 \omega t = 1\)

A2 Ch18 Electric Fields (E-field)

Constant \(k = \dfrac{1}{4 \pi \epsilon_o} \)
Electric potential (J/C), \(V = \dfrac{kQ}{r} \)
E-field strength, \(E = - \dfrac{dV}{dr} = \dfrac{kQ}{r^2} \)
E-field parallel plates, \( E=\dfrac{F}{Q} = \dfrac{V}{d} \)
Electric force \(F_e = qE = \dfrac{kQq}{r^2} \)
E-potential energy, \( U = -\int{F_{e} \; dr} = \dfrac{kQq}{r} = qV \)
Work done (J) on charge, \(W = Fd = \Delta U = q \Delta V\)
Uniform E-field strength, \(E = - \text{potential gradient} = - \dfrac{dV}{dr} = - \dfrac{\Delta V}{\Delta x} \)

A2 Ch19 Capacitance

Capacitance (F), \(C = \dfrac{Q}{V} \)
Combined Parallel Capacitance, \( C = C_1 + C_2 + C_3 + ... \)
Combined Series Capacitance, \( C = \left( \dfrac{1}{C_1} + \dfrac{1}{C_2} + \dfrac{1}{C_3} + ...\right)^{-1} \)
Energy stored in capacitors, \( E = \dfrac{1}{2} QV = \dfrac{1}{2} CV^2 \)
Current (A), \(I = \frac{total \; charge \;}{time \; taken}\ = \dfrac{Q}{t} \)
Discharging capacitor charge, \(Q = Q_o e^{-\frac{t}{RC}} \)
Charging capacitor charge, \(Q = Q_o (1 - e^{-\frac{t}{RC}} ) \)

A2 Ch20 Magnetic Fields & Induction

Field from a straight wire, \(B = \dfrac{\mu_o I}{2 \pi r} \)
Field inside solenoid, \(B = \dfrac{\mu_o N I}{L} \)
Force on wire in field, \(F = BIL \sin \theta\)
Force per unit length between two wires, \(\dfrac{F}{L} = \dfrac{\mu_o I_1 I_2}{2 \pi x} \)
Force on charged particle, \(F = Bqv \sin \theta \)
Hall voltage, \(V_H = \dfrac{BId}{nAq} = \dfrac{BI}{ntq} \)
Current (A), \(I = \frac{total \; charge \;}{time \; taken}\ = \dfrac{Q}{t} = nAvq \)
Magnetic flux (Wb), \( \Phi = BA\)
Induced e.m.f, \(E = Blv = B\dfrac{dA}{dt}\)
Faraday's Law \(E = - \dfrac{d \Phi}{dt} = -\dfrac{\Delta N \Phi}{\Delta t} \)

A2 Ch21 Alternating Currents (AC)

Root-mean-square current, \(I_{rms} = \frac{I_o}{\sqrt{2}} \)
Root-mean-square voltage , \(V_{rms} = \frac{V_o}{\sqrt{2}} \)
Max power, \(P_{max} = I_o V_o \)
Average power, \(P_{avg} = I_{rms} V_{rms} = \frac{1}{2} I_o V_o \)
Oscillating current. \(I = I_o \sin (2 \pi f t) = I_o \sin \omega t \)
Oscillating voltage, \(V = V_o \sin (2 \pi f t) = V_o \sin \omega t \)
Oscillating Power, \(P = IV = \dfrac{V_o I_o}{2} [1 - \cos(4 \pi f t)] \)
Ideal transformer \( \dfrac{V_s}{V_p} = \dfrac{N_s}{N_p} = \dfrac{I_p}{I_s} \)

A2 Ch22 Quantum Physics

Energy of a photon, \(E = hf = \frac{hc}{\lambda} \)
Max k.e. of photoelectrons \(E_{K(max)} = hf - \phi \)
Radiation intensity \( = \dfrac{hfN}{At}\) so \(I \propto f_{photons} N_{photons}\)
Work function energy, \(\phi = hf_o = \frac{hc}{\lambda_o} \)
Threshold frequency, \(f_o = \frac{\phi}{h} \)
Threshold wavelength, \(\lambda_o = \frac{c}{f_o} \)
De Broglie wavelength, \(\lambda = \frac{h}{p} = \frac{h}{mv} \)
Emitted photon energy, \(\Delta E = E_1 - E_2 = hf \)

A2 Ch23 Nuclear Physics

Energy released. \( \Delta E = (\Delta m) c^2 \)
Mass excess = mass (in u) - nucleon number
Number of nuclei, \(N = N_o e^{\lambda t} \)
Radioactivity, \(A = A_o e^{-\lambda t} = \frac{dN}{dt} = - \lambda N \)
Half-life duration, \(t_{1/2} = \dfrac{\ln 2}{\lambda} = \dfrac{0.693}{\lambda} \)
Mass, \(m = \text{moles} \times \text{molecular weight} \)
Number of particles, \(N = \text{moles} \times N_A\)

A2 Ch24 Medical: Ultrasound

Acoustic impedance, \(Z = \rho c\)
Intensity reflection coefficient \( \dfrac{I_R}{I_O} = \dfrac{(Z_1 - Z_2)^2}{(Z_1+Z_2)^2} \)
Wave intensity in material, \(I = I_0 e^{-\mu x} \)
Decibel (dB) = \(10 \log_{10} \left( \dfrac{I_2}{I_1 } \right) \)

A2 Ch24 Medical: X-Ray

X-ray photon max frequency \(hf_{max} = eV \)
X-ray attenuation \(I = I_o e^{-\mu x} \)
Decibel (dB) = \(10 \log_{10} \left( \dfrac{I_2}{I_1 } \right) \)

A2 Ch25 Astronomy & Cosmology

Radiant flux intensity, \(F = \dfrac{L}{4 \pi d^2} \)
Wien's blackbody radiation, \(\lambda_{peak} \propto \dfrac{1}{T} \)
Stefan-Boltzmann's stellar radii, \(L = 4 \pi \sigma r^2 T^4\)
Doppler Shift \( \dfrac{\lambda_{star}}{\lambda_{lab}} = \dfrac{v}{c} \)
Hubble's Law \( v = H_0 d\)

Jump to sections:
[AS Chapters][A2 Chapters]

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Discontinued Chapters:

A2 Communications

Bandwidth \(= (f_{c} + f_{s}) - (f_{c} - f_{s}) = 2 f_s \)
NOTE: Maximum frequency of signal = \(f_s\)
Bitrate = sample rate × bit depth
Attenuation (dB) = \(10 \log_{10} \left( \dfrac{P_1}{P_2} \right) \)
Gain (dB) = \(10 \log_{10} \left( \dfrac{P_{out}}{P_{in} } \right) \)

A2 Electronics (Op-Amp)

Gain, \(G = \dfrac{V_{out}}{V_{in}} \)
Output voltage, \(V_{out} = G (V_{+} - V_{-}) \)
Voltage gain (inverting), \( \dfrac{V_{out}}{V_{in}} = -\dfrac{R_F}{R_{in} }\)
Voltage gain (non-inverting), \( \dfrac{V_{out}}{V_{in}} = 1 + \dfrac{R_f}{R_{in} }\)