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Electromagnetism: Formula Cheat Sheet
Every key formula for Electromagnetism, with what it means and where it comes from.
| Formula | What it means | Topic |
|---|---|---|
| C = Q / V | Capacitance equals charge stored per volt across the plates. | Capacitors and Capacitance |
| C = epsilon0 A / d | Capacitance of a parallel-plate capacitor, with plate area A and separation d. | Capacitors and Capacitance |
| U = 1/2 C V^2 | Energy stored in a charged capacitor. | Capacitors and Capacitance |
| F = k q1 q2 / r^2 | Magnitude of the electric force between two point charges; k = 8.99 x 10^9 N m^2/C^2. | Electric Charge and Coulomb's Law |
| q = n e | Total charge equals the number of elementary charges n times e = 1.60 x 10^-19 C. | Electric Charge and Coulomb's Law |
| E = F / q | Electric field strength equals force per unit test charge, in newtons per coulomb. | Electric Fields and Electric Potential |
| E = k Q / r^2 | Field strength at distance r from a point charge Q. | Electric Fields and Electric Potential |
| V = k Q / r | Electric potential at distance r from a point charge Q, in volts. | Electric Fields and Electric Potential |
| V = W / q | Potential equals work done per unit charge to bring a charge to that point. | Electric Fields and Electric Potential |
| e.m.f. = -N (change in flux) / (change in time) | Faraday's law: induced e.m.f. equals turns times rate of change of magnetic flux; the minus sign is Lenz's law. | Electromagnetic Induction and Faraday's Law |
| Flux = B A cos(theta) | Magnetic flux through an area A in field B; theta is the angle between B and the normal to the area. | Electromagnetic Induction and Faraday's Law |
| F = q v B sin(theta) | Force on a charge q moving at speed v in field B; theta is the angle between v and B. | Magnetism: Fields, Forces and Compasses |
| F = B I L sin(theta) | Force on a straight wire of length L carrying current I in field B. | Magnetism: Fields, Forces and Compasses |
| V = I R | Voltage equals current times resistance. | Ohm's Law and Electric Circuits |
| P = V I | Electrical power equals voltage times current. | Ohm's Law and Electric Circuits |
| P = I^2 R | Power dissipated in a resistor, useful when the current is known. | Ohm's Law and Electric Circuits |
| P = V^2 / R | Power dissipated in a resistor, useful when the voltage is known. | Ohm's Law and Electric Circuits |
| c = lambda f | Speed of light equals wavelength times frequency; c = 3.0 x 10^8 m/s in a vacuum. | The Electromagnetic Spectrum |
| E = h f | Energy of one photon, with Planck constant h = 6.63 x 10^-34 J s. | The Electromagnetic Spectrum |
| E = h c / lambda | Photon energy from wavelength, combining the two equations above. | The Electromagnetic Spectrum |
| U = ½ × C × V² | The energy stored in a capacitor grows with its capacitance and with the square of the voltage across it. | Capacitor Energy Calculator |
| F = k × q₁ × q₂ ÷ r² | The force between two charges grows with the charges and falls with the square of the distance between them. | Coulomb's Law Calculator |
| I = Q ÷ t | Current is the rate at which charge flows past a point, so more charge per second means a larger current. | Electric Current Calculator |
| E = k × Q ÷ r² | The field around a point charge weakens with the square of distance, and a test charge feels a force equal to q times E. | Electric Field Calculator |
| P = V × I | Electrical power is voltage times current, and it can also be found from current squared times resistance. | Electrical Power Calculator |
| ε = N × ΔΦ ÷ Δt | The induced voltage equals the number of turns times how fast the magnetic flux through the coil is changing. | Faraday's Law Calculator |
| F = q × v × B × sin θ | A magnetic field pushes sideways on moving charge, strongest when motion is at right angles to the field and zero when parallel. | Magnetic Force Calculator |
| V = I × R | Voltage across a resistor equals the current through it multiplied by its resistance. | Ohm's Law Calculator |
| R_series = R₁ + R₂ | Resistors in series simply add up, while resistors in parallel give a total smaller than the smallest single resistor. | Resistor Combination Calculator |