Electric charges & fields
Arrange point charges, map the field and the potential, then let a charge go and watch it fly.
Fields — Action at a distance: gravitational, electric and magnetic fields and the paths in them.
Experiments already set up on it
Speed from the potential it fell through
Release a charge from rest and time it past a ring: v² against 1/r₀ is a straight line.
- Energy is conserved: qΔV becomes ½mv²
- v² = (2kQq/m)(1/r₀ − 1/r_g), so v² against 1/r₀ is straight
- The gradient gives kQq/m
Speed against the charge pushed
v² against the test charge is a straight line: the energy is proportional to q.
- The work done is qΔV, so the kinetic energy is proportional to q
Mapping a dipole
Walk the meter along the line between two opposite charges and record E and V.
- Field adds as a vector, potential as a number
- Halfway between two opposite charges the potential is zero but the field is not
What you can measure
Each of these can be recorded over time, aggregated over a run, or read at the moment something happens — through an instrument that carries its own uncertainty.
- Timet (s)
- Field at the meterE (N/C)
- Field, xE_x (N/C)
- Field, yE_y (N/C)
- Potential at the meterV (V)
- Meter’s distance from charge 1r (m)
- Test charge xx (m)
- Test charge yy (m)
- Test charge distanced (m)
- Test charge speedv (m/s)
- Speed at the ringv_g (m/s)
- Kinetic energyE_k (J)
- Potential energyE_p (J)
- Total energyE_t (J)
What you can change
Any of these can be the thing you vary, and every combination of the ones you choose is run as its own trial.
- How many chargesn (1)
- Charge 1Q_1 (uC)
- Charge 1: xx1 (m)
- Charge 1: yy1 (m)
- Charge 2Q_2 (uC)
- Charge 2: xx2 (m)
- Charge 2: yy2 (m)
- Charge 3Q_3 (uC)
- Charge 3: xx3 (m)
- Charge 3: yy3 (m)
- Meter position: xprobeX (m)
- Meter position: yprobeY (m)
- Test chargeq (uC)
- Mass of the test chargem (kg)
- Released fromr_0 (m)
- Released atθ_0 (°)
- Timing ring atr_g (m)
- Coulomb constantk (N*m^2/C^2)
The equations behind it
What electric charges & fields is built on. Every one of these is in the lab's formula reference too, where it copies straight into a write-up.
Electric fields
- Coulomb’s law
- Coulomb constant
- Electric field strength
- Electric potential
- Electric potential energy
- Work done moving a charge
- Field between parallel plates
Also in Fields
Orbits & gravitation
Put a satellite in orbit and measure its period against its radius, for Kepler’s third law.
Charged particle in a field
Fire a charge through electric and magnetic fields and watch the path bend, for E = V/d and r = mv/qB.
Electromagnetic induction
Slide a rod along rails through a magnetic field: ε = BLv, and Lenz’s law brings it to a stop.
AC generator
Spin a coil in a magnetic field and measure the alternating emf: ε = NBAω sin ωt.
Parallel-plate capacitor
Two plates whose size, spacing and filling you choose: C against 1/d gives ε₀, and pushing the voltage too far breaks the material down.