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.
Fields — Action at a distance: gravitational, electric and magnetic fields and the paths in them.
Experiments already set up on it
Deflection against plate voltage
With no magnetic field, the deflection at the screen is proportional to V.
- E = V/d
- A charge in a uniform field follows a parabola
- y ∝ V
Radius against speed
In a magnetic field alone, r against v is a straight line of gradient m/qB.
- r = mv/qB
- A magnetic force does no work, so the speed never changes
Velocity selector
Crossed fields let exactly one speed through: the deflection crosses zero.
- Crossed fields balance when v = E/B
- Why a selector is swept with B rather than with v
Mass spectrometer
At a fixed speed and field, the radius is proportional to the mass.
- r = mv/qB with v, q and B fixed
- How isotopes are told apart
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)
- Time for one circleT_c (s)
- xx (m)
- yy (m)
- Velocity alongv_x (m/s)
- Velocity acrossv_y (m/s)
- Speedv (m/s)
- Directionθ (°)
- Deflection at the screeny_s (m)
- Offset leaving the fieldy_e (m)
- Radius from the beam’s pathr (m)
- Electric fieldE (V/m)
- Magnetic flux densityB (T)
- Force on the particleF (N)
- Speed that passes undeflectedv_{sel} (m/s)
- Kinetic energyE_k (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.
- Chargeq (1)
- Massm (1)
- Speed on entryv_0 (m/s)
- Angle on entryθ (°)
- Voltage across the platesV (V)
- Electric field strengthE (V/m)
- Magnetic flux densityB (T)
- Plate separationd (m)
- Length of the field regionL (m)
- Distance to the screenD (m)
- Elementary chargee (C)
- Atomic mass unitu (kg)
The equations behind it
What charged particle in a field 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
Magnetic effects
- Force on a current-carrying wire
- Force on a moving charge
- Radius of a charged particle’s path
- Field around a straight wire
Also in Fields
Orbits & gravitation
Put a satellite in orbit and measure its period against its radius, for Kepler’s third law.
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.
Electric charges & fields
Arrange point charges, map the field and the potential, then let a charge go and watch it fly.
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.