Electric Fields

Coulomb's law, field strength, electric potential, and the comparison between gravitational and electric fields.

Spec Points Covered
  • I can define electric field strengthThe force per unit positive chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). at a point in an electric field. Measured in N C⁻¹ or V m⁻¹. and use $E = F/Q$.
  • I can calculate the uniform field between parallel plates using $E = V/d$.
  • I can state and apply CoulombThe SI unit of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. One coulomb is the chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). transferred by a current of 1 A in 1 second.'s law for point charges.
  • I can derive and use E = $\frac{Q}{4\pi\epsilon_0 r^2}$ for a radial field.
  • I can define electric potentialThe work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (charge times potential difference). per unit positive charge in bringing a small test charge from infinity to that point. and use V = $\frac{Q}{4\pi\epsilon_0 r}$.
  • I can sketch electric field lines and equipotentials for uniform and radial fields.
  • I can explain why no work is done moving a charge along an equipotential.
  • I can describe Millikan's oil drop experiment and how it determined the elementary chargeThe magnitude of charge carried by a single electron or proton. Equal to 1.6 × 10⁻¹⁹ C..
  • I can compare and contrast electric and gravitational fields quantitatively.
  • I can calculate the work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (charge times potential difference). moving a charge in an electric field using W = QV and W = $Q\Delta$V.
Notes
01 Electric field strength Electric field strength → 02 Between two large parallel plates with a potential difference V and separation d, the field is $E = \frac{V}{d}$ → 03 Coulomb's law Coulomb's law → 04 A point charge (or charged sphere) creates a radial field - field lines spread outward $E = \frac{Q}{4\pi\epsilon_0 r^2}$ → 05 Electric potential Electric potential → 06 Field strength is the negative potential gradient $E = -\frac{dV}{dr}$ → 07 Millikan measured the elementary charge (e = 1 $QE = mg \quad \Rightarrow \quad Q = \frac{mg}{E} = \frac{mgd}{V}$ → 08 Both obey inverse-square laws → 09 Coulomb's law → 10 Millikan's oil drop →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
$$E = \frac{F}{Q}$$
$$E = \frac{V}{d}$$
$$F = \frac{Q_1 Q_2}{4\pi\epsilon_0 r^2}$$
$$E = \frac{Q}{4\pi\epsilon_0 r^2}$$
$$V = \frac{Q}{4\pi\epsilon_0 r}$$
$$W = Q\Delta V$$
$$E = -\frac{dV}{dr}$$