Electric fields

• What does Coulomb's law state about the force between two point charges?

  It is proportional to their charges and inversely proportional to distance squared.

  
• What is the formula for the force between two point charges according to Coulomb's law?

  $F =$$\frac{1}{4 \pi \epsilon_0} \frac{Q_1 Q_2}{r^2}$
• What does $\epsilon_0$ represent in Coulomb's law?


  Permittivity of free space
• How can air be treated in the context of Coulomb's law?

  As a vacuum
• Where can charge be assumed to act in a charged sphere?

  At the centre of the sphere
• What happens to the force between charges with the same sign?

  The force will be repulsive
• What happens to the force between charges with different signs?

  The force will be attractive
• Why are electrostatic forces between subatomic particles greater than gravitational forces?

  Because subatomic particle masses are very small
• What is the gravitational force formula between two protons 2 pm apart?

  $F =$$\frac{GMm}{r^2}$
• What is the electrostatic force formula between two protons?

  $F =$$\frac{1}{4 \pi \epsilon_0} \frac{Q_1 Q_2}{r^2}$
• What is the ratio of electric force to gravitational force between two protons?

  Approximately $1.36 \times 10^{36}$
• What is electric field strength (E)?

  Force per unit charge in an electric field
• How does electric field strength vary in different fields?

  Constant in uniform fields, varies in radial fields
• What are the formulas for calculating electric field strength?

  1. $E =$$\frac{F}{Q}$ (for uniform fields)
  2. $E =$$\frac{V}{d}$ (for radial fields)
  3. $E =$$\frac{1}{4 \pi \epsilon_0} \frac{Q}{r^2}$ (for radial fields)
• What do electric field lines represent?

  The direction of force on a positive charge

  
• How does a uniform electric field differ from a radial electric field?

  Uniform fields have parallel, equally spaced lines

  
• How can you calculate work done moving a charged particle in a uniform field?

  Using $Work =$$F \times d$
• How can you rearrange the formula for electric field strength to find force?

  $F =$$E \times Q$
• How can you rearrange the formula for electric field strength to find distance?

  $d =$$\frac{E}{\Delta V}$
• What is the absolute electric potential (V) at a point?

  Potential energy per unit charge of a positive charge
• How does electric potential change with distance from a charge?

  It decreases as distance increases
• What is the formula for electric potential in a radial field?

  $V =$$\frac{1}{4 \pi \epsilon_0} \frac{Q}{r}$
• How does the sign of charge affect electric potential?

  Positive charge gives positive potential, negative charge gives negative potential
• What does the gradient of a potential (V) against distance (r) graph represent?

  The value of electric field strength (E)

  
• What is electric potential difference ($\Delta V$)?


  Energy needed to move a unit charge between two points
• What happens when a charge moves along an equipotential surface?

  No work is done
• How are equipotential surfaces represented in uniform electric fields?

  As planes equally spaced and parallel to the plates
• How are equipotential surfaces represented around a point charge?

  As concentric circles
• How can you find electric potential difference from a graph of electric field strength against distance?

  By finding the area under the graph