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Module 4 - Electric Fields Section 3.4.3

Section 3.4.3 Electric fields

- questions in a 'flip-book' for you to try

Coulomb's law, force between two point charges in a vacuum, permittivity of free space εo

Compare the equation for gravitational force to' the expression for electrostatic force.

Note it is an inverse square relationship

Electric field strength E defined as force per unit charge.

Representation by electric field lines

Magnitude of E in a radial field and radial field


Java applet to show a field

Explain meaning of permittivity, include absolute and relative values.
Explain action of dielectric in terms of capacitance - preparation for next section

Application, e.g. estimation of forces at closest approach in Rutherford alpha particle scattering

Define E by comparison with gravitational fields, NB can have +ve and -ve charge and therefore attractive and repulsive forces.

Draw diagrams to show field patterns for radial and uniform fields.

Electric Potential

Understanding of the definition of absolute electric potential, including zero at infinity and of potential difference.

Work done in moving electric charge:

ΔW = QΔV

Magnitude of V in a radial field given by

Graphical representations of the variation of E and V with r

Refer to vector nature of E - compare with scalar for V

Calculations involving position of 'null' point

Resultant V as scalar sum of potentials due to charges.

Graphs for E and V against r

Similarities and differences between electric and gravitational fields

No quantitative comparisons required (....but they are good for your soul! )

Define E by comparison with gravitational fields, NB can have +ve and -ve charge and therefore attractive and repulsive forces.

Both obey inverse square law.

Motion of charged particles in an electric field

Trajectory of particle beams

Use analogy with motion in a gravitational field ie. projectile motion.
Recognise difference in behaviour between +ve and -ve charges.