 # Electric Fields Section 3.7.3 Section 3.7.1 Electric fields - questions in a 'flip-book' for you to try 3.7.3.1 Coulomb's law Force between two point charges in a vacuum, permittivity of free space εo Appreciation that air can be treated as a vacuum when calculating force between charges. For a charged sphere, charge may be considered to be at the centre. Comparison of magnitude of gravitational and electrostatic forces between subatomic particles. Note it is an inverse square relationship Compare the equation for gravitational force to the expression for electrostatic force. Similarities and differences between electric and gravitational fields: - electric can have +ve and -ve charge and therefore attractive and repulsive forces - gravitational only attractive - Both obey inverse square law. No quantitative comparisons required (....but they are good for your soul! ) MS 0.3, 2.3 Students can estimate the magnitude of the electrostatic force between various charge configurations. 3.7.3.2 Electric field strength Representation of electric fields by electric field lines E is Electric field strength. E defined as force per unit charge: E = F/Q Magnitude of E in a radial field and radial field Derivation from work done moving charge between plates: Fd = QΔV             Trajectory of moving charged particle entering a uniform electric field initially at right angles. 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. PS 1.2, 2.2 / AT b Students can investigate the patterns of various field configurations using conducting paper (2D) or electrolytic tank (3D). Motion of charged particles in an electric field eg. trajectory of particle beams - Use analogy with motion in a gravitational field ie. projectile motion. Recognise difference in behaviour between +ve and -ve charges. 3.7.3.3 Electric Potential Understanding of the definition of absolute electric potential, including zero at infinity and of potential difference. Work done in moving electric charge Q: ΔW = QΔV Equipotential surfaces. No work done moving charge along an equipotential surface. Magnitude of V in a radial field given by Graphical representations of the variation of E and V with r V related to E by E = ∆V/∆r ∆V from the area under graph of E against 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