EJC Physics H213 E Field - 1. Notes (full)

“Faraday Cage” demonstration at the Singapore Science Centre. An innocent audience-volunteer can often be found sitting unharmed inside the cage, as high-voltage, high-current electrical arcs sends lethal amounts of charges coursing down the metallic cage structure. Content • Concept of an electric field • Electric force between point charges • Electric field of a point charge • Uniform electric fields • Electric potential Learning Outcomes Candidates should be able to: (a) show an understanding of the concept of an electric field as an example of a field of force and define electric field strength at a point as the electric force exerted per unit positive charge placed at that point (b) represent an electric field by means of field lines (c) recognise the analogy between certain qualitative and quantitative aspects of electric field and gravitational field (d) recall and use Coulomb's law in the form 12 2 04 QF Q r= for the electric force between two point charges in free space or air (e) recall and use 2 04 QE r= for the electric field strength of a point charge in free space or air (f) calculate the electric field strength of the uniform field between charged parallel plates in terms of the potential difference and plate separation (g) calculate the forces on charges in uniform electric fields (h) describe the effect of a uniform electric field on the motion of charged particles (i) define the electric potential at a point as the work done per unit positive charge in bringing a small test charge from infinity to that point (j) state that the field strength of the electric field at a point is numerically equal to the potential gradient at that point (k) use the equation 04 V Q r = for the electric potential in the field of a point charge, in free space or air.

A “field” is a region in space where a force is experienced by an “entity” without contact. A mass experiences a force when placed in a gravitational field. Similarly, a charged particle experiences force when placed in an electric field. Forces are vector quantities so the direction has to be well-defined. The direction of an electric field is the direction of force on a positive charge, by convention and by definition. An isolated mass generates its gravitational field in the region surrounding the mass and it permeates all of space (“to infinity”). Similarly, a charged particle or a collection of charged bodies, generate electric field in the surrounding region, all the way to infinity. When an additional charge particle comes into this region, the additional charged particle interacts with the existing electric field and experiences an electric force. Specifically, the tangent at a point on the electric field line shows the direction of electric force that acts on a small stationary positive test charge if the charge is placed at that point and is free to move. The arrows on electric field lines point