Electric fields
Two charges push and pull on each other without ever touching — so how does one charge 'know' the other is there? The answer is the electric field: a region of space in which a charge feels a force. In this Advanced Subsidiary lesson we build the idea slowly and rigorously. We define electric field strength as the force per unit positive charge, E = F ÷ Q (in N/C, which is the same as V/m), and learn to draw a field with field lines. We meet the radial, non-uniform field of a point charge and the uniform field between charged parallel plates, where E = V ÷ d. We calculate the force on a charge with F = EQ, and finish with the examiner's favourite: firing a charged particle across a uniform field and watching it trace a parabola.
By the end you should be able to (NSSCAS Physics (AS) 3.1):
- Explain the concept of an electric field as an example of a field of force — a region in which a charge experiences a force due to another charge
- Represent an electric field by means of field lines, including the radial field of a positive and a negative point charge
- Define and use electric field strength as the force per unit positive charge, E = F ÷ Q, and state its unit (N/C, equivalently V/m)
- Recall and use E = ΔV ÷ Δd to calculate the field strength of the uniform field between charged parallel plates
- Calculate the force on a charge in a uniform electric field using F = EQ
- Describe the effect of a uniform electric field on the motion of a charged particle, explaining its parabolic path
- Compare radial (non-uniform) and uniform electric fields
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.