AQA Physics Gravitational Fields, Electric Fields and Capacitance Section 4 Electric Fields

Charged Bodies If two bodies are charged they experience a mutual force between each other along the line joining them. If the two bodies are oppositely charged they experience a mutual attractive force. If the two bodies have the same type of charge (positive or negative) they experience a mutual repulsive force. Q2Q2 r Q1Q1 This force (F) is a function of the amount of charge on each body (Q) and the distance between them (r): AB Where  0 is a constant known as the permittivity of free-space and is equal to 8.85x m -3 kg -1 s 4 A 2.

Electric Fields The size of an electric field (E) represents the force that would be experienced by 1 coulomb of charge placed in that electric field in a given position. A body with charge q placed in an electric field therefore experiences a force F. q The units of electric field are NC -1. Electric field lines represent the direction a small positive charge would move were it placed stationary in the electric field. The density of the electric field lines represents the strength of the electric field.

Electric Fields Electric fields exist between parallel plates with a potential difference across them. The field between the plates can be approximated to be uniform.

Charged Bodies An electric field exists around any charged body. The electric field lines below indicate that the central charge causing the field is positive and that the field is strongest near the charge. Q1Q1 A body with charge q placed in this electric field therefore experiences a force F: The electric field (E) at a distance (r) from this charged body is given by the equation: q

Electric Fields An electric field due to a point charge obeys the inverse square law. E r

Charged Bodies Two bodies charged can therefore be thought of by either: 1.A mutual attraction according to the equation: Q2Q2 r Q1Q1 AB

Charged Bodies Or… 2. A force experienced due to the fact that each charge is in the electric field of the other. Q2Q2 r Q1Q1 AB

Trajectories A charged body initially travelling perpendicular to an electric field performs parabolic motion. q F = Eq Vertical Motion Horizontal Motion