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# Electric Fields

Guide to electric fields for A2 Physics
by

## CNS Physics

on 1 October 2012

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#### Transcript of Electric Fields

A guide for A2 Physics Electric Fields Fields can not be seen but their influence can be detected by observing the effects on particles. In the case of Starry Night the stars may be considered to be charged particles Certain particles disturb the space around them in such a way that
other particles placed in that space will experience a force. Uniform electric field lines Electric Fields The force that a charged particle will feel is the electric field strength (E) multiplied by the amount of charge on a particle in coulombs (Q), as given by the equation: F = EQ Electric Field Strength Places in an electric field that all have the same potential energy can be connected by lines called equipotentials. Equipotentials Field Theory The field strength is measured by the force it exerts on things placed in it. A bit of extra curricular amusement for those who didn't see it before Field lines show the direction a positively charged particle will be pushed. The closer the lines are together the stronger the field
Stronger field = stronger force Starry Night by Van Gogh (1889)
is an (unintentional?) representation of field theory We can also find out how quickly a charge will accelerate by using Newton's 2nd law F=ma:

F=EQ=ma

So:

a = EQ/m (like contour lines on a map that connect equal height) The field is always perpendicular to the equipotential lines as the field is defined as a region which changes the potential. In a radial field the equipotentials are concentric spheres. Equipotentials between parallel plates are evenly spaced because the electric field is uniform Electric Potential Moving the positive charge closer to another (static) positive charge means that work is being done against the repulsive force. Therefore the moving positive charge gains potential energy. If the positive charge was then 'released' it would lose potential energy and gain kinetic energy.
(this is roughly analogous to the gravitational potential energy = kinetic energy) Electric potential, V, at any point in a field is the potential energy that each coulomb of positive charge would have if placed at that point in the field. p.d (V) = JC-1 so V = E/Q so E = VQ Parallel metal plates with a p.d. across them create a uniform electric field. The strength of a uniform electric field is a
measure of how rapidly the potential changes.
Field strength can be increased by:

increasing p.d. across the plates, or
moving the plates closer together

The equation which describes this relationship is: E (Vm-1) = V/d Can you explain why lightning conductors work using the idea of equipotentials and field strength? The video may help.... What is the link between equipotential lines and field strength? Coulomb's Law The closer two charges are the stronger the force between them Coulomb's law relates the force between two objects to their electrical charges This is an inverse square law. If the distance between the charges is doubled the force between them will be a quarter the size Two charges feel an attractive force of 36N.
What force will they feel if..

a. the distance between them is tripled?
b. the distance is doubled and one charge is tripled?
c. distance and both charges are tripled? If you calculate a negative force then the force is attractive.
If you calculate a force with a positive sign then the force is repulsive.
If one of the charges is neutral there will be no force between them
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