Electricity and Magnetism Mr D. Patterson

Outcomes explain using electric fields the connection between electrical work, charge and potential difference—this will include applying the relationships of electrical work and power: explain that point charges create radial electric fields describe, using diagrams, electric field distributions around simple combinations of charged points, spheres and plates describe, explain and use electric fields between parallel plates and within uniform conductors, to explain the forces on charged particles—this will include applying the relationships:

Non contact forces

Fields Non contact forces are explained using fields A field describes the size and direction of a force acting at each point

Electric Fields An electric field shows the direction of the force that a small, positive test charge would feel – They point radially outwards from a positive charge – They point radially towards a negative charge – Field lines cannot cross one another

Electric Fields

Quantifying the field Definition: An electric field is the force per charge imparted on a test charge q is the charge of the test charge F q

Quantifying the field E is the electric field (NC -1 ) F is the force felt by the test charge (N) q is the charge of the test charge (C)

Parallel Plates of Charge Parallel plates of opposite charges create uniform electric fields A test charge, q, will be feel a force F and therefore move a distance, d, from one plate to another Using W=Fd, V=E/q and E=F/q we get: The red Es are energy, not electric field!

To recap Electric Field strength This second part only applies to a uniform electric field, like that made by parallel plates E is the electric field (NC -1 ) F is the force felt by the test charge (N) q is the charge of the test charge (C) V is the potential difference (V or JC -1 ) d is the distance between parallel plates (m)

Example problem An electron is “fired” from a hot cathode by establishing a potential difference of 16 kV between parallel plates spaced 14 cm apart. What is the strength of the electric field and how fast is the electron travelling after it covers this distance? Ans: E = 1.14 x 10 5 NC -1 v = 7.50 x 10 7 ms -1

The real cause of electric current The drift speed of an electron in a circuit may be only a few centimetres a minute. So why does a light turn on straight away when a switch is closed? Electric Field

The real cause of electric current The electric field, which applies a force to the electrons travels at nearly the speed of light. Electrons average speed around a circuit is slow, but the electric field “signal” which causes the electrons in the light to move is quick.