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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
Electric Fields
Electric Fields
Electric Fields
Quantifying the field
• Definition: An electric field is the force per
charge imparted on a test charge
F
q
q is the charge of the test charge
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 105 NC-1
v = 7.50 x 107 ms-1
The real cause of electric current
Electric Field
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?
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.