Transcript Physics_A2_Unit4_23_ElectricFieldStrength

Book Reference : Pages 76-78
1.
To examine the concept of Electric field
strength (two ways)
2.
To examine the factors which affect the
nature of an electric field
Previously we saw that a field line in an
electric field is the path a small positive
charge would take if it were free to move.
If this charge is moved by the field then it
must be experiencing a force
The electric field strength, E at a point in an
electric field is defined as the force per unit
charge on a positive test charge placed at
that point
E=F/Q
Where E is the electric field strength (NC-1)*,
F is the Force experienced(N) and Q is the
charge in Coulombs (C)
Note that E is a vector quantity in the identical
direction to F and is the same as the direction of
the field line for a positive charge (opposite
direction for a negative charge)
* Other units to follow
Under normal conditions
air is a good insulator.
However, if the electric
field strength is high
enough then the air
becomes ionised
(electrons ripped from
the molecules). During a
thunderstorm this
happens suddenly and
lightning strikes
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Consider the force F
experienced by a small
charge +Q in the
uniform electric field
between a pair plates
separated by d metres
which have a potential
difference (Voltage V)
between them.
Note the field lines are : Parallel to each other,
perpendicular to the plates and go from the +ve plate to
the –ve plate
By rearranging our field strength equation
(E=F/Q) we can show that the force on Q is:
F = QE
If this charge is moved from one plate to
another, a distance of d metres by the force
F then work is done (W = f x d)
W = QE x d
Definition :
The potential difference V between two points
is the work done per unit charge when a small
charge is moved through the potential
difference
V = W/Q
V = W/Q = QEd/Q = Ed (rearrange)
E = V/d
[Note : alternative units for E: Vm-1)
Any object with a charge will have an
electric field. As the charge increases, so
does the strength of the electric field
The more concentrated the charge the
greater the field strength
So far for parallel plates we’ve seen the
charge spread evenly across parallel plates
With a V shaped plate
the field lines are
concentrated at the tip
since this is where the
charge is most
concentrated
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For a uniform field
between parallel plates
the charge is spread
evenly over the surface
Experimentally it is
possible to show that
the electric field
strength is proportional
to the charge per unit
area on the surface
E  Q/A
Like all proportionalities we can turn this
into an equation by introducing a suitable
constant of proportionality....
0E = Q/A
0 is called “epsilon nought” & has the value
8.85 x10-12 FaradsMetre-1 (Fm-1)
We’ll cover this later.... (Use of 0 is required
for A2 but further knowledge is not)
A Farad is the unit of capacitance and is 1
Coulomb per Volt
“Epsilon nought” is the permittivity of free
space and is the charge per unit volume on a
surface in a vacuum which results in a field
strength of 1 volt per metre between the
plates