Static Electricity
Understand the nature of electron charge and
field lines.
Success Criteria
• Can identify the symbol for electric charge.
• Can describe the accumulation of negative charge in terms of
loss or gain of electrons.
• Can recall the unit and symbol of electric charge.
• Can recall the amount of electrons in one Coulomb.
• Can work out the magnitude of a charge when given the
number of electrons present.
• Can draw the direction of field lines when given point charges.
• Can identify a situation where a uniform field is present and
one where a radial field is present
Electric Charge
• A positive electric charged is produced a point has
lost electrons.
• A negative charge is produced when there is a net
gain in electrons.
positive
loses
electrons
+
-
+
-
+
-
+
-
+
-
+
+
-
+
-
+
+
negative
gains
electrons
+
-
-
+
- -+
- + +
The Coulomb
• The symbol for the quantity charge is q and it is measured in
coulombs (C).
• A charge (q) of -1.00 C is equal to the charge of 6.241 x 1018
electrons.
• An electric charge can either be positive or negative.
Exercise
A balloon has a charge of - 3.50 μC. How many extra electrons
are on the surface of the balloon?
6
 3.5  C   3.5  10 C
6
 3.5  10 C
 1.0 C
 6.241  10
18
 2.18  10 electrons
13
When charging a ruler on a silk cloth 6.52 x 1011 electrons
leave the ruler. What its charge?
6.52  10
11
6.241  10
7
18
9
 1.04  10 C  104  10 C  104 nC
Field lines
• Point charges produce field lines. These field lines
flow from a positive charge to a negative charge.
-
+
+
-
Field lines
• Draw the electric field lines present in the
following situations.
B
A
-
+
C
-
-
+
+
Uniform Electric Field
• Two parallel plates form an electric field separated by a
distance, d and attached to a battery with a voltage, V produce a
Uniform Electric Field, E.
• At any point between the two plates the strength of the electric
field, E acting on a charge is constant.
At the edges
the electric
field is no
longer uniform,
it is radial.
V=12V
+
+
+
+
+
E=2400Vm-1
+
+
d=5mm
Can work out the strength of an electric field
Success Criteria
• Can define what electric field strength is.
• Can solve electric field problems involving the voltage, electric
field strength and distance between two plates.
• Can describe how an electric charge can be used to work out
the strength of an electric field.
• Can explain why the direction of the force on an electron is
opposite to the direction of the electric field.
• Can work out the strength of an electric field when given the
force and a charge.
Electric Field Strength
Electric field strength
measure in Vm-1 or NC-1
E
1
d
E  V
E 
V
Voltage measured in V
d
distance measured in m
For a constant voltage. The shorter the
distance (d) between two the stronger
the electric field(E).
For plates kept at a constant
distance. The larger the potential
difference/voltage (V) the
stronger the electric field(E).
Example Exercises
1
What is the strength of an
electric field produced when 15 V is
applied across two plates separated by
4.0 mm?
E 
V

d
4.0mm
15V
+ + + + + + +
15V
0 .0 0 4 0 0 m
-1
E=3750V m =3700V m
-1
2
What is the distance between two plate that produce a
2000 Vm-1 electric field when 3.5V is applied.
d 
V
E

3.5V
2000
3
d= 1.75  10 m = 1.8m m
3
What voltage is needed to
produce a 5200Vm-1 electric field
between two plates 6.0mm apart?
V  E  d= 5200V m
V = 31.2= 31V
1
3
 6.0  10 m
+ + + + + + +
4
If the voltage applied to two plates is kept constant what
happens to the electric field strength if the two plates are pulled
further apart?
1
•Since the voltage stays constant,
E
d
•The distance increases.
•The Electric field strength is inversely proportional to the distance
so it (E) deceases.
Force on a charge in an electric field
F
- F
+
+
+
+
+
+
+
+
A positive charge has an electrostatic force acting on it in the
direction of the electric field. This force, F is proportional to the
charge, q of the particle and the strength of the electric field,
E. The force acting on a negatively charged particle acts in the
opposite direction to the electric field.
Electric field strength
measured in Vm-1 or NC-1
Force measured in N
Charge
measured in C
FE
F q
If the charge is constant. If the electric
field, E increases the force will increase
proportionally.
If the electric field, E is constant. The
larger the charge, q the larger the force, F.
Example Exercises
1
What is the force acting on a
particle with a +9.23 nC charge place in
between two plates with a uniform
electric strength of 275 Vm-1?
F  E  q  275V m
1
9
 9.23  10 C
6
F= 2.54  10 N = 2.54  N
q= +9.23 nC
+ + + + + + +
E = 275 Vm-1
2
What is the strength of an electric field that applies a
24.5μN force to a particle with a charge of 300 nC?.
d 
V
E

3.5V
2000
3
d= 1.75  10 m = 1.8m m
3
If the electric field strength on a charge is doubled
what happens to the electrostatic force the charge feels?
•Since the charge stays constant,
FE
•The field strength increases.
•The force is proportional to the electric field strength so it (F)
increases.
4
What happens to the force on a charge in a uniform
electric field if the distance between the two plates is halved?
If we assume that the V stays constant, E ∝ 1/d
∴ when the distance halves, E doubles.
If we also assume q stays constant, F ∝ E
∴ when the electric field strength doubles the force doubles.
Define electric potential in terms of work and energy.
Use V = work/q and V = ∆Ep/q to calculate change in
electric potential energy
Success Criteria
• Can work out the change in potential energy using  E p  E qd
• Recognise that when a charge does work(W) by moving
against electric field lines it gains potential energy(∆Ep).
• Recognise that when a charge moves with the field lines it
loses potential energy and gains kinetic energy.
E p
• Can use V  q to work out the potential energy a charge has
• Can describe the relationship between work, Ek and Ep.