Electric field

Introduction to electricity
Electricity – phenomena associated with
interaction between electrically charged objects
Particles and electric charge
Electron - electrically negative particle
Proton – electrically positive particle
Neutron – electrically neutral partice
SI unit of charge is coulomb (C)
Electron, e = -1.602 x 10-19 C
Proton, p = +1.602 x 10-19 C
Each electric charge (q) can be expressed as
q = n x e,
where e - charge of one electron, n - number of excessive or
deficient electrons
The Law of Conservation of
Charge
 The
Law of conservation of charge states
that the net charge of an isolated system
remains constant.
Exercise :

If you shuffle across a carpeted floor on a dry
day and acquire a net charge of -2.0 C, will you
have a deficiency or excess of electrons? How
many missing or extra electrons will you have?
 Answer: you have acquired 1.3x1013 electrons.

In this example, (a) what type of charge does the
carpet acquire? (b) How much charge does the
carpet acquire? (c) Does it have an excess or
deficiency of electrons? (d) How many
electrons?
 What about shuffling across a wooden floor?
Distinguishing between electrical
conductors and insulators
• Conductors – valence electrons
of atoms are loosely bound
CONDUCTORS
Mercury
Silver
Iron
Carbon
• Insulators – valence electrons are
more tightly bound and are not
readily moved
Copper
INSULATORS
Wood
Glass
Rubber
Semi-conductors


Semi-conductors are a very useful intermediate class,
not as conductive as metals but considerably more
conductive than insulators.
By adding certain impurities to semi-conductors in the
appropriate concentrations the conductivity can be wellcontrolled.
SEMICONDUCTORS
Germanium
(Transistors)
Silicon
(computer chips)
Electrostatic charging
- a process by which an object receives a net charge
•Charging by friction
•Charging by contact or by conduction
•Charging by induction
Electrostatic charging

Charging by friction - this is useful for charging
insulators. If you rub one material with another,
electrons have a tendency to be transferred from one
material to the other.

Charging by conduction - useful for charging
conductors. If a charged object touches a conductor,
some charge will be transferred between the object
and the conductor, charging the conductor with the
same sign as the charge on the object.

Charging by induction - also useful for charging
conductors. Again, a charged object is used, but this
time it is only brought close to the conductor, and does
not touch it.
Electrostatic charging
 Examples:




Clothes or papers sticking together
Electrostatic air cleaners
Electrostatic copiers
Electrostatic spark discharge can cause an
explosion in the presence of flammable gases
The Law of Electric charges:
Like charges repel each other, and unlike
charges attract each other.
Coulomb’s Law
F1
F2
+q1
+q2
r
F1
+q1
F1
F2
-q2
F2
q1 q2
k
2
r
k = 8.99x109 Nm2/C2
Exercise:

In a certain organic molecule, the nuclei of two
carbon atoms are separated by a distance of
0.25 nm. What is the magnitude of the electric
repulsion between them?

The problem concerns electric interaction
(force) between two charged objects (two
nuclei of carbon)
Force between two charges is described by
Coulomb’s law

F
q1 q2
k
r2
k = 8.99x109 Nm2/C2
F
F
+q1
+q2
r
In a carbon nuclei there are six protons, so
q1 = q2 = 6 x 1.6 x 10-19 C
r = 0.25 nm = 0.25 x 10-9 m
F = 0.133 x 10-6 N = 0.133 N
Electric field
•The space surrounding an electric charge has a property called an
electric field.
•This electric field exerts a force on other electrically charged objects.
Electric field
•The electric field (E) is a vector field with SI units of newton per
coulomb (N C−1).
•The strength of the field at a given point is defined as the force
that would be exerted on a positive test charge of +1 coulomb
placed at that point; the direction of the field is given by the
direction of that force.
E
F
q
Electric field
- electric field lines
Electric field
- electric field lines
Electric field
- electric field lines
Charge separation by polarization
+ -+- - -+ -+
-+ + + Nonpolar molecule
+
+
+
_
_
++ +
___
_ ++ +
_
_
_
Induced molecular dipole
Electric dipole gives us a model for
permanently polarized molecules, like the
water molecule.
Conductors and elecrtic field

The electric field is zero inside a charged conductor
Conductors and elecrtic field

Any excess charge on an isolated conductor
resides entirely on the surface of the conductor
Conductors and elecrtic field

The electric field at the surface of a charged
conductor is perpendicular to the surface
Conductors and elecrtic field

Excess charge tends to accumulate at sharp
points, or locations of highest curvature.
Uniform electric field
between two parallel plates
E
4 kQ
A
A – surface area
of the plate
Q – charge on the
plate
Energy of a charge in an
electric field
Electric field
•A charge (q) moving in an electric field (E) along the lines
of electric field over a distance (d) aquires energy ( U).
•To move a charge (q) against the electric field (E) over a
distance (d) a work ( U) has to be done on it.
U
q E d
• One can say also that a charge (q) in an electric field (E)
has potential energy equal to that work (ΔU) .
•That potential of electric field to move electric charges and
to change the energy of electric charges is expressed using
a term:
voltage
VOLTAGE

Any difference in charge between two objects
will result in the development of a
DIFFERENCE OF POTENTIAL ( V) between
them. We call this difference of potential
VOLTAGE.

SI unit of electric potential difference is volt
 1V = 1J/1C

Potential difference in a uniform electric field is
V = Ed
Exercise 6:





A proton is moved from the negative to the positive plate
of a parallel-plate arrangement. The plates are 1.5 cm
apart, and the electric field is uniform with a magnitude
of 1500N/C.
(a) What is the force on the electron?
(b) What is the proton’s potential energy change?
(c) What is the potential difference between the plates?
(d) What is the potential difference between the negative
plate and a point midway between the plates?
E = 1500 N/C
_
_
_
_
+
+
+
+
E
U
F
q
- Electric field
q E d
- Potential energy of charge in
an electric field
ΔV = Ed - Potential difference in
a uniform electric field
1.5 cm
(a) F = E x q = 1500 N/C x 1.6 x 10-19 C = 2400 x 10-19 N
(b) U = 1.6 x 10-19 C x 1500N/C x 0.015 m = 36 x 10-19 Nm = 36 x 10-19 J
(c) V = 1500 N/C x 0.015 m = 22.5 Nm/C = 22.5 J/C = 22.5 V
(d) V1/2 = 1500 N/C x 0.5 x 0.015 m = 11.25 Nm/C = 11.25 V

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