Transcript 6.2

Topic 6: Fields and
Forces
Topic 6.2 Electric force and field
Electrification by Friction
Electric charge, or `electricity', can
come from batteries and generators.
But some materials become charged
when they are rubbed. Their charge
is sometimes called electrostatic
charge or `static electricity'. It
causes sparks and crackles when you
take off a pullover, and if you slide
out of a car seat and touch the door,
it may even give you a shock.
Two Types of Charge

Polythene and Perspex can be charged by
rubbing them with a dry, woollen cloth.
When two charged polythene rods are
brought close together, they repel (try to
push each other apart).
 The same thing happens with two charged
Perspex rods.

However, a charged polythene rod and a
charged Perspex rod attract each other.
 Experiments like this suggest that there
are two different and opposite types of
electric charge.
 These are called positive (+) charge and
negative (-) charge:

Conservation of Charge
Where charges come from
 Everything is made of tiny particles called
atoms.
 These have electric charge inside them.
 There is a central nucleus made up of
protons and neutrons.
 Orbiting the nucleus are much lighter
electrons

Electrons have a negative (-) charge.
 Protons have an equal positive (+)
charge.
 Neutrons have no charge.

Normally, atoms have equal numbers of
electrons and protons, so the net
(overall) charge on a material is zero.
 However, when two materials are
rubbed together, electrons may be
transferred from one to the other.
 One material ends up with more
electrons than normal and the other with
less.
 So one has a net negative charge, while
the other is left with a net positive
charge.
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Rubbing materials together does not make
electric charge. It just separates charges
that are already there.

Charge is always conserved in any action,
the distribution of charge is changed.
Conductors and Insulators

When some materials gain charge, they
lose it almost immediately. This is because
electrons flow through them or the
surrounding material until the balance of
negative and positive charge is restored.
Conductors
Conductors are materials that let
electrons pass through them.
 Metals are the best electrical conductors.
 Some of their electrons are so loosely held
to their atoms that they can pass freely
between them.
 These free electrons also make metals
good thermal conductors.


Most non-metals conduct charge poorly or
not at all, although carbon (in the form of
graphite) is an exception.
Insulators
Insulators are materials that hardly
conduct at all.
 Their electrons are tightly held to atoms
and are not free to move - although they
can be transferred by rubbing.
 Insulators are easy to charge by rubbing
because any electrons that get transferred
tend to stay where they are.

Semiconductors
Semiconductors are `in-between'
materials.
 They are poor conductors when cold, but
much better conductors when warm.

Electrostatic Induction
Attraction of uncharged objects
 A charged object will attract any
uncharged object close to it. For example,
the charged screen of a TV will attract
dust.

The previous diagram shows what
happens if a positively charged rod is
brought near a small piece of aluminium
foil.
 Electrons in the foil are pulled towards
the rod, which leaves the bottom of the
foil with a net positive charge.
 As a result, the top of the foil is attracted
to the rod, while the bottom is repelled.
 However, the attraction is stronger
because the attracting charges are
closer than the repelling ones.

Coulomb’s Law
 The
force between two point
charges is directly proportional to
the product of the charges and
inversely proportional to their
distance apart squared.
Equations
F  q1 q2 / r2
 Or F = k q1 q2 / r2

Where k = 1/4πε0
 ε0 is the permittivity of free space
 Therefore Coulomb’s Law can be written
as
 F = q1 q2 / 4πε0r2

Applying Coulomb’s Law
To determine the net force on a charge
due to two or more other charges, you
must use vector addition.
 Find the force and direction due to each of
the other charges in turn
 and then resolve these forces to get the
resultant force

Electric Field
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A resultant force changes motion.
Many everyday forces are pushes or pulls
between bodies in contact.
In other cases forces arise between bodies that
are separated from one another.
Electric, magnetic and gravitational effects
involve such action-at-a-distance forces and to
deal with them physicists find the idea of a field
of force, or simply a field, useful.

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Fields of these three types have common features as
well as important differences.
An electric field is a region where an electric charge
experiences a force.
If a very small, positive point charge Q, the test charge,
is placed at any point in an electric field and it
experiences a force F,
then the field strength E (also called the E-field) at that
point is defined by the equation
In words,the magnitude of E is the force
per unit charge and its direction is that
of F (i.e. of the force which acts on a
positive charge).
 Field strength E is thus a vector.
 If F is in newtons (N) and Q is in
coulombs (C) then the unit of E is the
newton per coulomb (N C-1).
 A commoner but equivalent unit is the
volt per metre (V m-1).

To determine the net field strength on a
charge due to two or more other
charges, use must use vector addition.
 Find the field strength and direction due
to each of the other charges in turn
 and then resolve these field strengths to
get the resultant field strength
 Remember that the direction of a field is
the direction in which a positive charge
would move

Uniform Electric Field

The field between two parallel plates can be
calculated by
Field Patterns
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An electric field can be represented and so visualized by
electric field lines.
These are drawn so that
(1) the field line at a point (or the tangent to it if it is
curved) gives the direction of E at that point, i.e. the
direction in which a positive charge would accelerate,
and (2) the number of lines per unit cross-section area is
proportional to E.
The field line is imaginary but the field it represents is
real.
+
Positive Charge