Transcript electricity & magnetism

Electricity
All of us agree the importance of electricity in our daily lives.
But what is electricity?
 Electric Charge and Electrical Forces:

Electrons have a negative electrical charge.

Protons have a positive electrical charge.

These charges interact to create an electrical force.

Like charges produce repulsive forces – so they
repel each other (e.g. electron and electron or
proton and proton repel each other).

Unlike charges produce attractive forces – so they
attract each other (e.g. electron and proton attract
each other).
 Electrostatic Charge:

Electrons move from atom to atom to create ions.
 positively charge ions result from the loss of electrons and are called
cations.
 Negatively charge ions result from the gain of electrons and are
called anions.

The charge on an ion is called an electrostatic charge.

An object becomes electrostatically charged by
 Friction,which transfers electrons between two objects in contact,
 Contact with a charged body which results in the transfer of
electrons,
 Induction which produces a charge redistribution of electrons in a
material.
Arbitrary numbers of
protons (+) and electrons
(-) on a comb and in hair
(A) before and
(B) after combing.
Combing transfers
electrons from the hair to
the comb by friction,
resulting in a negative
charge on the comb and
a positive charge on the
hair.
Charging
by
Induction
 Electrical Conductors and Insulators:

Electrical conductors are materials that allows free
movement of electrons inside
Metals are good conductors of electricity. Silver is the best
electrical conductor.

Electrical nonconductors (insulators) are materials that
do not allow movement of electrons easily.
Examples are wood, rubber etc.

Semiconductors are materials whose conductivity lies in
between those of conductors and insulators.
Examples are silicon, arsenic, germanium.
 Measuring Electrical Charges:
 The fundamental charge is the electrical charge on an
electron and has a magnitude of 1.6021892 X 10-19 C
 The electrical charge (q) is a discrete quantity and it is always
measured as
q=ne
where e is the fundamental charge.
 Conservation of charge is a fundamental principle which states
that charge can neither be created or destroyed but can only
move from one atom to another.
 Coulomb’s law:
Electrical force is directly proportional to the product of
the electrical charges and inversely proportional to the
square of the distance. This is known as Coulomb’s law.
Mathematically,
F k
q1 q 2
d
2
where,
 F is the electrical force,
 k is a constant and has the value of 9.00 x 109
Newtonmeters2/coulomb2 (9.00 x 10 9 Nm2/C2),
 q1 represents the electrical charge of object 1 and q2 represents the
electrical charge of object 2, and
 d is the distance between the two objects.
 Electrical force is a VECTOR quantity and is directed along the line of
action
 Force Fields:
 The configuration of space around an object is changed by the
presence of an electrical charge.
 The electrical charge produces a force field, called as
electrical field
Coulomb’s Law:
| F | = k | Q qo | / r2
Rearranged:
| F | = | qo [k Q/r2] |
Gives us:
F = qo E
where the electric field E is:
| E | = | k Q / r2 |
 A map of the electrical field can be created by bringing a
positive test charge into an electrical field.

When brought near a negative charge the test charge is
attracted to the unlike charge and when brought near a
positive charge the test charge is repelled.

You can draw vector arrows to indicate the direction of the
electrical field.

This is represented by drawing lines of force or electrical
field lines,
 These lines are closer together when the field is stronger
and farther apart when it is weaker.
A positive test charge is
used by convention to
identify the properties of
an electric field. The
vector arrow points in the
direction of the force that
the test charge would
experience.
Electric Lines of force diagram for
(A) a negative charge and (B) a
positive charge when the charges
have the same magnitude as the test
charge.
 Electrical Potential:
 An electrical charge has an electrical field that surrounds it.
 In order to move a second charge through this field work must
be done.
 Bringing a like charge particle into this field will require work
since like charges repel each other and bringing an opposite
charged particle into the field will require work to keep the
charges separated.
 In both of these cases the electrical potential is changed.
 The potential difference (PD) that is created by doing 1.00
joule of work in moving 1.00 coulomb of charge is defined as
1.00 volt.
 A volt is a measure of the potential difference between
two points,
 electric potential =
work done,
charge
Or,
PD=W
Q
 The voltage of an electrical charge is the energy transfer per
coulomb.
 The energy transfer can be measured by the work that is done
to move the charge or by the work that the charge can do
because of the position of the field.
Van de Graff
electrostatic
generator:
simulates lightning
from cloud to
ground
 ELECTRIC CURRENT:
 Electric current means the flow of charges which is analogous
to water flow
 It is the charge that flows, and the current is defined as the
flow of the charge.
 An electrical circuit contains some device that acts as a source of
energy as it gives charges a higher potential against an electrical
field.



The charges do work as they flow through the circuit to a
lower potential.
The charges flow through connecting wires to make a
continuous path.
A switch is a means of interrupting or completing the circuit.
 The source of the electrical potential is the voltage source.
A simple electric circuit has a voltage source (such as a generator
or battery) that maintains the electrical potential, some device (such as a
lamp or motor ) where work is done by the potential, and continuous
pathways for the current to flow.
 Voltage is a measure of the potential difference between two
places in a circuit.
 Voltage is measured in joules/coloumb.
 The rate at which an electrical current (I) flows is the charge
(q) that moves through a cross section of a conductor in a give
unit of time (t),
I = q/t.


the units of current are coulombs/second.
A coulomb/second is an ampere (amp).
What is the nature of the electric current carried by these conducting
lines?
It is an electric field that moves at near the speed of light. The field causes
a net motion of electrons that constitutes a flow of charge, a current.
(A) A metal conductor without
a current has immovable
positive ions surrounded by a
swarm of randomly moving
electrons.
(B) An electric field causes the
electrons to shift positions,
creating a separation charge as
the electrons move with a
zigzag motion from collisions
with stationary positive ions
and other electrons.
 Electrical Resistance:
 Electrical resistance is the resistance to movement of electrons
being accelerated with an energy loss.
 Materials have the property of reducing a current and that is
electrical resistance (R).
 Resistance is a ratio between the potential difference (V) between
two points and the resulting current (I).
R = V/I
 The ratio of volts/amp is called an ohm ().
 The relationship between voltage, current, and resistance is:
V =I R
This is known as Ohms Law.
 The magnitude of the electrical resistance of a conductor
depends on four variables:
 The length of the conductor.
 The cross-sectional area of the conductor.
 The material the conductor is made of.
 The temperature of the conductor.
Resistors in Series
 Resistors can be connected in series; that is, the current
flows through them one after another. The circuit here
shows three resistors connected in series, and the direction
of current is indicated by the arrow.
 Note that since there is only one path for the current to
travel, the current through each of the resistors is the same.
 I1= I2 = I3
 Also, the voltage drops across the resistors must add up to the
total voltage supplied by the battery:
 V total = V1+V2+V3
 R equivalent = R1 + R2 + R3
Resistors in Parallel
 Resistors can be connected such that they branch out from a
single point (known as a node), and join up again somewhere else
in the circuit. This is known as a parallel connection. Each of the
three resistors in the figure below is another path for current to
travel between points A and B.
 At A the potential must be the same for each resistor.




Similarly, at B the potential must also be the same for each
resistor.
So, between points A and B, the potential difference is the
same. That is, each of the three resistors in the parallel
circuit must have the same voltage.
V1 =V2 = V3
Also, the current splits as it travels from A to B. So, the sum
of the currents through the three branches is the same as the
current at A and at B (where the currents from the branch
reunite).
I = I1 +I2 + I3
 Electrical Power and Electrical Work:
 All electrical circuits have three parts in common.



A voltage source.
An electrical device
Conducting wires.
 The work done (W) by a voltage source is equal to the work
done by the electrical field in an electrical device,
Work = Power x Time.
 The electrical potential is measured in joules/coulomb and a
quantity of charge is measured in coulombs, so the electrical
work is measure in joules.
 A joule/second is a unit of power called the watt.
Power = current x potential
Or,
P=IV
This meter measures the amount of electric work done in the circuits,
usually over a time period of a month. The work is measured in kWhr.
Magnetism
All of us are familiar with magnets. In a magnet we have
magnetic poles – the north and the south pole.
 A North seeking pole is called the North Pole.
 A South seeking pole is called the South Pole.
Like magnetic poles repel and unlike magnetic poles
attract.
Every magnet has ends, or poles, about which the magnetic
properties seem to be concentrated. As this photo shows, more iron
filings are attracted to the poles, revealing their location.
 Magnetic Fields:
 A magnet that is moved in space near a second magnet
experiences a magnetic field.

A magnetic field can be represented by field lines.
 The strength of the magnetic field is greater where the lines
are closer together and weaker where they are farther apart.
Broken Magnet
These lines are a map of the magnetic field around a bar magnet.
The needle of a magnetic compass will follow the lines, with the north
end showing the direction of the field.
 The Source of Magnetic Fields:
 Permanent Magnets:

Moving electrons produce magnetic fields.

In most materials these magnetic fields cancel one another
and neutralize the overall magnetic effect.

In other materials such as iron, cobalt, and nickel, the
atoms behave as tiny magnets because of certain
orientations of the electrons inside the atom.

These atoms are grouped in a tiny region called the
magnetic domain.
Our Earth is a big magnet.

The Earth’s magnetic field is thought to originate with
moving charges.

The core is probably composed of iron and nickel, which
flows as the Earth rotates, creating electrical currents that
result in the Earth’s magnetic field.
The earth's magnetic field. Note
that the magnetic north pole and
the geographic North Pole are not
in the same place.
Note also that the magnetic north
pole acts as if the south pole of a
huge bar magnet were inside the
earth. You know that it must be a
magnetic south pole since the
north end of a magnetic compass is
attracted to it and opposite poles
attract.
A bar magnet cut into halves always makes new, complete
magnets with both a north and a south pole. The poles always come in
pairs. You can not separate a pair into single poles.
Electric Currents
and
Magnetism
Oersted discovered that a compass
needle below a wire (A) pointed
north when there was not a current,
(B) moved at right angles when a
current flowed one way, and
(C) moved at right angles in the
opposite direction when the current
was reversed.
(A) In a piece of iron, the magnetic domains have random arrangement
that cancels any overall magnetic effect (not magnetic).
(B) When an external magnetic field is applied to the iron, the magnetic
domains are realigned, and those parallel to the field grow in size at the
expense of the other domains, and the iron becomes magnetized.
A magnetic
compass shows
the presence and
direction of the
magnetic field
around a straight
length of currentcarrying wire.
When a current is run through a
cylindrical coil of wire, a
solenoid, it produces a magnetic
field like the magnetic field of a
bar magnet. The solenoid is
known as electromagnet.
 Applications of Electromagnets:
 Electric Meters:

The strength of the magnetic field produced by an
electromagnet is proportional to the electric current in
the electromagnet.

A galvanometer measures electrical current by
measuring the magnetic field.

A galvanometer can measure current, potential
difference, and resistance.
A galvanometer measures the direction and relative strength of
an electric current from the magnetic field it produces. A coil of wire
wrapped around an iron core becomes an electromagnet that rotates in
the field of a permanent magnet. The rotation moves pointer on a scale.
 Electric Motors:

An electrical motor is an electromagnetic device that
converts electrical energy into mechanical energy.

A motor has two working parts - a stationary magnet
called a field magnet and a cylindrical, movable
electromagnet called an armature.

The armature is on an axle and rotates in the magnetic field
of the field magnet.

The axle is used to do work.
Electromagnetic Induction
 Induced Current:
 If a loop of wire is moved in a magnetic field a voltage is
induced in the wire.
 The voltage is called an induced voltage and the resulting
current is called an induced current.
 The induction is called electromagnetic induction.
A current is induced in a
coil of wire moved
through a magnetic field.
The direction of the
current depends on the
direction of motion.
The magnitude of the induced voltage is proportional to:

The number of wire loops cutting across the magnetic
field lines.

The strength of the magnetic field.

The rate at which magnetic field lines are cut by the
wire.
 Applications:
 DC and AC Generators,
 Transformers (step-up and step-down).
Ampere’s Law:
Electric currents
create
magnetic fields.
Lorentz Force:
Charges moving
in a magnetic field
experience an
electromagnetic force.
Faraday’s Law of Induction:
A changing magnetic field
creates an electric field.
Lenz’s Law:
Induced electric currents
act so as to oppose the
motion that caused them.
Prepared by:
SAVNEET KAUR
XII SCIENCE
FAITH ACADEMY, NEW DELHI-5. YEAR: 2008-09