Electricity

1

• Electricity

2
All of us agree the importance of electricity in
our daily lives. But
what is electricity?
3

• Electric Charge

4

• 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).

5
A very highly simplified model of an atom has
most of the mass in a small, dense center called
the nucleus. The nucleus has positively charged
protons and neutral neutrons. Negatively charged
electrons move around the nucleus at much
greater distance. Ordinary atoms are neutral
because there is a balance between the number of
positively charged protons and negatively charged
electrons.
6

• 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.

7
(A) A neutral atom has no net charge because the
numbers of electrons and protons are balanced.
(B) Removing an electron produces a net positive
charge the charged atom is called a positive ion
(cation). (C) The addition of an electron
produces a net negative charge and a negative ion
(anion).
8
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.
9

• 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.

10
Charging by induction The comb has become
charged by friction, acquiring an excess of
electrons. The paper (A) normally has a random
distribution of () and (-) charges. (B) When
the charged comb is held close to the paper,
there is a reorientation of charges because of
the repulsion of the charges. This leaves a net
positive charge on the side close to the comb,
and since unlike charges attract, the paper is
attracted to the comb.
11

• Electrical Conductors and Insulators
• Electrical conductors are materials that can move
  electrons easily.
• Good conductors include metals. Copper is the
  best electrical conductor.
• Electrical nonconductors (insulators) are
  materials that do not move electrons easily.
• Examples are wood, rubber etc.
• Semiconductors are materials that sometimes
  behave as conductors and sometimes behave as
  insulators.
• Examples are silicon, arsenic, germanium.

12

• Measuring Electrical Charges
• The fundamental charge is the electrical charge
  on an electron and has a magnitude of 1.6021892 X
  10-19 C (Note that the electrical charge is
  measured in coulombs).
• A coulomb is the charge resulting from the
  transfer of 6.24 x 1018 of the charge carried by
  an electron.
• The magnitude of an electrical charge (q) is
  dependent upon how many electrons (n) have been
  moved to it or away from it.Mathematically,
• q n e
• where e is the fundamental charge.

13

• Coulombs law
• Electrical force is proportional to the
  product of the electrical charge and inversely
  proportional to the square of the distance. This
  is known as Coulombs law.Mathematically,
• where,
• F is the 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.

14

• Force Fields
• The condition of space around an object is
  changed by the presence of an electrical charge.
• The electrical charge produces a force field,
  that is called an electrical field since it is
  produced by electrical charge.

15

• A map of the electrical field can be made 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.

16
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.
17
Lines of force diagrams for (A) a negative charge
and (B) a positive charge when the charges have
the same magnitude as the test charge.
18

• 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.

19

• 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, PDW
• 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.

20
The falling water can do work in turning the
water wheel only as long as the pump maintains
the potential difference between the upper and
lower reservoirs.
21

• Electric Current

22

• Introduction
• Electric current means a flow of charge in the
  same way that a water current flows.
• It is the charge that flows, and the current is
  defined as the flow of the charge.

23

• The Electric CircuitAn 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.

24
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 follow.
25

• 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).

26
A simple electric circuit carrying a current of
1.00 coulomb per second through a cross section
of a conductor has a current of 1.00 amp.
27

• The Nature of Current
• Conventional current describes current as
  positive charges that flow from the positive to
  the negative terminal of a battery.
• The electron current description is the opposite
  of the conventional current.
• The electron current describes current as a drift
  of negative charges that flow from the negative
  to the positive terminal of a battery.
• It is actually the electron current that moves
  charges.

28
A conventional current describes positive charges
moving from the positive terminal () to the
negative terminal (-). An electron current
describes negative charges (-) moving from the
negative terminal (-) to the positive terminal
().
29

• The current that occurs when there is a voltage
  depends on
• The number of electrons that are moved through
  the unit volume of the conducting material.
• The fundamental charge on each electron.
• The drift velocity which depends on the
  properties of the conducting material and the
  temperature.
• The cross-sectional area of the conducting wire.

30

• It is the electron field, and not the electrons,
  which does the work.
• It is the electric field that accelerates
  electrons that are already in the conducting
  material.
• It is important to understand that
• An electric potential difference establishes, at
  nearly the speed of light, an electric field
  throughout a circuit.
• The field causes a net motion that constitutes a
  flow of charge.
• The average velocity of the electrons moving as a
  current is very slow, even thought he electric
  field that moves them travels with a speed close
  to the speed of light.

31
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.
32
(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.
33

• 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 (?).

34

• 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.

35
The four factors that influence the resistance of
an electrical conductor are the length of the
conductor, the cross-sectional area of the
conductor, the material the conductor is made of,
and the temperature of the conductor.
36

• 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 I V

37
What do you suppose it would cost to run each of
these appliances for one hour? (A) This light
bulb is designed to operate on a potential
difference of 120 volts and will do work at the
rate of 100 W. (B) The finishing sander does
work at the rate of 1.6 amp x 120 volts or 192 W.
(C) The garden shredder does work at the rate of
8 amps x 120 volts, or 960 W.
38
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.
39

• Magnetism

40

• 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.

41
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.
42

• 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.

43
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.
44

• 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.

45

• Our Earth is a big magnet.
• The Earths 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 Earths
  magnetic field.

46
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.
47
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.
48

• Electric Currents
• and
• Magnetism

49
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.
50
(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.
51
A magnetic compass shows the presence and
direction of the magnetic field around a straight
length of current-carrying wire.
52
Use (A) a right-hand rule of thumb to determine
the direction of a magnetic field around a
conventional current and (B) a left-hand rule of
thumb to determine the direction of a magnetic
field around an electron current.
53
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.
54

• 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.

55
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.
56

• 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.

57

• Electromagnetic Induction

58

• 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.

59

• 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).