Transcript Electricty and Magnetism

Electricity
All of us agree the importance of electricity in our daily
lives.
But what is electricity?
Electric Charge
• 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).
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.
– 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.
(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).
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.
• 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.
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.
– 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.
• 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
the charge carried by 6.24 x 1018 electrons.
– 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=ne
where e is the fundamental charge.
• Coulomb’s 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 Coulomb’s law.
Mathematically,
F k
q1 q 2
d
2
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.
• 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.
– 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.
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.
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.
• 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.
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.
Electric Current
• 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.
• The Electric Circuit:
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
follow.
– 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).
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.
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.
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.
Resistors in Series
• Resistors can be connected in series; that is, the current
flows through them one after another. The circuit in
Figure 1 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
Resistors in Series
• resistance for resistors connected in series.
• 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 ciruit. This is known as a parallel
connection. Each of the three resistors in Figure 1 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
Resistors in Parallel
• I = I1 +I2 + I3
• By Ohm's Law, this is equivalent to:
•
Resistors in Parallel
• we see that all the voltages are equal. So the
V's cancel out, and we are left with
• 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
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.
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.
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).