Transcript Electricity & Magnetism

ELECTRIC FIELD DIRECTIONS
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.
FIELD LINES
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.
POTENTIAL DIFFERENCE
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
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.
• 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.
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 (+).
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.
– 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.
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 ().
OHM’S LAW
The relationship between voltage, current, and
resistance is:
V=IR
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 crosssectional area of the conductor, the material the conductor
is made of, and the temperature of the conductor.
ELECTRIC POWER AND 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.
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.
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.