Download electrical current

ELECTRICITY!!
• Electricity
– From the word “Elektron”
– Greek for “amber”
– Electricity is simply the flow of electrons.
“Laws of Attraction”
• Opposite Charges Attract
+
-
• Like Charges Repel
+
+
If you remove the 2 batteries from
a working flashlight and
reinstall them backward so that
they make good contact inside,
will the flashlight still work?
Question on batteries
Question:
Observations About Flashlights
(and electrical circuits)
Observations about flashlights
•
•
•
•
They turn on and off with a switch
More batteries usually means brighter
The orientation of multiple batteries matters
Flashlights dim as batteries age
For a functioning battery we need:
•
•
•
•
battery,
switch,
light bulb,
wire.
In a flashlight we are
creating an:
Electrical
circuit
• An electrical current (electrons)
runs through all the parts of the
circuit (close circuit).
• No current flows when switch is
open (open circuit).
• Electrons carry energy from
batteries to the bulb.
• Short circuit: A path (short cut)
in which the light bulb is cut out.
A Battery
Battery
• Battery “pumps” charge from + end to – end
– Chemical potential energy is consumed
– Electrostatic potential energy is produced
• Current undergoes a rise in voltage
– Alkaline cell: 1.5 volt rise
– Lead-acid cell: 2.0 volt rise
– Lithium cell: 3.0 volt rise
• Chain of cells produces larger voltage rise
A Light Bulb
A Light Bulb
• Structure
– Contains a protected tungsten filament
– Filament conducts electricity, but poorly
• Filament barely lets charge flow through it
– Electrostatic potential energy (voltage) is consumed
– Thermal energy is produced
• Current undergoes a drop in voltage
– Two-cell alkaline flashlight: 3.0 volt drop
Electric Current
Electric Current
Water Analogy
Water Analogy
h
water flow
V
I
Current: number of electrons passing through per second
Water analogy: number of water molecules passing through per second
Electric current --> water
I
What determines the current through the circuit (Load)?
Ohm’s Law
V = I·R or
I = V/R
R
V
So, 10V across a 100ohm load = 0.1 Amp
Where 1 Amp = 1 coulomb/sec = 6.25 x 1018 e/sec
1Amp=62,500,000,000,000,000,000 electrons/sec
I
Examples
examples
1. A battery can produce 1.5
V. When connected to a
light bulb a current of 2 A
(Ampere) runs through the
bulb. What is the
resistance of the bulb?
2. A bulb in a lamp that is connected to a household outlet has a
resistance of 100 ohms. What current flows through it?
3. Your skin has a resistance of about 106 to 104 ohms (dry) and
103 ohms (wet) . What current runs through you when you
stick your finger in an outlet (conduction to ground)?
Electric shock
Electric shock
• The severity of an electric shock depends on the magnitude of
the current, how long it acts and through what part of the body it
passes.
• Can feel ~ 1 mA; pain at a few mA; severe contractions above
10 mA; heart muscle irregularities above 70 mA.
• Resistance of dry skin ~ 104 to 106 W; wet skin 103 W or less.
• A person in good contact with ground who touches a 120 V line
120V
I
 120mA
with wet hands can suffer a current
1000
A word about the sign convention….
Positive Charge
Positive Charge
• Current points in the direction of positive flow
• Flow is really negative charges (electrons)
• It’s hard to distinguish between:
– negative charge flowing to the right
– positive charge flowing to the left
• We pretend that current is flow of + charges
• It’s really – charges flowing the other way
Electrical Resistance
• So, we hook up a wire between the
terminals on a battery
• The question is, how much current flows?
• We have lots of free electrons able to move
in the metal composing the wire
• So, how much charge moves past any point
in the wire per second?
Electrical Resistance
• The amount of current that flows is
determined by how much resistance there is
to the flow of the charges
• What makes this resistance?
• Collisions of the electrons with each other
and with the “stationary” atoms of the metal
• Atoms are actually vibrating in the lattice
Electrical Resistance
• Resistance depends on the structure of the
material and the temperature
• The higher the temperature, the more the
atoms vibrate and the more electrons make
collisions with the atoms
• How much current flows was discovered by
George Ohm, and we honor him by naming
the unit of resistance an ohm
Ohm’s Law
• Ohm’s discovery was that current is
proportional to voltage and inversely
proportional to resistance
voltage
current 
resistance
volts
amperes 
ohms
Ohm’s Law
• Three forms are convenient
V
I
R
V  IR
V
R
I
Electric Shock
• We all know not to stick our finger into a
light bulb socket or to drop an electrical
appliance into our bath tub
• What causes the damage to our bodies?
• It is the amount of current that flows
through the body that can cause problems
• Human skin has resistance ranging from
100 ohms to 500,000 ohms (wet to dry)
Electric Shock
• We can use Ohm’s Law to calculate current
based on the size of the applied voltage
• You can feel 0.001 ampere (1 milliampere)
• 0.005 amperes hurts
• 0.010 amperes causes muscle spasms
• 0.015 amperes loss of muscle control
• 0.070 amperes disrupts heart rythyms (fatal)
Electric Shock
• There must be a potential difference
between one part of your body and another
• You must become a conductor of electricity
• OK for birds to sit on a 5000 V transmission
line as long as no part of the bird touches
something else
• Squirrels get across the transformers
Power
•
•
•
•
Power is energy per unit of time
Power is measured in joules/second or watts
Batteries are power sources
Loads are power consumers
Battery Power
power produced by the battery
• Current: units of charge pumped per second
• Voltage rise: energy given per unit of charge
current · voltage rise = power produced
P = Vrise·I
Vrise
I
Load Power
• Current is units of charge passed per second
• Voltage drop: energy taken per unit of charge
current · voltage drop = power received
Vdrop
P = Vdrop·I
I
Electrodynamics
Electrostatics
the study of electrical
the study of electrical
charges in motion
charges at rest
Two opposite types of charge exist, named
positive and negative by Benjamin Franklin.
charges
Charge is a
property of
matter.
Charged particles exist in atoms.
Electrons are responsible
for negative charge;
protons for positive charge;
neutrons have no charge.
ONLY ELECTRONS MOVE!!!!
Small amounts of ordinary matter contain
incredible amounts of subatomic particles!
When charged particles near one another,
They give rise to two different forces.
Electrostatic force
A force is a push or a pull on an object.
Basic Law of Electrostatics
Like charges repel; unlike charges attract
Link
An electric field extends outward through space from
every charged particle. When a charged particle moves
into the electric field of another, it is either pushed
or pulled.
Field lines point away from positive
and toward negative charges.
field
Static Electricity: The buildup of
electric charges on an object.
static
A Neutral object gains an electric charge when it gains or
loses electrons.
Three Methods of Charging an Object
1. Friction - Rubbing two objects together
2. Conduction-touching two objects together.
3. Induction-rearrangement of charges on the object.
Charging objects by friction.
Example: Charging a balloon by rubbing it on your hair and
then sticking it to a neutral wall.
Conduction-Touching of Objects
Certain Materials permit electrical charges to flow freely through them.
Conductors: Metals, Earth
Semi-Conductors: Silicon, Germanium
Non-Conductors: rubber, glass, wood,
plastic
INDUCTION
charging without touching of objects
charging a rod and electroscope
positively and negatively
by conduction and induction
electroscope
When charging by conduction,
the rod touches the electroscope.
The electroscope gets the same charge as the rod.
When charging by induction, the rod does not
touch the electroscope. The electroscope gets
the opposite charge of the rod.
Go here , here, here, and here
to view simulations of charging
links
an electroscope. Read more here.
COULOMB’S LAW
The force between two charged objects is
directly proportional to the product of their
charges and inversely proportional to their
separation distance squared.
link1, link2, link3, link4, link4
In equation form:
q
q
F = k 12 2
d
F is the force of attraction, measured in NEWTONS,
between charges q1 and q2
k is the Universal Electrostatic Constant, equal to
9.00 x 109 N m2/coul2
q1 and q2 are the attracting charges, measured in
Coulombs
d is the distance between the charges,
and is measured in METERS
The SI unit of charge
is the Coulomb,
named in honor of
Charles Augustin Coulomb.
1 C = charge on 6.25 x 1018 electrons (or protons)
1 e- = 1.60 x 10-19 Coul = elementary charge
Electric force is a vector and must be treated as such.
Electric Fields
An electric field exists in a region
if space if a charge placed in that
region experiences an electric force.
The magnitude of an electric field at any
given point is defined to be the ratio of
the force on a
charge at that
point to the
amount of charge.
E = F/Q
Electric field strength has units
of Newtons/Coulomb (N/C).
Click here to view a simulation
showing the magnitude and direction of the
electric force on a test charge when
placed near other charges.
Click here to view a simulation of a
charged particle moving through a region
occupied by other charges.
Voltage Sources
• If we want to move charge from one place
to another, we must apply a force to make it
move
• Another way of thinking about this is to say
that we must give the charges some
potential energy
Voltage Sources
• We can give an object gravitational
potential energy by lifting it into the air
• We have to do work on the object to lift it
into the air
• Doing work uses energy
• Conservation of Energy
Voltage Sources
Voltage Sources
• We need an pump that pumps charges!
• The simplest charge pump is a battery
• It uses chemical reactions to separate
charges and thus create electrical potential
energy
• More convenient to talk about PE/charge or
electric potential measured in volts
• So, a battery is a kind of electric pump
Voltage Sources
• Another kind of source is a generator
• A generator converts mechanical energy
into electrical energy
• In any case, what we need is a device to
separate charges!!!
Electric Current
• The movement of charge is called a current
• Metals have some electrons that are not
needed to bond the atoms together in the
solid
• They are pretty free to just roam about the
material and are not associated with any
individual metal atom
Electric Current
• Compare to the flow of water in a hose
Electric Current
• We measure the flow of water in
gallons/minute
• We measure the flow of charges in
coulombs/sec
• 1 coulomb/sec = 1 ampere
• Remember this is 6.25 x 1018 electrons
moving past a point in a wire per second
Electric Current
• In a wire, the electrons actually move quite
slowly, less than 0.01 meters/sec
• However, their electric field moves at the
speed of light!!!
• So, we can send signals down a wire very
quickly, because the information moves at
the speed of the changing electric field
which is at the speed of light
Electric Current
Electron flow
+
-
Electric Current
• In the 1700’s people figured out that
charges could move
• They had two choices
– Positive charge moves
– Negative charge moves
• They guessed WRONG!!!!!
• We know electrons move
Electric Current
• Hook up a battery and electrons flow from
minus terminal to plus terminal
• Ancients defined current as positive charge
flow
• Direction of “current” is from plus to minus
Electric Potential Difference
the change in
electric potential energy
per unit charge
V = W/Q
The SI unit of electric potential
difference is the VOLT, named in
honor of Alessandro Volta.
One VOLT
is the electric potential difference
between two points when one Joule of work
is done in moving one Coulomb of charge
between the points.
current
the flow of
charged particles;
can be positive or negative,
but usually negative (electrons)
through a conducting metal
electric cell - a device that
converts one form of energy
to electrical energy
Chemical cells convert chemical
energy into electrical energy.
Chemical cells can be
“wet” or “dry”.
Solar cells convert light energy
into electrical energy.
A generator converts mechanical
energy into electrical energy.
battery - two or more cells
connected in series or in parallel
Electric current is measured
in Amperes, in honor of
Andre Marie Ampere.
One Ampere is the flow of
one Coulomb of charge per second.
1 Amp = 1 Coulomb per second
= 1 C/s
I = Q/t
Ammeter
a device that measures current
Voltmeter
a device that measures
electric potential difference
power = work/time
.
= (work/charge) (charge/time)
.
= electric potential difference current
P (Watts) = V (Volts).I (Amps)
Analogies of simple circuits are these links:
Water circuit analogy link
Air flow link
Various link
Teaching with Analogies link1, link2
Resistance
determines the amount of current flow
= the ratio of potential difference to current
V
The SI unit of
resistance is the
IOhm, W, named
in honor of Georg Simon Ohm.
R=
One Ohm of resistance is the resistance
such that one Volt of potential difference
is needed to obtain a current of one Amp.
The resistance of a circuit element depends on:
1. the length of the conductor
as length increases, resistance increases proportionally
2. the cross-sectional area of the conductor
as area increases, resistance decreases proportionally
3. the resistivity of the conductor
as resistivity increases, resistance increases proportionally
Resistivity
The resistivity, r, of a conductor is equal to the
resistance of a wire 1 cm long and having
a cross-sectional area of 1 cm2.
R =
r
l
A
R = resistance, measured in Ohms
r = resistivity, usually in units of W.cm
l = length, measured in cm
A = cross-sectional area, measured in cm2
Ohm’s Law
The ratio of potential difference to current
is constant.
If R = V/I is a constant value
for a given resistor, then that
resistor is said to obey Ohm’s Law.
Click here and here to link to pages describing resistor
color codes.
Many circuit elements do not
obey Ohm’s Law. Resistors
that get hot, like light bulbs
and heating elements, do not
keep a constant resistance.
Resistance generally increases as
objects become hotter.
Click here and here to run
simulations of Ohm’s Law.
Series
Resistor Circuits
Resistor circuits
1. total resistance is the sum of the
separate resistors
RT = R1 + R2 + R3 + ...
2. current is the same through each resistor
IT = I1 = I2 = I3 = ...
3. total potential difference is the sum of each
VT = V1 + V2 + V3 + ...
In other words, in a series circuit,
resistance and voltage add,
but current stays the same.
R,
W
E = 12 V
R1
R3
R2
R1
8.0
R2
2.0
R3
5.0
RT =
VT =
IT =
PT =
V,
V
I,
A
P,
W
R,
W
E = 12 V
R1
R3
R2
V,
V
I,
A
P,
W
R1
8.0 6.4 0.80 5.1
R2
2.0 1.6 0.80 1.3
R3
5.0 4.0 0.80 3.2
RT = 15 Ω
VT = 12 V
IT = 0.80 A
PT = 9.6 W
Parallel
1. reciprocal of the total resistance is the
sum of the reciprocals of the separate
resistors
1/RT = 1/R1 + 1/R2 +1/R3 + ...
2. total current is the sum of the current
through each resistor
IT = I1 + I2 + I3 + ...
3. potential difference is the same across
each resistor
VT = V1 = V2 = V3 = ...
In other words, in a parallel circuit,
resistance adds as reciprocals,
voltage stays the same, and current splits.
R,
W
E = 12 V
R1
R1
12
R2
8.0
R3
12
R2
R3
RT =
VT =
IT =
PT =
V,
V
I,
A
P,
W
R,
W
E = 12 V
R1
V,
V
I,
A
R1
12 12
R2
8.0 12 1.5 18
R3
12 12
1.0 12
1.0 12
R2
R3
P,
W
RT = 3.42 Ω
VT = 12 V
IT = 3.50 A
PT = 42 W
Kirchhoff’s Rules
Loop Rule: The sum of the potential differences
around any closed circuit loop is zero.
Junction Rule: The sum of the currents
into any circuit junction is zero.
Go to link1, link2, link3, link4, link5, and link6 to view
pages and simulations examining
Kirchhoff’s Loop and Junction Rules.
The site linked
here allows you
to build and test
your own series,
parallel, and/or
combination circuits.
For a complete interactive tutorial on
electricity and magnetism, go here.
POWER
THE RATE OF DOING WORK OR
CONVERTING ENERGY.
Power = Work / Time
Power = Energy / Time
UNIT- WATT = JOULE PER SECOND
POWER
Watt Demonstration:
Lift a 1-N mass a distance of 1 meter,
and you have done 1 Joule of Work.
Do this once each second, and your
power output is 1 Watt.
POWER
Light bulbs are rated by
the power used.
POWER
Power = Current × Voltage
What are the units for Current, Voltage, and Power?
Ampere = coulomb / second
Volt = joule / coulomb
POWER
What is a Volt X Ampere ?
Volt X Ampere = joule/coulomb X coulomb/second
Volt X Ampere = joule/second = watt
POWER
A PAIR OF LEMON BATTERIES
GENERATED ABOUT 0.5
MILLIWATTS
AN XMAS BULB REQUIRES
ABOUT 0.5 TO 1 WATT
POWER
ACTIVITY
HAND HELD
GENERATOR
Electromagnetism
21-1 Magnetism from Electricity
• Electromagnetism – relationship between magnetism and
electricity
• Oersted – (1820) Danish physicist who first discovered that an
electric current produces a magnetic field.
Example of Oersted’s Discovery
•
Solenoid – long coil of wire with a current passing through
wrapped into many loops creating a magnetic field. Acts like a
magnet when a current is passing through. Strongest at the ends –
like a magnet.
•
Increase the strength of the magnetic field:
1) increasing the amount of coils and/or
2) increasing the amount of current.
OR by adding magnetic material between the wires.
• An electro-magnet – solenoid with magnetic material (iron nail)
inside the coils. Greatly increases the strength.
Strong temporary magnets
• Just as an electric current exerts a force on a magnet. A
magnetic field exerts a force on an electric current (charged
particles)
• Practical devices: Electric motors –
•converts electrical energy into mechanical energy. Contains a
coil of wire connected to a cylinder (armature). Armature free
to spin between the poles of a magnet.
Galvanometers – instruments used to detect small
currents (coil of wire connected to an electric circuit and
a needle)
Examples: ammeters and voltmeters
21-2 Electricity from Magnetism
•Faraday – 1831 Englishman – and Henry an American – first to prove that electricity can be produced from magnetism.
•Electromagnetic Induction - Faraday noticed that changing the magnetic field produced an electrical current
– induced current.
Faraday’s experiment
Another experiment illustrating
electromagnetic induction
• An electric current will be produced (induced) in a circuit exposed to a
changing magnetic field.
• The direction of the current depends on the direction of the moving
magnet.
•Generators – converts mechanical energy into electrical
energy – opposite of a motor (electrical energy into mechanical
energy)
•Consist of loops of wire connected to a rotating rod.
AC generator
DC generator
• In large generators turbines provide the mechanical energy to turn the axles.
Either moving wind, water or steam turn the turbines.
• Wind power, hydroelectric power, nuclear power or electricity from burning
of fossil fuels - coal and oil.
• Transformers – devices that increase or decrease the voltage of alternating current.
In US 60 hertz
• Consists of 2 coils wrapped around an iron core. AC current passes through first
coil, inducing a current in second wire.
• Step-up transformers – when primary coil has less loops than second coil. Voltage
in second coil greater than first coil – step-up transformer (increases voltage)
• Step-down transformers – more
loops in first coil, less in second –
voltage in secondary coil less.
•Power can be conserved if voltage is
stepped up before transmission and
stepped down before being used by
consumer
DC and AC
• Hook up a battery and we have direct
current, i.e. current flows ALWAYS in one
direction
• Modern generators in power plants reverse
the polarity of the output terminals 60 times
per second
• This is called alternating current
• It is what comes out of the wall plug
DC and AC