Download Chapter 2 Basic Chemistry

Chapter 17
Electricity
Electric Charge and Force
• Electric charge: an electrical property of
matter that creates electric and magnetic
forces
• An object has a negative charge, a positive
charge, or no charge at all
• Like charges repel, and opposite charges
attract
Electric Charge and Force
• Rub a balloon back and forth across your
hair
– The balloon will be attracted to your hair and
vice-versa
• Two balloons will repel each other
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
Electric charge
• Electric charge depends on an imbalance of
protons and electrons
– The difference in the number of protons and
electrons determines an object’s electric charge
• Negatively charged objects have more electrons than
protons
• Positively charged objects have fewer electrons than
protons
Electric Charge and Force
• The SI unit of electric charge is the coulomb
(C)
– Proton has a charge of: +1.6 x 10-19C
– Electron has a charge of: -1.6 x 10-19C
• An object with a charge of -1.0 has 6.25
1018 excess electrons
– 1.6 – 1.0 = 0.6 x 1019 = 6 x 1018 electrons
Electric Charge and Force
• Transfer of charge:
– When materials are rubbed together, electrons
can be transferred from one material to other
• Conductors allow charges to flow:
– Copper, aluminum
• Insulators do not allow charges to flow:
– Paper, cardboard, glass, plastic
Electric Charge and Force
• Charges can move within uncharged objects
without changing the overall charge of the
material
– Induction: redistribution of charge without
contact
– Contact: redistribution of charge with contact
– Friction redistribution of charge with rubbing
– Surface charge can be created on an insulator
• Charged comb and paper / water a charged balloon
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
Electric Charge and Force
• Electric field lines never cross one another
Current
• Just as a ball will roll downhill, a negative
charge will move away from another
negative charge
• Electrical potential energy is the ability to
move an electric charge from one point to
another
Current
• Potential difference is measured in volts
– 1 volt (V) is equivalent to 1 joule per coulomb
(1 J/C)
• J = Nxm / W = F x d
Current
• There is voltage across the terminals of a battery:
potential difference between the positive and
negative terminals
– Example: 1.5V small battery / 12V car battery
– Most batteries are electrochemical cells
• Convert chemical energy into electrical energy
• Contains an electrolyte (solution that conducts electricity) and
two electrodes (each a different conducting material: lead/lead
alloy/zinc)
– Dry cell: paste
– Wet cell: liquid
Current
Current
• The chemical reactions in the battery causes a
build up of electrons at the anode. This results in
an electrical difference between the anode and the
cathode. You can think of this difference as an
unstable build-up of the electrons. The electrons
wants to rearrange themselves to get rid of this
difference. But they do this in a certain way.
Electrons repel each other and try to go to a place
with fewer electrons.
Current
• In a battery, the only place to go is to the cathode.
But, the electrolyte keeps the electrons from going
straight from the anode to the cathode within the
battery. When the circuit is closed (a wire connects
the cathode and the anode) the electrons will be
able to get to the cathode. In the picture above, the
electrons go through the wire, lighting the light
bulb along the way. This is one way of describing
how electrical potential causes electrons to flow
through the circuit.
Current
• However, these electrochemical processes
change the chemicals in anode and cathode
to make them stop supplying electrons. So
there is a limited amount of power available
in a battery.
Current
• When you recharge a battery, you change
the direction of the flow of electrons using
another power source, such as solar panels.
The electrochemical processes happen in
reverse, and the anode and cathode are
restored to their original state and can again
provide full power.
Current
Current
• Lead–acid storage battery, 2 V. (usually 6 in series to give 12V
supply).
• The electrodes are initially hard lead–antimony alloy plates coated in a
paste of lead(II) sulphate encased in dilute sulphuric acid. During the
first charging some of the lead(II) sulphate is reduced lead(0) on one of
the electrodes (this will acts as the (–) anode in discharging).
Simultaneously in charging, lead(II) sulphate is oxidised to lead(IV)
oxide on the other electrode which acts as the cathode (+) in
discharging.
• Pb(s)|PbSO4(s)|H+(aq),HSO4–(aq)||PbO2(s)|PbSO4(s)|Pb(s)
– Note the standard conventions in common use
• the | notation indicates a phase boundary, solutes in the same phase are
separated by a comma
– the || notation 'divides' the two half cells and
» the oxidation state increases 'towards' it
Current
• (–) anode discharging reaction (i) Pb(s) + HSO4–(aq)
==> PbSO4(s) + H+(aq) + 2e–
• (+) cathode discharging reaction (ii) PbO2(s) + 3H+(aq)
+ HSO4–(aq) + 2e– ==> PbSO4(s) + 2H2O(l)
• working cell reaction (iii) PbO2(s) + 2H+(aq) + 2HSO4–
(aq) + Pb(s) ==> 2PbSO4(s) + 2H2O(l)
• oxidation state changes: (i) oxidation Pb(0) ==> Pb(II) :
(ii) reduction Pb(IV) ==> Pb(II)
• The charging reactions will be the opposite of (i) and (ii)
Current
• Advantages: Inexpensive, high power density (can car
starter motor as well as lights), long shelf life, readily
recharges, so has a long working life of many years.
• Disadvantages: Lead needs to be recycled to avoid
environmental contamination, sometimes generates
hydrogen gas at the cathode when charging (explosive in
air + spark) – though batteries seem to be made of a high
standard these days in completely sealed units that last
many years.
• Uses: Car batteries.
Current
• - The cell has one plate made of lead and another plate
made of lead dioxide, with a strong sulfuric acid
electrolyte in which the plates are immersed.
- Lead combines with SO4 (sulfate) to create PbSO4 (lead
sulfate), plus one electron.
- Lead dioxide, hydrogen ions and SO4 ions, plus electrons
from the lead plate, create PbSO4 and water on the lead
dioxide plate.
- As the battery discharges, both plates build up PbSO4
and water builds up in the acid. The characteristic voltage
is about 2 volts per cell, so by combining six cells you get a
12-volt battery.
Current
• A lead-acid battery has a nice feature — the
reaction is completely reversible. If you
apply current to the battery at the right
voltage, lead and lead dioxide form again
on the plates so you can reuse the battery
over and over.
Current
• Most car batteries constitute six galvanic
cells laid out in series. Each cell delivers 2.1
volts of electromotive force that, when
combined, produce a 12. volt battery
(commonly advertised as 12 volt)
– Series: one dies: battery dies
Current
• A galvanic cell, or voltaic cell, named after Luigi Galvani,
or Alessandro Volta respectively, is an electrochemical cell
that derives electrical energy from spontaneous redox
reactions taking place within the cell. It generally consists
of two different metals connected by a salt bridge, or
individual half-cells separated by a porous membrane.
• Volta was the inventor of the voltaic pile, the first electrical
battery. In common usage, the word "battery" has come to
include a single galvanic cell, but a battery properly
consists of multiple cells.
Current
Current
• A salt bridge, in electrochemistry, is a laboratory device
used to connect the oxidation and reduction half-cells of a
galvanic cell (voltaic cell), a type of electrochemical cell. It
maintains electrical neutrality within the , preventing the
cell rapidly running its reaction to equilibrium. If no salt
bridge were present, the solution in one half cell would
accumulate negative charge and the solution in other half
cell would accumulate positive charge as the reaction
proceeded, quickly preventing further reaction, and hence
production of electricity.[1]
• Salt bridges usually come in two types: glass tube and
filter paper.
Current
Current
• Each galvanic cell consists of a series of lead (Pb) and lead
dioxide (PbO2) plates which are submerged in an
electrolyte solution-a mix of sulfuric acid (35%) and water
(65%). This acidic bath triggers a reaction with the lead
dioxide plate (the positive electrode) which produces ions
and lead sulfate.
Current
• These sulfuric ions then react with adjacent lead plates
(the negative electrode) to produce hydrogen ions and lead
sulfate. This chemical reaction, in turn, generates electrons
that can flow out via conductive terminals as electricity to
power essential functions.
Current
Current
• So that is the reaction that is occurring in
each cell of the battery. For each atom of
lead converted to PbSO4 two electrons are
produced. These are absorbed by one piece
of PbO2 becoming PbSO4.
Current
• How Batteries Work: The Reverse Reaction:
Recharging the Battery
•
To obtain the reverse reactions all we need to do is turn the
arrows to face the opposite direction. If we do this we can
see that the PbSO4 is converted back into Pb on one plate
and PbO2 on the other. This is a forced reaction that can
only occur if extra electrons are pushed into the battery.
• The reaction is not permanent, with lead sulfate reforming
into lead dioxide and lead when the battery is recharged.
Overtime, the battery’s efficiency will deplete, with most
working optimally for about 4 years
Current
Current
• Resistance is caused by internal friction,
which slows the movement of charges
through a conducting material
• The resistance of the filament of a light bulb
determines how bright the bulb will be.
– The filament of a 40 W light bulb has a higher
resistance than the filament of a 100 W light
bulb
Current
• A 40 Watt light's filament (for example)
operates at much the same temperature as
the filament in a 100 Watt light. The
difference between them is that the 100
Watt light has a thicker filament and uses
more current. But their filament
temperatures are much the same, so their
rate of evaporation of their filaments is
similar.
Current
Current
• Ohm’s Law
• Resistance = voltage /
current
• R=V/I
• Cover the variable that
you require and
perform the resulting
calculation in the
triangle
Current
• The headlights of a typical car are powered
by a 12 volt battery. What is the resistance
of the headlights if they draw 3.0 A of
current when they are turned on?
• R = V / I = 12 V / 3 A = 4.0 ohm
Current
• Conductors have low resistance
• Insulators have high resistance
• Semiconductors conduct under certain
conditions
– Electrical properties between those of an
insulator and a conductor
– Naturally insulator but when specific materials
are added their ability to conduct increases
Current
Current
• The atoms in a semiconductor are materials from either
group IV of the periodic table, or from a combination of
group III and group V (called III-V semiconductors), or of
combinations from group II and group VI (called II-VI
semiconductors). Because different semiconductors are
made up of elements from different groups in the periodic
table, properties vary between semiconductors. Silicon,
which is a group IV, is the most commonly used
semiconductor material as it forms the basis for integrated
circuit (IC) chips and is the most mature technology and
most solar cells are also silicon based. A full periodic table
is given in the page Periodic Table. Several of the material
properties of silicon are given in the page Silicon Material
Parameters
Current
• Some materials can become superconductors
– Some metals and compounds have zero resistance when
their temperature falls below a critical temperature and
pressure
• Temperature/pressure depends on the material
– Usually very high or low temperatures
– Usually ambient (pressure of surrounding
material) or high pressure
– The search continues for superconductors at
room temperature
Current
Circuits
• Circuit: a closed-loop path
for electrons to follow
– Because charges are
moving, there is a current in
the circuit
• The conducting path
produced when a load,
such as a string of light
bulbs, is connected across
a source of voltage (outlet)
is called a closed circuit
Circuits
• Switches interrupt the flow of charges in a circuit
Circuits
• Schematic diagrams
are used to represent
circuits
Circuits
Circuits
Circuits
Circuits
• A resistor is a passive two-terminal electrical component that
implements electrical resistance as a circuit element. Resistors act to
reduce current flow, and, at the same time, act to lower voltage levels
within circuits. Resistors may have fixed resistances or variable
resistances, such as those found in thermistors, varistors, trimmers,
photoresistors, humistors, piezoresistors and potentiometers.
• The current through a resistor is in direct proportion to the voltage
across the resistor's terminals. This relationship is represented by
Ohm's law:
–
• where I is the current through the conductor in units of amperes, V is
the potential difference measured across the conductor in units of volts,
and R is the resistance of the conductor in units of ohms (symbol: Ω).
Circuits
Circuits
Circuits
• Series circuits have a single path for current
– Voltage is divided among the devices
– Current in each device is the same but the
resistance can be different
• Therefore, the voltage across each device can be
different
– If one element along the path is removed, the
circuit will not work
Circuits
Circuits
Circuits
• Parallel circuits have multiple paths for
current
– Voltage across each device is the same
– The current does not have to be the same
• The sum of the currents in all of the devices equals
the total current
• If one bulb is removed the charges would still move
through the other loop
Circuits
Circuits
Circuits
Circuits
• When a charge moves in a circuit, the charge loses energy.
Some of this energy is transformed into useful work, such
as the turning of a motor, and some is lost as heat.
• The rate at which electrical energy is changed to other
forms of energy is called electric power
• Power = current x voltage
• P = IV
• SI unit for power is the watt (W)
– 1 W = 1J./s
– 1kW = 1,000W
Circuits
• Electric companies measure energy in kilowatt-hours
– Charge for energy, not power used in the home
– One kilowatt-hour (kW x h) is the energy delivered in 1 h at the
rate of 1kW
• 1 kW x h = 3.6 x 106J
• Price could be 5-20 cents per kilowatt-hour
Circuits
Circuits
Circuits
Circuits
• When a hair dryer is plugged into a 120 V
outlet, the hair dryer has a 9.1 A current in
it. What is the hair dryer’s power rating?
• Power = current x voltage
• P = IV
• P = 9.1A x 120V
• P = 1.1 x 103W
Circuits
• High currents in overloaded circuits can cause fires
• Large power overloads may potentially destroy electrical
equipment, or in more serious cases, cause a fire. A fuse
and circuit breaker both serve to protect an overloaded
electrical circuit by interrupting the continuity, or the flow
of electricity. How they interrupt the flow of electricity is
very different, however. A fuse is made up of a piece of
metal that melts when overheated; a circuit breaker has an
internal switch mechanism that is tripped by an unsafe
surge of electricity. Fuses tend to be quicker to interrupt
the flow of power, but must be replaced after they melt,
while circuit breakers can usually simply be reset.
Circuits