Download Electric Charge

Unit D:
Electrical Principles
and Technologies
1.1 Static Electricity
• Benjamin Franklin (1706- 1790)
introduced the theory of positive and
negative charges
Atomic Theory and Electricity
• Protons (+) and neutrons (0) are contained in the nucleus
• Electrons (-) orbit around the nucleus
• atoms are neutral because there is a balance between the
number of positively charged protons and negatively
charged electrons.
Three Laws of electrical charges:
1. opposite charges are attracted to each other
2. like charges repel
3. charged objects attract neutral objects
(A) Initially, there are
equal numbers of
protons (+) and
electrons (-) on the
comb and hair. No
net charge.
(B) 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.
Try it out with a balloon . . .
• 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
Charge separation occurs when a
charged object is brought close to a
neutral object.
Charge Separation
The comb has become charged
by friction, acquiring an excess
of electrons.
(A) The paper 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 as the comb electrons repel the paper
electrons. 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.
• Static electricity happens when there
is an imbalance of electrons between
two objects.
• Electrical Discharge - the movement
of charges from one object to
another because of this imbalance.
Protons of a neutral object attract the
charged electrons of the other, and
the electrons “jump” to the neutral
object.
• Eg.When you feel a shock when
touching metal
• Lightning - lightning is a discharge
from the static electricity formed by
raindrops rubbing against one another
Van De Graaff Generators (VDGs)
• A rubber belt rubs on a
piece of metal, acquiring a
negative charge by friction
building up electrons
- the negative charges are
then transferred to the
sphere.
• When you touch the sphere, the
negative charge builds up on you. Your
hair stands up because the like charges
in your hair repel.
• Van Der Graaf Demo
• P 277 Static Demo
• Now do Check and reflect P.278 #1-9
1.2 Current Electricity
– Electric current is a steady flow of charged
particles(electrons) 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 difference between static and current electricity is
that current is the continuous flow of electrons, while
static is simply the rapid discharge of electrons.
• Amperes (A):
– Often called “amp” for short
– Measures the rate at which electrical
current flows
• A simple electric circuit has a voltage source
(generator or battery) that maintains the electrical
potential, a device (lamp, motor, etc.) where work is
done, and a continuous pathways for the current to
follow( wires).
Every Electric Circuit Must Contain At
Least Three Elements:
1. Energy source: A voltage source of electricity
(battery/cell)
2. Conductor: for transmitting electrons
(eg. copper wire)
3. a load or form of resistance: resistance can
come in many forms. It might be a light bulb,
an electric motor or resistor.
There is also often a switch to control current
flow.
Elements
• When a circuit is closed
electrons leave the
negative side of the
battery and will flow
towards the positive
side. The flow of
electrons past a fixed
point is called a
“current” and is
measured in amperes
or amps
of a Circuit
• Voltage is the force
that pushes
electrons through
the circuit. The
greater the voltage
and current, the
more work that a
circuit can do.
Current, Resistance and Voltage can be compared to water
travelling through a garden hose.
• Voltage (V):
– A measure of the potential difference
between two places in a circuit.
– In other words, voltage tells us how much
energy each charged particle carries.
– The higher the voltage, the more energy
each particle has.
• For safety purposes, the voltage of most
household appliances is relatively low.
• The voltage of industrial devices and
transmission lines is relatively high.
• Do p279 Electrical Current Lab
• Sketch # 4 and 5
1.2
• Do page 283 1-9
1.3 Electrical Safety
Coming in contact with a power transmission
line can prove to be deadly. By touching it, a
short circuit can occur, because the
electricity is trying to find a path to the
ground - you can complete the circuit, but it
may be fatal.
The Dangers of Electrical Shock
High voltage power lines carry 50,000V of electricity.
However, amperage is more important to consider.
• 0.001A will likely not be felt at all, 0.015A to 0.020A will
cause a painful shock and loss of muscle control (which
means you will not be able to let go of the line).
• Current as low as 0.1A can be fatal.
• Electrical Dangers vary, depending on the situation. When
the current can flow easily, it is more dangerous.
• Insulators (such as wood, rubber and air) hamper the flow
of electricity, but moisture is a good conductor of electricity,
so avoid water when working with electricity.
Protecting Yourself From Electrical Shock
The Canadian Standards Council issues
labels to identify the amount of voltage
required to operate electrical devices and the
maximum current they use.
Electrical Safety Pointers...
•Never handle electrical devices if you are wet or near water
•Don't use devices that have a frayed or exposed power cord
•Always unplug an electrical device before disassembling it
•Don't put anything into an electrical outlet - except a proper
plug for an electrical device
•Don't overload an electrical circuit, by trying to operate too
many devices at once
•Avoid power lines
•Don't bypass safety precautions when you are in a hurry
•Pull on the plug, not the wire
•Never remove the third prong from a 3 prong plug
Plugs, Fuses and Breakers
The third prong of a 3 prong plug is a ground wire,
connected to the ground wire of the building, in case of
a short circuit.
Fuses and circuit breakers interrupt a circuit when
there is too much current flowing through it.
Fuses contain a thin piece of metal, which is designed
to melt if the current is too high.
Circuit breakers, on the other hand, trip a spring
mechanism, which shuts off the flow of electricity
through the circuit, when there is too much current. It
can be reused over and over (provided the cause of the
increased flow is corrected).
The Danger of Lightning
A lightning strike can have
30,000A - more than enough to
kill you. Avoid being the target
of a lightning strike by staying
low to the ground (horizon) and
away from trees.
• Lightning can also do a lot of
damage to a building. Metal
lightning rods, which are
connected to the ground with a
grounding wire, are fixed on
the roof of many buildings to
prevent damage to the building
during an electrical storm.
• Do Check and Reflect P.287 #1-9
1.4 Cells and Batteries
• An electrochemical cell supplies a
steady current.
• The electricity comes from chemical
reactions within the cell.
Dry Cells
Electricity-producing cells referred to
dry cells are 'dry' because the
chemicals used are in a paste.
Volatic Cell Animation
The chemical reaction in a cell releases
free electrons which travel from the
negative terminal of the cell, through
the device which uses the electricity,
and back to the positive terminal of the
cell.
Dry cells are made up of two different
metals, called electrodes in an
electrolyte paste.
An electrolyte is a paste or liquid
that conducts electricity because it
contains chemicals that form ions.
The electrolyte reacts with the
electrodes, making one electrode
positive and the other negative.
These electrodes are connected to
the terminals.
Dry Cell
Wet Cells
These are called 'wet' because the electrolyte is a
liquid (usually an acid). In a car battery, the
electrolyte is sulfuric acid.
Electrons travel from the negative electrode (zinc)
through the device and on to the positive electrode
(copper).
Rechargeable Cells
Non-rechargeable cells are called
primary cells.
The chemical reactions in a secondary
cell (rechargeable) can be reversed by
using an external electrical source to
rejuvenate the cell.
The cheapest and most common
secondary cells are Nickel Oxide and
Cadmium (Ni-Cad). The reactants are
restored, but the electrodes wear out
over time.
• Batteries
Connecting cells together creates a battery, which is
a sealed case with only two terminals.
• Electrochemistry
Allesandro Volta made the first practical battery
around 1800, by piling zinc and copper plates on top
of each other, separating them with electrolytesoaked paper discs. Humphrey Davy filled an entire
room with 2000 cells to make one massive battery.
His work led to a whole new field of science called
electrochemistry, the study of chemical reactions
involving electricity.
Electrolysis:electricity is used to split molecules
into their elements.
Many industries use electrolysis to separate
useful elements from solutions.
Electrolysis of Water
Electroplating: Silver and Gold
plating can make items look
expensive.
A thin coating is used to cover (and
often strengthen) the material.
This is often used to protect the
metal from corrosion.
Other Electrochemical Applications
Anodizing and Electrorefining are other examples
of electrochemical processes
• Anodizing is a process that coats aluminum parts
with a layer of aluminum oxide, which is much
harder than aluminum. It is used in products such
as screen doors, airplanes, car parts, kitchenware
and jewellery.
• Electrorefining is used to remove impurities from
metal. Another process used by automobile
companies bonds special paints onto car parts.
• Do Check and Reflect P.294 # 1-10
• And Section Review P.295 #1-11