Download on Electrons

I. Electric Charge and Force
A. Electric Charge
1. Type of Charges
a. positive
b. negative
c. neutral (lack of charge)
2. Source of charge: The Atom
All matter is made up of atoms. Each atom contain a central "nucleus".
The nucleus contains protons and neutrons, and the nucleus is considered to be
at rest. Electrons move around the nucleus in the empty space of the atom.
In a nuetral atom, the number of protons equal the number of electrons
a. Atomic Structure
Protons and electrons have equal and opposite electric charge. By convention,
electrons are said to have a negative charge. Similarly, protons are said to have a
positive charge. Neutrons have no charge.
Atoms are electrically neutral...not because they contain no overall charge...they have
equal numbers of protons and electrons...their total charge adds up to zero.
Electric Forces
•
•
•
•
•
•
•
Like charges repel, unlike charges attract
Two electrons will tend to repel each other because both have a negative electrical
charge.
Two protons will also tend to repel each other because they both have a positive
charge.
Electrons and protons will be attracted to each other because of their unlike
charges.
Since the electron is much smaller and lighter than a proton, when they are
attracted to each other due to their unlike charges, the electron usually does most
of the moving.
This is because the protons have more mass and are harder to get moving.
Although electrons are very small, their negative electrical charges are still quite
strong.
Another important fact about the electrical charges of protons and electrons is
that the farther away they are from each other, the less force their electric fields
have on each other. Similarly, the closer they are to each other, the more force
they will experience from each other due to this invisible force field called an
electric field.
Like energy and momentum, charge is neither created nor destroyed, it is conserved.
Opposite charges attract and like charges repel. As a result negatively charged
electrons are attracted to the positive nucleus
An electron is said to have a charge of - and
a proton a charge of +.
An atom in it's normal state has no
charge. This is due to the fact that atoms
A
have only neutrons
B
have no protons or electrons
C
have equal numbers of protons and electrons
answer
1
A
Electron
B
Neutron
C
Proton
D
Nucleus
answer
What object moves freely around the
atom?
2
What symbol is used for the charge on a proton?
A
+
B
-
C
o
answer
3
What the atom DOESNT look like:
This is NOT what an atom looks like!!!
If an atom was magnified so that the nucleus
was the size of a dime on the 50 yard line of a
football field, the first electrons would be at
the edge of the stadium…nothing exists in
between. The remaining electrons could be
miles away (depending on the element/size of
the atom)
And the electrons would STILL be too small to
see. Atoms are almost all empty space.
Since everything (including us) is made of atoms, that means everything (including
us) is mostly empty space.
More on Electrons
The nuclei of atoms are much more massive than electrons. Each proton
or neutron is 2000 times more massive than an electron with each
nucleus containing at least one proton.
That's one reason that when electric charge moves it's usually the result
of electrons moving, not protons.
The other reason is that in solids, the nuclei are locked together so they
can't move...regardless of their mass.
b. ions
Some atoms are charged because they have more protons or
electrons----When that happens----the atom gains a
charge and is called an “ion”
Ions are atoms with charges
b. ions
Some atoms are charged because they have more protons or
electrons----When that happens----the atom gains a
charge and is called an “ion”
Ions are atoms with charges
3. Build up of Charge
Solids are a form of matter whose nuclei form a fixed structure.
Nuclei, and their protons, are "locked" into position.
Some electrons are bound more tightly to their atoms than other electrons.
It depends on the make up of the material. For example, human hair easily
gives up electrons while plastic is good at “stealing” electrons.
Static electricity occurs when material obtains and holds onto a charge
(either positive or negative)
4. Movement of Charge
A nucleus, and the protons, are "locked" into position. Protons are never removed or added
to atoms (unless we are working in the field of nuclear physics—not this unit)
In solids….
Some materials have electrons that are bound more tightly to their atoms than other
electrons. In other materials, electrons are free to move.
In conductors, some electrons are free to move through the solid. In insulators, no electrons
are free to move
Current electricity is the movement of charges through a conductor in a particular direction
(like a flow of water)
Static electricity is the build up of charges on material without “flowing”
a. Conductors
In conductors, electrons move freely inside the solid. Like charges repel, therefore the
electrons tend to want to spread as far apart as possible.
b. Insulators
Insulators are materials that have strongly bound electrons which cannot move
within the solid. Different insulators have varying levels of insulation capabilities.
However, electrons can still be picked up or removed from the material (just can’t
move with in it)
c. electrolytes
Electrolytes are liquids that conduct electricity.
Most often, water has charged partilces dissolved in it. These charge can move
through the water to conduct.
Water itself is an insulator. (pure water does not conduct, tap water does)
II. Electrical Potential Difference
A. Movement of Charges
• Like charges repel and opposite charges attract
• Repulsion and attraction are forces that cause charges to move
• Force causing motion over a distance is known as ___________ which can be converted
into ___________.
• Which way will charges move?….
B. Electrical Potential
Placing a positive charge near a negative charge is a form of potential energy. They
want to move toward each other. It takes work to keep them apart.
Like charges desperately want to move away from each other. It takes work to keep
them together.
A force needs to separate opposite charges and a force needs to keep like charges
together.
Charges that are forces to be apart are together contain potential energy (think of
GPE or EPE).
The amount of potential energy per charge is called electrical potential
C. Potential Difference (voltage)
Two charges separate from each other will move.
Like charges move away from each other
Opposite charges move towards each others
The difference in the potential energy they have from before and after the movement is the
potential difference, which we call voltage
Charges are being forced to stay
close to each other
When free to move the charges will
separate and each have less potential
energy
They want to separate; right now
they have potential energy
When free to move the charges will
separate and each have less potential
energy
Each charge has high potential
energy
Each charge has less potential energy
The difference is potential energy for each charge before and after the move is
called Potential Difference, or Voltage and is measured in volts.
III. Current
A. Definition
Current is the movement of charges in a single path; it measures
the number of charges that flow past a point in a given time
(C/s—renamed amperes)
This requires a conducting path (a metal wires)
III. Current
B. Cause of current
1. Charges that are separated or put near each other have
electrical potential energy
2. Charges move through a wire to go from high potential energy
to low potential energy: which is voltage
III. Current
C. Factors Affecting Current
1. Resistance
a. Defined: resistance is hindrance to the flow of electricity
b. Factors affecting resistance
• Diameter of a wire (the greater the diameter, the lower
the resistivity)
• Length (the longer the wires, the more the resistance)
• Temperature (generally, higher temp is lower
conductivity)
• Type of material (varies)
III. Current
C. Factors Affecting Current
2. Ohm’s Law
Current
The letter "I" is the symbol for current. It is defined as the amount of charge
that flows past a location in a conductor per unit time.
Current flows through a conductor
It is measured in amperes (amps or A)
Voltage
The letter “V" is the symbol for voltage. It is defined as the is the difference in
electrical potential between two points in a circuit. It's the push or pressure
behind current flow through a circuit.
Batteries and outlets provide the force that pushes the electrons through the
conductor
It is measured in volts (V)
Batteries
D, C, AA, & AAA have the
same voltage, however they
differ in the amount of power
they deliver. (meaning how
fast)
9V
D
C
1.5 V
AAA
AA
For instance, D batteries can
deliver more current and
therefore more power.
Resistance
The letter “R" is the symbol for resistance. Resistance determines how much
current will flow through a component. Resistors are used to control voltage
and current levels. A very high resistance allows a small amount of current to
flow. A very low resistance allows a large amount of current to flow
Appliances are filled with resistors
It is measured in ohms (Ω)
What is Ohm's Law: Ohm's shows the relationship between electric voltage,
current and resistance.
What is voltage?
An analogy would be a huge water tank filled with thousands of gallons of water
high on a hill. The difference between the pressure of water in the tank and the
water that comes out of a pipe connected at the bottom leading to a faucet is
determined by the size of the pipe and the size of the outlet of the faucet. This
difference of pressure between the two can be thought of as potential Voltage.
What is current? An analogy would be the amount of flow determined by the
pressure (voltage) of the water thru the pipes leading to a faucet. The term current
refers to the quantity, volume or intensity of electrical flow, as opposed to voltage,
which refers to the force or "pressure" causing the current flow.
What is resistance? An analogy would be the size of the water pipes and the size of
the faucet. The larger the pipe and the faucet (less resistance), the more water that
comes out! The smaller the pipe and faucet, (more resistance), the less water that
comes out! This can be thought of as resistance to the flow of the water current.
All three of these: voltage, current and resistance directly interact in Ohm's law.
Change any two of them and you effect the third.
Current, Resistance & Voltage
Raising resistance reduces current.
Raising voltage increases current.
We can combine these relationships in what we call "Ohm's Law".
I=
V
R
Another way to write this is that:
OR
V = IR
IV. Circuits
A. Definition
• In electronics, a circuit is
a path between two or
more points along which
an electrical current can
be carried.
IV. Circuits
B. Parts
Drawing realistic pictures of circuits can be very difficult. For this reason, we have
common symbols to represent each piece.
Wire
Switch
Resistor
Battery
Bulb
*Note: Circuit diagrams do not show where each part is physically located.
IV. Circuits
C. Types of circuits
1. Series
• In series circuits, current can only take one path.
• The amount of current is the same at all points in a
series circuit.
Resistance in a series circuit
• Each resistance in a series
circuit adds to the total
resistance of the circuit.
Rtotal = R1 + R2 + R3...
Total resistance
(ohms)
Individual resistances (W)
• Light bulbs, resistors, motors, and heaters usually have much
greater resistance than wires and batteries.
Voltage in a series circuit
• Each separate resistance creates a
voltage drop as the current passes
through.
• As current flows along a series circuit,
each type of resistor transforms
some of the electrical energy into
another form of energy
• Ohm’s law is used to calculate the
voltage drop across each resistor.
IV. Circuits
C. Types of circuits
2. Parallel
• In parallel circuits the current can take more than one path.
• Because there are multiple branches, the current is not the
same at all points in a parallel circuit.
• Sometimes these paths are called branches.
• The current through a branch is also called the branch current.
• When analyzing a parallel circuit, remember that the current
always has to go somewhere.
• The total current in the circuit is the sum of the currents in all
the branches.
• At every branch point the current flowing out must equal the
current flowing in.
• This rule is known as Kirchhoff’s current law.
Voltage and current in a parallel circuit
• In a parallel circuit the voltage is the same across each branch
because each branch has a low resistance path back to the
battery.
• The amount of current in each branch in a parallel circuit is
not necessarily the same.
• The resistance in each branch determines the current in that
branch.
Advantages of parallel circuits
Parallel circuits have two big advantages over series circuits:
1.
Each device in the circuit sees the full battery voltage.
2.
Each device in the circuit may be turned off
independently without stopping the current flowing to other
devices in the circuit.
Calculate current
• Two bulbs with different resistances are connected in
parallel to batteries with a total voltage of 3 volts.
• Calculate the total current supplied by the battery.
Adding resistance in parallel circuits
• A circuit contains a 2 ohm resistor and a 4 ohm resistor in
parallel.
• Calculate the total resistance of the circuit.
D. Power
Power (watts)
P = VI
Voltage (volts)
Current (amps)
Calculate power
• A light bulb with a
resistance of 1.5Ω is
connected to a 1.5-volt
battery in the circuit shown
at right.
• Calculate the power used
by the light bulb.
Paying for electricity
• Electric companies charge for the
number of kilowatt-hours used
during a set period of time, often a
month.
• One kilowatt-hour (kWh) means that
a kilowatt of power has been used
for one hour.
• Since power multiplied by time is
energy, a kilowatt-hour is a unit of
energy.
• One kilowatt-hour is 3.6 x 106 joules.
Calculate power
• Your electric company charges 14 cents per kilowatt-hour.
Your coffee maker has a power rating of 1,050 watts.
• How much does it cost to use the coffee maker one hour
per day for a month?
E. Safety Features
E. Safety Features
1. Insulators
• Electrical wires, primarily made of copper, are
surrounding by plastic and rubber….why?
2. Grounding
 Many electrical appliances have metal cases,
including cookers, washing machines and
refrigerators – the grounding wire creates a safe
route for the current to flow through if the live
wire touches the casing
 You will get an electric shock if the live wire inside
an appliance, such as a cooker, comes loose and
touches the metal casing
 The grounding terminal is connected to the metal
casing so that the current goes through the earth
wire instead of causing an electric shock
 A strong current surges through the earth
grounding because it has a very low resistance –
this breaks the fuse and disconnects the appliance
3. 4.) Fuses & Circuit Breakers
• Fuses and circuit breakers protect electrical circuits and appliances
3. Fuses
• The features of a plug are: – The case is made from tough plastic or rubber, because these
materials are good electrical insulators
– The three pins are made from brass, which is a good conductor of
electricity
– There is a fuse bbetween the live terminal and the live pin
•
•
•
There is a fuse between the live terminal and the
live pin
The fuse breaks the circuit if too much current flows
The cable is secured in the plug by a cable grip – this
should grip the cable itself, and not the individual
wires inside it
3. Fuses
•
The fuse breaks the circuit if a fault in an appliance causes too much
current flow, protecting the wiring and the appliance
•
The fuse contains a piece of wire that melts easily – if the current going
through the fuse is too great, the wire heats up until it melts and breaks
the circuit
•
Fuses in plugs are made in standard ratings (3A, 5A, 13A etc…)
•
The fuse should be rated at a slightly higher current than the device
needs: – If the device works at 3A, use a 5A fuse
– If the device works at 10A, use a 13A fuse etc…
4. Circuit Breakers
•
The circuit breaker does the same job as the fuse, but works slightly
differently – a spring-loaded push switch is held in the closed position by
a spring-loaded soft iron bolt
•
An electromagnet is arranged so that it can pull the bolt away from the
switch
•
If the current increases beyond a set limit, the electromagnet pulls the
bolt towards itself, which releases the push switch into the open position
Circuit working
Circuit broken