Download Topic 1 - Electrical Charges

Electricity Unit Notes
Topic 1 - Electrical Charges:
We see examples of electric charges in daily life: storm clouds, electric currents in batteries, combing
your hair. The electrical devices that we use on a daily basis use the energy of moving charges.
Charged Materials: materials that attract and repel other materials
• A charge on a material can change its characteristics after rubbing or touching.
• Electric charges are produced when many combinations of materials are rubbed, touched or
moved close together and then separated.
• Static electricity: charges that are produced by rubbing or touching usually remain stationary
but in some cases will actually move. They are called unbalanced charges.
• Types of Charges:
• Negative (-)
• Positive (+)
• Neutral (0)
The Law of Electric Charges:
This law describes the behavior between charged and uncharged objects.
1. Unlike charges attract.
2. Like charges repel
3. Charged objects attract uncharged (neutral) objects.
Conductors, Insulators and In-Between:
• Insulators: materials that do not allow charges to move freely on or through them. ex. Rubber;
non-metals
• Conductors: materials that allow charges to move freely. ex. Metals
• Semi-conductors: materials with higher conductivity than insulators but lower conductivity
than metals.
• Superconductors: materials that offer little, if any, resistance to the flow of charges. Ex: metal
alloys, ceramics. These take up less space and carry higher charges than ordinary conductors.
Good Conductors
Aluminum
Copper
Nickel
Some Common Conductors and Insulators:
Fair Conductors
Insulators
Silicon
Cotton
Human Body
Rubber
Humid Air, Salty Water
Plastic
Neutralizing Unbalanced Charges:
• Discharge: electrons either enter an object to make up for a shortage, or an excess of electrons
leave the object.
• Neutralized: when the accumulated charge becomes balanced.
• Grounding: connecting an object to Earth with a conducting wire is an easy way to neutralize
conductive materials.
What do you think are some uses for materials with each type of electrical conductivity? Where would you
expect to find materials like these in typical electrical devices?
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Electricity Unit Notes
Topic 2 - Circuits:
For electrons to flow and do work, there must be a complete pathway for them to flow. When
you have a battery connected to wires, to a light bulb, and re-connected to the battery you
have a simple electric circuit.
Four subsystems of an electric circuit:
• a source (of current) – ex. battery
• a conductor (path) – ex. wire
• a load (requires energy) – items along the circuit that convert electricity into other
forms of energy (ex. light bulbs, motors, heaters, speakers)
• a control (switch). – turn the circuit or devices on or off.
Electrons can flow when the circuit is closed. The flow is interrupted when the circuit is
open. Electrons come out of the negative terminal and then return via the positive terminal.
Rules for drawing circuit diagrams:
• Draw with a pencil and ruler using graph or unlined paper.
• Place components in a rectangular or square arrangement
• Make conductors straight lines with right-angled (“square”) corners.
• If possible arrange your diagram so conductors do not cross.
• Draw neatly, making symbols a consistent size.
conducting wire
lamp
cell
switch
battery
resistor
Draw an example of a simple circuit with a battery, a switch and a light bulb, using the circuit
symbols:
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Electricity Unit Notes
Current
Voltage
Resistance
Description
the amount of charge that
passes a point in a conducting
wire every second.
also called potential difference, this
is the difference in energy per unit of
charge between one point in the
circuit and another point in the
circuit.
Resistance is the
property that hinders the
motion of electric charge.
Measurement
Galvanometer – measures
very weak currents.
measured with a voltmeter.
measured with an
ohmmeter.
V
Volt (V)
R
Ohm (Ω)
Ammeter – measures larger
currents.
Symbol
Units
I
Amperes (A)
Resistors can:
- change energy into heat
- control current or voltage
Variable resistors have no set resistance. Their resistance changes depending on temperature,
light, and voltage.
Types of Circuits:
Series: One current path moving through all
Parallel: More than one current path. Made up of
components in a circuit
many branches.
Resistance depends on:
Factor
Length
Cross-Sectional Area
Temperature
Material
Effect
Resistance increases with length
Resistance decreases with area. If the cross sectional area doubles,
resistance is half as great.
As temperature increases the resistance increases
Due to the structure of their atoms some metals allow electrons to
move more freely than others.
Supporting Video on YouTube: Electricity and Circuits by ScienceOnline
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Electricity Unit Notes
Topic 3 – Ohm’s Law:
Voltage, Current and Resistance are closely related in a circuit.
- Current – movement of electrons
- Voltage – what makes electrons move
- Resistance – opposes the movement of electrons
We describe the relationship between resistance, voltage and current in a circuit with “Ohm’s
Law”.
Ohm’s Law:
Voltage = Current X Resistance
V = IR
How to Convert Units:
Example:
What is 1kV in V? 1 X 1000 = 1000 V
What is 1mV in V? 1 ÷ 1000 = 0.001 V
Convert the Following:
1000 mV  _________________V
1.5 A  ________________ mA
3 k Ω  ________________ Ω
2.5 kV  _____________________ mV
How to Solve an Ohms Law Problem:
1) List the values that are known and the unknown value.
2) Decide which form of the V=IR equation to use. Use the triangle to help you with this.
3) Place the given values into the equation. Make sure you convert to proper units (ie. mA
to A)
4) Calculate. Make sure to write down the units too!
Name:
Symbol:
Unit:
Unit symbol:
Voltage
V
volt
V
Current
I
ampere
A
Resistance
R
ohm
Ω
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Electricity Unit Notes
Practice Problems:
1. What is the resistance of a light bulb if a 12 V battery sends a current of 2.4 A through it?
2. A toaster with a resistance of 145 Ω is connected to a 120 V source. What current will
flow through the toaster?
3. What is the potential difference across a 1 500 Ω resistor carrying a current of 75 mA?
4. An extension cord rated at 15 A is connected to a 120 V supply. What is the resistance
with which the cord can safely be used?
5. If a circuit with a 20 V power source has a resistance of 100 mΩ, what will the current
running through it be?
6. A power source of 1000 mV is connected to a lamp, and a current of 2.0 A flows. What is
the resistance of the lamp?
Topic 4 – Energy:
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Electricity Unit Notes
Energy:
• The ability to do work.
• Energy appears in many forms, ex. electrical, light, heat, kinetic (energy of motion)
• Some electric devices convert one form of energy into another or directly to electrical
energy.
Device
hair dryer
Energy Conversion
Starting Form
Final Form
electrical energy
heat energy
solar cell
light energy
electrical energy
speaker
electrical energy
kinetic energy
ex.
ex.
ex.
Check your understanding:
Read the descriptions of how a piezoelectric crystal is used in two different scenarios. Then,
identify the energy conversion (start form and end form) that occurs.
1. Sound is produced when an electric current causes vibrations in a tiny crystal – the
piezoelectric effect. When a piezoelectric crystal such as quartz is connected to a
potential difference, the crystal expands or contracts slightly. Any material touching the
crystal experiences pressure, which creates the sound.
ENERGY CONVERSION: ______________________ energy  _______________________ energy
2. While a piezoelectric crystal is compressed or pulled – a potential difference builds up
on opposite sides of the crystal. Conductors attached to the crystal can then be
connected to a circuit. The crystal acts as the source of energy.
ENERGY CONVERSION: ______________________ energy  _______________________ energy
Topic 5 – Electrochemical Cells:
An electrochemical cell is a setup used for creating electricity in a conductor separating two
reactions. The current is caused by the reactions releasing and accepting electrons in to the
different ends of the conductor. The most common example of an electrochemical cell is the
standard battery. Batteries are usually several cells connected in series, within a sealed case.
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Electricity Unit Notes
The parts of an electrochemical cell are:
-
two metal electrodes – these give up or accept electrons,
which travel in between the electrodes to produce a
current.
electrolyte: This can be liquid or a paste. It conducts
electricity. The electrodes need to be immersed in an
electrolyte in order for a current to be produced.
Types of Cells:
Primary: Nonrechargeable, dead
once reactants are
used up
Secondary:
Rechargeable cell
Wet Cell
Dry Cell
Description:
Uses a liquid electrolyte (usually an acid).
Uses a paste electrolyte. These are usually
what we refer to as ‘batteries’.
Example:
car batteries
AA, AAA batteries
for flashlights
These cells can be hooked up to an external
electrical source to rejuvenate the cell.
laptop/cell phone
batteries
Check your understanding: What is the difference between a primary and secondary cell? What is
the difference between a wet and dry cell?
Topic 6 - Generators and Motors:
Electric Generators:
 Depend on the conversion of mechanical energy into electrical energy.
 Mechanical Energy-> Any system with moving parts ie. windmill, waterwheel
 Based on the fact that magnetism and electricity are related.
Electromagnets
 Michael Faraday -> discovered the Basic Principles of
Electromagnetism (early 1800s)
 Moving a magnet close to a wire produced a voltage.
 Could move a wire over a magnet or magnet over a wire
to produce this change.
 An electromagnet is an iron core surrounded by a wire with a
current moving through it.
 The current creates a north and south
pole on the iron core by aligning charges.
 The strength of an electromagnet is
affected by the:
 1) type and size of core
 2) strength of current
 3) number of coils
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Electricity Unit Notes
Electromagnet Demo: Observations
Input
Generator Mechanical Energy
Motor
Electrical Energy
Output
Electrical Energy
Mechanical Energy
DC Generator: (Also called a Dynamo)
A device that changes mechanical energy into
electrical energy.
Made of:
1.
2.
3.
4.
Armature -> Rotating loop of metal
Brushes -> Makes connection from
battery to the armature
Field Magnet
Split-Ring Commutator -> metal
ring with two breaks
How it works:
1. An external force is used to move something that is attached to an armature (ie.
wind).
2. The armature spins in the presence of an external magnet, and a current is
produced.
3. The current produced is not constant.
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Electricity Unit Notes
DC Motor:
A device that changes electrical energy
into mechanical energy.
Made of:
1.
2.
3.
4.
Magnet
Armature
Commutator
Electricity Source
How it works:
1. Electrons flow through the commutator to the armature. This electricity flow
causes the armature to become magnetized.
2. The N-pole of the armature is
repelled by the N-pole of the
magnet turning the motor.
3. As the N-pole of the armature
moves close to the S-pole of
the magnet the commutator
cuts off the electricity
(because of two breaks in its
surface)
4. The current direction is then
switched so the N-pole of the
armature becomes the S-pole
and is then repelled by the Spole of the magnet.
Supporting Video on Youtube: Magnetism: Motors and Generators:
http://www.youtube.com/watch?v=d_aTC0iKO68&feature=related
Check your Understanding:
1. What are two ways that an electromagnet can be made stronger?
2. In what ways are generators and motors similar? Different?
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Electricity Unit Notes
Topic 7 - Electricity in the Home:
Transmission of Electricity through the Power
Grid
 The power grid is a complex network that
links generating stations with electric energy
users such as homes and factories.
 Alternating current for use in homes and
industry is produced by large electric
generators in power stations.
 Transformers – used to step up the voltage for
efficient transmission over long distance.
 At the destination other transformers will
step down the voltage so that it can be used in
homes, etc.
 Semiconductors are installed in the master
panel to limit the amount of electric current in
each circuit and prevent fires
From the Grid into Your Home
Electrical power enters a meter on the side of your
house where electrical usage is recorded. Power is then routed into the service panel (usually
in the basement). The main circuit breaker shuts off all the power in the house at once, in case
of an overload. The individual circuit breakers in the service panel control the branch circuits,
located throughout the entire house.
Fuses:
 a thin piece of metal alloy that will melt if the current that passes through it goes over a
certain limit. This stops the flowing of the current.
 Needs to be replaced
Circuit Breakers:
 Consists of a bimetallic strip connected to the switch.
 This will bend if the current gets higher. Therefore the higher the current the greater
the bending. If it is too high, then it will trip the switch and break the circuit.
 This does not need to be replaced every time this occurs
Paying For Electrical Energy
 Power companies use the customer’s electricity meter to find out how much the
electricity will cost.
 Electric energy is measured in a kilowatt hour (kWh)
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Electricity Unit Notes
CALCULATIONS: Power
Power-> Energy per unit of time. Power is measured in Watts (W)
Name:
Symbol:
Unit:
Unit symbol:
Power
P
watt
W
Energy
E
joules
J
time
t
seconds
s
We can calculate power in two ways:
How to solve a Power Problem:
1) List the values that are known and the unknown value.
2) Decide which form of the P=IV or E=Pt equation to use. Use the triangle to help you
with this.
3) Place the given values into the equation. Make sure you convert to proper units (ie. mW
to W)
4) Calculate. Make sure to write down the units too!
SAMPLE PROBLEM:
A current of 13.6 A passes through an electric baseboard heater when it is connected to a 110 V
wall outlet. What is the power of the heater?
PRACTICE:
1. What is the power (in watts and kilowatts) of a hair dryer that requires 10 A of current
to operate on a 120 V circuit?
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Electricity Unit Notes
2. A 900 W microwave oven requires 7.5 A of current to run. What is the voltage of the
circuit to which the microwave is connected?
3. A flashlight using two 1.4 V cells contains a bulb that can withstand up to 0.5 A of
current. What would be the maximum power of the bulb?
4. A 60 W light bulb is hooked up to a power source of 9 V. What current will run through
the bulb?
5. A machine uses 30J of energy in 10 seconds. How much power does it use?
6. A device needs 5 W of power for 5 seconds. How much energy does it use?
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Electricity Unit Notes
CALCULATIONS: Efficiency
No real device is a perfectly efficient energy converter. Some input energy is always converted
into waste heat. The efficiency of a device can be expressed as a percentage.
Efficiency = Useful energy output x 100% = Eout x 100%
Total energy input
Ein
To determine energy input or output – need to calculate the power of the device.
How to solve an efficiency problem:
1) Determine what values are associated with Ein and Eout.. Sometimes, the question will tell
you, sometimes you will need to figure it out. Ein will have to do with the energy (power)
going into the device. Eout will be how much energy goes towards performing it’s task.
2) List the values that are known and the unknown value, under each heading, Ein and Eout.
3) Use any power formulas that you might need.
4) Once you have two Energy values, in J, put them into the efficiency equation and solve.
5) The units for efficiency is percent. Your final answer should be less that 100%!
SAMPLE PROBLEM:
Calculate the efficiency of a 1000W kettle that takes 240 s to boil water. To heat the water to
boiling point, it takes 196 000 J of energy. What is the efficiency of the kettle?
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Electricity Unit Notes
PRACTICE:
1. What is the efficiency of a lamp that uses 500 J to produce 250 J of light energy?
2. Find the efficiency of a 23 W fluorescent tube that is used 240 minutes per day and in that
time produces 66 240 J of useful light energy. (hint: convert the time to seconds)
3. A 100 W incandescent bulb also produces 66 240 J over a 4.0 h period. What is the
efficiency of this bulb? (hint: convert the time to seconds)
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Electricity Unit Notes
Topic 8 – Electricity Production:
Type of Energy
Generation:
Thermo-electric
Generating
Plants
How it works:
Pros:
Fuel is burned and used to heat
water. This creates steam which turns
a steam turbine, which will generate
electricity. Usually fossil fuels (coal,
oil, natural gas) are used as fuel.
Relatively
cheap and easy
to produce
electricity.
Cons:
Produces
greenhouse
gases, causes acid
rain, resource
collection can
harm
environment as
well.
Hydro-electric
Use dams to collect large amounts of
No ‘pollution’
Has long term
Plants
water. Water pressure is used to turn created.
effects on
a turbine, which will generate
environment,
electricity.
causes flooding,
harms aquatic
ecosystems.
Atomic Energy - Utilizes nuclear fission (a reaction
Does not
Produces
Nuclear Fission
that splits an atom). Uranium is
produce
radioactive
bombarded with neutrons so that it
greenhouse
waste, and
splits into 2 smaller atoms.
gases. Produces reactors can be
vast amounts of incredibly
energy.
harmful in
emergency
situations (ie.
Chernobyl)
Atomic Energy – Nuclear fusion joins small atoms to
Produces a lot
Expensive,
Nuclear Fusion
form larger ones. This is done by the
of energy.
extremely
sun and other stars.
difficult and not
fully understood.
Choose an Alternative Energy Source: Solar, Wind, Geothermal, Tidal. Complete research on it,
and outline: 1) How it works (include the energy conversions), 2) Where it is used in Canada
(be specific), and 3) Pros and Cons.
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