current electricity. the movement of electric charge from one place to another. the movement of...
TRANSCRIPT
Current Electricity
Current Electricity
The movement of electric charge from one place to another.
Voltage
Potential (Voltage)- IS THE ELECTRIC POTENTIAL PER CHARGE
MOVING BETWEEN TERMINALS. THIS IS LIKE THE ELECTRIC PRESSURE
PUSHING THE ELECTRONS. VOLTAGE DOES NOT MOVE, IT PUSHES THE
ELECTRONS.
http://faraday.physics.utoronto.ca/IYearLab/Intros/DCI/Flash/WaterAnalogy.html
Low Voltage
High Voltage
Low Voltage
High Voltage
Voltage
Potential (Voltage) cont’d- Measured in volts
(V) Measured using a
voltmeter.
Current
Current- The measure of the rate at which
electric charges move past a given point in a circuit.
Measures the amount of electricity passing a point.
Measured in amperes (A) Measured using an:
Ammeter- larger currents Galvanometer- smaller currents
High Current
Low Current
High Current
Low Current
Resistance
Resistance The measure of an objects opposition to
the passage of a steady electrical current
Measured in ohm’s (Ω) Measured using an ohmmeter
Low Resistance High
Resistance
Low Resistance
Ohm’s law
Created by Georg Ohm (1789- 1854) “the potential difference between
two points on a conductor is directly related to the electric current flowing through the conductor”
Potential difference= Electric current x electrical resistance
V = I x R
Example One
What is the voltage drop across a tungsten filament in a 100-W light bulb? The resistance of the filament is 144 Ω and a current of 0.833 A is flowing through it.
Example Two
An electric toaster is connected to a 120-V outlet in the kitchen. If the heating element in the toaster has a resistance of 14 Ω, calculate the current flowing through it.
Problem solving
?Textbook (10-10)
Electrical circuits
Electrical circuit- Controlled path of flowing electricity in a complete
circle
Contain 4 parts1. Source- Where electricity comes from.2. Load- Where the electrical energy is transferred.3. Control- What starts and stops the electricity. 4. Connectors- The path where the electricity runs.
Source
Control
Load
Connector
Electrical circuit- Source
Cells- Converts chemical energy into electrical energy.
Batteries- combination of 2 or more cells
Generators-a device that converts movement into electrical energy
Photoelectric cells- a cell that converts light directly into electrical energy
Cell
Battery
Generator
Photoelectric Cell
Electrical circuit- Source cont’dCells
Primary cells- Disposable cells Secondary cell- reusable cells
All cells contain:Electrodes- Metal plates that are placed in the
electrolyteElectrolytes- Chemicals that conduct electric currentPositive terminal- Place where positive charges
collectNegative terminal- Place where negative charges
collect
Electrical Sources
Cells can be: Wet cells- electrolyte is a liquid
Easy to make with available chemicals Hard to transport and quite large
Electrolyte
Electrodes
Electrical Sources
Dry cells- Electrolyte is a paste Easy to transport and very compact Special and sometimes more dangerous chemicals are
required.
Electric circuit- Load
Anything that converts electrical energy into the form of energy required Light bulb (light energy) Toaster (heat energy) Television (light and sound energy) Computer (light and sound energy) Fan (mechanical energy) Music player (sound energy) Motor (mechanical energy)
Electrical circuits- Control
A device that controls the flow of electrical energy Switches
Single pole switches Double pole switches 3-way switches
Buttons Keys Timers Bimetallic strips Variable resistors (dimmer switches)
Electrical circuits-Connectors
A conducting wire that provides a controlled path for electric current to flow to each part of the circuit Conductor- A substance where electrons can
move freely from one atom to another. (electric current)
Insulator- A substance where electrons cannot move freely from one atom to another. (Static electricity)
Superconductor- Ceramics that conduct electricity with no resistance at low temperatures. (bullet trains)
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Conductor
Insulator
Superconductor
Electrical circuits
Open circuit- circuit is not connected, switch is open, no electricity is flowing
Closed circuit- circuit is connected, switch is closed, electricity is flowing
Short Circuit- Circuit where there is not a load attached to the circuit, no resistance. Can be very dangerous, connectors can become overheated and burn, cells will use up the potential very rapidly.
Open CircuitClosed Circuit
Short Circuit
Electrical circuits
Electrical circuits can be made in two different ways.
1. Series circuit- One path of electric charge
Electrical circuits
2. Parallel circuit- 2 or more paths for electric charge to follow (branches)
Electrical circuits(10-7)
Connecting cells in: Series- the potentials of the cells are
added together ie. Three 1.5V cells connected end to end
has a potential of 4.5V
+
-
+ +
- -
= 4.5 V
Electrical Circuits
Parallel- the cells will last longer Potential remains the same
+
-
+
-
+
-
= 1.5V
Electrical circuits
Connecting loads in: Series-
Circuit potential remains the same (total) Resistance is additive Circuits current changes according to Ohm’s
law
1.5V
3 Ω
4 Ω
V= 1.5V
R= 3Ω + 4Ω
= 7Ω
I = ?
V = I x R
I = V/R
I = 1.5V/ 7Ω
I = 0.21 A
1.5V
5 Ω
7 Ω
V= 1.5V
R= 5Ω + 7Ω
= 12Ω
I = ?
V = I x R
I = V/R
I = 1.5V/ 12ΩI = 0.14 A
Electrical circuits
Parallel- Circuits potential remains the same (total) Resistance decreases Circuits current increases according to
Ohm’s law
Electrical circuits
Pro’s and cons of series and parallel circuits. Series
Pro’s Simple to make and easy to follow.
Con’s Limited control over the circuit and when one load
is broken, the entire circuit won’t work.
Electrical circuits
Pro’s and cons of series and parallel circuits. Parallel
Pro’s Lots of control over the circuit and not all loads
have to be working at the same time Con’s
Much more complex and difficult to follow.
Multimeter A multimeter is a device that can measure
potential, current and resistance in one machine.
When using a multimeter in a circuit, it must be connected properly to get a correct reading
When using as a voltmeter or ohmmeter, it must be connected in parallel to the circuit
When used as an ammeter, it must be connected in a series in the circuit
Voltmetre or ohmmetre
-Connect in parallelMultimetre
Ammetre-Connect in series Multimetr
e
Circuit schematics
Schematic circuit diagrams are used to show how electrical circuits are connected on paper.
Special symbols are used to indicate the different parts of the circuit.
- Cell
- Battery (2 cells)
- Light
M - Motor
V - Voltmeter
A - Ammeter
- Switch (1 pole)
Making a Circuit Schematic Diagram• Create a circuit that has one cell powering 1 light that is controlled by a switch.
• Create a circuit with a 3 cell battery that has 2 lights connected in series all controlled by one switch.
• Create a circuit with a 3 cell battery that has 2 lights connected in parallel all controlled by one switch, with another switch controlling just one of the lights.
Energy
Energy
Energy- The ability to do work. There are many forms of energy
Light Sound Movement (mechanical) Heat Electricity Nuclear Chemical
Thermodynamics
Thermodynamics is the study of moving energy.
The first law of Thermodynamics says that: Energy cannot be created or destroyed. Energy can only be transformed from
one form to another.
Electricity
Light
Energy Transformations
Toaster- Electrical energy is converted to heat energy. The heat is produced by resistance
inside the toaster (friction)
Electricity
Heat
Energy Transformations
Light Bulb- Electrical energy is converted to light energy Light is produced by the resistance
inside the light bulb.
Electricity
Light
Energy Transformations
Speaker- Electrical energy is converted into sound energy. Sound is produced by having electricity
turn a magnet on and off. (electromagnet)
Electricity
Sound
Energy Transformations
Electric motor- Electrical energy is converted into motion (mechanical) energy. Motion is produced by creating an alternating
magnetic field. (electromagnet)
Electricity
Motion
Electromagnets
A coiled wire carrying electrical current produces a magnetic field around it. It acts like a magnet.
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Electromagnets
If a piece of metal is inserted into the coil, the metal will become magnetized.
The magnetism will only last when electricity is running through the circuit.
With an open circuit, there is no magnetic field.With a closed circuit, there is an electric field.
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Electromagnet
Metal plate
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Permanent Magnet
Brushes
Wire coil
Running electricity through a wire coil produces a magnetic fieldThe similar poles ‘repel’ each other causing the wire coil to rotate.When the gap in the wire coil reaches the brushes, the magnetic field disappears.The wire coil will continue to rotate in the same direction, which will produce a new magnetic field in the same direction.This process continues producing a continuously rotating coil.
Thermodynamics
The second law of thermodynamics tells us that every time energy is transformed from one type of energy to the next type, some of the energy is transformed into an unusable form. Not all the energy is converted.
Electricity
LightHeat
Sound
Energy Loss
Toaster- Electrical energy is converted to heat energy. Energy is lost in the production of light
and sound.
Electricity
HeatLight
Sound
Energy Loss
Light Bulb- Electrical energy is converted to light energy Energy is lost as heat
Electricity
Light
Heat
Energy Loss
Speaker- Electrical energy is converted into sound energy. Energy is lost as heat and motion.
Electricity
Sound
Heat
Motion
Energy Loss
Electric motor- Electrical energy is converted into motion (mechanical) energy. Energy is lost as heat and sound.
Electricity
Motion
Heat
Sound
Efficiency
Efficiency is the comparison between the amount of useful energy produced (output energy) and the original amount of energy used (input energy).
% efficiency = Useful Output Energy x 100%
Input Energy
Efficiency
Energy is measured in Joules (J) A regular 60W fluorescent light bulb
produces 400J of light energy but it consumes nearly 2000J of electrical energy. What is the percent efficiency of a 60W fluorescent light bulb?
Output energy = 400JInput Energy = 2000J% efficiency = ?
% efficiency = Useful Output Energy x 100% Input Energy
% efficiency = 400J x 100%2000J
% efficiency = 20%
Can you ever have an appliance than is 100%
efficient?
No
Cost of electricity
You pay for electricity based on the amount of power you use.
Power is the measure of the voltage used and the current it was used at.
P = V x I Power is measured in Watts (W)
A kW is 1000W
Cost of electricity
On every electrical appliance in Canada, at least 2 out of the following 3 items have to be included. Power Voltage Current
Only 2 have to be included because the 3rd can be calculated using the equation P = V x I.
How much power is used by a light bulb that uses 120V of potential and runs at a current of 0.83A?
V= 120V I= 0.83A P= ? P = V x I P = 120V x 0.83A P= 99.6W
What is the current used by a 60W light bulb that runs on 120V of electricity?
P= 60W V= 120V I= ? P = V x I I = P / V I= 60W / 120V I= 0.50A
Cost of electricity
The cost of your electricity is based on the amount of power you use (in kW), how long you use it (in hours), and the cost of the electricity (¢10.22/kW·h in Saskatchewan).
Cost = Power x time x rateCost = P x t x rate
How much does it cost to leave a 100W light bulb on for 8 hours in a day?
P= 100W= 0.100kW t= 8h Rate= ¢10.22/ kW·h Cost=? Cost= P x t x rate Cost= 0.100kW x 8h x ¢10.22/kW·h Cost= ¢8.2
How much does it cost to leave an equivalent 27W fluorescent light bulb on for 8 hours in a day?
P= 27W= 0.027kW t= 8h Rate= ¢10.22/ kW·h Cost=? Cost= P x t x rate Cost= 0.027kW x 8h x ¢10.22/kW·h Cost= ¢2.2
Comparing Incandescent lights to fluorescent light
For every 100W incandescent light bulb you have replaced with a 27W fluorescent light bulb, you will save ¢6 for every 8h of use.
If you replace 4 bulbs, that could be ¢24 a day.
Over 1 year that could save $87.60!
Generating electricity
We have discussed several sources of electrical energy; Cells Batteries Photoelectric cell Generator
Generating electricity
The majority of our daily electricity comes from electricity generating stations.
All generating stations work on the same principals.
We discussed earlier how current electricity in a coil produces a magnetic field. The reverse process is used to produce current electricity.
A moving magnetic field produces current electricity.
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NIn the presence of a magnet, the electrons are drawn in by the magnetic field.
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When the magnet moves, the electrons will be pulled along with it. The electrons are now moving, this is current electricity.
Generating electricity
A generator uses the same materials as an electric motor, but everything happens in reverse. Electric motor- current electricity
produces the spinning coil. Generator- a spinning coil produces
current electricity
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The electrons are drawn to one side of the magnet so the electrons flow in one direction to that side of the magnet.
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As the coil rotates, the electrons are again drawn to the one side of the magnet so the electrons make the trip again
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rotate, current electricity continues to flow.
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Generating Electricity
There are two types of electricity that can be created: Direct current (DC)- Electricity flows in
one direction (cells, generator shown) Alternating current (AC)- Electricity
switches directions as the coil rotates.
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Direct Current (DC)
Alternating Current (AC)
Generating Electricity
There are many different ways to turn the wire coil inside the generator.
The coil is attached to a turbine, and the turbine is rotated by: Steam (coal, nuclear, natural gas,
biomass) Water Wind
Transporting Electricity
Electricity has to be moved from the generating stations to peoples houses.
Most large scale generators produce around 25 000 V of electricity.
The electricity is then moved to a transformer at the power plant which boosts the voltage to 400 000 V. Electricity travels more efficiently at higher voltages.
The high voltage electricity is then carries by thick transmission cables made of copper or aluminum.Copper and aluminum are used because of their low resistance.
The power lines go into substations near businesses, factories and homes. Here transformers change the very high voltage electricity back into lower voltage electricity.
From these substations, electricity in different power levels is used to run factories, streetcars and mass transit, light street lights and stop lights, and is sent to your neighborhood.
In your neighborhood, another small transformer mounted on pole or in a utility box converts the power to even lower levels to be used in your house.
Electricity that comes into our homes has been reduced to around 240 V.Older homes have the electricity brought in by overhead lines, newer homes have underground lines.
Before the electricity is brought into your house, it is passed through a power meter to record the amount of electricity being consumed.
After passing through the power meter it enters the house through the breaker box. There is one main breaker switch that controls all the electricity in the house. A breaker is a safety device that shuts the power off if the current becomes to high.
Main breaker switch
Inside the breaker box, the electricity can be kept at 240 V for some appliances (Oven, Furnace, etc.) or reduced to 120 V for lights and plugs. Each circuit in the house is controlled by a separate cicuit breaker.
Individual circuit breakers