Unit D - Electricity

Week 2

2.2 MODELING AND MEASURING ELECTRICITY

I can…

• Measure voltage and amperages in circuits and calculate resistance using Ohm’s Law (V=IR)

• Read page 304

• In your notes, write how currents can be compared to a Waterfall.

• Write how water flow can be used to describe resistance

Types of resistors

• There are many types of resistors for different things ex. Electronics in a TV

• Why would we add a resistor (that doesn’t use the energy usefully) into a current?

• The most common types of resistors are wire-wound and carbon-composition

• They vary from 0.1 Ω to 200 kΩ

4 factors affecting resistance in wire

• Factor Effect• 1) Length -Resistance

INCREASES with length

- So: if the length doubles so does the resistance

• Factor Effect• 2) Cross sectional - Resistance

DECREASES with area - So, if the

cross sectional area doubles the resistance is half as great

• Factor Effect• 3) Temperature -As the temperature of

wire increases the resistance INCREASES

• Factor Effect• 4) Material - Some metals allow

electrons to move more freely than others

Measuring Electricity

• There are 2 ways that we measure electricity:– 1) Amperes: how many electrons going by– Units: Amps (A)– Measured using: ammeter– 2) Voltage: how much energy each electron has– Units: Volts (V)– Measured using: voltmeter

– Extra tool: Galvanometer- measures volts and amps

More about voltage

• Voltage is also called potential difference• Named after Alessandro Volta• Some common voltages– Flashlights- 6 V or less– Car Batteries- 12 V– Wall Sockets- 120 V– Major Power Lines- 100 kV

Measuring Amps and volts

• Teacher DemoHow Many Volts are there in a…• C Battery• D Battery• AAA BatteryNotes- when using a voltmeter you must attach the red terminal to the positive side of the battery and attach the black side to the negative side of the batter• AND: sometimes you have to change the readings from

mV to V

Ohm’s law

• George Simon Ohm experimented with different substances, and found a relationship between voltages, current and resistance

• He found that if the temperature remained the same then– 1) The resistance of a conductor stays constant– 2) Current is directly proportional to the voltage

applied• So… if you increase the voltage, you increase the

current too and vice versa

The math of ohm’s law

• V= I x R• V= voltage volts (V) (voltmeter)• I= current amps (A) (ammeter)• R= resistance ohms (Ω) – (ohmmeter)

Steps to solving ohm’s law questions

• 1) Identify the known quantities• 2) Identify the unknown quantities• 3) Rearrange the formula if necessary• 4) Fill in the numbers for the variable and

solve

Steps to solving ohm’s law questions

Example 1

• 1) A motor has an internal resistance of 40 Ω. The motor is in a circuit with a current of 4.0 A. What is the voltage?

Example 2

• A 30 V battery creates a current through a 15 Ω resistor. How much current is created?

Example 3

• An electric stove is connected to a 240 V outlet. If the current flowing through the stove is 20 A, what is the resistance of the heating element?

Example 4

• A hair dryer takes 1200 mA to run and it gives a resistance of 40 Ω. What voltage is necessary?

To do:

• Ohm’s Law Worksheet- To hand in

Assignment Questions• How much current is created if a 12 V battery passes current

through a 10 Ohm resistor?• An electric shaver that is plugged into a 110 V wall outlet has a

resistance of 6,000 Ohms. How much current flows through this device?

• A current of 12 A flows through a vacuum cleaner that is plugged into a 120 V household circuit. What is the total resistance of the vacuum cleaner?

• A current of 2.0 Amperes flows through a resistor rated at 30 Ohms connected to a battery. What is the voltage of the battery?

• An electric garage heater draws 30 A from a special 220 V circuit designed for heavy duty heaters. What is the heaters resistance?

Review

• What are the four factors that affect resistance?

3.1 ENERGY FORMS AND TRANSFORMATIONS

I can…

• Describe energy conversions (mechanical, chemical, thermal (heat), electrical) in household appliances and energy transport systems (power generating station to your home)

Think about it!

• What types of energy do we know of?

4 common forms of energy

• 1) Chemical Energy- the energy stored in chemicals. This is a form of potential or stored energy. This energy is released when chemicals react

• Draw or write 3 things that contain chemical energy

• 2) Electrical Energy- the energy of charged particles. Electrons are negatively charged. Electrical energy is transferred when electrons travel from place to place

• Draw or write 3 things that contain electrical energy

• 3) Mechanical Energy- the energy possessed by an object because of its motion or its potential to move

• Draw or write 3 things that contain mechanical energy

• 4) Thermal Energy- the total kinetic energy of all particles in a substance (its temperature). The faster a particle moves, the more kinetic energy it has.

• Draw or write 3 things that contain thermal energy

Transformations involving chemical and electrical energy

• There are many ways to convert energy forms into the other forms• Using this table, come up with devices that you use every day that follow

the same energy transformation• The Law of Conservation of Energy – Energy can not be created or

destroyed, but can change from one form to another. (PAT/EXAM Question!)

Input Energy Device (Converter) Output Energy

Electrical Thermal

Chemical Electrical, light and thermal

Electrical Mechanical

Chemical Electrical, mechanical and sound

Lets read!

• Page 323• What is a thermocouple?

• When is a thermocouple useful?

• What energy conversions do thermocouples do?

• What energy conversion to heaters and ovens do?

Bill nye’s electricity video

3.2 ENERGY TRANSFORMATIONS INVOLVING ELECTRICAL AND MECHANICAL ENERGY

I can…

• Explain the parts in a motor and generator and suggest how to modify these parts to increase output.

• Predict the effect of changes in the orientation and placement of magnets, commutator and armature in a St.Louis motor or in a personally-built model of a motor.

introduction

• In 1820 Hans Christian Oersted discovered that a wire with a current running through it created a magnetic field

• Michael Faraday used the connection between electricity and magnetism to make the first motor in 1831

• However, he used an open container or mercury in his design. Why wouldn’t we used that today?

Electromagnets• Electromagnets can be created by winding a wire

into a coil• An electromagnet is only a magnet when electricity is

run through the wire • It is possible to change the poles of the

electromagnet by running the electricity the other way

• How is that different from a permanent magnet?

• An electromagnet will line up in a magnetic field made from permanent magnets

Electric motors

• An electromagnet will line up in a magnetic field made from permanent magnets

• How can we keep an electromagnet moving within an electrical field?

• The N and S of the electromagnet needs to be switched every half turn so that the ends never align and there is always attraction to somewhere new– called changing polarity

Parts of an electric motor

• A commutator and brushes are used to reverse the flow of electricity through the electromagnet core

• The commutator is a split ring that stops the flow of electricity for a brief moment so that magnetic force can flip

• The armature keeps spinning during that time because of momentum

• What is momentum?

• The commutator keeps spinning and reconnects to the brushes (usually made of carbon bars) and therefore the motor will keep spinning

• There brushes touch the commutator by brushing against it

Direct and alternating current

• Direct current (DC) only flows in one direction• Many devices such as cell phones and computers use

DC• Alternating current (AC) flows back and forth 60

times a second• AC current flows into your house• Devices that need DC often come with their own

power supply

transformers

• Power companies use AC because it can be transformed into different voltages without energy loss

• Power comes from generating stations at a high voltage and is transformed to a lower voltage through a transformer

• More coils mean more voltage- we can “step up” to higher voltage or “step down” to a lower voltage

Generating electricity

• Michael Faraday also discovered that he could create an electrical current by moving a wire through a magnetic field (the motion created the electricity)

• A hand-held generator moves a coil of wire in permanent magnets and creates a current

• The faster you turn the more current is made

Generating in real life• Generators are machines that convert energy

from one form to another• Most electric power stations use generators• How would hydroelectric dams use a

generator?

• How would a wind turbine use a generator?

• How would a coal burning plant use a generator?

Let’s read!

• About Generating AC and DC on page 331

3.3 MEASURING ENERGY INPUT AND OUTPUT

I Can…

• Apply appropriate units, measures and devices in determining and describing quantities of energy transformed by an electrical device. P=IV, E = Pt

• Compare energy inputs and outputs of a device, and calculate its efficiency.

Power

• Power is the rate at which a device converts energy

• Measured in watts which is equal to J/s

• P = I x V• P= power (watts)• I= current (amps)• V= voltage (volts)

Steps for solving power problems

• 1) Identify known quantities• 2) Identify unknown quantities• 3) Rearrange the formula if necessary• 4) Fill in the numbers and solve the problem

Example 1

• If a blender uses 40 V when there is a current of 3.4 A, what power did it use?

Example 2

• A hair dryer has a power rating of 1000 W. It is plugged into a 120 V outlet. What is the current flowing through the hair dryer?

Example 3

• A remote control car used 345 mV and had a power rating of 560 W. How much current runs through it?

energy

• In order to discover how much energy a device uses we need to know how long it uses power for

• The formula for energy is• E = P x t• E= energy (Joules- J)• P= power (W)• t= time (seconds)

Steps for solving energy problems

• 1) Identify known quantities• 2) Identify unknown quantities• 3) Rearrange the formula if necessary• 4) Fill in the numbers and solve the problem

Example 1

• A microwave oven has power rating of 800 W. If you cook a roast in this oven for 30 minutes on high, how many Joules of electrical energy are converted by the microwave?

Example 2

• If 6200 kJ of energy are used to run a TV for a week and the power rating of a TV is 300 W, how many seconds, minutes and hours was the TV on for?

Kilowatt hours

• A kilowatt hour is the unit used to measure how much energy households or businesses use- usually in a hour and then have to pay for

• To find kilowatt hours you use the same formula

• E(kWh)= P(kW) x t(h) but use kilowatt instead of watts and hours instead of seconds

Example 1

• A microwave oven has a power rating of 800 W. If you cook a roast in this oven for 30 minutes on high, how many kilowatt hours did it use during that time?

Example 2

• A CFL light bulb that uses 25 watts of power is used for 125 hours. It costs 15 cents/kWh to run one light bulb. Calculate the kilowatt hours that the light bulb uses, then calculate how much it will cost to run the light bulb. (3)

Energy dissipation

• The Law of Conservation of Energy says: Energy can neither be created or destroyed

• However, the output energy of an energy transformer is always less than its input

• Where does the energy go?

Example

• You create an electric pump with a motor to pump water from an irrigation ditch to irrigate a crop. You find that the motor used 100 kJ of electrical energy for every 75 kJ of work done rating water to the field. Where did the other 25kJ go?

Understanding efficiency

• The efficiency of a device is a ratio of useful energy to the total energy that went in

• The more useful energy that comes out the more efficient the device is

• We use percent efficiency

• Percent efficiency = joules of useful outputjoules of input energy x100

Example:

• Calculate the percent efficiency of an incandescent light bulb that produces 2 500 J of light energy from 50 000 J of electrical energy.

Comparing efficiencies

• We can compare efficiencies of devices which will tell us about their energy cost and their environmental impact

• New technologies often help make devices more environmentally friendly and make them more efficient

Videos

• https://www.youtube.com/watch?v=ByCOTG2-mhg 90 seconds!

• https://www.youtube.com/watch?v=H5s1ia50-aw

Additional Assignment Question

A washing machine has a power-rating of 2500W. If a unit of electricity costs 24p, how much does it cost to run the appliance for 90 minutes?

A vacuum cleaner has a power-rating of 1.2kW. If a unit of electricity costs 20p, how much does it cost to run the appliance for a year if it is switched on for 1 hour each day