Science 9 Unit D Booklet 2

Modeling and Measuring Electricity

•

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

• Write how water flow can be used to describe resistance

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

The most common types of resistors are wire-wound and carbon-composition. They vary from 0.1 Ω to 200 kΩ

Factors that Affect Resistance

Factor Effect1

2

3

4

Measuring Electricity

Measurement Unit Measured Using1

2

What can a Galvanometer be used for?

Voltage is also called _______________________________. It is 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

Demo:

How Many Volts are there in a…

D Battery

AA Battery

Notes- 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

Ohms Law

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

He found that if the temperature remained the same when

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 Formula for Ohms law is ___________________________________________________

• V= voltage volts (V) (voltmeter)

• I= current amps (A) (ammeter)

• R= resistance ohms (Ω) – (ohmmeter)

Steps to Solving Ohms Law Problems

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

Examples

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?

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

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?

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

Please complete the last page of the booklet – this will be marked

Energy Forms and Transformations

Think about it!

• What types of energy do we know of?

Type Definition 3 Examples1. Chemical Energy

2. Electrical Energy

3. Mechanical Energy

4. Thermal Energy

Energy Transformations

• 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

Take a Break! Read the paragraphs below

What is a thermocouple?

• When is a thermocouple useful?

• What energy conversions do thermocouples do?

• What energy conversion to heaters and ovens do?

Energy Transformations Involving Electrical and Mechanical Energy

• 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

An electromagnet is only a magnet when __________________________________________________

• It is possible to change the poles of the electromagnet by

______________________________________________________________________________

• How is that different from a permanent magnet?

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

Parts of an Electromagnetic Motor:

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

• The commutator is a

• The armature keeps spinning during that time because of momentum

• The commutator keeps spinning and reconnects to

• There brushes touch the commutator

DC Electromotor DC Electric Generator

Direct and Alternating Current

• Direct current (DC) only flows:

• Many devices such as cell phones and computers use DC

• Alternating current (AC) flows:

• 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 _______________________________ and is transformed to a _________________________ through a _________________________________

Generators

• 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 ________________________________________

• The faster you turn the _______________________________________

Most electric power stations use generators

• Generators are:

• How would hydroelectric dams use a generator?

• How would a wind turbine use a generator?

• How would a coal burning plant use a generator?

Measuring Energy Input and Output

• Power is the rate __________________________________________________________

• Measured in watts which is equal to J/s. The Formula is:

• P= power (watts)

• I= current (amps)

• V= voltage (volts)

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?

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= 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

• 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?

• 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?

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

• 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?

• 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.

Energy Dissipation

• The Law of Conservation of Energy says:

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

• Where does the energy go? 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?

Efficiency

• The efficiency of a device is a ratio of:

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

• We use percent efficiency

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.

• 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

Unit D Assignment – Calculations for Electricity – HAND IN! This is for Marks!

Name:_________________________

1. How much current is created if a 12 V battery passes current through a 10 Ohm resistor?

2. 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?

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

4. An electric drill draws 3.2 amps of electric current at 120 volts. What is the power output of this drill?

5. An aquarium light bulb uses 210 J of energy in 30 s. How much power does it use?

6. How many joules of energy would be used by a 60-W light bulb in 1 hour?

7. Determine the percent efficiency of a 60 W fluorescent light bulb that uses 2000J of electrical energy to produce 400J of light energy.

8. Calculate the percent efficiency of an electric motor that uses 15 000 J of energy to produce 11 500 J of useful energy

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