Science 30 Electric Circuits Chapter 6 - Sign In Workbook... · Science 30 Electric Circuits...

Science 30 – Workbook Electrical Circuits

Science 30

Electric Circuits

Chapter 6

Name _____________________________________

McKennitt 2014

Section 6 Contents

6.1 – Current Electricity

6.2 - Circuits

6.3 – Electromagnetic Induction


6.1 – Current Electricity

Terminology

Current Electricity - Electrons moving in a continuous path.

Current – The flow of electrical charge by electrons (charge/second).

Measured in amperes (A)

Voltage – The electric potential difference between two points.

Measured in Volts (V)

Power – Rate at doing work (Energy/time). Measured in Watts (W)

Resistance – opposition to the passage of current through a

conductor. Measured in Ohms (Ω)

Energy – The capacity for doing work. Measured in Joules (J) or

kilowatt-hours (kWh)

Analogy of Circuits to Flow of Water


Types of Current

Direct Current: a flow of charges that does no increase or decrease and flows in a single direction. Often it

is used to power things using batteries or electronics that need a steady stream of power.

Works well over short distances. Loses a lot of energy over long distances.

Cells, batteries and regulated power supplies provide steady DC which is ideal for electronic circuits.

Alternating Current: a flow of charges that reverse directions at regular intervals.

Examples include: Power that comes from wall outlets.

Power lines use AC as it can travel farther without losing energy.

Question: Where do you see devices like this?


Quick Tip! Many problems may use more than

one formula. List what you have and then select

the most appropriate formula to solve the

equations.

Example Calculations

1. A circuit has a resistance of 4.00 Ω. Cells provide a voltage of 50.0V.

a) What amount of current passes through the circuit? b) What amount of power is this?

c) For 20.0 seconds, how much energy is transferred in the circuit?


Practice: Try these on your own.

If a circuit has 2.50 A of current flowing through a wire that provides 28.0 Ω of resistance. What voltage is needed from the battery?

How many seconds must a 740W portable Bluetooth speaker need to run to use 5.80 kJ of energy?

If the power rating is 300 W, what is the resistance when the current is 1.40 A?

Think of a waterfall as an analogy for voltage, current, and resistance. What parts of the waterfall would represent each component?


Energy Costs

-traditional unit for billing is the kilowatt hour (kWh) is a measure of Energy. Not Power!

Examples:

How much energy is consumed if a 500W television runs for 12 days straight? Answer in Joules and kWh.

Using the price of electricity for April, calculate how much it would cost Mr.

McKennitt to run his bumpin’ boom box (1800W) from now until SPA’s

grad (use 17 days)


Lesson 6.1 HW

1. Answer the following current electricity questions

a) Electricity transferred from a power plant through transformers and power lines travels large

distances in Canada. This type of current is _______________.

b) Electricity used in circuits where there needs to be a steady supply of electricity over a short

distance is this type of current _____________.

c) Sketch what Voltage over Time graphs look like for both direct and alternating current.

2. An automobile headlight has an average resistance of 24.0 Ω. Car batteries provide a voltage of 12.0V.

What amount of current passes through the headlight?

3. In a portable radio, 0.500 A of current are flowing through a conductor that provides 18.0 Ω of

resistance. What Voltage is provided by the battery?

4. What is the resistance of hair dryer plugged into a 110 V outlet with a 10.0 A current flowing through it?


5. A portable CD player, operating with four 1.50 V cells connected in series, provides a resistance of 15 000 Ω. What amount of current is flowing through the CD player?

6. A digital recorder plugged into a 300 Watt stereo with a current of 0.500 A, what is the voltage in the circuit?

7. An electric heater draws 10.00 A from a 120.0 V source. What is the heater’s power in watts?

8. If the power rating is 500 W, what is the resistance when the voltage is 20 V?


9. A kettle rated at 2000 W operates for a total of 120.0 min in a typical week.

a. Determine the energy consumed in joules.

b. Determine the energy consumed in kilowatt-hours.

10. Most models of TVs and VCRs use electrical energy even when they are turned off. This stand-by power is used to run clocks in VCRs and to provide an “instant on” feature, allowing home electronics to become operational with a click of the remote control.

Average values for stand-by power are about 3.00 W for a VCR and 5.00 W for a TV. Since this power is required 24 h a day, the electrical energy consumption is significant.

a. Determine the electrical energy required to supply 8.00 W of stand-by power for both a TV and a VCR during one year (365 days year). Express your answer in kilowatt-hours.

b. If the price of electricity is 9.30¢/kWh, determine the cost in dollars of providing stand-by power to the VCR and TV for 365 days.

c. There are about 2.0 million TVs and VCRs that operate with stand-by power in Alberta. Use this fact to estimate the total annual cost of maintaining stand-by power for all of these devices in Alberta.


11. A household clock rated at 5.00 W is operated for 365 days. Calculate the total electrical energy cost if the charge for electrical energy is 8.70¢/kWih.

6.1 HW Solutions

1) See your notes

2) 0.500 A

3) 9.00 Ω

4) 11.0 Ω

5) 4.0 X 10-4 A

6) 600V

7) 1200W

8) 0.800 Ω

9 a) 1.44 X 107 J 9b) 4.00 kWh

10 a) 70.1 kWh 10b) $6.52 10c) $13 million

11) $3.81


6.2 - Circuits

Schematic Diagram: a simplified drawing that shows parts of the circuit.

Components of a Circuit

Component Description Examples

SOURCE Provides energy and supply of electrons for the

circuit

CONDUCTOR

provides a path for the current

SWITCH Controls the current flow, turning it off and on,

directing it to different paths of the circuit

LOAD Converts electrical energy into another form of

energy

Schematic Symbols – YOU NEED TO KNOW ALL OF THESE!


Types of Circuits

Series Circuits: Parallel Circuits:

Series Circuits: All the electrons flow through _______________________

of paths.

Parallel Circuit -The electrons can flow through _____________________

paths.

House Wiring: Practical electric circuits for the home are a mix of series

and parallel circuits.

In a circuit we will use a multi-meter to measure the Current, Voltage, and Resistance of a Circuit.

- ________________________ (also can use an ammeter as part of the circuit)

- ________________________ (also can use a ohmmeter as part of the circuit)

- _________________________ (also use a voltmeter outside the testing area)

** When using these to measuring devices:

- Ammeters/ohmmeters are measured as part of the circuit.

- A voltmeter will need to measure the voltage drop across part of the circuit (i.e. it is outside the

circuit)


Draw the following (schematic symbols on previous page):

Series Circuits:

A basic circuit wired in series with 1.5V cell, switch, and one resistor.

A circuit wired in series with two 1.5V cells, switch, and three light bulbs.

A circuit wired in series with AC current , switch, an ammeter, and a light bulb. Add a

voltmeter measuring one light bulb.

A circuit wired in series with three 1.5V cells, switch, an ohmmeter, and two resistors. Add two

voltmeters, each measuring across each resistor.

Question: If you remove any load in a series circuit, what happens?


Parallel Circuits:

Note: When drawing parallel circuits, don’t split the power sources (keep them all one the same ‘leg’ of the

circuit)

A basic parallel circuit with a 1.5V cell and two resistors (each resistor on its own leg of the circuit).

A parallel circuit with three 9V cells, two light bulbs on one leg of the circuit, and two light bulbs on

the other leg of the circuit.

Question: If you remove a light bulb from below, what would happen?


Resistance in the Circuits

Large power overloads are dangerous, potentially destroying electrical equipment or causing a fire. In your

house, circuit breakers and fuses act as protection:

Fuses have a small piece of metal inside. If the current gets too high, the metal melts electricity

cannot flow until the fuse is replaced.

Circuit breakers have an electromagnet, which trips the circuit breaker when an unsafe limit is

reached. Once tripped, the switches simply turn off until it is reset.

In a series circuit – there is only one path, so resistance of every load (resistor, light bulb, appliances,

etc..) will add to the total resistance. Voltage will also add together (Vtot = V1 + V2 + V3…)

For example

What would the total resistance below be?

What would the current be in the circuit?

What would the power be?

Does it matter where you put the ammeter to measure current?

http://www.wisegeek.com/what-is-a-circuit-breaker.htm


For parallel circuits, adding extra branches to the circuit actually reduces the resistance. This is because

current is able to flow much easier through multiple pathways than just one. The voltage of each branch

will also have the same amount of voltage (Vtot = V1 = V2 = ….)

Example Problems:

Calculate the total resistance in the circuit.

Example Problem #2 – Calculate the total resistance of a parallel circuit with two branches – one branch

with a 4.0 ohm resistor and another branch with a 10.0 ohm resistor.


6.2 Homework

1. Draw a schematic diagram with a circuit wired in series with four 3.5V cells, switch, and two light

bulbs, a toaster, and a miniature blue fan. Add a voltmeter that measures the potential difference of the

toaster.

2. Draw a schematic diagram with a parallel circuit with three 9V cells, one light bulb and switch on one

leg of the circuit, and two light bulbs and a switch the other leg of the circuit. Add a master switch that

shuts off all the electricity in the circuit. Put an ammeter somewhere in the circuit.

3. Identify the major advantage of parallel circuits over series circuits.


4. Calculate the total resistance in the following circuit: [2 marks]

5. Calculate the total resistance and then the total voltage in the following circuit: [2 marks]

6. If the total resistance of a series circuit is 80 ohms and there are four identical resistors, what is the

resistance of each resistor?

7. Calculate the total resistance and then find the current running through the ammeter. [3 marks]


8. In a parallel circuit, there are three resistors, each on its own branch of the parallel circuit. If each

resistor is measured at 0.140 ohms, what is the total resistance of the circuit?

9. In a parallel circuit, there are three resistors, each on its own branch of the parallel circuit. If

resistor1=25.0 ohm, resistor2=70.0 ohm, and resistor3=100.0 ohm, what is the total resistance of the

parallel circuit?

10. The total resistance in a circuit containing three resistors is 2.0 ohm. If the values of two of these

resistors are 4.5 ohm and 9.0 ohm, what is the value of the third resistor?

Answers:

4) 35.0 Ω

5) R = 7.5 Ω and V=60V

6) R = 20 Ω

7) R = 4.4 Ω and I = 5.6 A

8) R = 0.0467 Ω

9) R = 15.6 Ω

10) 6.0 Ω


6.3 – Electromagnetic Induction

_____________________________ – one of the fundamental forces in our universe (another is gravity..)

________________________________ – to ________________________________/field in wires.

There are four main effects resulting from the interactions of electricity and magnetism. We already know

two of them….

_____________________________________________________________ with a force inversely

proportional to the square of the distance between them: unlike charges attract, like ones repel.

____________________________ (or states of polarization at individual points) attract or repel one

another in a similar way and always come in pairs: every North Pole is paired with a south pole.

Experiment #1 – Michael Faraday Wraps a Compass in Wire

Describe what happens when the switch is

closed/opened:

Why does this happen?

http://micro.magnet.fsu.edu/electromag/java/faraday/index.html


Experiment #2 – Magnetic Field Induction by a Moving Magnet

Describe what happens when the switch is

closed/opened:

Why does this happen?

Major Discovery: Electromagnetic Induction

An electric current in a wire creates a circular magnetic field around the wire, its direction depending on

that of the current.

A current is induced in a loop of wire when it is moved towards or away from a magnetic field with the

direction of current depending on that of the movement.

http://micro.magnet.fsu.edu/electromag/java/faraday2/index.html

http://www.brainpop.com/technology/energytechnology/electromagneticinduction/


Applications of Electromagnetic Induction

Three applications of electromagnetic induction we will look at include: motors, generators, and transformers.

Motors – ____________________________________________________________________________

Generators – ________________________________________________________________________

Motor

Motors ______________________________________ to run. Think about your car without a battery…

SEE p. 358 for description – NEED TO KNOW PARTS and STEPS

http://www.walter-fendt.de/ph14e/electricmotor.htm

Armature – part of the motor that spins. It is a coil of

wire or a rotating shaft.

Commutator – acts as a bridge between the armature and

the brush to provide current. Is often split in order to

switch on/off the flow of electricity.

Magnets – Permanent magnets provide

attraction/repulsion for the moving current and magnetic

field.

Brush – makes contact with the Commutator

Source – Electricity is needed to run the current.

The key component to the motor is the split-ring commutator that will switch the flow of electrons and

reverse the magnetic field. This will allow the armature to keep spinning continually.

http://www.walter-fendt.de/ph14e/electricmotor.htm


Generators

Generators _______________________. Think about generators in power plants or for portable power!

See p. 361 of your text.

Just like with a motor, spinning of magnetic/electric field produces current. This time, electricity is not powering the shaft, but an outside source (mechanical energy) http://www.vjc.moe.edu.sg/fasttrack/physics/generator.htm http://www.seedsfoundation.ca/energyliteracyseries3.html

Transformers

How could 500kV transmitted from the power plant to the home be a

problem? (Homes use about 240V or 120V)

Voltage is changed by devices called TRANSFORMERS.

Transformers increase (step up) or decrease (step down) the voltage in a circuit. This is done using

electromagnets made from coils of wire and iron. A basic transformer setup is displayed below:

Transformers only work with alternating current because this provides a constant change of flux and thus

a constant induced voltage.

Key components

- Iron Core

- 2 coils of wire: Primary coil (input) and Secondary coil (output)

An input voltage is supplied to the primary coil. This causes a voltage to be induced in the right of the

transformer. The voltage induced on this, the secondary coil, depends on the number of turns in the wire of

the coil. To increase the output voltage from the input voltage you increase the amount of coils on right

compared to the left. The opposite causes the voltage to decrease.

http://upload.wikimedia.org/wikipedia/commons/4/4f/Modern_Steam_Turbine_Generator.jpg

http://www.vjc.moe.edu.sg/fasttrack/physics/generator.htm

http://www.seedsfoundation.ca/energyliteracyseries3.html


This gives rise to the formula:

Two types of transformers

Step up – Increases the voltage (low number of loops higher number of loops)

Step down – Lowers the voltage (high number of loops lower number of loops)

Ex. #2 Neon sign at La Bezt 240V input uses a transformer to increase the voltage to 12000V.

a) Is this step up or step down transformer? _________________________

b) There are 125 turns (also called loops) on the primary, how many on the secondary?

c) Requires 25.0A to operate, determine the original current before the transformer?

Example #3

A socket supplies a TV with 240V however the TV circuit only requires 12V.

The TV uses a transformer with 400 turns on the primary coil to step down the voltage.

How many turns are required on the secondary coil?


6.3 Homework:

1. Sketch a simple St. Louis motor and label the parts (use armature, permanent magnets, conducting

wire).

2. Explain how the components of a motor allow the motor to continuously spin.

3. Sketch the simple electromagnet and label the parts.

4. Explain the term: electromagnetic induction and a practical use of an electromagnet.


5. An ideal transformer has 100 turns of wire in the primary coil and 1000 turns of wire in the secondary coil. If the voltage and current in the primary coil is 120 V and 10.0 A, respectively, determine

a. the voltage and current in the secondary

b. the power in the primary and the secondary

6. The doorbell of a home requires 10.0 V to operate. A transformer is used to connect a 120-V primary circuit within the home to the doorbell. The doorbell transformer has 500 turns on the primary coil and supplies the doorbell with 900 mA of current.

a. Determine whether the transformer features a step-up or step-down design. ___________________

b.Calculate the number of turns on the secondary coil.

c. Calculate the current that is drawn from the 120-V household circuit to operate the doorbell in mA.

Answers:

5a) Vs=1200V and Is = 1.00 A 5b) 1200W for both

6a) Step Down 6b) 42 turns/loops 6c) 75 m

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