any path along which electrons can flow is a circuit

35 Electric Circuits

Any path along which electrons can flow is a circuit.


Mechanical things seem to be easier to figure out for most people than electrical things. Maybe this is because most people have had experience playing with blocks and mechanical toys. Hands-on laboratory experience aids your understanding of electric circuits. The experience can be a lot of fun, too!


In a flashlight, when the switch is turned on to complete an electric circuit, the mobile conduction electrons already in the wires and the filament begin to drift through the circuit.

35.1 A Battery and a Bulb


A flashlight consists of a reflector cap, a light bulb, batteries, and a barrel-shaped housing with a switch.



There are several ways to connect the battery and bulb from a flashlight so that the bulb lights up.

1. The important thing is that there must be a complete path, or circuit, that

• includes the bulb filament ( load with resistance)• runs from the positive terminal at the top of the battery • runs to the negative terminal at the bottom of the battery



2. Electrons- real flow• from the negative part of the battery through the wire • to the side (or bottom) of the bulb• through the filament inside the bulb• out the bottom (or side) • through the wire to the positive part of the battery

The current then passes through the battery to complete the circuit.

3. Conventional Flow of Electrons from positive part of the battery to negative .



a. Unsuccessful ways to light a bulb.



a. Unsuccessful ways to light a bulb.

b. Successful ways to light a bulb.



The flow of charge in a circuit is very much like the flow of water in a closed system of pipes.

In a flashlight, the battery is analogous to a pump, the wires are analogous to the pipes, and the bulb is analogous to any device that operates when the water is flowing.

When a valve in the line is opened and the pump is operating, water already in the pipes starts to flow.



Neither the water nor the electrons concentrate in certain places.

They flow continuously around a loop, or circuit.

When the switch is turned on, the mobile conduction electrons in the wires and the filament begin to drift through the circuit.



Electrons do not pile up inside a bulb, but instead flow through its filament.



What happens to the mobile conduction electrons when you turn on a flashlight?



For a continuous flow of electrons, there must be a complete circuit with no gaps.

35.2 Electric Circuits


4. Any path along which electrons can flow is a circuit.

A gap is usually provided by an electric switch that can be opened or closed to either cut off or allow electron flow.



The water analogy is useful but has some limitations.• A break in a water pipe results in a leak, but a

break in an electric circuit results in a complete stop in the flow.

• Opening a switch stops the flow of electricity. An electric circuit must be closed for electricity to flow. Opening a water faucet, on the other hand, starts the flow of water.



Most circuits have more than one device that receives electrical energy.

These devices are commonly connected in a circuit in one of two ways, series or parallel.

5. When connected in series, the devices in a circuit form a single pathway for electron flow.

6. When connected in parallel, the devices in a circuit form branches, each of which is a separate path for electron flow.



How can a circuit achieve a continuous flow of electrons?



If one device fails in a series circuit, current in the whole circuit ceases and none of the devices will work.

35.3 Series Circuits


If three lamps are connected in series with a battery, they form a series circuit. Charge flows through each in turn.

When the switch is closed, a current exists almost immediately in all three lamps.

The current does not “pile up” in any lamp but flows through each lamp. Electrons in all parts of the circuit begin to move at once.



Eventually the electrons move all the way around the circuit.

6. A break anywhere in the path results in an open circuit, and the flow of electrons ceases.

Burning out of one of the lamp filaments or simply opening the switch could cause such a break.



In this simple series circuit, a 9-volt battery provides 3 volts across each lamp.

35.3 7.Series Circuits


For series connections:• Electric current has a single pathway through the circuit. • The total resistance to current in the circuit is the sum of

the individual resistances along the circuit path.• The current is equal to the voltage supplied by the source

divided by the total resistance of the circuit. This is Ohm’s law.

• The voltage drop, or potential difference, across each device depends directly on its resistance.

• The sum of the voltage drops across the individual devices is equal to the total voltage supplied by the source.

35.3 8. Series Circuits


The main disadvantage of a series circuit is that when one device fails, the current in the whole circuit stops.

Some cheap light strings are connected in series. When one lamp burns out, you have to replace it or no lights work.

35.3 9. Disadvantage of Series Circuits


think!What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit?



think!What happens to the light intensity of each lamp in a series circuit when more lamps are added to the circuit?

Answer:

The addition of more lamps results in a greater circuit resistance. This decreases the current in the circuit (and in each lamp), which causes dimming of the lamps.



think!A series circuit has three bulbs. If the current through one of the bulbs is 1 A, can you tell what the current is through each of the other two bulbs? If the voltage across bulb 1 is 2 V, and across bulb 2 is 4 V, what is the voltage across bulb 3?



think!A series circuit has three bulbs. If the current through one of the bulbs is 1 A, can you tell what the current is through each of the other two bulbs? If the voltage across bulb 1 is 2 V, and across bulb 2 is 4 V, what is the voltage across bulb 3?

Answer:

The same current, 1 A, passes through every part of a series circuit. Each coulomb of charge has 9 J of electrical potential energy (9 V = 9 J/C). If it spends 2 J in one bulb and 4 in another, it must spend 3 J in the last bulb. 3 J/C = 3 V



What happens to current in other lamps if one lamp in a series circuit burns out?



10. In a parallel circuit, each device operates independent of the other devices. A break in any one path does not interrupt the flow of charge in the other paths.

35.4 Parallel Circuits


In a parallel circuit having three lamps, each electric device has its own path from one terminal of the battery to the other.

There are separate pathways for current, one through each lamp.

In contrast to a series circuit, the parallel circuit is completed whether all, two, or only one lamp is lit.

10. A break in any one path does not interrupt the flow of charge in the other paths.



In this parallel circuit, a 9-volt battery provides 9 volts across each activated lamp. (Note the open switch in the lower branch.)

35.4 11. Drawing of a Parallel Circuits


Major characteristics of parallel connections:• Each device connects the same two points A and B of

the circuit. The voltage is therefore the same across each device.

• The total current divides among the parallel branches. • The amount of current in each branch is inversely

proportional to the resistance of the branch. • The total current is the sum of the currents in its

branches.• As the number of parallel branches is increased, the

total current through the battery increases.

35.4 12. Parallel Circuits


From the battery’s perspective, the overall resistance of the circuit is decreased.

This means the overall resistance of the circuit is less than the resistance of any one of the branches.

35.4 12. Parallel Circuits


think!What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit?



think!What happens to the light intensity of each lamp in a parallel circuit when more lamps are added in parallel to the circuit?

Answer:

The light intensity for each lamp is unchanged as other lamps are introduced (or removed). Although changes of resistance and current occur for the circuit as a whole, no changes occur in any individual branch in the circuit.



What happens if one device in a parallel circuit fails?



In a schematic diagram, resistance is shown by a zigzag line, and ideal resistance-free wires are shown with solid straight lines. A battery is represented with a set of short and long parallel lines.

35.5 Schematic Diagrams


Electric circuits are frequently described by simple diagrams, called schematic diagrams.

• Resistance is shown by a zigzag line, and ideal resistance-free wires are shown with solid straight lines.

• A battery is shown by a set of short and long parallel lines, the positive terminal with a long line and the negative terminal with a short line.

35.5 13. Schematic Diagrams


These schematic diagrams represent

a. a circuit with three lamps in series, and



These schematic diagrams represent

a. a circuit with three lamps in series, and

b. a circuit with three lamps in parallel.

35.5 14. Schematic Diagrams of Series and Parallel


What symbols are used to represent resistance, wires, and batteries in schematic diagrams?



The equivalent resistance of resistors connected in series is the sum of their values. The equivalent resistance for a pair of equal resistors in parallel is half the value of either resistor.

35.6 Combining Resistors in a Compound Circuit


Sometimes it is useful to know the equivalent resistance of a circuit that has several resistors in its network.

15. The equivalent resistance is the value of the single resistor that would comprise the same load to the battery or power source.

The equivalent resistance of resistors connected in series is the sum of their values. For example, the equivalent resistance for a pair of 1-ohm resistors in series is simply 2 ohms.



The equivalent resistance for a pair of equal resistors in parallel is half the value of either resistor.

The equivalent resistance for a pair of 1-ohm resistors in parallel is 0.5 ohm.

The equivalent resistance is less because the current has “twice the path width” when it takes the parallel path.

35.6 16. Example of Combining Resistors in a Compound Circuit


The equivalent resistance of two 8-ohm resistors in series is 16 ohms.



a. The equivalent resistance of two 8-ohm resistors in series is 16 ohms.

b. The equivalent resistance of two 8-ohm resistors in parallel is 4 ohms.

35.6 18. Combining Resistors in a Compound Circuit


For the combination of three 8-ohm resistors, the two resistors in parallel are equivalent to a single 4-ohm resistor.

They are in series with an 8-ohm resistor, adding to produce an equivalent resistance of 12 ohms.

If a 12-volt battery were connected to these resistors, the current through the battery would be 1 ampere.

(In practice it would be less, for there is resistance inside the battery as well, called the battery’s internal resistance.)



Schematic diagrams for an arrangement of various electric devices. The equivalent resistance of the circuit is 10 ohms.

35.6 19. Combining Resistors in a Compound Circuit


More Examples

20.Star Connection

21.Ladder Connection


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: Experiment

Analyzing Series and Parallel CircuitsUsing Ohm’s Law

I. Purpose • Learn about resistors and Ohm’s Law• Learn how to calculate resistance, current and voltage in

each circuit arrangement

• Discover how to use electrical circuits with: -resistors -breadboard-Multimeter (Am-meter, Volt-meter, Ohm-meter)

-connectors (alligator leads/clips)• Conduct experiments using resistors in: -series and parallel arrangements

• Measure resistance, current and voltage in each circuit arrangement

• Learn how to analyze test results


II. Procedures A. Series Connection

1. Connector 1 resistor using the breadboard.2. Measure the resistance across it using an ohmmeter.3. Set the ohmmeter to 20 X. Values indicated on the

screen of the ohmmeter will be multiplied by 1000.4. Add one resistor at a time up to 4 resistors of the same

value.5. Measure the resistance across two, three and four

resistors in series .6. Record . 7. Calculate the % difference = / Expected value –

Measured Value / divided by Measured Value


III. A. Data and Results

Expected Measured %

value value difference

R1 =

R1 + R2 =

R1 + R2+R3 =

R1 + R2 + R3 + R3 =


II.B Procedures Parallel Connection

1. Connect 1 resistor equals to 1,500 ohms in the breadboard.

2. Connect another resistor of the same value in parallel .

3. Measure the resistance across the parallel connection using an ohmmeter at X20K

4. Connect two parallel resistors in series with each other and measure the resistance at terminal A and B . See the circuit below .

5. Calculate the % difference .


IIIB. Data and Results Parallel

Expected Measured %

Value Value difference

R1

R1//R2

R//R2 + R3//R4


IV. Analysis and Conclusion

1. How are series and parallel connections done using a breadboard?

2. Explain how an ohmmeter is used in the circuit.

3. How is the total resistance or equivalent resistance calculated in a series and parallel circuits?

4. How is the % difference calculated ?5. Enumerate the things you have learned from

this experiment .


Experiment : Voltmeter and Ammeter in Series Circuit

I. Purpose : To measure the voltage across the battery

using a voltmeter.To measure Voltage drops across each

resistor using a voltmeter.To verify if the voltage drops equals the

voltage of the battery .To measure current in the series circuit

using an ammeter.


II. Materials

2 resistors – 1,500 ohms each

Battery = 6v

Multi meter

Alligator wires

breadboard


III. Procedures and Results :1. Set the voltmeter to VDC 20 V range. Vbattery=___v2. Measure the voltage of the battery by connecting the

red probe of the voltmeter to the + positive of the battery and the black probe to the – negative of the battery . Record.

3. Connect 2 resistors in series using the breadboard.4. Connect the + terminal of the battery and one terminal

of the series connection using a red alligator clip5. Connect the – terminal of the battery and the other

end of the series connection using a black alligator clip.


6. Using the voltmeter , measure the voltage across one resistor R1 by connecting the red voltmeter probe to the terminal of the resistor that is positive and the black probe of the voltmeter to the other terminal of R1. Record as VR1 = _____volts

7. Repeat procedure 6 with R2 VR2 = ______ volts 8. Calculate the total voltage by Vs = VR1 + VR2 = _____volts + _____volts Vs = _____ volts 9. Is the total voltage equal to the voltage of the battery ? Yes or No ? ________


IV. Procedures and Results for Ammeter:1. Set your multi meter to DCA 20 m range .2. Disconnect the wire that connects the + positive

of the battery and one end of the series circuit . 3. Insert the ammeter by connecting the positive

red probe of the ammeter to the +positive of the battery .

Connect the black negative probe of the ammeter to one terminal of the series that was disconnected.

4. Record the current reading .I = _____ A


Analysis and Conclusion

1. How do you set up an ammeter?2. How do you set up a voltmeter?3. How do you measure a current in a series circuit using

an ammeter ?4. How do you measure the voltage drops (voltage

across the resistors) and the voltage of the battery using a voltmeter?

5. What is the relationship of the voltage drops and the source voltage?

6. Based on your results, what is the current flowing from the battery ? Current through the resistors ?


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What's a Resistor?

A resistor is anything that electricity can not travel through easily. When electricity is forced through a resistor, often the energy in the electricity is changed into another form of energy, such as light or heat. The reason a light bulb glows is that electricity is forced through tungsten, which is a resistor. The energy is released as light and heat.

An Ohm is a unit used to measure the resistance value of a resistor. It is the name of scientist Georg Ohm who did research on materials with resistive characteristics.

A conductor is the opposite of a resistor. Electricity travels easily and efficiently through a conductor.


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Resistor Types


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Breadboard

Solid lines are impregnatedwith flat conducting stripped copper wires.

Wire inserted in a front view hole makes contact with one of the copper wires in the back.

A breadboard is a piece of a plastic board perforated with small holes and conduction channels that allow electronic parts to be mounted and connected into circuits.


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This is the Voltmeter symbol.

This is the switch symbol.

This is the resistor symbol.

This is the Ammeter symbol.

This is the battery symbol.

Electric Circuit Diagram

Engineers draw circuit diagrams to help them design the actual circuits. These diagrams depict the arrangement of components.

R


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• What is a SHORT? SHORT is a term used by engineers to describe the effect of inadvertent connection of two power supply outlets (plus and minus). A short circuit means that there is a short path between battery terminals, or almost no resistance between them. This causes unintended heating and sometimes “arcs and sparks” that can damage equipment.

•What is an OPEN or CLOSED circuit? OPEN is a term used by engineers to describe the circuit condition where current is not flowing (e.g., a switch is in the open position or a light bulb has a broken filament). A circuit must be CLOSED for current to flow allowing the circuit to perform its intended function (e.g., a light switch is in the closed position allowing the light to function).

Definitions


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Ohm’s Law

• Ohm’s Law establishes the relationship between voltage V, resistance R and electric current I in an electric circuit.

• When a voltage, V is applied in the same circuit an electric current, I flows and it is limited only by the circuit resistance, R.

or V = R x I (Eq. No. 2-1)

or Voltage = Resistance x Current

R is the resistance, measured in Ohms. Its symbol is V is the voltage measured in Volts. I is the current measured in Amperes. 1 Ohm = 1 Volt/Ampere

Ohm’s Law states that in a circuit with a given resistance the electric current varies proportionally to the applied voltage.


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More about Resistance, R

Electrical resistance is a property of a conductor that opposes the flow of electric current through it. An object of uniform cross section has a resistance proportional to its resistivity and length and is inversely proportional to its cross-sectional area.

Resistance, R = Resistivity, ρ x ----------------- (Eq. No. 2-2)

Units: Resistivity, ρ [ m] Length, l [m] Area, A [m2]

Length, l

Area, A

Example: If a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω m. ⋅


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Table 1 Material Resistivity, ρ (Ω·m) at 20 °C

Carbon (graphene) 1×10−8

Silver 1.59×10−8

Copper 1.68×10−8

Annealed copper 1.72×10−8

Gold 2.44×10−8

Aluminium 2.82×10−8

Calcium 3.36×10−8

Tungsten 5.60×10−8

Zinc 5.90×10−8

Nickel 6.99×10−8

Lithium 9.28×10−8

Iron 1.0×10−7

Platinum 1.06×10−7

Tin 1.09×10−7

Carbon steel (1010) 1.43x10−7

Lead 2.2×10−7

Titanium 4.20×10−7

Sea water 2×10−1

Drinking water 2×101 to 2×103

Glass 10×1010 to 10×1014


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Resistors In Series

• SERIES CIRCUITS

– The current that flows in a series circuit flows through every component in the circuit.

– All the components in a series connection carry the same current.

Rtot = R1 + R2 + R3 +………. Rn (Eq. No. 2-3)


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R1

I

R2

V1

V2

R3 V3

V

We must use Ohm’s Law to calculate the total resistance Rtot.

Vtot = V1 + V2 + V3

= R1 x I + R2 x I + R3 x I = (R1+ R2+ R3) x I ; Vtot = Rtot x I and we can deduce that Rtot

Rtot = R1+ R2+ R3 (See Eq. No. 2-3 )

Example:R1 = 1500 Ohm; R2 = 1500 Ohm; R3 = 1500 Ohm; V = 6 Volt;Calculate the total circuit resistance R tot, I and voltages V1, V2, V3Rtot = 1500 + 1500 +1500 = 4500 OhmI = V/Rtot = 6/450 = 1.33 mAV1 = 1500 x 1.33 = 1995 mV; V2 = 1500 x 1.33 = 1995 mV; V3 = 1500 x 1.33 = 1995 mV

Resistors in SeriesDerive equation for calculating the total circuit resistance and

voltage drop across each resistor


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Resistors in Parallel

• PARALLEL CIRCUITS

– The current in each individual resistor is found using Ohm’s Law. Factoring out the voltage gives:

– To find the total resistance of all components, add the reciprocals of the resistances (the conductances) and take the reciprocal of the sum.

R1

R2

Rn

I1

I2

In

Iin Iout

(Eq. No. 2-4


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1. Itot = V / Rt ; I1 = V /R1 ; I2 = V /R2 ; I3 = V /R3

Itot = V x (1/R1 + 1/R2 + 1/R3) therefore1/Rtot = 1/R1 + 1/R2 + 1/R3

We’ll first calculate the resistance of R1 and R2 in parallel i.e.. R1,2 where

2. Itot = V / R1,2 and1/R1,2 = 1/R1 + 1/R2

= R1 /(R2 x R1) + R2 /(R2 x R1) = (R1 + R2)/(R2 x R1) and the reverse of 1/R1,2

R1,2 = (R1 x R2) /(R1 + R2) (Eq. No. 2-5) We can use the same equation to calculate the total circuit resistance

3. R1,2,3= (R1,2 x R3) /(R1,2 + R3)

Resistors in ParallelDerive equation for calculating the total circuit resistance and the

current in each resistor

We must use Ohm’s Law to calculate the total resistance Rtot.R1

Itot

R2

I1 I2R3

I3

Itot

R1 R2

I1 I2

V

V


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Calculation of Series and Parallel Resistances

R1 and R2 in Series R1 and R2 in Parallel

Example no. 1: R1 = 1500 Ohm = 1.5 KOhm R2 = 1500 Ohm = 1.5 Kohm

R1,2 = R1+ R2 R1,2 = (R1 x R2) /(R1 + R2)

R1,2 = 3,000 Ohm = 3 Kohm

Example no. 2: R1 = 1500 Ohm = 1.5 KOhm R2 = 470 Ohm = 0.47 KOhm

R1,2 = 1.5 x 0.47 1.97

= 0.36 Kohm= 360 Ohm

R1,2 =1,970 Ohm =1.97 Kohm

R1,2 = _1.5 x 1.5 3

= 0.75 Kohm= 750 Ohm

Resistors in SERIES increase total circuit resistance.

Resistors in PARALLEL reduce total circuit resistance.


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Test No. 1 Resistors in Series

Four resistors are connected inSERIES with each other

Connect the four 1500 Ohm resistorsR1, R2, R3 and R4 as shown.

Measure the resistance using the 20K resistance scale on the meter.

Expected Actual

R1 = _______;_______ R1 + R2 = _______;_______R1 + R2 + R3 = _______;_______R1 + R2 + R3 + R4 = _______;_______

b5 to b10

a1 to a5

a10 to a15

b15 to b20

R1

R2

R3

R4

Note: Enter your Test No. 1 four resistors data on the Data Sheet Pg.18


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Two Resistors are connected in PARALLEL with each other.

Each of the two pairs is in SERIES with the other pairs.

Connect the four 1500 Ohm resistors R1, R2, R3 and R4 as shown. Measure the resistance using the 20K resistance scale.

Expected

Actual(R1 & R2) = ______ ______(R1 & R2) + (R3 & R4)=______ ______

R2R1

a1 to a5 b1 to b5

Test No. 2 Resistors in Parallel

R3 R4

Note: Enter your Test 2 data on the Data Sheet (pg. 18)


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Calculate the Current

Power supply voltage: 5.8 to 6.4 V (Why the variation?)

Resistor R1: 1425 to 1575 Ohm (Why the variation?)

Current, I = VR1

= ?

I = V RV

R1= _6_

1500= 0.004 A = 4.0 mA (nominal)

R1 V

a5

a1

BI

Note: Enter your calculated current value under Test 3 on the Data Sheet (pgs. 16 and 18)

See Eq. No. 2-1)


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Test No. 3 Measure the Current

•Set the Am-meter dial at 20m on the DCA scale•Connect the Am-meter BLACK probe alligator with R1 at (a1)•Touch the Am-meter RED probe with the Power Supply (Battery +)

•Measure the current: I = mA•Compare with the calculated value of the current: I = mA

R1 R

a5

a1

B

Note: Enter you Test 3 data on the Data Sheet (pg. 18)


7979

Test No. 4 Measure the Current

R1 R1,2

a5

a1

BR2

Note: Enter your Test No. 4 data on the Data Sheet (pg. 18)


8004/20/23

Expected Actual

Test No. 1 Four resistors in SERIES ____ Ohm _______Ohm

Test No. 2 Two resistors in PARALLEL with each other ____ Ohm _______Ohm

Test No. 3 Measure the current in the circuit with one resistor ____ mA _______mA

Test No. 4 Measure the current with two resistors in parallel ____ mA _______mA

Explain why is there a difference in voltage between No. 1 and 2 tests? ___________

____________________________________________________________________

____________________________________________________________________

Explain why is there a difference between the expected and actual current values in

Test No. 3? __________________________________________________________

____________________________________________________________________

Explain why is there a difference between No. 3 and No. 4 tests?____________________________________________________________________

____________________________________________________________________

Experiment No. 2 Resistors and Ohms LawData Sheet with Estimated and Actual Test Results

Student Name: ______________________


Review of Terminology

• Resistor A material that resists the flow of electrical current.• Resistivity A property of a conductor that opposes electric current flow. • Conductor A material through which electricity flows easily.• Insulator A material that has exceptionally high resistance, such that

it essentially stops the flow of electricity.• Ohm’s Law The relationship between voltage, current, and resistance.

In equation form, it is simply I =V/R or V=IR.• Circuit Diagram A drawing depicting an arrangement of components.• Open Circuit No current flows. The electrical circuit is incomplete.• Closed Circuit Current allowed to flow. The circuit is complete.• Short Circuit An unintended closed circuit.• Breadboard A device that allows mounting and connection of electrical

components in circuits.• Ohm-meter A way to measure resistance (R).• Am-meter A way to measure current (I).• Volt-meter A way to measure voltage (V).

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You learned:

-How to calculate resistances in series and parallel circuits

-Ohm’s law (I=V/R or its equivalent form V=IR):

~If we know two among three variables (voltage, current and resistance), Ohm’s Law allows us to calculate the unknown.

-Electric units of measurement: Ohm (resistance), Ampere (current), Volt (voltage)

NEW CONCEPT: Power is the rate of doing work. The power dissipated in a resistor is the product of voltage and current (VI) or V2/R (KW)

Summary


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Name Mark True or False.

1. In class we use a breadboard to mount and connect electronic parts. (T) (F)

2. An open circuit means electric current is not flowing. (T) (F)

3. A short circuit means the wire connectors are not long enough. (T) (F)

4. Ohm's law lets you calculate the current through a resistor from the measured voltage across the resistor and the known resistance. (T) (F)

5. Current flowing through a resistor varies with the voltage applied across the resistor. (T) (F)

6. The Volt and the Ohm are units in the Metric System, also known as Systeme Internationale or SI. (T) (F)

7. The current flowing through a resistor is always the same. (T) (F)

8. The current flowing through a resistor may vary if something else changes in the circuit. (T) (F)

9. Ohm's law defines the relationship between current, resistance and voltage. (T) (F)

10. Ohm is the name of a scientist. (T) (F) .

11. An Ohm is a measure of electrical resistance. (T) (F)

12. Ohm's Law is the name of an important law in electrical science. (T) (F)

13. Two 100 Ohm resistors in series, replacing a single 100 Ohm resistor, increase circuit resistance. (T) (F)

14. Two 100 Ohm resistors in parallel, replacing a single 100 Ohm resistor, increase circuit resistance. (T) (F)

15. One kilogram is 1000 grams, so one kilo-Ohm is 10,000 Ohms. (T) (F)

16. One Mega-byte is 1,000,000 bytes, so one Meg-Ohm is 1,000,000 Ohms, or 106 Ohms. (T) (F)

Homework, Experiment No. 2 Resistors and Ohm’s Law, Sheet 1


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Homework, Experiment No. 2 Resistors and Ohm’s Law, Sheet 2

17. In the electric circuit shown below calculate first the total circuit resistance and the voltages across each resistor R1, R2 and R3 based on these parameters:V= 10 VDC; R1 =300 Ohms; R2 = 600 Ohms; R3 = 600 Ohms; What is the total electric power delivered in Watts?

R1

I=?

R2

V1=?

V2 =?R3

V3=?

V=10V

18. Calculate the resistance of a copper wire that is 10,000 m long and has an area of 1 cm2; Hint: use Eq. No. 2-2 and Table 1.

Cm cm cm2

Type


22. More Circuit Diagram


think!In the circuit shown below, what is the current in amperes through the pair of 10-ohm resistors? Through each of the 8-ohm resistors?



think!In the circuit shown below, what is the current in amperes through the pair of 10-ohm resistors? Through each of the 8-ohm resistors?

Answer:

The total resistance of the middle branch is 20 Ω. Since the voltage is 60 V, the current = (voltage)/(resistance) = (60V)/(2 Ω) = 3 A. The current through the pair of 8-Ω resistors is 3 A, and the current through each is therefore 1.5 A.



What is the equivalent resistance of resistors in series? Of equal resistors in parallel?



To prevent overloading in circuits, fuses or circuit breakers are connected in series along the supply line.

35.7 Parallel Circuits and Overloading


Electric current is fed into a home by two wires called lines. About 110 to 120 volts are impressed on these lines at the power utility.

These lines are very low in resistance and are connected to wall outlets in each room.

The voltage is applied to appliances and other devices that are connected in parallel by plugs to these lines.



As more devices are connected to the lines, more pathways are provided for current.

The additional pathways lower the combined resistance of the circuit. Therefore, a greater amount of current occurs in the lines.

Lines that carry more than a safe amount of current are said to be overloaded, and may heat sufficiently to melt the insulation and start a fire.



Consider a line connected to a toaster that draws 8 amps, a heater that draws 10 amps, and a lamp that draws 2 amps.

• If the toaster is operating, the total line current is 8 amperes. • When the heater is also operating, the total line current increases to

18 amperes.• If you turn on the lamp, the line current increases to 20 amperes.



To prevent overloading in circuits, fuses or circuit breakers are connected in series along the supply line.

The entire line current must pass through the fuse.

If the fuse is rated at 20 amperes, it will pass up to 20 amperes.

A current above 20 amperes will melt the fuse ribbon, which “blows out” and breaks the circuit.



Before a blown fuse is replaced, the cause of overloading should be determined and remedied.

Insulation that separates the wires in a circuit can wear away and allow the wires to touch.

This effectively shortens the path of the circuit, and is called a short circuit.

A short circuit draws a dangerously large current because it bypasses the normal circuit resistance.



Circuits may also be protected by circuit breakers, which use magnets or bimetallic strips to open the switch.

Utility companies use circuit breakers to protect their lines all the way back to the generators.

Circuit breakers are used in modern buildings because they do not have to be replaced each time the circuit is opened.



How can you prevent overloading in circuits?



1. In a light bulb, the amount of current in the filament is

a. slightly less than the current in the connecting wires.

b. the same as the current in the connecting wires.

c. slightly greater than the current in the connecting wires.

d. twice as great as the current that is in the connecting wires.

Assessment Questions


1. In a light bulb, the amount of current in the filament is

a. slightly less than the current in the connecting wires.

b. the same as the current in the connecting wires.

c. slightly greater than the current in the connecting wires.

d. twice as great as the current that is in the connecting wires.

Answer: B



2. The flow of charge in an electric circuit is

a. much like the flow of water in a system of pipes.

b. very different from water flow in pipes.

c. like an electric valve.

d. like an electric pump.



2. The flow of charge in an electric circuit is

a. much like the flow of water in a system of pipes.

b. very different from water flow in pipes.

c. like an electric valve.

d. like an electric pump.

Answer: A



3. In a series circuit, if the current in one lamp is 2 amperes, the current in the battery is

a. half, 1 A.

b. 2 A.

c. not necessarily 2 A, depending on internal battery resistance.

d. more than 2 A.



3. In a series circuit, if the current in one lamp is 2 amperes, the current in the battery is

a. half, 1 A.

b. 2 A.

c. not necessarily 2 A, depending on internal battery resistance.

d. more than 2 A.

Answer: B



4. In a circuit with two lamps in parallel, if the current in one lamp is 2 amperes, the current in the battery is

a. half, 1 A.

b. 2 A.

c. more than 2 A.

d. cannot be calculated from the information given



4. In a circuit with two lamps in parallel, if the current in one lamp is 2 amperes, the current in the battery is

a. half, 1 A.

b. 2 A.

c. more than 2 A.

d. cannot be calculated from the information given

Answer: C



5. In a circuit diagram there may be

a. no switches.

b. at most, one switch.

c. two switches.

d. any number of switches.



5. In a circuit diagram there may be

a. no switches.

b. at most, one switch.

c. two switches.

d. any number of switches.

Answer: D



6. Consider a compound circuit consisting of a pair of 6-ohm resistors in parallel, which are in series with two 6-ohm resistors in series. The equivalent resistance of the circuit is

a. 9 ohms.

b. 12 ohms.

c. 15 ohms.

d. 24 ohms.



6. Consider a compound circuit consisting of a pair of 6-ohm resistors in parallel, which are in series with two 6-ohm resistors in series. The equivalent resistance of the circuit is

a. 9 ohms.

b. 12 ohms.

c. 15 ohms.

d. 24 ohms.

Answer: C



7. One way to prevent overloading in your home circuit is to

a. operate fewer devices at the same time.

b. change the wiring from parallel to series for troublesome devices.

c. find a way to bypass the fuse.

d. find a way to bypass the circuit breaker.



7. One way to prevent overloading in your home circuit is to

a. operate fewer devices at the same time.

b. change the wiring from parallel to series for troublesome devices.

c. find a way to bypass the fuse.

d. find a way to bypass the circuit breaker.

Answer: A


any path along which electrons can flow is a circuit

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