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Unit Plan for Electricity and Magnetism

Nov 08, 2014

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This document outlines a unit plan for teaching electricity and magnetism at grade 10 to grade 11 students.
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Page 1: Unit Plan for Electricity and Magnetism

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Page 2: Unit Plan for Electricity and Magnetism

NAME

SUBJECT PHYSICS

GRADE 11

DURATION THREE (3) WEEKS

UNIT TOPIC CIRCUITS AND COMPONENTS

TOPICS AND

SUBTOPICS

1) Circuit Diagrams

(a) Circuit symbols

(b) Types of electric circuits

i. Closed circuit

ii. Open circuit

iii. Short circuit

(c) Series and parallel connection of conductors

2) Cells

(a) The zinc-carbon cell

(b) Primary and Secondary cells

(c) Recharging a secondary cell

3) I -V Relationships

(a) Variation of current with voltage in:

(b) Metallic conductors at constant temperature

(c) Filament lamps

(d) Semiconductor diodes

(e) Electrolytes

4) Resistance, R

(a) The ohm

(b) Resistors

(c) The resistance of ammeter and voltmeter

(d) Resistors connected in series

(e) Resistors connected in parallel

5) Electricity in the Home

(a) Reasons for using parallel circuits in the home

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Page 3: Unit Plan for Electricity and Magnetism

(b) Electrical Safety devices

i. Fuses

ii. Circuit breakers

iii. Earth wire

(c) International insulation colour code

Hazards of mains electricity

SUMMARY NOTES

Circuit Diagrams

Circuit symbols

A circuit diagram is a simple and clear way to record how a circuit is constructed. Special

symbols are used to represent all the common devices that are used in electrical circuits and to

show how they are connected.

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Page 4: Unit Plan for Electricity and Magnetism

Types of electric circuits

A closed circuit is any continuous path round which an electric current flows.

Figure 1

This circuit reveals three features that are common to all electric circuits. All closed electric

circuits:

have a source of potential difference (sometimes called a voltage source or an e.m.f.);

have a complete conducting path for current flow;

contain resistance; the lamp provides the resistance in this case.

An open circuit is one in which there is a break in the conduction path. The lamp in Figure 2

does not light because there is a break in the circuit.

Figure 2

A short circuit occurs when a voltage source or potential drop has a closed path of low

resistance across its ends. The lamp in Figure 3 does not light because almost no current flows

through it. Most of the current flows through the short circuit.

Figure 3

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Page 5: Unit Plan for Electricity and Magnetism

N. B.: Short-circuiting is a fire hazard as large amounts of heat are normally

generated.

Series and parallel connection of conductors

These lamps are connected in series.

The lamps share the voltage from the battery, so each has equal brightness.

If one lamp is removed, the others go out because the circuit is broken.

These lamps are connected in parallel.

Each lamp gets the full voltage from the battery because each is connected directly to it,

so each glows brightly

If one lamp is removed, the others keep working because they are still part of an

unbroken circuit.

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Page 6: Unit Plan for Electricity and Magnetism

Cells

The zinc-carbon dry cell

Carbon rod: positive electrode and current collector

Zinc case: negative electrode

Ammonium chloride solution: electrolyte

Manganese dioxide: stops polarization

Primary and Secondary cells

Characteristics of Primary cells

Eg. Simple cell, dry cell (zinc-carbon cell)

chemical reactions are not easily reversible

cannot normally be recharged

must be replaced with a new one when fully discharged

portable

produce small currents

usually have lower emf than secondary cells

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Page 7: Unit Plan for Electricity and Magnetism

have larger internal resistance than secondary cells

cheaper to produce

easier to install

Characteristics of Secondary cells

eg lead-acid cells in car batteries

must be charged before use

can be recharged over and over again

the chemical reactions are reversible

not as portable as primary cells

can produce large currents

usually have higher emf than primary cells

have smaller internal resistance than primary cells

more expensive to produce

harder to install

Recharging a secondary cell

To charge (or recharge) secondary cells:

low charging currents should be used

the positive terminal of a d. c. supply must be connected to the positive terminal of the

cell or battery

the emf (voltage) of the d. c. supply must be greater than that of the cell or battery

The diagram below shows a simple charging circuit.

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Page 8: Unit Plan for Electricity and Magnetism

N. B.: Overcharging produces gases, which can be dangerous.

I -V Relationships

Variation of current with voltage in:

(a) Metallic conductors at constant temperature

Current is directly proportional to voltage and resistance is constant. This conductor closely

obeys Ohm’s law

(b) Filament lamps

Graph obeys ohm’s law at low currents (straight line). Increasing resistance due to rising

temperatures is responsible for the curve part of the graph.

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Page 9: Unit Plan for Electricity and Magnetism

(c) Semiconductor diodes

A semiconductor diode is a non-ohmic device

(d) Electrolytes

Resistance, R

The resistance of a conductor is its impedance to current flow.

Resistance of a conductor depends on its:

Length

Cross-sectional area

Type of material

Temperature

resis tan ce= voltagecurrent , S. I. Unit of resistance is the ohm ()

The ohm

The resistance of a device is one ohm if the current through it is one ampere when the potential

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Page 10: Unit Plan for Electricity and Magnetism

difference across its ends is one volt.

Resistors are devices that oppose, control, or limit the current in electrical circuits.

Types of resistors

Fixed resistors have one specific value, eg. 5 ohms or 1 million ohms.

Variable resistors (rheostats) are used for varying current.

Thermistors have a high resistance when cold but a much lower resistance when hot.

Light-dependent resistors (LDRs) have a high resistance in the dark but a low

resistance in the light.

Diodes have an extremely high resistance in one direction but a low resistance in the

other. In effect, they allow current to flow in one direction only.

The resistance of ammeter and voltmeter

Ammeters should have as low a resistance as possible so that the total circuit resistance is not

increased. A greater circuit resistance would reduce the circuit current.

The ideal voltmeter should have infinitely large resistance so that no current flows through it.

Any current flowing through a voltmeter means an increase in the total circuit current, which

will affect the voltmeter reading.

Resistors connected in series

The resistors R1, R2 and R3 in Figure 5 are of different values. They are connected in series.

Figure 4

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For series circuits:

the same current flows through all components

the total resistance, Rs = R1 + R2 + R3;

the total voltage or emf (Vs ) of the power supply is the sum of the individual voltages

across each resistor

Vs = VR1 + VR2 + VR3

Vs = IRs

if there is a break at any point in the circuit, no current flows;

the total power dissipated is equal to the sum of the powers dissipated in the individual

resistors.

Ps = P1 + P2 + P3,

Ps = IVs

Resistors connected in Parallel

For the parallel arrangement of resistors of Figure 6

Figure 5

the potential difference across each parallel branch or load resistor is the same and is

equal to the potential difference of the battery;

the reciprocal of the total resistance equals the sum of the reciprocals of the individual

resistances:

1RP

= 1R1

+ 1R2

+ 1R3

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the total current is the sum of the currents in the individual branches: Ip = I1 + I2 + I3

the potential difference across each resistor branch is the same;

the total resistance for a parallel combination of resistors is always less than the value

of the smallest resistance;

the total power dissipated equals the sum of the powers dissipated in the individual

resistors;

Ps = P1 + P2 + P3

Ps = Ip V

a break in any branch of a parallel circuit does not affect other branches. However, the

total current decreases;

adding an extra branch in parallel with those already present increases the total current

taken from the cell or battery.

Electricity in the Home

Reasons for using parallel circuits in the home

Appliances will use only as much current as is needed

multiple appliances can be operated independently of each other

it is safer

Electrical Safety devices

i. Fuses

Fuses use the heating effect of an electric current to break or open a circuit when an

excessive current flows thus reducing the danger it poses. Fuses may be made of tinned

copper wire. They rated according to the maximum safe current they can carry. The fuse

should blow when a fault develops so that this safe current is not exceeded. Three-pin plugs

are usually fitted with either 3A, 5A, or 13A fuses.

ii. Circuit breakers

Circuit breakers are resetable switches that protect equipment from excessive currents by

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opening or tripping when a specified safe current is exceeded.

iii. Earth wire

This is a safety wire. It connects the metal body of an appliance to earth and stop it becoming

live. If the live wire becomes loose and touches the metal body, a current immediately flows to

earth and blows the fuse.

International insulation colour code

Live wire: brown insulation

Neutral wire: blue insulation

Earth: green/yellow insulation

Adverse effects of incorrect or fluctuating voltage

electrical fires

damage appliances

decreased power output of appliances

electrocutionGENERAL

OBJECTIVE

S

At the end of the unit students will:

1) have a working knowledge of electrical circuits and components

2) have a conceptual understanding of electrical quantities and the relations between

them.

3) be aware of applications of electricity in the home

SPECIFIC

OBJECTIVE

S

At the end of each lesson students will be able to:

Circuit Diagrams

1) use circuit symbols for cells, switches, wires, fuses, fixed and variable resistors,

filament lamps, Voltmeters, ammeters and semiconductor diodes;

Cells

2) draw a diagram of a zinc-carbon cell and explain the functions of its various parts;

3) state the difference between primary and secondary cells and their relative advantages

and disadvantages;

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Page 14: Unit Plan for Electricity and Magnetism

4) draw a circuit diagram to show how a secondary cell can be recharged;

I - V Relationships

5) describe experiments using an ammeter and a voltmeter to investigate the relationship

between current and potential difference for:

(a) metallic conductors at constant temperature;

(b) filament lamps;

(c) semiconductor diodes;

(d) solutions of copper sulphate in water using copper electrodes;

6) analyze I-V graphs and make inferences about the I-V relationship.

Resistance, R

7) explain the concept of resistance;

8) use the relationship

R=VI to solve problems;

9) explain why it is necessary for an ammeter to have a very low resistance;

10) explain why it is necessary for voltmeter to have very high resistance;

11) differentiate between series and parallel circuits;

12) recall that current in a series circuit is the same everywhere in the circuit and apply

this concept to solve problems;

13) recall that the sum of the currents in the branches of a parallel circuit is equal to the

current entering or leaving the parallel section and apply this concept to solve

problems;

14) recall that the sum of the potential differences across any number of components in

series is equal to the potential difference across all those components and apply this

concept to solve problems;

15) recall that the potential difference across any number of components in parallel

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is the same and apply this concept to solve problems;

16) recall and use the formulae:

R s=R1+R2+R3+⋯ for resistors in series; and

1RP

= 1R1

+ 1R2

+ 1R3

+⋯

for resistors in parallel;

Electricity in the Home

17) discuss the reasons for using parallel connections of domestic appliances;

18) explain the purpose of a fuse or circuit breaker;

19) select a fuse or circuit breaker of suitable current rating for a given appliance;

20) explain the function of the earth wire;

21) recall international insulation colour code;

22) state the adverse effects of connecting electrical appliances to incorrect or fluctuating

voltage supplies.

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Page 16: Unit Plan for Electricity and Magnetism

Classification of specific objectives in the cognitive domain according to Bloom’s Taxonomy

TAXONOM

Y

SPECIFIC OBJECTIVES

Knowledge 2) describe experiments using an ammeter and a voltmeter to investigate the

relationship between current and potential difference for:

(a) metallic conductors at constant temperature;

(b) filament lamps;

(c) semiconductor diodes;

(d) solutions of copper sulphate in water using copper electrodes;

3) recall that current in a series circuit is the same everywhere in the circuit and

apply this concept to solve problems;

4) recall that the sum of the currents in the branches of a parallel circuit is equal

to the current entering or leaving the parallel section and apply this concept to

solve problems;

5) recall that the sum of the potential differences across any number of

components in series is equal to the potential difference across all those

components and apply this concept to solve problems;

6) recall that the potential difference across any number of components in

parallel is the same and apply this concept to solve problems;

7) recall the formulae:R s=R1+R2+R3+⋯ for resistors in series; and

1RP

= 1R1

+ 1R2

+ 1R3

+⋯ for resistors in parallel;

8) select a fuse or circuit breaker of suitable current rating for a given appliance;

9) recall international insulation colour code;

10) state the adverse effects of connecting electrical appliances to incorrect or

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fluctuating voltage supplies.

11) draw a diagram of a zinc-carbon cell

12) draw a circuit diagram to show how a secondary cell can be recharged;

Comprehension Students will be able to:

13) explain the concept of resistance;

14) explain why it is necessary for an ammeter to have a very low resistance;

15) explain why it is necessary for voltmeter to have very high resistance;

16) explain the purpose of a fuse or circuit breaker;

17) explain the function of the earth wire;

18) explain the functions of the various parts of a zinc-carbon cell

Application Students will be able to:

19) use circuit symbols for cells, switches, wires, fuses, fixed and variable

resistors, filament lamps, Voltmeters, ammeters and semiconductor diodes;

20) use the relationship

R=VI to solve problems;

21) use the formulae: R s=R1+R2+R3+⋯ for resistors in series; and

1RP

= 1R1

+ 1R2

+ 1R3

+⋯ for resistors in parallel

Analysis Students will be able to:

22) analyze I-V graphs and make inferences about the I-V relationship

23) differentiate between series and parallel circuits;

24) discuss the reasons for using parallel connections of domestic appliances;

Synthesis

ACTIVITIES

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Page 18: Unit Plan for Electricity and Magnetism

Topic: Circuit Diagrams

Teaching strategy: Guided Discovery

Activity1: Testing an electric circuit

1. Connect up a circuit as shown in fig.6 including a single 1.5-volt battery (called a cell), a switch and a

lamp.

Figure 6

2. Press the switch to close the circuit.

3. What happened to the lamp when the switch was closed?

4. Investigate whether the lamp will stay lit if the circuit is broken at other points such as A or B, or by

disconnecting a wire or by unscrewing the bulb in its holder. fig. 7

Figure 7

5. What happened to the lamp when the circuit was broken?18

Page 19: Unit Plan for Electricity and Magnetism

6. What condition the circuit must be in for the lamp to light?

7. What is the name given to the circuit in figure 6?

8. What is the name given to the circuit in figure 7?

9. Complete or close the circuit again so that the lamp lights.

10. Now connect a piece of copper wire between points A and B and notes what happens.

11. What happens to the lamp?

12. Carefully feel the wire with your fingers then disconnect the wire.

13. How does the wire feel? Why does it feel this way?

14. Why is it important not to leave the wire connected in the wire for long?

15. What is the name given to this circuit in figure 8 in which the copper wire bypasses the lamp?

Figure 8

Activity2: Investigating Series and Parallel Connection of Conductors

1. Using three 1.25 volt lamps as conductors, connect up the circuit in Figure 9 on a circuit board.

2. Compare the brightness of the lamps.

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Page 20: Unit Plan for Electricity and Magnetism

Figure 9

3. Give an explanation for your observation in (2).

4. Now unscrew any one of the lamps and watch what happens. These lamps are said to be connected in

series.

5. Give an explanation for your observation in (4).

When lit, the lamps have equal brightness. This shows that the same current flows through all the lamps when

they are connected in series. If any one lamp fails or comes loose in its holder the whole circuit is broken and,

with no current anywhere in the circuit, all the lamps go off. This happens in some Christmas tree lamp circuits,

but not all. Lamps connected in series in a circuit follow one after the other like a series of events.

6. Connect the same three lamps together as shown in figure 10 using only one 1.5-volt cell in this circuit to

avoid burning out the lamps.

Figure 10

7. Again compare the brightness of the lamps, then unscrew any one of them and note what happens.

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In parallel connection the current in a circuit divides up and only part of it flows in each conductor. When

lamps are connected in parallel, if one fails it does not affect the other lamps; less current flows in the

circuit as a whole.

8. Using three similar lamps as before, connect them to two 1. 5 volt cells as in figure 11, and again

compare their brightness.

Figure 91

TOPIC: CELLS

Guided Discovery

Activity3: Drawing a labeled diagram of the zinc-carbon dry cell (a primary cell)

1) The teacher will cut away a section of a zinc-carbon dry cell and ask the students if they can identify

each section. If they cannot the teacher will identify them for the students.

2) The teacher will then explain the function of each part of the dry cell.

3) The teacher will then place a large chart showing an unlabelled diagram of the zinc-carbon dry cell

before the class.

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4) The teacher will provide two sets of flash cards – one set with the labeling and the other set with the

functions. Individual students will be allowed to identify parts of the dry cell by placing a flash card

on the appropriate part of the diagram.

5) Individual students will be asked to match the various parts of the dry cell with its function.

6) Finally the students will be asked to draw the diagram in their notebooks.

Activity 4: Making and testing a lead-acid storage cell (a secondary cell)

1) Arrange two lead plates in a small beaker so that they hang in some dilute sulphuric acid, but do not

touch each other.

2) Connect up the charging circuit as shown in fig. 12.

3) Use a direct current source of about 4V (e.g. 3 dry cells in series) and, if available, a centre-zero

ammeter of range - I to +1A.

The ammeter is used to show the direction of the current during charging and discharging.

4) Allow the charging current to flow for about five minutes then look carefully at the lead plates.

5) Now replace the battery with a filament lamp of I.25V rating and watch what happens, fig. 13.

6) Repeat the charging process for a longer time, then reconnect the lamp. What difference has this

made?

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Figure 102

Figure 113

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Page 24: Unit Plan for Electricity and Magnetism

Activity 5: Investigating and comparing the characteristics of primary and secondary cells

1) Students will use a voltmeter to measure the terminal voltage (V) of a zinc-carbon dry cell and the

lead-acid storage cell.

2) Students will use an ammeter to measure the current (I) of a zinc-carbon dry cell and the lead-acid

storage cell.

3) Students will calculate the internal resistance of both cells using the formula: r=V

I

4) Students will compare the portability of both cells.

5) Are both cells rechargeable? Students will examine the labeling of a zinc-carbon dry cell to sensitize

them to the potential hazard of attempting to recharge a dry cell.

6) Students will tabulate all results.

7) The students will list the relative advantages and disadvantages of both cells.

TOPIC: I-V RELATIONSHIPS

Activity 6 Investigating the relationship between current and potential difference for a metallic

conductor (constantan wire) at a constant temperature.

Student instruction information sheet

Aim: To investigate the relationship between current and potential difference in a coil of

constantan wire.

Apparatus/materials: Ammeter

Voltmeter

Rheostat or variable resistor

12 volt d.c. supply

Connecting wires

Fixed resistor (constantan wire)

Switch

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Page 25: Unit Plan for Electricity and Magnetism

Procedure: 1) Set up the apparatus as shown in the circuit diagram.

2) Adjust the rheostat or variable resistor to a high enough value to get a low value

of current on the ammeter.

3) Record the current through the fixed resistor and the potential difference across

it.

4) Repeat step 2 to obtain increasing values of current, each time recording the

current and the potential difference until a total of six sets of ammeter readings

and voltmeter readings are obtained.

5) Display your data in a suitable format.

6) Plot a graph of current versus potential difference.

7) State the relationship between current and potential difference giving reason(s)

for your answer. Compare this result with Ohm’s law.

8) Compare this relationship to the I-V relationships of a filament lamp,

semiconductor diode and aqueous copper (II) sulphate obtained from your

physics textbook. Include a sketch of the graphs these other relationships in your

answer.

9) List the precautions taken.

10) Comment on sources of error.

11) State the conclusion.

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Page 26: Unit Plan for Electricity and Magnetism

Topic: Resistance

Activity 7: What is resistance ?

1) The teacher will use the analogy of water moving through a water hose to concretize the concept of

electrical resistance. The opposition to water flow can be likened to the opposition of current flow

through a conductor.

2) The teacher will then ask the students to come up with everyday examples of situations where

resistance exists. Students will be asked to suggest words that convey the notion of resistance. Words

like opposition, impedance, struggle, conflict, etc.

3) The students will be given 5 minutes to formulate a definition for electrical resistance, after which it

will be discussed. The teacher will then state the definition.

4) The teacher will introduce the relationship R=V

I and state the S.I. unit of resistance.

5) The teacher will then state the definition of the ohm ().

Activity 8 : Calculating resistance, potential difference (or voltage) and current using the relationship R=V

I

1) The teacher will guide students’ in solving the following problems:

a. If a current of 6A flows through a car headlamp when it is connected to a 12 V car battery,

providing a voltage of 12 V across the lamp, what is its resistance?

b. What voltage would be needed to drive a current of 0.6 A through a torch lamp of resistance

24.5?

2) Students will give their inputs while one student write the solution on the chalkboard.

Activity 9: Calculating resistance, voltage, current and power in a series and parallel circuit

1) The teacher will guide students in solving the following problems:

(a) A battery of 20 V is connected to a series arrangement of 5, 3 and 2 resistors.

(i). Draw a circuit diagram to represent this information.

What is:

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(ii). the effective resistance in the circuit?

(iii). the current flowing?

(iv). the potential difference across each resistor?

(v). the total power dissipated in the circuit?

(b)

Determine

(i). the total resistance in the circuit above

(ii). the current drawn from the battery,

(iii). the potential difference across each of the resistors,

(iv). the current through the 30 resistor,

(v). the current through the 25 resistor.

(vi). Calculate the total current

(vii). Calculate the total power dissipated

30

10

25

24V

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(c) The teacher will guide students in drawing equivalent circuits for the following circuits.

40V20

3

15

30

40

60

50

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Electricity in the Home

Activity 9: Class discussion on the reasons fro using parallel connections for domestic appliances.

1) Students will be shown a large diagram on a chart showing the type of circuitry used in the home.

2) The teacher will ask the students why are fuses or circuit breaker needed in this type of circuit.

3) The teacher will ask the students to identify the three different types of wire in the circuit by the colour.

4) By examining the power ratings on various appliances the student will be guided to select a fuse or

circuit breaker of suitable current rating for a particular appliance.

5) The teacher will guide the students to discover the function of the earth wire by giving them a short

comic strip of a boy handling a washing machine with and without the facility of an earth wire.

6) The teacher will draw from the students’ experiences with fluctuating voltages at home. The students

will be asked to describe what happens to the lights, washing machine of refrigerator when the voltage is

fluctuating. Elicit from the students the possible adverse effect of connecting electrical appliances to

incorrect or fluctuating voltage supplies.

7) The teacher can bring in a resource person from the light and power company to address the class for

half hour on electricity in the home. The teacher and students will have relevant questions beforehand to

ask the resource person.

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(MULTIPLE CHOICE)

1)

Which of the following statements for the parallel circuit in the diagram above would be true?

I. The potential difference across each of the resistors is the same.

II. The potential difference between X and Y is the same as that across the 2~ resistor.

III. The current through each of the resistors is different.

(a) I only (b) III only (c) I and III only (d) I, II and III

2)

In the circuit shown above, which lamps will be lit when the switch is closed?

PHYSICS

UNIT – CIRCUITS AND COMPONENTS

TEST ITEMS TO ASSESS ACHIEVEMENT OF THE OBJECTIVES

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Page 31: Unit Plan for Electricity and Magnetism

(a) P, Q and R (b) P and Q only (c) P and R only (d) R only

3)

The diagram above shows the cross-section of a dry cell. Which of the labelled parts is the moist electrolyte?

4) Which of the following statements would be true concerning a domestic wiring system?

I. Switches are always connected in the live wire.

II. Fuses are included to prevent short-circuiting.

III. The lights are connected in parallel with one another.

(a) I only (b) I and III only (c) II and III only (d) I, II and III

Item 5 refers to the following experiment.

A circuit is set up to investigate how the current through copper sulphate solution varies with the applied

potential difference. Carbon electrodes are used.

5) Which of the following graphs should he the correct representation of the outcome of this investigation?

(a)

(b)

(c)

(d)

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6) When an excessive current passes through a fuse, which of the following is the sequence of events?

(a) Wire gets hot current is cut off wire melts

(b)Wire gets hot wire melts current is cut off

(c) Wire melts current is cut off wire gets hot

(d) Wire melts wire gets hot current is cut off

7) The potential difference across the ends of a device is 72 V. The current through the device is 8 A. The

resistance of the device, in ohms, is:

(a) 9 (b) 64 (c) 80 (d) 576.

8) A battery sends a current of 3.5 A through a resistance of 12. This battery also sends a current of 10.5 A

through another resistor. What is the value of this resistor?

(a) 42 Ω (b) 15.5 Ω (c) 4 Ω (d) 2 Ω

9) The resistance of a cylindrical metallic conductor is directly proportional to its:

(a) density (b) diameter (c) length (d) mass.

10) If the potential difference across a device (of constant resistance) is halved, the current through it is:

(a) unchanged

(b) halved

(c) doubled (d) quadrupled.

11) Three identical resistors in parallel have an effective resistance of 15 ohms. What would their effective

resistance be if they are connected in series?

(a) l5ohms (b) l35ohms (c) 45ohms (d) 2l0ohms

12) The body of an electric appliance is connected to earth in order to:

(a) complete the circuit

(b) prevent the breaker from tripping

(c) protect the user from shock

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(d) limit the current flowing.

13) Which of the following statements concerning circuit breakers is correct. Circuit breakers:

(a) limit the current flowing in electric circuits

(b) should be connected to the neutral of the a.c. supply

(c) allow current to flow readily to earth in the event of a fault developing in a circuit

(d) should be made of alloys of high melting temperature.

14) What applied voltage produces a current of 5 mA through a 2 MΩ resistor?

(a) 10MV (b) 10kV (c) 10mV (d) 10μV

15) Two 50, 0.5 W resistors are connected in parallel. Their combined resistance and wattage (power) is:

(a) 100, 1W

(b) 100, 0.5W

(c) 25, 1W

(d) 25, 0.5W

16) Four resistors are connected in series to a 100 V source. The values of the resistors are 28, 46 , 52 ,

and x. The current drawn from the battery is 0.5A. The value of x is:

(a) 52 (b) 74 (c) 100 (d) 200

17) Which of the following is an ohmic device?

(a) a diode

(b) a thermistor

(c) a colour-coded resistor

(d) a transistor

18) A 2 V lead-acid cell has an internal resistance of 0.05 . What current would flow if the cell is short-

circuited?

(a) 1600A (b) 0.5A (c) 25A (d) 40A

Questions 19 and 20

An ammeter in series to a cell of e.m.f. E and negligible internal resistance and a 30 resistor, reads 0.3 A.

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19) The e.m.f. of the cell is:

(a) 1.5V (b) 9V (c) 3.0V (d) 12V.

20) If the 30 resistor is replaced by a 45 resistor, the ammeter would read:

(a) 0.2A (b) 2.0A (c) 5A (d) 10A.

A motorcar headlamp is rated 12 V, 36W.

21) What is the working resistance of the device?

(a) 24 (b) 4 (c) 3 (d) 0.33

Questions 22 and 23 refer to the diagram below.

22) The current flowing through resistor X is:

(a) 120mA (b) 210mA (c) 180mA (d) 300mA.

23) The ratio of the resistance of X to the resistance of Y is:

(a) 120/300

(b) 180/300

(c) 180/120

(d) 120/180.

24) Which of statements I-IV are true? An ammeter:

(i). is connected in series with the current to be measured

(ii). has a low value resistor in parallel with its operating coil

(iii). significantly affects the p.d. across other circuit components

(iv). has a fixed range or ranges, which cannot be extended.

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(a) (i) and (ii) (b) (ii) and (iii) (c) (iii) and (iv) (d) (i) and (iv)

Questions 25 and 26 are related to the diagram below.

25) What is the effective resistance, in ohms, in this circuit?

(a)

12×312+6

(b)

12×612+6

(c) 12 6

(d) 12 + 3

26) The current drawn from the battery, in amperes, is:

(a) 6 (b) 4 (c) 2 (d) 1.33.

Questions 27 and 28

27) What is the value of the current flowing through the 60 resistor?

(a) 0.5A (b) 5A (c) 20A (d) 30A

28) What is the potential difference across the points PQ?

(a) 5V (b) 25V (c) 50V (d) 300V

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29) Which of the following statements is NOT true?

(a) Semiconductor diodes obey Ohm's law.

(b) Conductors have free electrons.

(c) Germanium is an example of semiconductor material.

(d) Semiconductors conduct better at higher temperatures.

30) A battery of e.m.f. 40V is connected across the series combination of fixed resistors as shown. What is the

potential difference between points I and III?

(a) 3 V

(b) 7V

(c) 12V

(d) 28 V

31) What is the potential difference across the points PQ?

(a) 5V (b) 25V (c) 50V (d) 300V

32) Which of the following statements is not true?

(a) Semiconductor diodes obey Ohm's law.

(b) Conductors have free electrons.

(c) Germanium is an example of semiconductor material.

(d) Semiconductors conduct better at higher temperatures.

40 V

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33) A battery of e.m.f. 40V is connected across the series combination of fixed resistors as shown. What is the

potential difference between points I and III?

40 V

(e) 3 V

(f) 7V

(g) 12V

(h) 28 V

END OF MULTIPLE -CHOICE

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STRUCTURED QUESTIONS

1)

(a) An electric stove, connected to a 240 V a.c. Supply has TWO 'burners', EACH with a resistance 30 Ω, and

TWO others, EACH with a resistance of 15 Ω. The FOUR elements can be operated independently.

(i). Of a 30 Ω and 15 Ω burner, which one takes the greater current and which delivers the greater

power?

(ii). Draw a diagram to show how the burners are connected, and find the equivalent resistance of the

circuit when they are all switched on at the same time. (The symbol for a heating element is the

same as the symbol for a resistor.)

(iii). The Stove is connected, via a circuit breaker, to a special high current circuit in the kitchen.

What would be the minimum current rating for the circuit breaker?

(b) Each of the lamps in the circuit below takes a current of 35 mA from the 240 V mains supply. The lamps are

all lit at the same time.

(i). How much current flows in the wire, PQ?

(ii). What is a suitable current rating for the fuse in this circuit?

(iii). An ac. voltmeter is connected, in turn, across

- Q and R

- P and S

- P and Q

What would be the voltmeter reading In EACH case?

240 V

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(iv) Lamp X burns out. How much current will now flow in Lamp Y?

2) The combined resistance RT for various series combinations of resistors is given in the table below.

Combination of Resistors R1/Ω R2/Ω R3/Ω R4/Ω R5/Ω RT/Ω

A 7. 0 - 4.0 - - 11

B - 55 - 32 - 87

C 8. 4 - 220 - 9. 2 237. 6

D 200 310 440 - - 950

E 200 310 440 760 1000 2710

(a) Based on the results in the table above, write a general formula for series connected resistors. Show

clearly the link between the formula and the data.

(b) What is the effect, if any, on RT for combination C if the positions of the resistors are interchanged?

(c) Design an experiment which involves the use of an ammeter and a voltmeter to test the validity of

the formula you arrive at in (a) above.

(d) Calculate the series resistance RT of 2.5MΩ, 495 kΩ and 1.5MΩ.

(e) Two resistors connected in series have a combined resistance of 300Ω. One of the resistors has a

value of 158 Ω. What is the value of the other?

(f) A certain lamp has a resistance of 48 Ω. What is the combined resistance of 5 such lamps? How

many such lamps, connected in series, have an effective resistance of 432 Ω?

(g) The external resistance of a circuit consists of a 550 Ω and a 720 Ω resistor connected in series.

What is the value of the single resistor which must be added to this combination to make the

effective resistance 1800 Ω?

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3) For an ohmic device at constant temperature, the readings of an ammeter (current) and of a voltmeter

(potential difference) suitably connected to it were:

V/V I/A

0. 28 0. 1

0. 55 Ix

0. 83 0. 3

1. 10 0. 4

Vy 0. 5

1. 65 0. 6

1. 93 0. 7

2. 20 0. 8

(a) What is meant by the term 'an ohmic device'?

(b) Plot a graph of voltmeter reading, V (y-axis) against ammeter reading, I (x-axis). Use the graph paper

provided.

(c) Use the graph to find the values of

(I). Vy and

(II). Ix

(d) What is the relationship between current and potential difference according to your graph?

(e) Use your graph to determine the resistance of the ohmic device.

4) The diagram gives a possible circuit for recharging a run-down battery.

(a) Why is a.c. not used, directly, to recharge the battery?

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The battery should be charged at a low steady current.

(b) How is the current controlled in this circuit?

(c) Why does the current tend to decrease as the battery is being charged?

(d) Why are the charging supply and the battery to be recharged connected with opposing polarities?

(e) Assuming that the battery is a lead-acid battery, outline five steps for its care and maintenance.

(f)State three ways in which a lead-acid battery differs from a dry cell.

(g) Explain the function of ANY THREE parts of a zinc-carbon dry cell.

5) A student connects three 240 V, 75 W devices as shown.

(a) Why is the fuse connected to the live wire?

(b) What is the function of the earth connection?

(c) What is the total resistance in the circuit, as connected?

(d) What current does each device need to function normally?

(e) What current is flowing in the circuit as connected?

(f) In the light of your answers to (d) and (e) criticize the circuit arrangement.

240 V

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(g) Draw a suitable diagram of the circuit arrangement that would work. Include in your diagram

components that would enable the devices to operate independently.

6) A pupil takes a series of readings for the current (I) through, and the corresponding p.d. (V) across, a resistor

R which is in the form of a coil of resistance wire. The results are shown on the graph below.

(a) Draw the complete circuit used in the experiment.

(b) What is the value of the resistance over the range 0-6 V? .

(c) Why does the shape of the graph change when the p.d. is greater than 6 V?

(d) Name TWO electrical components which do not obey Ohm's law.

(e) Explain why the mains wiring of an electric hot plate has thick wires, while the wires used for an

electric light bulb are much thinner.

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7)

(a) What is the total resistance in the circuit above?

(b) Calculate the total current flowing in the circuit.

(c) What would an ammeter placed at A read?

(d) Explain why it is necessary for an ammeter to have a very low resistance and a voltmeter to have

very high resistance.

(e) Draw the equivalent series circuit for the circuit above.

(f) What current would the cell deliver if short-circuited by connecting a copper wire across its

terminals?

(g) Calculate the energy dissipated in the circuit in one minute.

END OF TEST ITEMS

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