1

OBJECTIVES

Electricity is fascinating to learn about and experiment with. Scientists have learned, and

still learn about it. Man has learned to control and use electricity, even though he still

does not completely understand their nature. Although, electricity acts mysteriously at

times, there is no reason to consider it to be supernatural.

Various laws and principles of electricity reflect the realities and philosophy of life also.

These will also be discussed along with the electrical properties.

Electricity in the wires and outlets usually found in homes and schools is too dangerous

for experiments, but, one or a few low volt batteries are safe to experiment with. In the

light of the above points, the content discussed later is intended to enable students to:

1. Link scientific concepts with psycho-socio-spiritual aspects of life

2. Illustrate the importance of hardships and challenges in life.

3. Tell the need for becoming an asset for the society.

4. Hypothesize solutions for preventing undesirable events happening around

them in the world.

5. Apply systems perspective to electrical concepts

6. Perform various experiments related to electricity.

7. Draw and make various series and parallel circuits using wires, switches,

ammeters, voltmeters, resistances, low volt batteries, etc.

8. Identify the three principle parts of an atom.

9. List the laws of charges and describe their importance to current flow.

10. Explain electron current flow

11. Establish contrasts between direct and alternating current.

12. Describe current flow through a conductor and discuss why heat is produced.

13. Write various terms, units, symbols and formulae of concepts related to

electricity

14. Solve numerical related to calculations of current, voltage, resistance,

power, etc.

APPENDIX G2:

SYSTEMS APPROACH BASED RESOURCE MATERIAL OF ELECTRICITY FOR CLASS 10

2

Concept Map of Electricity

Introduction

It has been over 250 years since Ben Franklin discovered two kinds of electrical charges.

It’s been almost 200 years since Hans Oersted and Michael Faraday developed ideas of

electric fields, currents, and magnetism. At about the same time, British

electrician/engineer William Sturgeon invented the electromagnet. Because of these men

and other pioneers in the fields of magnetism and electricity, we enjoy watching

television, listening to our stereos, surfing the Internet on our computers, and heating

our food in microwave ovens. Let us understand various concepts of electricity, as

enlisted in the concept map (Figure 1) now.

3

Understanding Electricity Using Systems Perspective

We all come across such incidents in life which motivate us to progress in life. Same

way, many times we face challenges, which temporarily act as obstacles in our path of

progress. But, these hindrances have a special role to play. In fact, a reflection on various

scientific principles, indicates that hindrances help in maintaining balance in many

situations. Everywhere around us we observe two major forces acting on all types of

systems: one which pushes the systems forward, the other which hiders the forward

movement and slows down the speed. But, can this hindering force always be

considered as a drawback, or as we believe everything has a positive side, can this also

have some usefulness? Lets try to solve this mystery through the example of electrical

systems.

The foundational subsystem of electrical systems (bulbs, heaters, fans, iron, etc.) is an

electric circuit consisting of three essential elements: (i) a push, (ii) a flow of matter in a

closed circuit and (iii) an obstacle. Through the push, energy is transferred from the

outside into the system, and through the obstacle it is removed from the system. The

push supports forward movement of matter in system, whereas, the obstacle hinders

this forward movement. Now, considering an electric circuit as a system, in electrical

terminology the push can be considered as a voltage V (potential difference), the flow as

current I, and the obstacle as resistance R. The speed of the motion of matter (i. e. the

charged particles) will increase with the increase in the push, and decrease if the obstacle

is made stronger. Thus, in mathematical terms it can be assumed that I (current) is

proportional to V (voltage / potential difference) and inversely proportional to R

(resistance).

A simple electric circuit has a voltage source (such as a

generator or battery) that maintains the electrical

potential, some device (such as a lamp or motor ) where

work is done by the potential, and continuous pathways

for the current to follow.

Figure 2 Simple electric circuits

4

There is one more important aspect related to an electric circuit system. Can you try to

guess it?

Whenever there is any motion of matter, then energy (kinetic) gets associated with it.

Similarly, when the charged particles move in a closed circuit, these produce electrical

energy which can be converted to other forms mainly as heat and light energy.

Now can you make out the positive side of resistance in an electric circuit? It is simple. It

is due to this resistance only, that the energy produced due to the moving charged

particles in a circuit, can be put to use and converted into different forms of energy.

So the lesson learnt is that we must never give in the face of challenges, but, try to see

how these can benefit us and help in improving us for becoming assets for the society.

We must work hard with determination and conviction in ourselves to overcome these

challenges.

Figure 3 Systems perspective to understanding electricity

5

Types of Electric Circuits

Circuits typically contain a voltage source, a wire conductor, and one or more devices

which use the electrical energy. A circuit having resistances in series (Fig. 4) is called

series circuit, while the circuit having resistances attached in parallel is called a parallel

circuit. (Fig. 5).

Fig.4 A series circuit Fig 5. A parallel circuit

Using Water Pressure Analogy for Understanding the Electrical Concepts:

Voltage, Current and Resistance.

The flow of electricity in current electricity has electrical pressure or voltage. Electric charges flow from an area of high voltage to an area of low voltage, similar to the flow of water (fig 6 & 7).

Fig 6

Fig 7

6

The pressure of the water flowing through the pipes can be compared to the voltage

(electric potential) flowing through the wires of the circuit. The unit used to measure

voltage is volts (V). Water flows from a reservoir of higher pressure to the reservoir of

lower pressure; flow stops when the pressure difference ceases (Fig 8). Water continues

to flow because a difference in pressure is maintained with the pump.

Fig 8 Flow of water from a high pressure reservoir to a low pressure one.

Just as water current is flow of water molecules, electric current is the flow of electric

charge. In circuits, electrons make up the flow of charge (Fig 9).

Fig. 9

Ampere is the unit of current (I) which is a measure of how much electrical charge (Q)

is flowing in a circuit in time (t) and is measured in units of amps (Fig 10).

Fig 10

The current varies depending on the force behind the current and the resistance to flow.

7

Fig. 11 Current is inversely proportional to resistance.

Potential difference, or voltage, is the electrical potential energy per coulomb of charge.

Fig. 12 Current is directly proportional to voltage

Resistance (R) is the opposition to the flow of an electric current, causing the electrical

energy to be converted to thermal energy or light. The unit for measuring resistance is

the ohm (Ω).

Fig. 13 Motion of charged particles in a thick and thin wire

The resistance of short, thick piece of wires is less than that of long, thin pieces.

8

Lets Experiment

Make an electric circuit using two ammeters (used for measuring current in a circuit),

voltmeter, a low volt battery, LED / bulb, connecting wires, a switch / key, etc. to

make the LED / bulb glow. Draw the diagram of the circuit (in the space provided) in

which the bulb glows. Also, notice when do the ammeters show deflection.

Conclusion based on the experiment: Since, for measuring current using an ammeter,

the whole amount of current flowing in a circuit should pass through it, hence, ammeter

always needs to be connected in series to be able to measure current. Current increases

as the voltage increase. Due to charge conservation, same current flows into and out of

light bulb.

Post Lab Exercise Q1. Why we use voltmeter in parallel and ammeter in series in a circuit?

Q2. What is the function of volt meter?

Q3. What are the units of voltage, current and resistance? Also write their symbols.

Q4. By using the readings taken from the experiment, draw a graph between the voltage

and current. Is it a straight line?

9

Lets Revise

1. What is an electric current?

Ans: Current: The flow of electrons from one place to another. Measured in amperes (amps) It has Kinetic energy

2. What is resistance?

Ans: The opposition to the flow of an electric current, producing heat. The greater the resistance, the less current gets through. Good conductors have low resistance. Measured in ohms.

3. Which all factors influence resistance?

Ans: Material of wire – aluminum and copper have low resistance Thickness – the thicker the wire the lower the resistance Length – shorter wire has lower resistance Temperature – lower temperature has lower resistance

4. What is voltage?

Ans: The measure of energy given to the charge flowing in a circuit. The greater the voltage, the greater the force or “pressure” that drives the charge

through the circuit.

5. What is Ohm’s Law?

Ohm’s Law defines the relationship between voltage, current, and resistance. It was developed by German physicist Georg Ohm, for whom both Ohm’s Law

and the unit of measure for resistance were named. It states that the voltage (V) in a circuit is equal to the product of the current (I) and the resistance (R), or V = I R

10

CALCULATIONS

Solution: I = 3 V / 2 Ω = 1.5 Amp.

1. Complete the following grid:

2. Calculate the value of resistance of a bulb drawing a current of 1.5 amp from a battery of 3 volts.

(Ans: 4.5 ohms)

11

ELECTRICAL POWER (P) AND ENERGY (J)

All electrical circuits have three parts in common.

• A voltage source.

• An electrical device or resistor

• Conducting wires.

The work done (W) or the energy (J) required by a voltage source is equal to the work

done by the electrical field in an electrical device or resistor

Work (J) = Power (P) x Time (t)

It can be said that electrical energy is a measure of the amount of power used and the

time of use. Electrical energy is the product of the power and the time.

The electrical potential is measured in joules/coulomb and a quantity of charge is

measured in coulombs, so the electrical work or energy is measured in joules.

A joule/second is a unit of power (P) called the watt.

Power (P) = current (I) x potential / Volt (V) or P = I V

How is Electrical Power calculated?

Example Problem 1: How much power is used in a circuit which is 110 volts and has a

current of 1.36 amps?

Solution:

P = I V

Power = (1.36 amps) (110 V) = 150 W

Example Problem 2: The current flowing through an appliance connected to a 120-V

source is 2 A. How many kilowatt-hours of electrical energy does the appliance use in

4h?

12

Solution:

E = P X time

P = I V

P = (2A) (120 V) = 240 W

E = (240 W) (4 h) = 960Wh = 0.96 kWh

CALCULATE 1. A bulb is rated as 25 Watt -250 Volt. On connecting this bulb to a mains of

250 volts, what will be the value of current (I) flowing in it, and how much resistance (R) will the bulb provide to this flow of current?

(Ans: I=0.1 amp; R=2500 ohms)

2. An electric heater is drawing a current of 6 amperes from an electric source of 250 volts. Calculate the electric energy consumed when heater is used for ten minutes.

(Ans = 9,00,000 Joules)

13

A Closer Look at Electricity

Although, till now basic concepts of electricity have been discussed, now, let us try to

understand the details related electricity with reference to atom, electrons, protons, etc.

The Atom

An atom is the smallest particle into which an element can be divided without losing its

identity. A group of identical atoms is called an element. All matter is composed of

atoms. Whether we are discussing metal, wood, or glass, they are all made up of atoms.

Before we go any further, let’s take a step back and look at some useful terms. The

smallest particle of a substance that still has all of its characteristics is called a molecule.

A molecule consists of two or more atoms. If a molecule of metal is divided into smaller

parts, it is no longer metal.

There are over a hundred different types of atoms. There are as many elements as there

are atoms. A few of the more common elements are iron, nitrogen, and oxygen;

important elements in terms of electrical conductivity are copper, silver, and gold. Since

there are so many different materials in the world, most materials are made up of more

than one element. When different kinds of atoms combine chemically, they form

materials referred to as compounds. Two very familiar compounds are table salt and

water. Table salt is made up of the elements sodium and chloride; water is made up of

the elements hydrogen and oxygen.

Since we need to develop an understanding of what electricity is, let’s take a closer look

at the atom. There are three principle parts of the atom. They are the electron, proton,

and neutron. An electron is an elementary particle carrying one unit of negative

electrical charge. The proton is found in the nucleus and has a positive electrical charge

equal to the negative charge of an electron. Neutrons and protons have about the same

weight, but the neutron has no electrical charge. Figure 14 illustrates a helium atom. The

nucleus is the very dense region consisting of protons and neutrons at the center of an

atom. The electrons revolve around the nucleus in an elliptical, or oval-shaped, path.

Figure 14 Helium atom illustrating electrons, protons, and neutrons.

14

Figure 15 Positive charge of a proton with lines of force extending outward.

We saw in Figure 14 that the electrons orbit outside the nucleus. Although an electron is

about three times larger than a proton, the proton weighs over 1800 times more. Since

electrons are negatively charged, the lines of force produced by an electron come in from

all directions. See Figure 15.

Figure 16 Electron lines of force.

Electrons revolve around the nucleus of the atom similar to the way the earth rotates

around the sun. In atoms that contain more than one electron (all atoms except

hydrogen), each electron has its own orbit. It is possible for two or more atoms to share

common space; in many materials, closely spaced atoms share both electrons and space.

Figure 16 shows the electron orbits of an atom.

Charges were discovered by Ben Franklin

15

Figure 17 Electron orbits.

The two electrons closest to the nucleus are said to occupy the first shell, or orbit of the

atom. This first shell can only have two electrons. Atoms that have more than two

electrons, like iron with 26 electrons, must have a second, third, and fourth shell, or

orbit. Have you ever heard the expression, “Opposites attract and likes repel?” Well,

much like the magnetic poles of a magnet that will repel if like poles are brought

together and attract if unlike poles are brought together, the law of charges states that

opposite charges attract and like charges repel. This basic law will help you understand

the importance of attraction and repulsion of electrons and protons in the atom.

Electric Charge

Electrons and protons exhibit electric charges of opposite polarity. Polarity refers to the

type (positive or negative) of charge. The proton possesses a positive charge and the

electron is negatively charged. As already illustrated these charges produce lines of

force. A basic law of physics, known as the law of charges, states that opposite charges

attract and like charges repel. Figures 18 and 19 illustrate this law.

Fig. 18 Unlike charges attract each other Fig. 19 Like charges repel each other

16

When considering the electric charge of an atom, the neutron can be ignored since it has

no electric charge. An atom such as helium, illustrated in Figure 14 in its natural state, is

electrically neutral. Helium has 2 protons in the nucleus and 2 electrons orbiting the

nucleus. Even though the electrons and protons are electrically charged, the net electric

charge is zero.

Electricity is not magic. Current is simply electrons in motion. Next, we will learn how

those electrons are put into motion.

Electricity – Electrons in Motion

In this section we will try to look into the facts of valence shell of an atom, where

charges come together, free electrons can escape, and positive or negative ions are

created. Static electricity will also be discussed; this transfer of electrons from one object

to another has both practical and challenging applications.

Valence Electrons

The outermost shell of an atom is referred to as the valence shell, or ring. Electrons

located in this valence shell are called valance electrons. These electrons are atomic

particles involved in chemical reactions and electric currents. The valence shell cannot

hold more than 8 electrons. A conductor, for example, is comprised of a material that

contains between 1 and 3 valence electrons. A conductor is a material that readily

conducts electricity.

One of the forces that help keep electrons in orbit is the force of attraction between

unlike charges. The closer together two particles of opposite electric charges are, the

greater the electrical attraction between them. So, the attraction between the proton in

the nucleus and the orbiting electron decreases as the electron gets farther from the

nucleus. Because of this, valence electrons are more loosely held than the electrons in the

inner shells. Here are a few characteristics of valence electrons:

They can be more easily removed from the atom than electrons in the inner shell.

Valence electrons possess more energy than electrons in the inner shells because

the farther an electron is from the nucleus, the more energy it possesses.

17

Lets revise

1. What is a conductor?

2. When does the attraction between the proton in the nucleus and the orbiting

electron decrease?

Free Electrons and Electron Flow

Valence electrons that have been temporarily separated from an atom are called free

electrons. They are not attached to any particular atom. Only the valence electrons in the

outermost orbit can become free electrons. Recall that the nucleus and the attraction of

the protons keep electrons in the inner shells held tightly. A valence electron can be

freed from an atom when energy is added to the atom. This energy permits the loosely

held valence electron to escape the force of attraction between the electron and the

nucleus. A free electron possesses more energy than it did as a valence electron.

Electrical current is the flow of electrons. Heating an atom or subjecting an atom to an

electric field provides the necessary energy to free the valence electron. After traveling a

short distance, the free electron enters the valence orbit of a different atom. When it

returns to orbit, some or all of the gained energy is released in the form of heat. This is

why conductors become warm when current flows through them. If current through a

conductor becomes excessive, a fire may result.

Lets revise

1. What are free electrons?

2. Define electrical current.

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Ions

The departure of a valence electron leaves a previously neutral atom with an excess of

positive charge, or more protons than electrons. Atoms which have more or less than

their normal complement of electrons are referred to as ions. When an atom has an

excess of electrons, it becomes a negative ion; when an atom loses electrons, it becomes a

positive ion. The amount of energy necessary to cause a valence electron to become free

is different with each element.

The number of an atom’s valence electrons determines how much energy is required to

create a free electron. Typically, if there are fewer electrons in the valence orbit, less

energy is required to free that valence electron.

Lets revise

1. How does an ion become positive?

2. How does an ion become negative?

Direct Current

A direct current (dc) power source provides a voltage whose polarity and output voltage

never change direction. Dc power sources provide two terminal devices, one terminal

negative and the other positive, with respect to each other. Direct current sources of

energy include batteries, dc generators, and electronic power supplies.

Lets revise

1. What never changes in direct current?

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Alternating Current

Alternating current (ac) power sources produce a voltage that periodically reverses

polarity and magnitude. An example of an ac power source is an ac generator.

Alternating current has one major advantage over direct current; the voltage can be

increased or decreased by using a transformer. A transformer is an electrical device by

which alternating current of one voltage is changed to another voltage.

When electric current runs through a conductor, a magnetic field is produced.

Magnetism makes motors, transformers, and even your doorbell possible. In

electromagnetism we will discuss magnetism, electromagnets, and some of the devices

that work because of this force.

Lets revise

1. What is one major advantage alternating current has over direct current?

Heating Effect of Current

Heating effect of electricity is one of the widely used effects in the world.

Heating effect of electricity is one of the widely used effects in the world. When electric

current is passed through a conductor, it generates heat due to the resistance it offers to

the current flow. The work done in overcoming the resistance is generated as heat. This

is studied by James Prescott Joule and he enunciated various factors that affect the heat

generated. The heat produced by a heating element is directly proportional to the square

of the electric current (I) passing through the conductor, directly proportional to the

resistance (R) of the conductor, time (t) for which current passes through the conductor.

It is given by the expression H = I2Rt and is well known as Joule’s Law.

The orbiting electrons have a negative charge, the protons have a positive charge, and

the neutrons display no charge. Because the neutron has no charge, the nucleus has a net

20

positive charge. The element type is determined by how many protons are in the

nucleus. Helium, as already noted has 2 protons in its nucleus; copper has 29. The

atomic number of an element is equal to the number of protons in the nucleus.

Applications of the heating effect of electric current include appliances like electric

immersion water heater, electric iron box, etc. All of these have a heating element in it.

Heating elements are generally made of specific alloys like, nichrome, manganin,

constantan etc. A good heating element has high resistivity and high melting point. An

electric fuse is an example for the application of heating effect of electric current. The

rating of 3 A of an electric fuse implies the maximum current it can sustain is three

ampere

21

SUMMARY

Electricity is a form of energy, which is used to make electrical appliances work

Electric current is the rate of flow of electrical charge.

The path that the current flows through is known as a circuit.

There are two types of circuits: Series and Parallel. In Series Circuits the

components are lined up along one path and if the circuit is broken, all the

components turn off. In Parallel Circuits, there are several branching paths to the

components. If the circuit is broken at any one branch, only the components on

that branch will turn off.

Ampere is the unit of current (I) which is a measure of how much electrical

charge (Q) is flowing in a circuit in time (t) and is measured in units of amps

The potential difference across any two points is the amount of energy needed to

move one unit charge of electricity from one point to the other.

Resistance in a circuit (R) = Potential Difference (V) / Current (I)

The unit for measuring resistance is the ohm (Ω).

Resistors are also beneficial as they allow electrical energy to be converted into

other forms of energy: heat, light and mechanical.

Atoms are composed of protons (+), electrons (-) and neutrons. The nucleus

contains the protons and neutrons and the electrons surround the nucleus.

The coulomb (C) is the unit of electric charge.

The basic quantity of electric charge (e) is 1.6 x 10-19 C

There are two types of currents: Direct (DC) and Alternating current (AC). In

Direct Current (DC) electrons flow in the same direction in a wire. In an

Alternating Current (AC), electrons flow in different directions in a wire

Electrical energy can be transformed into other forms of energy to produce many

effects

Wire heats up as a current passes through it

Electrical energy is converted into heat energy

Heat produced depends on the resistance of the wire

Greater resistance, greater heat

Heating appliances have coils of wires made of high resistance materials such as

nichrome

Heating elements heat up when current passes through them

Heat produced is then used for ironing, toasting, cooking and drying

The power of an electric current is the rate at which it does work and is equal to

the product of the current and the voltage of a circuit:

The unit of electric power is the watt. The commercial unit of electric energy is

the kilowatthour (kWh).

22

TEST YOUR KNOWLEDGE

1. What are the parts of a circuit?

2. What are circuit diagrams?

3. What is the difference between a series and parallel circuit?

4. How do you calculate the voltage, current, and resistance?

5. A motor with an operating resistance of 30 Ω is connected to a voltage source.

The current in the motor is 4.0 A. What is the voltage of the source?

6. What is the charge on an electron?

7. If a particle gained 1.2 x 107 electrons, what is the charge on the particle?

8. If a particle losses 1.5 x 104 electrons, what is the charge on the particle?

9. If a particle has a charge of +45 mC, has it gained or lost electrons? How many

electrons has it gained or lost?

10. A motor with an operating resistance of 30 Ω is connected to a voltage source.

The current in the motor is 4.0 A. What is the voltage of the source?

11. A current of 0.5 A flows through a lamp when it is connected to a 120 V source.

a. What is the resistance of the lamp?

b. What is the wattage of the lamp?

Multiple Choice

Identify the choice that best completes the statement or answers the question.

____ 1. Coulomb's law says that the force between any two charges depends

a. directly on the size of the charges.

b. inversely on the square of the distance between the charges.

c. both A and B d. none of the above

____ 2. When the distance between two charges is halved, the electrical force between

the charges

a. doubles. b. reduces to one fourth.

c. halves. d. quadruples. e. none of the above

____ 3. A positive ion has

a. more electrons than protons. b. more protons than electrons.

c. a +1 charge always. d. one proton.

____ 4. In a good insulator, electrons are usually

a. not moving at all. b. free to move around after an impurity has been added.

c. free to move around. d. tightly bound in place. e. semi-free to move around.

23

____ 5. When a charged cloud passes overhead, the ground below is charged by

a. induction. b. polarization. c. deduction. d. electrification.

____ 6. Two charged particles held close to each other are released. As they move, the

force on each particle increases. Therefore, the particles have

a. opposite signs. b. the same sign. c. charges that cannot be determined

____ 7. The SI unit of charge is the:

a. ohm. b. joule. c. coulomb. d. ampere. e. newton.

____ 8. The electrostatic force between two charges located 8 meters apart is 0.10 N.

What will the force be between these charges when they are located 2 meters apart?

a. 0.01 N b. 0.03 N c. 0.1 N d. 0.8 N e. 1.6 N

____ 9. An ampere is a:

a. unit of resistance. b. unit of current. c. type of charge. d. voltage. e. current.

____ 10. Compared to thin wires, electrical resistance in thick wires is:

a. less. b. the same. c. greater.

____ 11. For most conductors, as their temperature increases, their resistance

a. decreases. b. increases. c. stays the same.

____ 12. Electrical resistance is measured in:

a. volts. b. joules. c. watts. d. none of the above

____ 13. Electric power is defined as:

a. current times voltage. b. current divided by voltage.

c. current times resistance. d. resistance times voltage.

e. voltage divided by current.

____ 14. Power outlets in our homes typically have a potential difference of

a. 30 V. b. 120 A. c. 60 A. d. 240 V. e. 120 V.

____ 15. When an 8-V battery is connected to a resistor, 2 A of current flows in the

resistor. What is the resistor's value?

a. 2 ohms b. 4 ohms c. 8 ohms d. 16 ohms e. more than 16 ohms

24

____ 16. The current through a 5-ohm resistor connected to a 150-V power supply is

a. 1A. b. 10 A. c. 30 A. d. 150 A. e. none of the above

____17. A 15-ohm resistor has a 5-A current in it. What is the voltage across the resistor?

a. 5 V b. 15 V c. 20 V d. 25 V e. more than 25 V

____ 18. How much power is used by a 12.0-V car battery that draws 0.5 A of current?

a. 0.5 W b. 6 W c. 12 W d. 24 W e. 30 W

____ 19. A 60-W light bulb and a 100-W light bulb are both rated at 120 V. Which light

bulb has the larger resistance?

a. the 60-W bulb b. the 100-W bulb c. Both have the same resistance.

e. none of the above

____ 20. In order for current to flow in a circuit, you must have

a. a switch that is open. b. a complete path for the current.

c. two light bulbs in parallel. d. two light bulbs in series. e. all of the above

____ 21. When two light bulbs are connected in series, the

a. current through each light bulb is proportional to the resistance of the bulb.

b. same amount of current always flows through each bulb.

c. neither A nor B

____ 22. When resistors are put in series next to each other, their overall resistance is

a. the same as the resistance of one of the resistors.

b. larger than the resistance of any individual resistor.

c. smaller than the resistance of any of the resistors.

____ 23. As more lamps are put into a series circuit, the overall current in the circuit

a. stays the same. b. increases. c. decreases.

____ 24. Compared to the resistance of two resistors connected in series, the same two

resistors connected in parallel have

a. less resistance. b. more resistance. c. the same resistance.

____ 25. Fuses and circuit breakers are used to

a. protect us. b. prevent overloading.

c. keep wires from getting overheated.

d. break the circuit when too much current is being used.

e. all of the above

25

____ 26. A short circuit occurs when

a. the positive wire is connected directly to the negative wire.

b. very short wires are used in the circuit.

c. current lasts in the circuit for only a short time.

d. all of the above e. none of the above

____ 27. A 60-W light bulb and a 100-W light bulb are both connected in parallel to a

120-V outlet. Which light bulb has more current in it?

a. the 100-W bulb b. the 60-W bulb c. Both have the same current.

_____ 28. A lighted bulb consumes:

(a) Mechanical energy (b) Nuclear energy (c) Potential energy (d) Electrical energy

_____ 29. To measure current in the circuit an ammeter is always connected;

(a) in series (b) in parallel (c) in any way (d) parallel to voltmeter

______ 30. The unit of resistance is;

(a) Ohm (b) Ampere (c) Volt (d) Coulomb