ELECTRIC CURRENT AND ITS EFFECT.

 NOTES

Heating Effect of Electric Current

Conductors and insulators

• Substances or materials that offer comparatively less opposition to electric current through them are called as conductors. Eg: copper, iron, water, etc.
• Substances, which offer larger opposition to electric current through them are called as insulators. Eg: rubber, wood, sand, etc.

Heating effects of electric current

• A current carrying wire gets hot whenever electric passes through it. This is the heating effect of electric current.
• The amount of heat produced in the wire depends on the material, thickness and length of the conductor.
• If a large amount of electric current passes through the wire, the wire may become so hot that it may melt and break.
• Heating effects of electricity are applied for many household appliances such as electric heaters, toasters, water heaters.
• They are also used in bulbs and used as safety devices(fuses).

• When wires are made of a material that melts on the passage of electric current are called as a fuse. Electrical fuses are used as a safety device that breaks a circuit in times of overloading. They prevent the cause of fire and damage to electrical circuits.

• An electromagnet is an artificial magnet, which produces a magnetic field around a conductor due to the passage of electric current through it.

VIDEO CLICK OPN THE ABOVE LINK TO SEE HOW ELECTRIC BELL WORKS;

WORKING OF ELECTRIC BELL

Exercise Questions

1. Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery

Solution:

2. Draw the circuit diagram to represent the circuit shown in Fig.14.21.

Solution:

3. Fig.14.22 shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.

Solution:

4. The bulb in the circuit shown in Fig.14.23 does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Solution:

In the circuit above bulb is connected on either side.

5. Name any two effects of electric current.

Solution:

i) Heating effect of electric current

ii) Magnetic effect of electric current

6. When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.

Solution:

When the current is switched on through a wire, magnetic field is created around it hence we see deflection in the compass needle kept nearby.

7. Will the compass needle show deflection when the switch in the circuit shown by Fig.14.24 is closed?

Solution:

No , compass needle does not show deflection when the circuit is a closed, magnetic field is not created until current is flowing through the circuit.

8. Fill in the blanks:

(a) Longer line in the symbol for a cell represents its terminal.

(b) The combination of two or more cells is called a .

(c) When current is switched ‘on’ in a room heater, it .

(d) The safety device based on the heating effect of electric current is called a .

Solution:

(a) Longer line in the symbol for a cell represents its positive terminal.

(b) The combination of two or more cells is called a battery.

(c) When current is switched ‘on’ in a room heater, it produces heat .

(d) The safety device based on the heating effect of electric current is called a fuse .

9. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)

(b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)

(c) An electromagnet does not attract a piece of iron. (T/F)

(d) An electric bell has an electromagnet. (T/F)

Solution:

a) False

b) True

c) False

d) True

10. Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.

Solution:

No, because plastic does not have magnetic property to get attracted to a magnet hence magnet cannot be used to separate plastic bags.

11. An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.

Solution:

It is not a wise idea to replace fuse by a piece of wire, as it has very low melting point. In case of metal piece, melting point will be high and the circuit will be intact in case there is overload or overheat.

12. Zubeda made an electric circuit using a cell holder shown in Fig. 14.4, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.

Solution:

Reasons maybe two

i) The connecting wire may be loose

ii) The electric cell may be used up

iii) switch may not be functioning well

iv) cell power has been exhausted

13. In the circuit shown in Fig. 14.25

(i) Would any of the bulb glow when the switch is in the ‘OFF’ position?

(ii) What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?

Solution:

i) No, the bulb will not glow as the circuit is not complete when the switch is off

ii) If the switch is On, all the bulbs glow simultaneously.

ELECTRICITY AND CIRCUITS

 NOTES.

Electric current

The flow of electric charges in a circuit is called an electric current. The direction is taken from the positive terminal to the negative terminal of the battery in an external circuit.

Electric Cell

• A cell is a source of electric power, created inside by internal chemical reactions.
• When the chemicals inside the cell are exhausted, the cell needs to be replaced by a new one.
• Cells have two terminals: a positive and a negative terminal. In order for current to flow, the positive terminal must be connected to the negative terminal and this polarity must be maintained.

  Electric circuit

  • A closed-loop path, which the current takes is known as an electric circuit.
  • When the path of the circuit is closed, current flows through it.
  • When there is a break in the path (switch is open) then, the circuit is open and not conducting so the current does not flow.

  

• Conductors and insulators

  • Any material that allows the electric current to pass through it is called as the conductor. Eg: metals like copper
  • Materials that do not allow the free flow of current through it are known as bad conductors or insulators E.g.: Rubber, plastic

@An electric bulb is made up of thin filament which heats up on the passage of electricity and glow .this thin filament is made up of tungsten which has high melting point.

EXCERCISE

1. Fill in the blanks:
(a) A device that is used to break an electric circuit is called_______________
(b) An electric cell has___________
Ans:
(a) switch
(b) two

2. Mark ‘True’ or ‘False’ for the following statements:
(a) Electric current can flow through metals.
(b) Instead of metal wires, a jute string can be used to make a circuit.
(c) Electric current can pass through a sheet of thermocol.
Ans:
(a) True
(b) False
(c) False.

3.Explain why the bulb would not glow in the arrangement shown in fig

Ans: The bulb would not glow in the arrangement shown in figure because the one end of tester/screw driver is made up of plastic which does not allow the electric current to flow through it.

4. Complete the drawing shown in Fig. 12.9 to indicate where the free ends of the two wires should be joined to make the bulb glow.

Ans:

5.What is the purpose of using an electric switch? Name some electrical gadgets that have switches built into them.
Ans: Electric switch is used to make electric circuit open or closed for a particular appliance and hence with the help of a switch we can use an appliance according

6. Would the bulb glow after completing the circuit shown in Fig. 12.9 if instead of safety pin we use an eraser?
Ans: No, since eraser is an insulator so it does not allow the current to pass. Hence the bulb will not glow.

7. Would the bulb glow in the circuit shown in Fig. 12.11.

Ans: Yes, the electric circuit is closed so the bulb will glow.

Using the“conduction tester” on an object it was found that the bulb begins to glow. Is that object a conductor or an insulator? Explain.
Ans: Yes, if the object is good conductor of electricity then current will pass through conduction tester and the bulb will glow. Hence the object will be a conductor of electricity.

9. Why should an electrician use rubber gloves while repairing an electric switch at your home? Explain.
Ans: Our body is good conductor of electricity and rubber is insulator. During repairing work if the body comes in contact with current carrying wire then there will not be any accident as rubber does not allow the passage of current through it. Hence electrician uses rubber gloves while repairing an electric switch.

10. The handles of the tools like screwdrivers and pliers used by electricians for repair work usually have plastic or rubber covers on them. Can you explain why?
Ans: Plastic or rubber is an insulator which does not allow electric current to pass through it. The handles of the tools like screwdrivers and pliers used by electricians for repair have covering of plastic or rubber so that electric current may not pass through these tools to the body of the electrician to harm him.

LIGHT SHADOWS AND REFLECTIONS

 NOTES

Source of light: An object which emits light, is called a source of light. For example, sun, torch, etc.

Non-luminous objects: These are the objects which do not emit light of their own. Such a body becomes visible when light falls on it. For example, the moon, the planets, etc.

Ray of light: A straight thin beam of light from a source to an object is called a ray of light.

Obstacle: An object which comes to the path of the light is called an obstacle.

Formation of a shadow

• All the opaque objects seem to form a dark shadow of their own.
• We need a source of light, an opaque object in the way, and a screen to see a shadow.
• Screen: This is a surface on which the shadow is formed. It may be a butter paper or simply ground.
• Shadows give us some information about shapes of objects.
• The colour of the opaque object does not affect the colour of the shadow.
• All the space behind the opaque object, up to some distance behind it seems to be filled with the shadow.

Rectilinear propagation: Light travels in a straight line. It is called rectilinear propagation of light.

Image formation by a plane mirror: We are able to see images through a mirror. Image formed by a mirror (flat) has following features:

• Reflected image retains the colour of the object.
• Image is erect but laterally inverted.

Lateral inversion: Right side of the object appears as left side in the image formed by a plane mirror. For example, if we show our right hand, image in the mirror will show as left hand.

In a mirror, if you see another person, surely the other person can also see you in that mirror.

Luminous: Objects that give out or emit light of their own are called luminous objects.

Mirror: A smooth shining surface, which rebounds the light back in same or in different directions is called a mirror.

Opaque objects: If an object completely stops the passage of all the light falling on it, it is an opaque object.

Pinhole camera: It is a device which forms a photograph-like image of a bright object on a screen.

Reflection of light: When a ray of light falls on a smooth and polished surface, light returns back in the same medium. It is called reflection.

Shadow: Opaque objects do not allow light to pass through them and cast dark patches behind them. These dark patches are called shadows.

Translucent objects: Some objects allow only a part of light falling on them to pass through, such objects are called translucent objects. For example, a single thin sheet of paper.

Transparent objects: Those objects which allow all the light to pass through them are called transparent objects.

Characteristics of a Shadow:
A shadow has the following three characteristics:

• It is always black, regardless of the colour of the object used to make the shadow
• It only shows the shape or outline of the object and not the details.
• The size of a shadow varies depending on the distance between the object and the source of light, and the distance between the object and the screen.

Reflection Surfaces
We say light is reflected when it bounces off a surface. Reflection of light helps us to see most of the things around us.
Reflection of light by a surface depends on the nature of the surface. A rough and bumpy surface (also called an irregular surface) reflects a parallel beam of light incident upon it in different directions (Fig. 13.5). A good example of a rough surface is bark of a tree and blanket. This kind of reflection is called diffused reflection.

A smooth surface (a highly polished surface) reflects a parallel beam of light incident upon it in one direction. (Fig. 13.6). A good example of a smooth surface is a mirror. When you stand in front of a mirror, you can see yourself in the mirror. This is called your image.

NCERT QUESTION ANSWERS AND EXCERCISES

1. Rearrange the boxes given below to make a sentence that helps us understand opaque objects.

Ans:

2. Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous:
Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the fame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon.
Ans:

3. Can you think of creating a shape that would give a circular shadow if held in one way and a rectangular shadow if held in another way?
Ans: Yes, there are many things which give a circular shadow if held in one way and a rectangular shadow if held in another way. For example: a cylinder, a circular disc etc.

4. In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of yourself in the mirror?
Ans: No, in a completely dark room no image will be formed because there is no light in the room so no reflection of light takes place and no image will be formed.

SOUND

 NOTES

Sound is produced by vibrating the objects and it is carried in all directions with the help of a medium.

It cannot travel in a vacuum.

Frequency: The number of oscillations or vibrations per second is called the frequency of oscillation.

The loudness is expressed in a unit called decibel (dB). It also depends on the amplitude of the sound.

Amplitude: The maximum distance to which a vibrating body moves on either side of its mean position is called the amplitude of vibration.

Audible Frequencies: For human ears, the range of audible frequencies is roughly from 20 to 20,000 Hz.

Eardrum: A thin membrane which is stretched tightly to receive the waves bf sound at the end of ear canal.

Hertz: The frequency is expressed in hertz (Hz).

Pitch of the Sound: Higher the frequency of vibration, the higher is the pitch, or shrill of the sound.

Shrillness: The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher we can say that sound is shrill.

Time Period: The time taken by a pendulum to complete one oscillation is called the time period.

Vibration: The to and fro or back and forth motion of an object is termed as vibration.

NCERT EXCERCISE

Question 1.
Choose the correct answer.
Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids, and gases
Answer:
(d) solids, liquids, and gases.

Question 2.
Voice of which of the following is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Answer:
(c) A man

In the following statements, tick ‘T’ against those which are true and ‘F’ against those which are false.

1. Sound cannot travel in a vacuum.
2. The number of oscillations per second of a vibrating object is called its time period.
3. If the amplitude of vibration is large, the sound is feeble.
4. For human ears, the audible range is 20 Hz to 20,000 Hz.
5. The lower the frequency of vibration, the higher is the pitch.
6. Unwanted or unpleasant sound is termed as music.
7. Noise pollution may cause partial hearing impairment.

Answer:

1. True
2. False
3. False
4. True
5. False
6. False
7. True

Question 4.
Fill in the blanks with suitable words.

1. Time taken by an object to complete one oscillation is called _______
2. Loudness is determined by the ________ of vibration.
3. The unit of frequency is ________
4. Unwanted sound is called _______
5. The shrillness of a sound is determined by the ______ of vibration.

Answer:

1. Time period
2. Amplitude
3. Hertz (Hz)
4. Noise
5. Frequency

Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
No. of oscillation = 40
Total time is taken = 4 seconds

Question 6.
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Answer:
Number of vibrations per second = 500

QUESTION 7

Identify the part which vibrates to produce sound in the following instruments.

1. Dholak
2. Sitar
3. Flute

Answer:

1. Stretched membrane
2. String of sitar
3. Air column

Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Answer:
The type of sound which are unpleasant to listen is known as noise whereas music is a pleasant sound, which produces a sensation.
Yes, music can become noise when it’s too loud.

Question 9.
List the sources of noise pollution in your surroundings.
Answer:
Following are the major sources of noise pollution:

• Sound of vehicles
• Sound of kitchen appliances
• Sound of bursting crackers
• Sound of loudspeakers, TV, transistors

Question 10.
Explain in what way noise pollution is harmful to humans.
Answer:
Noise pollution causes:
(a) Lack of sleep
(b) Anxiety
(c) Hypertension
and these are harmful to health.

Question 11.
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Answer:
I would suggest my parents buy a house three lanes away from the roadside because house on the roadside would be much noisy in both days and night due to running vehicles. Whereas, a house three lanes away would be comparatively quieter as the intensity of noise decreases with the distance between the source and the listener.

Question 12.
Sketch larynx and explain its function in your own words.
Answer:
Larynx is also known as voice box. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for passage of air (Fig. 13.12). When lung force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose.

When the vocal cords are tight and thin, the type or quality of voice is different from that when they are loose and thick.

QUESTION13

Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Answer:
The speed of light is more than that of the speed of sound. Thus, due to more speed of light it reaches us before sound. So, lightning is seen earlier and thunder is heard later.

MOTION AND TIME

 NOTES

SPEED-The distance travelled by an object per unit time (either in one hour, in one minute or in one second) is known as speed of the object.

SPEED =DISTANCE / TIME

NONUNIFORM AND UBIFORM MOTION-

If the speed of an object moving along a straight line beeps changing, its motion is said to be non-uniform motion.
On the other hand, an object moving along a straight line with a constant speed is said to be in uniform motion. In this case, the average speed is the same as the actual speed.

Note
(i) A sundial measures time by the position of the shadow cast by the sun.
(ii) The device which uses the flow of sand from one glass bulb to another in order to measure time is known as a sand clock
(iii) A device which uses the rate at which water drip from one vessel to another measure time interval is known as a water clock

imple Pendulum
A simple pendulum consists of a small metal ball called bob which is suspended by a long thread from rigid support such that bob is free to swing back and forth. The to and fro motion of a simple pendulum is an example of periodic or oscillatory motion. Galileo was the first person to study the motion of a pendulum.

Speedometer and Odometer
The speedometer is an instrument on a vehicle’s dashboard which indicates the speed of the vehicle when it is moving. This instrument tells us the speed of a running vehicle at that instant of time in kilometre per hour.

QUESTION ANSWERS AND EXCRCISE

Q.1.Classify the following as motion along a straight line, circular or oscillatory motion :
(i) Motion of your hands while running.
(ii) Motion of a horse pulling a cart on a straight road.
(iii) Motion of a child in a merry-go-round. •
(iv) Motion of a child on a see-saw.
(v) Motion of the hammer of an electric bell.
(vi) Motion of a train on a straight bridge.
Ans.(i) Oscillatory motion
(ii) Linear motion
(iii) Circular motion
(iv) Oscillatory motion
(v) Oscillatory motion
(vi) Linear motion

.2.Which of the following are not correct?
(i) The basic unit of time is second.
(ii) Every object moves with a constant speed.
(iii) Distances between two cities are measured in kilometers.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h
Ans. (ii), (iv), (v)

Q.3.A simple pendulum takes 32s to complete 20 oscillations, what is the time period of the pendulum?
Ans. Time taken to complete 20 oscillations = 32 s
Time taken to complete 1 oscillation = 32/20 s = 1.6 s
Time period of a pendulum is time taken by it to complete 1 oscillation.
Time period of pendulum is 1.6 seconds.

Q.4.The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Ans.Distance = 240 km
Time taken = 4 hours
Speed =Distance covered/time taken = 240km/4 hammer
=60 km/h
Speed of train =60 km/h

Q.5.The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by the car, if at 08:50 AM, the odometer reading has changed to 57336.0 km? Calculate the speed of the car in km/min during this time. Express the speed in km/h also.
Ans.Distance = 57336.0 km – 57321 km =15 km
Speed in km/min =15km/20 min = 3/4 km/min

Speed in km/hr =15 km/1/3 hr
=(15 x 3) km/hr
=45 km/hr.

Q.6.Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/min, calculate the distance between her house and the school.
Ans.Time taken =15 min
Speed = 2 m/min .
Distance = speed x time = 2 x 15 = 30 m
Distance between Salma’s school and her house is 30 m.

Q.7.Show the shape of the distance-time graph for the motion in the following cases :
(i) A car moving with a constant speed.
(ii) A car parked on a side road
Ans.

Q.8. Which of the following relations is correct?
(i) Speed = Distance Time
(ii) Speed = Distance/Time
(iii)Speed – Time/Distance
(iv)Speed = 1/Distance Time
Ans. (ii) Speed = Distance/Time is correct

Q.9.The basic unit of speed is:
(i) km/min (ii) m/min (iii) km/h (iv) m/s
Ans. (iv) m/s

Q.10.A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is:
(i) 100 km (ii) 25 km (iii) 15 km (iv) 10 km
Ans.(ii) 25 km

Q.11.Suppose the two photographs, shown in fig. 13.1 and fig. 13.2 of NCERT had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs, calculate the speed of the blue car.
Ans.0.1 cm/s or 10 m/s

Q.12.Fig 13.10 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?

Ans. ‘A’ is moving faster.

Q.13.Which of the following distance-time graphs shows a truck moving with speed which is not constant?

Ans. (iii)