science work sheets correction

8/13/2019 Science Work Sheets Correction

1/161Pg.No|

Symbols of Electric Components

You will learn

How to represent an

electric circuit using

standard symbols

You already know

Components of an

electric circuit

Class: Subject: Topic:VII Science Electric Current and Its Effects

PONDER THIS:If you were asked to showcase a project at the National Science Fair based

on an application of Electric Current, what would it be?

Did you know? Magicians use electromagnets in their famous Levitation act.

The list of Subtopics

Symbols of Electric

Components

Heating Effect ofElectric Current

Magnetic Effect of

Electric Current

Electromagnet

Why these? Why should I care? Common Mistakes

Important from

School and

Competitiveexamination point-

of-view

This Chapter forms

a basis for higher

order concepts ofElectricity, and

Magnetic Effects of

Electric Current in

Classes IX and X.

Home appliances

such as electric

iron, electric kettle

etc. make use of the

heating effect of

electric current.

Earphones, hard

disks, railway

technology in

Japan, MRI

scanners and a lot

more such devices

all run onapplications of

electromagnets.

Mistakes in circuit

diagram

representationHeating effect of

electric current

applications in home

appliances

How the circuit

breaks in an electric

bell


2/162Pg.No|

Electric circuits are an easy way to represent ______________________.

Common electric components are signified using _________________.

The symbols for some regular electric components are:

Complete the table given below:

Electric Components Symbols

Electric cell

Electric bulb

Switch in ON position

Battery

Wire

In the symbol for an electric cell, the longer line represents the

_______________________________, whereas the shorter thicker parallel line

represents the _______________________________.

What is the difference between an electric cell and a battery?

________________________________________________________________________________________________________________________________________________________________________________________________________________________

Where have you used batteries?

________________________________________________________________________

When you insert electric cells side by side, like in the picture shown below

(as in a TV remote control), the _____________________ terminal of one electric

cell is connected to _______________________ terminal of the next electric cell.

The compartment in the TV remote control (or any other device that uses

batteries), shown in the picture, which holds the cells is called the

________________________.


3/163Pg.No|

Practice with circuits:

Here is a sample electric circuit and its representation using symbols.

{NOTE that in the circuit above, it does not matter what order you show the

elements in. It is important to show all elements with their correct symbols

and connected properly. The key/ switch can be placed anywhere in the

circuit. Since the lamp is glowing in the first circuit, in the second circuit

also it must be glowing. Therefore, the circuit must be closed and the switch

must be ON.}

Explain why the light bulbs will not light in the arrangements below:


4/164Pg.No|

From the above exercise, we have learnt that:

When the switch is in ON position, the circuit from the positive terminal of the battery

to the negative terminal is complete. The circuit is then said to be _______________ and

the _______________ flows throughout the circuit instantly.

When the switch is in OFF position, the circuit is incomplete. It is said to be _____________

and no __________________ flows through any part of the circuit.

For any circuit to be complete, it is notsufficient for the circuit to be closed, the

circuit needs to be connected to all these components:

a batteryor a cell (a source of electric power),

connecting wires,

a device (such as an electric bulb) through which the electric power may be

released as light or heat, and

a keyor a switch which may be turned on and off.

PONDER THIS:In cases where there is no device, how do you know that electric

current flows through the circuit?


5/165Pg.No|

You will learn

Principle of electric bulb

Principle of electric fuse

You already know

What an electric bulb is

Where a fuse is used

Heating Effects of Electric Current

You have seen and used an electric bulb before.

In the bulb there is a thin wire, called the __________________, which glows when an

electric current passes through it.

Why shouldnt we touch a lighted electric bulb?

________________________________________________________________________________________________________________________________________________

What happens when an excess current passes through the bulb?

________________________________________________________________________________________________________________________________________________

You have seen your mother heating water for your warm bath by using geysers, electric

kettles or immersion rods. You have also seen her ironing your school uniform.

How do these appliances work?

How does the electricity passed through these objects convert to heat energy?

This property of electric current is called its ____________________________________.

All of these appliances contain a coil of wire, called an __________________.

The amount of heat produced in a wire depends on:

________________________________________________________________________________________________________________________________________________


6/166Pg.No|

Knowing this helps us because

________________________________________________________________________________________________________________________________________________

________________________________________________________________________________________________________________________________________________

Where do we want wires that become hot and where do we want wires which do notbecome very hot?

________________________________________________________________________________________________________________________________________________________________________________________________________________________

________________________________________________________________________________________________________________________________________________

Some wires offer very little resistance, like those made of good conductors of electricity

(like silver, aluminium etc.). Those made of lesser conductors like the tungsten filament

used in electric bulbs, offer more resistance. Thus, electric work is done in overcomingthe friction, which is released as heat and light energy.

Some wires made from special materials melt quickly, resulting in

_________________________________________________________________________________________________________________________________________

These wires are used for making ________________________, which are safety devices

that __________________________________________________

MCBs or _______________________________________ are also being used these days in

places of fuses.

These are ___________________________________________________________ when current

in a circuit exceeds the safe limit.

What causes overload in the circuit?

Excessive currents in electric circuits are caused if there is

__________________________________________________, due to wear and tear of insulation

on the wires.

Also, connection of many devices ________________________________ causes overload.


7/167Pg.No|

You will learn

Magnetic effects of electric

current

You already know

Poles of magnet

Attraction and repulsion in magnets

Magnetic Effects of Electric Current

A Danish physicist and chemist, Hans Christian Oersted, during a lecture, noticed that a

compass needle deflected when an electric current from a battery was switched on and off.

You already know that a compass needle is made up of a ______________________, which

aligns itself in the __________________________________________, and deflects whenever

brought near a _______________.

From this, we learn that, when _____________________________ passes through a wire, it

behaves like a ________________.

This is called the ___________________________ of electric current.

You will learn

Applications of electromagnets

Electric bell

You already know

Magnetic and non-magnetic materials

Electromagnet

You have just learnt about the magnetic effect of electric current.An electric current can,

hence, be used to make magnets.

When a current wire is wound closely around an iron nail, like a coil, and currentpassed through it, some pins placed near the coil attract the nail.

This shows that the coil behaves like a ________________ when electric current flows

through it.

On switching off the current, the pins no longer attract the coil.

This shows that when the electric current is switched off, the coil

____________________________________.

Such coils are called ________________________.


8/168Pg.No|

These electromagnets can be made very strong and can lift very heavy loads. For e.g. in

cranes - where they are used to separate magnetic material from junk.

Many toys also make use of electromagnets.

Name at least two toys you know that use electromagnets and explain how they are used:

All of you have seen and used electric bells at your homes.

It makes use of an _________________________.

The circuit of an electric bell consists of:

When the iron strip is in contact with the _____________, the current flows through the

coil which becomes an ____________________________, which pulls the iron strip.

In the process, the ________________ at the end of the strip strikes the gong of the bell

to produce a sound.


9/169Pg.No|

However, when the electromagnet pulls the iron strip, it also

______________________________________.

The current through the coil stops flowing and the coil is no longer an

_______________________________. It no longer ________________ the iron strip.

The iron strip comes back to its original position and touches the

________________________ again.

This completes the circuit. The current flows in the coil and the __________________

strikes the gong again.

This process is repeated in quick succession. The hammer strikes the gong every time the

circuit is completed.


10/1610Pg.No|

1.

When current flows through the coil, the iron rod is attracted downward because the

(A) Current through the coil produces a gravitational field

(B) Current through the coil produces a magnetic field

(C) Iron rod and the coil carry opposite charges

(D) Iron rod and the coil carry like charges

..............................................................................................................................................

2. The figure shows an electromagnetic train signal. When it is switched on, the signal remains

in the upward position, whereas when it is switched off, the signal remains in the downward

position as shown in the figure.

Which part of the electromagnetic train signal is the electromagnet?

(A) A (B) B (C) C (D) D

..............................................................................................................................................

3. Steve constructs an automatic door latch as shown in the figure. When the switch is closed,

the electromagnet attracts the door latch toward itself and the door can be opened. When

the switch is opened again, the door latch comes back to its original position using the spring

attached between the core of the electromagnet and the door latch.

Steve can improve the structure of the automatic door latch by

(A) Using a wooden or plastic door latch

(B) Increasing the number of turns in the electromagnet(C) Using glass rod as the core of the electromagnet

(D) Increasing the distance between the door knob and the electromagnet

FORMATIVE WORKSHEETS


11/1611Pg.No|

4. Clarke constructs a device by passing current through a piece of long insulated wire wrapped

around an iron core. He uses this device for picking paper clips.

What is the name of the device constructed by Clarke?

(A) Electric generator (B) Magnetic compass

(C) Electromagnet (D) Galvanometer

..............................................................................................................................................

5. An electric iron works on the principle of i effect of electric current. The component of an

iron that works on this principle is its ii.

The information in which alternative completes the given statements?

(A) i ii

chemical base

(B) i ii

heating element

(C) i ii

heating base

(D) i ii

chemical element

..............................................................................................................................................

6. How many poles does an electromagnet have?

(A) One (B) Two (C) Three (D) Four

..............................................................................................................................................

7. An electric fuse of 3 A rating is connected in a branch circuit. Suddenly, a current of

amount 4 A flows through the circuit that consists of a bulb.

Which of the following events could be a possibility in the given circuit?

(A) The bulb will get fused

(B) The fuse will melt and break the circuit

(C) The fuse will absorb 1 A current and will pass 3 A current only

(D) The bulb will continue to work normally getting a 4 A current

..............................................................................................................................................

8. Electric fuses are

(A) Safety devices (B) Heating devices

(C) Switching devices (D) Controlling devices

..............................................................................................................................................


12/1612Pg.No|

9. Which of the following devices works on the principle of heating effect of current?

(A) Switch (B) Battery

(C) Electric fuse (D) Socket board indicator

..............................................................................................................................................

10. While reading about the heating effect of current, Raju decides to list some home electrical

devices that work on that principle. He lists some devices (as shown in the given table).

His father notices that one device is listed incorrectly in the table.

Electrical device:

Electric kettle, Motor, Toaster, Geyser, Hair dryer

Which device is listed incorrectly by Raju in the table?

(A) Motor (B) Toaster

(C) Hair dryer (D) Electric kettle

..............................................................................................................................................

11. The given figure shows a simple circuit consisting of a bulb and a switch.

On closing the switch, the bulb will

(A) Glow

(B) Not glow

(C) Glow after some time

(D) Glow only for a short time..............................................................................................................................................

12. The given figure shows four wires, a switch, a bulb, and a battery.

The bulb will glow when the given components are connected as

(A) (B)

(C) (D)

..............................................................................................................................................


13/1613Pg.No|

13. The given figure shows a simple circuit involving a battery, a switch, and a bulb.

If the terminals of the battery are reversed, then the bulb will

(A) Glow

(B) Not glow

(C) Glow for a short time

(D) Glow after some time..............................................................................................................................................

14. A simple electrical circuit containing two bulbs and a battery is shown in the given

figure.

If bulb 1 fuses on closing the switch, then bulb 2 will

(A) Keep glowing (B) Stop glowing

(C) Glow brightly (D) Glow dimly

..............................................................................................................................................

15. The symbol for a fuse is

(A) (B)

(C) (D)A

..............................................................................................................................................

16. When negative terminal of a cell is connected to the positive terminal of the next cell are

said to be in(A) Series (B) Parallel

(C) Both (A) and (B) (D) Neither (A) nor (B)

..............................................................................................................................................


14/1614Pg.No|

CONCEPTIVE WORKSHEETS

1. A pin is placed near a wire loop, as shown in the given figure. When current flows

through the wire loop, the pin gets attracted toward the loop.

Which force attracts the pin toward the loop?

(A) Electric (B) Magnetic

(C) Frictional (D) Gravitational

.....................................................................................................................................................................................

2. Electromagnets are used in electrical devices because they

(A) Are cheap(B) Are strong

(C) Have a long life

(D) Can be switched on and off

.....................................................................................................................................................................................

3. Which of the following characteristics is associated with an ideal fuse element?

(A) It should be cheap(B) It should have very low resistance

(C) It should have a low melting point

(D) It should show the magnetic effect of current

.....................................................................................................................................................................................

4. Consider the following statements about an electric bell:

I. An electric bell works on the principle of magnetic effect of current

II. An electric bell consists of an electromagnet

III. An electric bell produces sound continuously

Among the given statements,

(A) Only I is correct

(B) Only III is correct

(C) Both I and II are correct

(D) Both II and III are correct

.....................................................................................................................................................................................


15/1615Pg.No|

5. The given figure shows a simple electric circuit that consists of a coil of wire wounded

around an iron rod. Some iron filings are placed near one end of the rod. When the

switch is closed, some iron filings get attracted towards the rod.

The iron filings will get attracted towards the iron rod

(A) For a very long time

(B) As long as the current flows through the coil

(C) Until the polarity of the battery is not changed

(D) Until the sheet containing the iron filings is not removed

.....................................................................................................................................................................................

6. The given figure shows a simple circuit containing four bulbs and five switches along

with a battery.

What is the minimum number of switches that must be closed to light at least one bulb?

(A) 1 (B) 2 (C) 3 (D) 4

.....................................................................................................................................................................................

7. The given figure shows a simple circuit that consists of a bulb, a battery, and two switches.

In the given circuit, when switch I is closed, the bulb will

(A) Glow (B) Flicker

(C) Not glow (D) Flicker after some time

.....................................................................................................................................................................................


16/16

8. Robert constructs an electrical circuit as shown in the given figure. Four different points

are labeled as P, Q,R, and S on the wooden rod.

In the given circuit, the bulb will glow when the positive terminal of the battery is connected

to the point

(A) P (B) Q (C) R (D) S

.....................................................................................................................................................................................

9. In which of the following circuits can the electrical bulb be operated using the switch in

the circuit?

(A) (B)

(C) (D)

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