Electric current and circuits class 7

 NOTES PREPARED BY

ASHAQ HUSSAIN BHAT 

GL TEACHER SCHOOL EDUCATION 

DEPARTMENT JAMMU AND KASHMIR

ELECTRIC CURRENT AND CIRCUITS (14)

Points to Remember

• Some electric components can be represented by symbols.

• A cell or a battery is a source of energy,

• A cell is represented by two parallel lines, one short and other long.

• A battery is a combination of two or more cells.

• Batteries are used in devices such as torches, transistors, toys, T.V. remote controls.

• Electric bulb has a thin wire, called filament which glows when an electric current passes through it.

• Electric current has heating effect.

• The coil of wire used in a heating device is known as 'element'...

• Electric iron, immersion heater, hot plate, geyser, electric kettle, dryer are some electric devices having heating effect of current.

• The amount of heat produced depends on the length, material, and thickness of wire used.

• Electric fuses are used to prevent fires and damages to electric appliances.

• Electric fuse is generally made of special materials which melt quickly and can break the circuit easily when large currents greater than the maximum limit of current are passed through them.

• Wire carrying electric current behaves like a magnet.

• Electromagnets have many applications.

• Electric bell works on an electromagnet.

SOME IMPORTANT DEFINITIONS

Switch. A simple device used to make or break a circuit is called a switch.

Conductors. Materials which allow current to pass through them are called conductors.

Insulators. Materials which do not allow current to pass through them are called insulators

Electric circuit. The unbroken (continuous) path of current from one terminal to another terminal of the source of current (battery) via a switch and a bulb is known as electric circuit.

Bulb. A simple device which converts electric energy in to light energy 

Filament. A small piece of thin tungsten wire inside a bulb which glows to give light when current is allowed to pass through it, is called filament.

Battery. It is an electrochemical cell, which converts its chemical energy into electrical energy. It may also be considered to be combination of two or more cells.

Electric cell. It is a source of current which supplies electrical energy from the chemicals stored in it.

Electric current. The rate of flow of charge from the positive (+ve) terminal of the electric cell to its negative (-ve) terminal.

TEXT BOOK EXERCISES

A. Fill in the blanks :

1. A drawing of an electrical circuit with standard symbols is called a/an ............ (circuit diagram / electric diagram)

2. Hot plates, electric toasters and electric irons get hot when switched on because of the .......... effect of current. (Magnetic / heating)

3. A fuse is a...........device.(Safety / heating)

4. A wire twisted in the form of a circle is called a/an..........(coil / electromagnet)

5. An............. consists of a soft iron core with an insulated wire wound around it.(Electromagnet / Electric Magnet)

Ans. 1. Circuit diagram 

2. Heating

3. Safety

4. Coil

5. Electromagnet

B. Choose the correct answer :

1. A circuit diagram is:

(a) a picture of a circuit.

(b) a drawing of a circuit with standard pictures for the different electrical components.

(c) a diagram of an electrical circuit with standard symbols for the different electrical components.

(d) a difficult representation of an electrical circuit.

Ans. (b) a drawing of a circuit with standard pictures for the different electrical components.

2. When an electric current passes through a wire, the wire gets hot. This is called:

(a) Joule heating (b) Conduction

(c) electricity (d) thermal heating.

Ans. (d) thermal heating.

3. An electric fuse is

(a) a safety device

(b)an appliance

(c) used to produce electric current

(d) used to heat a room.

Ans. (a) a safety device.

4. When we bring a magnetic compass near a current carrying wire :

(a) it deflects the magnetic needle of the compass

(b)it makes the magnetic needle point north

(c) it makes the magnetic needle point south

(d) it has no effect on the magnetic needle.

Ans. (a) it deflects the magnetic needle of the compass.

5. A coil refers to :

(a) an electrical wire

(b) a fuse

(c) a current carrying conductor

d) a wire twisted in the form of a circle,

Ans. (d) a wire twisted in the form of a circle.

6. An electromagnet acts like a magnet

(a) when a current is passed through the coil

(b) all the time

(e) because it has magnetic core

(d) only if current does not pass through the coil.

Ans. (a) when a current is passed through the coil.

7. In an electric bell, we have:

(a) an electromagnet 

(b) a hammer

(c) an interrupter

(d) All of these.

Ans. (d) All of these.

8. In an electric bell, which of these gets attracted to the electromagnet?

(a) the hammer 

(b) the gong

(e) the soft iron strip 

(d) the screw.

Ans. (c) the soft iron strip. 

9. Which of these does not use heating effect of current ?

(a) electric toaster 

(b) electric fan

(c) electric iron

(d) room heater,

Ans. (b) electric fan.

10. An electric fuse wire melts if the amount of current flowing through it :

(a) more than a minimum amount

(b) less than a minimum amount

(c) more than a maximum amount.

(d) less than a maximum amount.

Ans. (c) more than a maximum amount. 

C. Answer the following questions: 

Q. 1. What is an electrical circuit?

Ans. Electric Circuit. The complete (unbroken) path of current from one terminal of a source of current (say a cell or battery) through a bulb back to the other terminal (of the cell or battery), is called electric circuit.

Q. 2. Draw an electrical circuit with an electrical cell, a bulb and an 'ON' switch.

Q. 3. Why does an electric bulb get hot if it is kept on for a little while?

Ans. When electric bulb is kept 'ON' for a while, it becomes hot. It is because the current tends to flow through the filament of the bulb while the material of the filament offers resistance to its flow. As a result of this the bulb becomes hot. This kind of heating is called ohmic heating.

Q. 4. Name three appliances that use the heating effect of electric current.

Ans. Appliances in which heating effect is used. The following appliances use the heating effect of current:

(1) Electric toasters

(2) Electric irons

(3) Hot plates

(4) Electric ovens

Q. 5. What is an electric fuse?

Ans. Electric fuse. It is a safety device which is used in an electric circuit to prevent electric appliances from getting damaged as a result of excessive passage of current through them

Q. 6. Draw an electric fuse and explain briefly how it works?

Ans. Electric fuse. It is a safety device used in electric circuit to prevent the appliance from being damaged due to excessive heating.

Principle. A wire gets heated up if electric current passes through it. The amount of heating caused depends on the amount of current flowing through the wire. The greater the current, the more is the heating caused.the electric fuse is based on this principle of heating effect of current.

Q. 7. How can you show that an electric current has a magnetic effect?

Ans. 1. Take an insulated wire.

2. Remove the insulation from the two ends of the wire.

3. Connect one end of the wire to the negative terminal of a cell.

4. Now connect other end of the wire to the positive terminal of the cell.

5. Place the magnetic compass near the wire. 

6. You will notice that the needle of the magnetic compass gets deflected, when current flows through the wire. So to say. it behaves like a magnet. This shows that current produces magnetic effect.

Q. 8. What is an electromagnet ?

bor Ans. Electromagnet. A piece of soft iron around which a wire is wrapped (known as core) behaves like a magnet when current is passed through the wire, is called electromagnet.

Q. 9. Name two factors on which the strength of an electromagnet depends.

Ans. The strength of an electromagnet depends on :

(i) Number of turns of the wire wound around the core.

(ii) Amount of current passing through it.

Q. 10. Explain the working of an electric bell using diagrams.

Ans. Principle. The principle underlying the working of electric bell is that a current carrying coil with a soft iron core placed inside it behaves like a strong magnet.

Construction. It consists of a copper coil wound on an iron core. When an electric current flows through this coil, it becomes an electromagnet. A bar of iron called armature is kept close to the electromagnet. The armature has a hammer at one end and there is gong of the bell facing the armature.

Working. When we push the button or switch of the electric bell, the electric current starts flowing through the wire, which now acts as an electromagnet. The electromagnet attracts the and the armature and hammer strikes the gong producing sound. When the electromagnet attracts the armature, the circuit breaks and the current stops flowing through the coil. Due to this the electromagnet loses its magnetism and the armature is then pulled back by a spring attached to it. When the armature comes back to the normal  position, the circuit again gets completed and current again flows through the coil and the cycle is repeated again and again. Thus, the hammer strikes the gong of the bell a number of times producing sound.

TEXT BOOK QUESTIONS LET'S ANSWER

I Match the following:

Column 'A'.                                                                                    Column 'B'

1. Circuit diagram.        (a) Heating of a conductor due to an electric current

2. Joule heating.            (b) Fuse

3. Safety device.             (c) Fuse wire

4. Melts easily.               (d) A drawing of an electrical circuit with standard symbols

Ans.   

1---------d

2---------a

3---------b

4---------c

(II) Draw the standard symbol used to represent each of the following components in a circuit diagram.

1. Electric cell

2. An Open switch

3. A closed switch

4. A bulb

5. A wire

(III) Fill in the blanks :

1. A magnet has two poles, namely the......... pole.and ........pole

2. Like poles of a magnet.........each other and unlike poles.........each other.

3. A magnet when suspended freely aligns it self in the........direction

Ans. 

1. North, South

2. Repel, attract

3. North-South

LET'S ANSWER

State whether the following statements are True or False.

1. An electromagnet attracts a safety pin even when the current is turned off.

2. The strength of an electromagnet does not depend on the number of turns of the coil wrapped around it.

3. An electromagnet is used in a loudspeaker to amplify the signals.

4. In an electric bell, the hammer hits the gong when the electromagnet attracts the soft iron strip.

5. When the soft iron strip gets attracted to the electromagnet, it gets detached from the hammer.

Ans. 1. False 2. False 3. True 4. True 5. False.

ADDITIONAL IMPORTANT QUESTIONS

LONG ANSWER QUESTIONS

Q. 1. Give an activity to show the heating effect of current.

Ans. Heating effect of current

1. Remove the insulation from both ends of the wire to about 1 cm length.

2. Clean the ends with sand paper.

3. Connect one end of one wire with the negative terminal of the cell and the other end to the threaded metallic part of the bulb. Secure it with adhesive tape.

4. Connect one end of the other wire to the positive terminal of the cell and other end of the wire to the bulb. Secure it with adhesive tape when you will see that the bulb begins to glow. If it does not glow then check for proper connections till it glows.

5. Touch the glass part of the bulb to know how hot the bulb is.

6. Leave the bulb glowing for some time and then touch the glass part of the bulb as before. You will notice that the bulb has become warmer. This is because the passage of current has heated the filament of the bulb.

Q. 2. What are the uses of electromagnets ?

Ans. Uses of Electromagnets. We use some devices in our daily life which make use of electromagnets.

1. Electromagnets are used in electrical overhead cranes to pick up cars or heavy machine parts made of iron in scrap yards or factories.

2. Electromagnets are used to fill furnaces with iron scraps.

3. The receiver of telephone also makes use of electromagnet.

4. Electromagnets are also used in electric motors, electric fans, washing machines and refrigerators etc.

5. Electromagnets are also used in loudspeaker to amplify signals.