Electric Current And Circuits, Class 7th Notes

THESE NOTES ARE PREPARED ACCORDING TO JKBOSE SYLLABI IN SIMPLIFIED AND CONCISE PATTERN

Exercises

A. Fill in the blanks

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


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


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


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


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




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: (c) a diagram of an electrical circuit with standard symbols 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: (a) Joule heating



3.  An electric fuse is 

(a) a safety device    (b) an appliance
(c) used to produce electric current

 (d) use 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
(c) because it has a magnetic core
(d) only if a 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
(c) 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:


1.  What is an electrical circuit? 

Ans: An electric circuit is the unbroken path through which an electric current flows.

It is represented in form of circuit diagrams.



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

Ans: 

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

Ans: The electric bulb gets hot due to heating effect of current.

When electricity passes through a conductor, it produces heat in it.


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

Ans: The appliances that use heating effect of electric current are:
Electric Iron, Water geyser, electric heater.




5. What is an electric fuse?

Ans: An electric fuse is a safety device used in curcuit to prevent the appliances from getting damaged.

If more than certain maximum amount of current passes through the fuse, it breaks.



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

Ans: 

Working: The fuse allows only certain maximum amount of current to pass through it. 

If more current passes through the fuse, it will break and stop flownof current.



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

Ans: When we connect the wire to the two terminals of a 1.5 V cell and put a magnetic compass near it.

The magnetic compass deflects, thus showing that electric current has magnetic effect.


8.   What is an electromagnet?

Ans: An electromagnet is a temporary magnet made by wrapping a wire around soft iron core and allowing the current to pass through it.



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

Ans: The strength of electromagnet depends on following two factors:

1. Number of turns of the wire around soft iron core
2. Amount of current passing through it.



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

Ans: Electric bell uses electromagnet, soft iron strip, hammer, gong, screw etc. 

Step 1: When we push the switch of the bell, the current passes through electromagnet.

Step 2: The electromagnet attracts soft iron strip. The hammer attached to iron strip hits gong.

Step 3: As soon as the iron strip moves towards electromagnet, it gets detached from the screw.
It breaks the circuit.

Step 4. The iron strip returns to its initial position and touches the screw again.

Step 5: The circuit gets closed again and iron strip gets attracted to electromagnet again

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