NCERT 7th Class (CBSE) Science: Electric Current and its effects

Question: Draw a circuit diagram of dry cell connected to a bulb through a switch. Mark the positive and negative terminals of the cell and the direction of flow of current in the diagram.

Answer: 

Question: How can you increase the magnetic strength of a solenoid?

Answer: The strength of the solenoid can be increased by:

1. Increasing the number of turns in the solenoid.
2. Increasing the current in the solenoid.
3. Winding the solenoid around a magnetic material.

Question: What is an electromagnet? Give three uses of electromagnet?

Answer: An electromagnet is a coil of insulated wire wound around a piece of a magnetic substance such as soft iron, with acts as a magnet only as long as the current is flowing in motors, washing machines etc.

Question: Explain  the working of a simple electric bell.

Answer: When the switch is pressed on, current flows in the electromagnet. It then attracts the iron rod towards itself, causing  the hammer to strike the gong. At the same time, the armature loses contact with the screw and the current is switched off. This causes the electromagnet to lose its magnetism and the armature springs back to its original position to close the circuit once again.

Question: Fuse wires or miniature circuit breakers are not used in a circuit containing electric cells why?

Answer: fuse wires or miniature circuit breakers are not used in a circuit containing electric cells because a dry cell does not has a high amount of voltage and there is no danger.

Question: Can you use a copper wire to make the filament of an electric bulb? Why?

Answer: The filament of an electric bulb is made up of tungsten which allows very high resistance thus it doesn’t get heated up and is not useful to give heat and light energy.

Question: Mohit used a fuse wire a specification of 5 amperes in a circuit connected to an air conditioner which draws a current of 10 amperes. What will happen when the air conditioner is switched  on? What will you advise him to do?

Answer: When the air conditioner is switched on then the fuse will melt and circuit will open. I will advise him to use a wire of 10 amperes or more than that.