Electric Current and its Effects

Q39. Draw the circuit diagram to represent the circuit shown below.

 Image from NCERT

Ans. Here, the switch is in the ‘OFF’ position as one of the board pin is not attached to the safety pin. Thus, the circuit is incomplete. It is said to be open and no current flows through any part of the circuit. Bulb glows only when the switch is in the ‘ON’ position and the electric circuit is closed.

Q40. The bulb in the circuit shown in Fig. does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Ans. The bulb in the circuit is not glowing because the two cells are not connected properly. In order to make the bulb glow, the negative terminal of one cell should be connected to positive terminal of the other cell.

Q41. Zubeda made an electric circuit using a cell holder shown in Fig., 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.

Ans. Possible defects

                           i.        Rubber band used may not be tight enough to hold the metal strips tightly. It is necessary that the rubber bands hold the metal strips tightly.

                          ii.        The two cells may not be connected properly. The negative terminal of one cell should be connected with the positive terminal of the other cell.

                         iii.        The two cells may not be in proper contact. Current will not flow if circuit is not complete.

                         iv.        The bulb may be fused. Fuse bulb do not complete the circuit. Hence bulb will not glow.

Q42. Write an experiment to show the magnetic effect of current.

Ans. Take the cardboard tray from inside a discarded matchbox. Wrap an electric wire a few times around the cardboard tray. Place a small compass needle inside it. Now connect the free ends of this wire to an electric cell through a switch. Now, move the switch to the ‘ON’ position. We will observe that compass needle gets deflected when the current flows in a nearby wire. So, when electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current.

Q43. Write an experiment to make an electromagnet.

Ans. Take around 75 cm long piece of insulated (plastic or cloth covered or enamelled) flexible wire and an iron nail, say about 6–10 cm long. Wind the wire tightly around the nail in the form of a coil. Connect the free ends of the wire to the terminals of a cell through a switch. Place some pins on or near the end of the nail. Now switch on the current. The pins cling to the end of the nail. The coil in the above activity behaves like a magnet when electric current flows through it. When the electric current is switched off, the coil generally loses its magnetism. Such coils are called electromagnets.

Q44. How does an electric bell work?
Or
Explain the working of an electric bell.

Ans. The circuit of an electric bell consists of a coil of wire wound on an iron piece. The coil acts as an electromagnet. An iron strip with a hammer at one end is kept close to the electromagnet. There is a contact screw near the iron strip. When the iron strip is in contact with the screw, the current flows through the coil which becomes an electromagnet. It, then, pulls the iron strip. In the process, the hammer at the end of the strip strikes the gong of the bell to produce a sound. However, when the electromagnet pulls the iron strip, it also breaks the circuit. The current through the coil stops flowing. The coil is no longer an electromagnet. It no longer attracts the iron strip. The iron strip comes back to its original position and touches the contact screw again. This completes the circuit. The current flows in the coil and the hammer strikes the gong again. This process is repeated in quick succession. The hammer strikes the gong every time the circuit is completed. This is how the bell rings.