Class 8 Science Chapter 4 current electricity and magnetism question answer

 Current Electricity and Magnetism

Q.1) Write proper words from the following group of words in the blanks.

[magnetism , 4.5V , 3.0V , Gravitational attraction , Potential difference , potential , higher , lower , 0V ]

a. Water in the waterfall flows from a higher level to the lower level because of gravitational force .

b. In an electric circuit, electron flow from a point of lower potential to the point of higher potential.

c. The difference between the electrostatic potential of the positive end the negative end of an electric cell is the potential difference of the cell . 

d. Three electrician of potential difference 1.5 V each have been connected as a battery. the potential difference of the battery will be 4.5 V .

e. an electric current flowing in a wire creates magnetism around the wire.

Q.2) A battery is to be formed by joining 3 dry cells them with connecting wires. show how will you connect the wires by drawing a diagram. 

Answer- 

Q.3) In an electric circuit, a battery and a bulb have been connected and the battery consist of to sell up equal potential then which test will you perform in the order to find out the reason for the bulk not glowing ?

Answer - If you can see the filament of the bulb, check whether it is intact or broken. Check whether the cells are connected in a proper manner:

Or in a wrong way:
If they are connected in a wrong way as shown above, the total potential difference will be 2V + (-2V) = zero.

Q.4) Electrics cells having 2V potential difference each have been connected in the form of a battery. what will be the total potential depress of the battery In both cases . 

Answer-

(i) Total potential difference = 2 + 2 + 2 = 6 V

(ii) Total potential difference = 2 + 2 + 2 + 2 = 8 V

Q.5) Describe the construction, working and usefulness of a dry Cell, with the help of a diagram.

Take a lead dry cell & remove its outer coating. Inside you will find a whitish, metal layer. This is the Zinc (Zn) metal layer. This is the negative terminal of the cell. Now, carefully break open this layer. There is another layer inside. An electrolyte is filled between these two layers. The electrolyte contains negatively charged & positively charged ions. There are carries electricity. The electrolyte is a wet pulp of Zinc Chloride (ZnCl₂) & Ammonium Chloride (NH₄Cl). There is a graphite rod at the center of the cell. A paste of Manganese di Oxide (MNO₂) is filled outside the rod. Because electric charge is produced on the two terminals & an electric current flows in the circuit.

Q.6) Describe the construction and working of an electric bell with the help of a diagram.

the construction of an electric bell and also the electric circuit. The bell consists of an electromagnet, contact screw, iron strip, metal striker and metal gong. A coil of copper wire wound around an iron piece works as an electromagnet and an iron strip along with a striker is fitted near it. A contact screw touches the strip . The circuit is closed with the key. The current flows in the circuit when the screw is in contact with the iron strip. The current- carrying coil becomes a magnet and attracts the iron strip towards it. As a result, the metal striker hits the metal gong producing sound. At the same time, the screw loses the contact with the strip. The circuit is now incomplete. Hence, no current flows in the circuit. Therefore, the electromagnet loses its magnetism and the electric iron strip returns to its initial condition, making contact with the screw. As the circuit is now completed, the electromagnet attracts the iron strip and the striker strikes the gong producing sound. The action repeats itself and the bell continues to ring till the circuit is broken by opening the key in the circuit. The working of the electric bell is thus based on the magnetic effect of electric current.