Solutions-Class 8-Science-Chapter-4-Current Electricity and Magnetism-Maharashtra Board

A. Water in the waterfall flows from a higher level to the lower level because of potential differences.

B. In an electric circuit, electron flow a from of point of higherpotential to the point of lowerpotential.

C. The differences between the electrostatic potential of the positive end the negative end of an electric cell is the potentialof the cell.

D. Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential differences of the battery will be 1.5 + 1.5 + 1.5 = 4.5V.

E. An electric current flowing in a wire creates magnetic fieldaround the wire.

We can perform following tests to find the reason why bulb is not glowing:

• Check how the terminals of the batteries are connected to each other: Ensure that the positive terminal of one battery is connected to negative terminal of other battery.

If the batteries are connected in correct way and even then the bulb does not glow, go for the next tests given below.

• Check for the broken wires in the circuit: Ensure that the wires used for connecting the various electrical components are not broken in between i.e. ensure that the circuit is closed.
• Check how the connecting wires are connected to the bulb: Ensure that the bulb is connected to the batteries using the connecting wires as shown below.

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

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

Construction of dry cell
Outer metal covering: Take the dry cell Remove its outer coating, inside you will find a whitish metal layer covering. The metal covering is made up of zinc metal and is whitish in appearance. It acts as a negative terminal of the cell.

Electrolyte: Inside the Zinc metal, there is the electrolyte filled between two layers. Electrolyte is a wet pulp of Zinc chloride (ZnCl₂) and Ammonium chloride (NH₄Cl). It is the charge carrier of electricity as it contains negatively charged and positively charged ions.
Metal rod: There is a graphite rod at the centre of the cell. A paste of Manganese dioxide (MnO₂) is filled outside the rod. It acts as the positive terminal of the cell.

Working of dry cell: Chemical reactions take place between the electrolyte, zinc container and graphide rod. Because of this, electric charge is produced on the two terminals of the cell and electric current flows in the circuit.

Usefulness of dry cell: They are handy and portable, It can held in any dirction. The life of dry cell is longer than cells using liquid electrolyte. Dry cells can be use in Torch, Mobile,  Radio set, TV remote controls, Toys, etc.

Construction of an electric bell
Following are the components of electric bell:

• Electromagnet: A coil of copper wire  wound around an iron piece which acts a magnet when current flows through it.
• Striker in touch with contact screw through an iron strip
• A metal gong
• A key or a switch

Working of an electric bell

• When the switch is 'ON' and the screw is in contact with the iron strip, then electric current flows through the copper wire which gets magnetised because of electromagnetism.
• This magnetised copper wire (or the electromagnet) attracts the iron strip towards it, letting the striker hit the gong and thus sound is produced.
• As soon as the striker hits the gong, the screw loses its contact with the iron strip and therefore, current stops in the circuit.
• At this point, the electromagnet loses its magnetism and the iron strip moves back and comes in contact with the contact screw.
• The electric current is then restored in the circuit and again the striker hits the gong by the above process. This action repeats itself and the bell rings.

Thus the working of the electric bell is based on magnetic effect of electric current.