Science textbook Solutions for Class 8 Science Chapter 4 – Current Electricity and Magnetism

Important points to remember :

  1. This electric  level  deciding the direction  of flow of electric  charges is called electrostatic potential. 
  2. Potential difference  :  Similar to the height of a waterfall,  the temperature  difference  of hot and cold bodies, the difference  between the potential of two points, 
  3. Lithium  ion  cells are used in modern equipments  for  example  smart  phone, laptop etc. 
  4. These cells can be recharged. More electrical  energy can be stored  in  these  cells  as compared to that in Nicd cells.

Question 1: Write proper words from the following group of words in the blanks.
(magnetism, 4.5V, 3.0V, gravitational attraction, potential differences, potential, higher, lower, 0V)

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 higher potential to the point of lower potential.

C. The differences between the electrostatic potential of the positive end the negative end of an electric cell is the potential of 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 = 4.5 V.

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

Question 2: A battery is to be formed by joining 3 dry cells with connecting wires. Show how will you connect the wires by drawing a diagram.
Answer :

Question 3: In an electric circuit, a battery and a bulb have been connected and the battery consists of two cells of equal potential difference. If the bulb is not glowing, then which tests will you perform in order to find out the reason for the bulb not glowing?
Answer :

  1. Check how the terminals of the batteries are connected to each other: 
  2. Ensure that the positive terminal of one battery is connected to negative terminal of other battery. If the batteries are connected in this this way and even then the bulb does not glow, go for the next test given below.
  3. Check for the broken wires in the circuit: Ensure that the wires used for connecting the various electrical components are nor broken in between i.e. ensure that the circuit is closed. Even after ensuring that the wires are not broken in between, the bulb does not glow, move to the next test.
  4. 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. Even now, if the bulb does not glow, replace the bulb or the batteries with a new one.

Question 4: Electric cells having 2 V potential difference each have been connected in the form of a battery. What will be the total potential difference 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

Question 5: Describe the construction, working and usefulness of a dry cell, with the help of a diagram. 
Answer :

  1. Take  a  lead  dry  cell  and  remove  its  outer coating.  Inside you will  find a whitish, metal layer. This is the Zinc  (Zn) metal  layer. 
  2. 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 elctrolyte  contains negatively charged and positively  charged ions. These are the carriers of electricity.  
  3. The electrolyte is a wet  pulp  of Zinc  chloride  (ZnCl2) and Ammonium chloride  (NH4Cl). There is a graphite  rod at the centre  of the cell.  This is  positive  terminal  of  the  cell.  A  paste  of Manganese dioxide  (MnO2) is filled  outside the  rod.  Because  of  the  chemical  reactions of all  these  chemicals,  electrical  charge  is produced  on the  two terminals  (graphite  rod and zinc layer) and an electric  current flows in the circuit. 
  4. Due to the wet pulp used in this cell, the chemical  reaction proceeds very slowly. 
  5. Hence a large electric current can not be obtained from this. Compared to the electric  cells using liquids, the shelf life of dry cells is longer. 
  6. Dry cells are very convenient  to use as these can be held in any direction with respect to ground and can be used in mobile instruments.

Question 6: Describe the construction and working of an electric bell with the help of a diagram.
Answer :

  1. A  copper wire is wound  around an iron piece. This coil  acts as an electromagnet.  An iron strip along with  a striker  is fitted  near to the  electromagnet. 
  2. A  contact  screw is in  touch  with  the  strip.
  3. The  current  flows in  the  circuit  when  screw  is  in  contact  with  the  strip,  and hence  the coil becomes  a magnet  and attracts  the iron strip towards it.  
  4. Therefore,  the  striker hits the  gong and the sound is  created.  However, at  the  same  time,  the  contact screw loses the contact with the strip and the current in the circuit  stops. 
  5. In this situation,  the electromagnet  loses its magnetism and the iron strip moves back and comes in contact  with  the  contact  screw. 
  6. The  electric  current  is then immediately  restored and again the striker hits the gong by the above process. This action  repeats itself  and the bell rings.

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