Electricity Worksheet-20

Electricity Worksheet-20

 

  1. Two wires are of the same material but of different lengths and areas of cross-section. Will their resistivity be same or different ?

 

  1. Why is nichrome used as a heating element ?

 

  1. What is the difference between a resistor and resistance ?

 

  1. Why are constantan and manganin used for making standard resistances ?

 

  1. Why is lead-tin alloy used for making fuse ?

 

  1. Why are copper and aluminium wires used as transmission lines for electric power ?

 

  1. What are superconductors ?

 

  1. A wire having a resistance of 18 Ω is cut into three equal parts. What is the equivalent resistance if these three parts are joined in parallel ?

 

  1. What is the ratio of equivalent resistance of series combination of n equal resistance to the equivalent resistance in parallel combination of these n resistances ?

 

  1. What is the resistance of a human body ? How much voltage is dangerous to it ?

 

Answer:

  1. Resistivity depends on the material of the conductor and not on its dimensions. Hence, the two wires will have the same resistivity. But their resistances are different.

 

  1. Nichrome has high resistivity and high melting point. It does not react with air when it is red hot (800°C).

 

  1. A resistor is an object whereas resistance is its property due to which it obstructs the flow of electrons through it.

 

  1. The resistivity of these materials is moderate and almost independent of temperature.

 

  1. It has low melting point and a fuse made of this alloy melts when the current exceeds a particular

level.

 

  1. These metals possess low resistivity and as such there is less power loss (I2R) due to heating effect of current.

 

  1. Superconductors are materials which have zero resistance at a particular temperature, for example., mercury is super­conductor at 4.2 K.

 

  1. Resistance of each part, r = 18/3 = 6 Ω

Since these three parts are joined in parallel, the equivalent resistance, R = r/3 = 6/3 = 2 Ω

 

  1. Equivalent resistance of series combination of n equal resistances where each resistance equal to r,

          Rs = nr

Equivalent resistance of parallel combination of n equal resistance each resistance equal to r, 

         

         

 

  1. The resistance of a dry human body is approximately 50k Ω and in case it is wet, the resistance is about 10k Ω. A current of 1 mA is enough to send a shock through a human body. A voltage of about 100 V is injurious and 220 V may prove fatal.