Q.No.13.9: - What is Wheatstone bridge? How it be used to determine an unknown resistance?

Q.No.13.9: - What is Wheatstone bridge? How it be used to determine an unknown resistance?

Ans: - Wheatstone bridge is used to determine unknown resistance of a wire. The circuit is consists of four resistances R₁, R₂, R₃, R₄ connected in such a way as to form a loop or mesh ABCDA. A battery of emf E is connected between points A and C. A galvanometer of resistance R_g is connected between the points B and D.

          When the bridge is balanced, it satisfied the following relation

                                        R₁/R₂      =       R₃/R₄

Or,                                   R₄           =        R₂.R₃/R₁

If the value of R₁, R₂, and R₃ are known then R₄ can be calculated. The galvanometer should not show any deflection by adjusting the potential between terminals B and D. (Sorry for Diagram. Pleas concert with Text Book for diagram.)

Q.No.13.6: - Is the filament resistance lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?

Q.No.13.6: - Is the filament resistance lower or higher in a 500W, 220V light bulb than in a 100W, 220V bulb?

Ans: -

Power of light bulb                    =        P₁       =        500W

Potential difference        =        V₁       =        220V

Now apply,

                                        P₁       =        V₁²/R₁

                                        R₁       =        V₁²/P₁

                                        R₁       =        (220)²/500

                                        R₁       =        48400/500

                                        R₁       =        96.8 Ω

For second Bulb

Power           P₂       =        100W

Voltage        V₂       =        220V

Now, 

                    R₂       =        V₁²/P₂

                    R₂       =        (220)₂/100

                    R₂       =        48400/100

                    R₂       =        484 Ω

It means that the filament resistance of 100W, 220V light bulb is higher than that of other.

Q.No.13.5: - What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s law?

Q.No.13.5: - What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s law?

Ans: - According to Ohm when we increase voltage then current will also increases in direct proportion but resistance of the conductor must remain same and hence temperature also must remain same or constant. But when we test the filament of a lighted bulb that either it obeys Ohm’s law or not then with the passage of time its temperature increases and resistance also increases. These are the difficulties in testing whether filament of a lighted bulb obeys Ohm’s law or not.

Q.No.13.4: - Why does the resistance of a conductor rise with temperature?

Q.No.13.4: - Why does the resistance of a conductor rise with temperature?

Ans: - Electrical resistance is due to the collision of free electrons with atoms of the conductors. Atoms in the conductors are in vibratory motion when electrons collide with atoms they transfer some of their energy to the atoms and atoms starts vibrating with larger amplitude as the amplitude of vibration increases the probability of collision of electrons also increases. As a result increases resistance. When temperature increases then also amplitude of vibration increases and then probability of electrons collision also increases and then increases resistance.

Q.No.13.3: - What are the resistances of the resistors given in the figure A and B? What is the tolerance of each? Explain what is meant by tolerance?

Q.No.13.3: - What are the resistances of the resistors given in the figure A and B? What is the tolerance of each? Explain what is meant by tolerance?

(Sorry for Diagram but I’m trying to describe this figure here)

1^st band is Brown 2^nd is Green 3^rd is Red 4^th is Gold. In B fig. 1^st band is Yellow 2^nd is White 3^rd is Orange and 4^th is Silver.

Ans: - For Figure A

First band brown   =        1

            Second band Green =     5

             Third band Red (have number two so)= 00

             Now,       R   =   1500Ω

             Fourth band Gold which shows tolerance = ±5%

So,                       R   =   1500Ω±5%

For Figure B

                    First Band Yellow  =        4

                    Second band White=        9

                    Third band orange =        000 (no. of zeros)

                              R     =     49000 Ω

                    Fourth band silver =        ±10%

Now resistance is

                              R     =     49000±10% Ω

Tolerance: -

       Tolerance means the possible variation from the marked value.

Q.No.13.2: - Do bends in a wire affect its electrical resistance?

Q.No.13.2: - Do bends in a wire affect its electrical resistance?

Ans: - The resistance of wire is

                                       R   =  ρ.L/A

Where “ρ” is the resistivity of the conductor, L is the length and A is the area of cross section of the wire. So according to this equation we can say that bends in a wire will never affect the electrical resistance because we bending the wire its ρ, L and A remain same so its resistance remain same.