# Important Questions and Answers: Direct Current Circuit

**1. What do you mean by resistivity of a metal?**

**Ans:** The resistance of a conductor having length 1 metre and cross sectional area 1m2 is called resistivity. The unit of resistivity is ohm metre.

**2. Why do we use connecting wires made of copper?**

**Ans:** Copper is preferred as connecting wire because of the following reasons:

- Copper has high electric conductivity.
- Copper possess diamagnetic property. So, it does not obstruct the flow of current.
- It is comparatively cheaper than other metal.

**3. What are the factor in which the resistance of a conductor depend?**

**Ans:** The resistance of a conductor depends upon its length and area. The resistance is directly proportional to its length and inversely proportional to the cross sectional area.

**4. Why are the alloys such as constantan and manganin used to make standard resistor?**

**Ans:** Standard resistors are made up of alloys like constantan and managanin because these alloys can resist to high temperature and pressure. Even these alloys have low temperature coefficient. This means when large amount of current is passed through these alloys, there is a negligible increase in temperature.

**5.** **The drift velocity of the electron is very small, but a bulb glows instantly as we turn on the switch. Why?**

**Ans:** As we turn on the switch, electric field is developed throughout the circuit nearly at the speed of light. Thus all the electron throughout the circuit comes in motion at once at the same speed. Thus a bulb glows instantly as we turn on the switch.

**6. Why is an ammeter connected in series in a circuit?**

**Ans:** We know that ammeter is a low resistance device. When it is connected in series, the current flowing through the circuit remains unaffected. Hence, it can read exact amount of current passing through it.

But, when it is connected in parallel, its resistance decreases to large extent. So, very high amount of current flows through the circuit and may damage ammeter and even shows wrong reading.

**7. Why is a voltmeter connected in parallel in a circuit?**

**Ans:** A voltmeter is a high resistance device. When it is connected in parallel, it draws very small amount of current from the circuit. So, it can measure actual potential difference of the two terminals of the circuit.

But, when it is connected in series, the resistance of the circuit increases and very small amount of current is passed through it. So, it cannot measure the exact p.d. of the circuit.

**8. What is shunt ? What is its significance?**

**Ans:** A shunt is a small resistance connected to a connected in parallel to galvanometer. It is generally used for increasing the range of galvanometer and current in the circuit.

**9. Differentiate between ohmic and non ohmic conductors. **

Ohmic Conductors |
Non-ohmic conductors |

Those conductors which obey ohm’s law are ohmic conductor. | Those conductors which does not obey ohm’s law are non-ohmic conductor. |

The resistance of ohmic conductor doesn’t charge as per voltage. | The resistance of the non-ohmic conductor change as per voltage. |

The graph between V and I is straight line passing through origin. | The graph is not straight line passing through origin. |

Examples: Metallic conductors like copper,iron,zinc etc. | Examples: Semiconductor like silicon,germanium etc. |

**10. If you are given n wires each of resistance R ****Ω. What is the ratio of maximum to minimum resistance that can be obtained from these wires?**

**Ans:** We know that the resistance becomes maximum when they are connected in series and minimum when they are connected in parallel.

In series: R_{s} = R_{1 }+ R_{2} + R_{3} + ……n

If R_{1 }= R_{2} = R_{3} …… = R

Then R_{s} = nR……(i)

In parallel, $$\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} + …..n$$

If R1 = R2 = R3……= R, Then:

$$\frac{1}{R_p} = \frac{n}{R}$$

$$or, Rp = \frac{R}{n}……(ii)$$

Dividing equation (i) by (ii), we get:

$$So, \frac{R_s}{R_p} = \frac{nR}{\frac{R}{n}}$$

$$\therefore\frac{R_s}{R_p} = n^2$$

So, the required ratio is n^{2}.

**11. If you are given 2 wires each of resistance R. What is the ratio of maximum to minimum resistance that can be drawn from the wires?**

We know that the resistance becomes maximum when they are connected in series and minimum when they are connected in parallel.

Since the two wires have same resistance, the maximum resistance is given by:

R_{max} = R + R = 2R……(i)

The minimum resistance is given by:

$$R_{min} = \frac{R \times R}{R + R} =\frac{R}{2}……(ii)$$

Dividing equation (i) by (ii), we get:

$$\frac{R_{max}}{R{min}} = \frac{2R}{R} \times 2 = 4$$

So the required ratio is 4:1.

**12. A wire is stretched double to its length. What happens to its resistance and resistivity?**

**Ans:** We know that, the resistance of a wire having length l and cross sectional area A is given by:

$$R = \rho \frac{l}{A}$$

Where ρ is the resistivity of the material of the wire.

Dividing both numerator and denominator by l, we get:

$$R = \rho \frac{l}{A} \times \frac{l}{l}$$

$$or, R = \frac{\rho l^2}{V} [\text{Since A} \times \text{l is V]}$$

ρ and V are constant for a given wire. So,

$$R \propto L^2$$

$$ or, \frac{R_2}{R_1} = \frac{L_1^2}{L_2^2}……(i)$$

When a wire is stretched double to its length, then:

Initial reisistance (R_{1}) = 2 Ω

Initial length(l_{1}) = l

Final length (l_{2}) = 2l

Final reistance (R_{2}) = ?

From equation (i), we have:

$$ or, \frac{R_2}{2} = \frac{(1)^2}{(2)^2} = \frac{1}{4} $$

So, the resistance increases by four times. Since the resistivity of the conductor depends upon the material of the conductor, so it remains same.

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