10th Chapter: Electricity and its effects |

Very Short Answer Questions 1 mark each with answers |

Q.1 What is represented by joule/coulomb? |

Answer. It represents potential difference. |

Q.2 A charge of 2C moves between two plates, maintained at a p.d of 1V. What is the energy acquired by the charge? |

Answer. W=QV=2×1=2J |

Q.3 Why are copper wires used as connecting wires? |

Answer. The electrical resistivity of copper is low |

Q.4 A wire of resistivity 10 ohm meter is stretched to double its length. What is its new resistivity? |

Answer. It remains same because resistivity depends on nature of material. |

Q.5 What is the resistance of connecting wire? |

Answer. The resistance of a connecting wire, which is made of good conductor, is negligible. |

Q.6 What should be the resistance of an ammeter? |

Answer. The resistance of an ammeter should be very small and for an ideal ammeter, its value is zero. |

Q.7 What should be the resistance of a Voltmeter? |

Answer. The resistance of a voltmeter should be very high and for an ideal voltmeter, its value is infinity. |

Q.8 Which has more resistance: 100W bulb or 60W bulb? |

Answer. As R is inversely proportional to P for constant V. Thus, the resistance of 60W bulb is more. |

Q.9 How will you join three resistances, each of 2 ohm so that the effective resistance is 3 ohm? |

Answer. A parallel combination of two resistances (which will be 1 ohm) joined in series with the third resistance (2 ohm) |

Q.10 What happens to the current in a circuit if its resistance is doubled? |

Answer. As I inversely proportional to R, the current is reduced to half of its previous value. |

Q.11 What happens to the resistance of a circuit if the current through it is doubled? |

Answer. The resistance of the circuit does not depend on the current through it., it will remain the same. |

Q.12 How does the resistance of a wire depend upon its radius? |

Answer. As R is inversely proportional to A, R is inversely proportional to square of radius r. |

Short Answer Questions 2 mark each with answers |

Q.1. Two wires are of the same length, same radius, but one of them is of copper and the other is of iron. Which will have more resistance? |

Answer.A and l are same R depends only on resistivity and it is more for iron so iron has more resistance. |

Q.2. Two wires of same material and same length have radii r1 and r2. Compare their resistances. |

Answer. R inversely proportional to square of radius r. R1/R2 =(r2/r1)2 |

Q.3. Given n resistors each of resistors Rs. How will you combine them to get the |

( i ) maximum and |

(ii) Minimum effective resistance? What is the ratio of the maximum to minimum resistance? |

Answer. (i)For maximum resistance Rs= nr (Equivalent of series combination) |

(ii)For minimum resistance Rp= r/n (Equivalent of parallel combination) Rs/Rp= n2 |

Q.4. A wire of length L and resistance R is stretched so that its length its doubled. How will its (a) Resistance change (b) Resistivity change ? |

Answer. (a) If the original length of the wire is l and its cross-sectional area is A, then |

R = l/A. When length becomes 2l, cross-sectional area reduces to A/2 because |

volume does not change. The new resistance =(2l)/A/2=4 l/A = 4R |

(b) Resistivity does not change |

Q.5. How much work is done in moving a charge of 3 coulumb from a point at the volts 115 to a point at 125 volts? |

Answer. Potential difference V= 125 - 115=10 volts Charge Q = 3 coulumb |

V= W/Q W=Vx Q 10x3= 30 Joules. |

Q.6. Ammeter burns out when connected in parallel. Give reasons. |

Answer. Ammeter consists of a wire of low resistance when connected in parallel, a large amount of current passes through it hence gets burnt i.e. short circuited. |

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