**Ch. No. 4. THE ELECTRIC SPARK**

**Q1. Choose the correct alternative and rewrite the following:**

^{3}A

**b.**

**10**

^{ – 3 }A^{6}A

^{ – 6 }A

**a.**

**Series**

^{6}Joule

**b.**

**3.6 x 10**

^{6}Joule^{6}Joule

^{5}Joule

**b.**

**4A**

**Q2. State True or False. If false correct it.**

**1.**

**The SI unit of charge is volt.**

**2.**

**A voltmeter is always connected in series with the device.**

**3.**

**The conventional direction of flow of current is from positive terminal to negative terminal of the cell.**

**4.**

**Silver and copper are good conductors.**

**5.**

**Resistivity of pure meal is more than alloys.**

**6.**

**The electric bulb consists of the filament whose melting point is low.**

**Q3. Match the column.**

I | II | III |

Heat generated | Is used to reduce effective resistance in a circuit | V = IR |

Resistance in parallel | Proportional to the square of current | |

Resistivity | Gives relation between V and I | |

Ohm’s law | Depends on the material of the conductor |

**Ans.**

I | II | III |

Heat generated | Proportional to the square of current | |

Resistance in parallel | Is used to reduce effective resistance in a circuit | |

Resistivity | Depends on the material of the conductor | |

Ohm’s law | Gives relation between V and I |

**Q4. Give scientific reasons.**

**1.**

**The material used for fuse has low melting point.**

**2.**

**Wood and glass are good insulators.**

**3.**

**The melting point of filament of a bulb is very high.**

**4.**

**Connecting wires in a circuit are made of copper and aluminium.**

**Q5. Define**

**1 volt**: The potential difference between two points is said to be 1 volt if 1 joule of work is done in moving 1 coulomb of electric charge from one point to another.

**1 ampere**: 1 coulomb of charge passing through a cross – section of a conductor in 1 second is one ampere. It is the S.I. unit of an electric current.

**1 ohm**: If one ampere current flows through the conductor, and 1 volt potential difference is applied across it, then its resistance is 1 ohm.

**Potential**: Electric potential is the electrical level.

**Resistivity**: The resistivity of a conductor is defined as the resistance of a conductor of unit length and the unit area of the cross – section.

**Electric power**: Electric power is the rate at which electric energy is consumed. It is the electrical work done per unit time.

**Q6. Differentiate between.**

**1.**

**Resistances in series and parallel.**

Resistance in series | Resistance in parallel |

1. If a number of resistances are connected in such a way that the same current flows through each resistance, then the arrangement is called resistances in series. 2. The effective resistance is a series combination is greater than the individual resistances. 3. This combination is used to increase resistance in a circuit. 4. This combination decreases the current in the circuit. | 1. If a number of resistances are connected between two common points such that the potential difference across each is the same then that arrangement is called resistances in parallel. 2. The effective resistance of the combination is less than the individual resistances. 3. This combination is used to decrease resistance in the circuit. 4. This combination increases the current in the circuit. |

**2.**

**Conductors and insulators.**

Conductors | Insulators |

1. Those substances through which electricity can flow are called conductors. 2. Electrical resistances of conductors are very low. 3. They contain large number of free electrons. 4. Generally metals are conductors. E.g. silver, copper, aluminium | 1. Those substances through which electricity cannot flow are called insulators. 2. Electrical resistances of insulators are infinitely very high. 3. They do not contain free electrons. 4. Generally non – metals are insulators. E.g. wood, rubber, plastic |

**3.**

**Resistance and resistivity.**

Resistance | Resistivity |

1. The property of the conductor due to which it opposes flow of current through it is called resistance. 2. The SI unit of resistance is Ohm ( 3. The resistance of a conductor is inversely proportional to its area of cross section. This means that a thick wire has less resistance. | 1. Resistivity of a conductor is the resistance of a conductor of unit length and unit area of cross section. 2. The SI unit of resistivity is Ohm metre ( 3. Resistivity is independent of the shape and size of the conductor. |

**4.**

**High resistance and low resistance.**

High resistance | Low resistance |

1. A high resistance indicates a material that hardly allows the movement of electrons. 2. It is due to the less number of free flowing electrons in the outer most orbit of an element. 3. Substances with infinitely high electrical resistance are insulators. 4. High resistance provides low conductivity. | 1. A low resistance indicates a material that readily allows the movement of electrons. 2. It is due to large number of electrons in the outer most orbit of an element. 3. Substances with low electrical resistances are good conductors. 4. Low resistance provides high conductivity. |

**Q7. State the laws:**

**Ohm’s law**: Ohm’s law states that the electric current flowing in a metallic conductor is directly proportional to the potential difference across its terminals, provided physical conditions of the conductor such as length, area of cross section, temperature and material remain constant.

**Joule’s law**: - Joule’s law states that the quantity of heat generated (H) is a conductor of Resistance (R), when a current (I) flows through it for a time (t) is directly proportional to:

**Q8. Find the following.**

**a.**

**Find the expression for the resistors connected in series and parallel.**

**: - If the number of resistance are connected one after another in such a way that the same current flows through each resistance, then the arrangement is called resistance in series.**

__Resistors connected in series__

_{1, }R

_{2 }and R

_{3 }are three resistances connected in a series combination and let R be their equivalent resistance.

_{1, }V

_{2 }and V

_{3}be the P.D. across resistances R

_{1, }R

_{2 }and R

_{3 }respectively.

__Conclusion__**: If the numbers of resistance are connected between two common points, such that the potential difference across each resistance is the same, then the arrangement is called resistance in parallel.**

__Resistors connected in parallel___{1,}R

_{2 }and R

_{3}are connected in parallel between the points A and B. Let R be the equivalent resistance of the parallel combination.

_{1, }R

_{2 }be I

_{2}, and R

_{3}be I

_{3}and that of R be I.

__Conclusion__**b.**

**Find the expression for resistivity of a material.**

__Conclusion__**Q9. Answer the following:**

**1.**

**P and Q are the two wires of same length and different cross sectional areas and made of same metal. Name the property which is same for both the wires and that which is different for both the wires.**

**2.**

**Resistivity of some material is given below. State which one will be the best conductor.**

Material | Copper | Aluminium | Silver | Nickel |

Resistivity (Ωm) | 1.62 x 10 ^{ – 8 } | 2.63 x 10 ^{ – 8 } | 1.60 x 10 ^{ – 8} | 6.48 x 10 ^{ – 8} |

^{ – 8}Ωm), which means that silver offers the least resistance to the flow of current through it. Thus, silver metal is the best conductor of electricity.

**3.**

**If the resistance of wire A is four times the resistance of wire B, find the ratio of their cross sectional areas.**

**4.**

**Two dissimilar bulbs are connected in series, which bulb will be brighter? (Hint: consider the resistance of the bulb).**

**Write the SI unit of the following quantities**

Particulars | Units |

Charge | The coulomb |

Current | The ampere |

Potential difference | The volt |

Resistance | The ohm |

Resistivity | the ohm metre |

Energy | The joule |

Power | The watt |