6 STATIC ELECTRICITY I. Tick ( ) the most appropriate answer. 1. When an ebonite rod is rubbed with fur, the charge acquired by the fur is: (a) negative (b) positive (c) both positive and negative (d) none of these 2. When a negatively charged body is brought near a suspended positively charged ball, the ball gets: (a) attracted (b) repelled (c) stay at same place (d) none of these 3. Aluminium metal is a: (a) good conductor of electricity (b) magnetic substance (c) bad conductor of electricity (d) bad conductor of heat 4. A positively charged body has: (a) excess of electrons (b) deficiency of electrons (c) no change in number of electrons (d) none of these 5. When an uncharged conductor gets electrically charged when brought near a charged body the process is called (a) conduction (b) convection (c) induction (d) none of these 6. When a glass rod is rubbed with silk, both glass rod and silk : (a) acquire equal and similar charges (b) acquire equal and opposite charges (c) acquire unequal and similar charges (d) acquire unequal and opposite charges Ans. 1. (b) 2. (a) 3. (a) 4. (b) 5. (c) 6. (b) II. Fill in the blanks. 1. Cat s skin acquires charge, when rubbed with ebonite rod, because its electrons are transferred to rod. 1
2. Similar charges each other, whereas opposite charges each other. 3. It is electrons in the outermost of an atom, which are responsible for electrification. 4. When a negatively charged body is placed in contact with the charged disc of G.L.E. its diverge. 5. In an atom number of is equal to number of electrons. 6. An uncharged insulator can be charged by with another body. 7. An uncharged conductor can be charged by. 8. is the surest test of electrification. 9. A G.L.E. can be charged by touching a charged glass rod with its cap. 10. A negatively charged body has of electrons. 11. The process of charging objects like glass and amber is known as. 12. Ebonite rod acquires charge when rubbed with wool. 13. A body which has lost electrons becomes. 14. is an instrument used for testing whether a body is charged or not. 15. is a device that protects buildings from a lightning strike. 16. is used in the chimneys of factories to prevent air pollution. 17. A glass rod is rubbed with silk, the charge on the glass rod is and the charge on the silk is. 18. An ebonite rod is rubbed with fur. The charge on the ebonite rod is and the charge on the fur is. Ans. 1. Positive, ebonite 2. repel, attract 3. free, shell 4. negatively, leaves 5. Protons 6. rubbing 7. induction 8. repulsion 9. positively 10. excess 11. electrification 12. negative 2
13. positively charged 14. electroscope 15. lightning conductor 16. electrostatic precipitator 17. positive, negative 18. negative, positive III. Statements given below are incorrect. Write correct statements. 1. Glass wool is good conductor of electricity. Ans. Glass wool is bad conductor of electricity. 2. positive electrification is due to the excess of electrons. Ans. Positive electrification is due to deficiency of electrons. 3. When silk is rubbed with glass, the silk gets positively charged. Ans. When silk is rubbed with glass, the silk gets negatively charged. 4. Similar charges attract each other. Ans. Similar charges repel each other. 5. When ebonite rod is rubbed with wool, they get oppositely charged due to induction. Ans. When ebonite rod is rubbed with silk, they get oppositely charged due to conduction. 6. Lightning conductor always develops the similar charges as is the charge in the clouds. Ans. Lightning conductor always develops the opposite charges as is the charge in the clouds. 7. A gold leaf electroscope can be charged by both conduction only. Ans. A gold leaf electroscope can be charged by conduction and induction. 8. The electrons flow from a positively charged body to negatively charged body when connected with a copper wire. Ans. The electrons flow from a negatively charged body to the positively charged body when connected with a copper wire. 9. The fur gets negatively charged because of deficiency of electrons. Ans. The fur gets negatively charged because of excess of electrons. 10. Bad conductors have excess of free electrons. Ans. Good conductors have excess of free electrons. 11. Similar charges attract each other and opposite charges repel each other. Ans. Similar charges repel each other while opposite charges attract each other. 3
12. An uncharged electroscope can tell us the nature of charge on a body. Ans. A charged electroscope can tell us the nature of charge on a body. 13. Lightning is caused due to discharge of similar charged clouds. Ans. Lightning is caused due to discharge of oppositely charged clouds. 14. Lightning do not strikes at high rise buildings. Ans. Lightning strikes at high rise buildings. 15. An insulator allows readily an electric current to pass through it. Ans. A conductor allows readily an electric current to pass through it. IV. (a) Match the columns. Column A Column B 1. Electric current (a) unit of current. 2. Conductor (b) device that prevents a circuit from overheating 3. Insulator (c) flow of electrons along a conductor. 4. Ampere (d) substance which allows electricity to pass through it. 5. Fuse (e) non-conductor. Ans. 1. (c), 2. (d), 3. (e), 4. (a), 5. (b) (b) Column A Column B 1. Two similar charges (a) acquires a positive charge 2. Two dissimilar charges (b) insulator 3. When a glass rod is rubbed with silk, it (c) conductor 4. When an ebonite rod is rubbed with fur, it (d) acquires a negative charge 5. Silver (e) repal each other 6. Sulphur (f) attract each other Ans. 1. (e), 2. (f), 3. (a), 4. (d), 5. (c), 6. (b) V. Find the odd one out. Give a reason for your choice. 1. Mica, copper, wood, shellac, sealing wax. Ans. Copper. Copper is a conductor while mica, wood, shellac and sealing wax are insulators. 4
2. Mercury, sulphur, graphite, copper. Ans. Sulphur. Sulphur is an insulator while mercury, graphite and copper are conductors. 3. Induction, conduction, magnet, charged body. Ans. Magnet. Induction, conduction and charged body are all related to static electricity while magnet is not. 4. Mercury, graphite, oxygen, human body, steel. Ans. Oxygen. Oxygen is an insulator while mercury, graphite, human body and steel are conductors. 5. Sulphur, rubber, wood, hydrogen, iron. Ans. Iron. Iron is a conductor while sulphur, rubber, wood and hydrogen are insulators. 6. Electron, atom, proton, neutron. Ans. Atom. An atom is the smallest unit of an element, which may or may not have an independent existence, but always take part in a chemical reaction. Atom is made up of electron, proton and neutron. 7. Chloride ion, oxide ion, nitride ion, magnesium ion. Ans. Magnesiumion. Magnesium ion is positively charged ion while chloride ion, oxide ion and nitride ions are negatively charged ions. VI. Give reasons for the following. 1. When you touch the metal disc of an electroscope with a charged glass rod the metal leaves diverge. Ans. When we touch the metal disc of an electroscope with a charged glass rod the metal leaves diverge because of the transfer of free electrons. 2. One should not use an umbrella while crossing an open field in a thunderstorm. Ans. One should not use an umbrella while crossing an open field in a thunderstorm because when a charged cloud passes over an umbrella, it induces a charge in it, and there is an electrical discharge between an umbrella and the cloud. Which may be very dangerous. 3. In an electroscope the disc, the connecting rod and the leaves are all made of metal. 5
Ans. In an electroscope the disc, the connecting rod and the leaves are all made up of metals because metals are good conductors of electricity. 4. The metal wires in the bottom part of the electroscope are earthed. Ans. The metal wires in the bottom part of the electroscope are earthed because if there is any charge inside the bottle, it immediately flows to the earth making the inside of the bottle neutral. 5. The diverging leaves of a charged electroscope collapse, the moment you touch the disc of the electroscope with your finger. Ans. The diverging leaves of a charged electroscope collapse, the moment you touch the disc of the electroscope with your finger because no electric charge is present on it as the charge will flow from hand to earth through the body. VII. State the difference between the following. 1. Proton and electron. Ans. Proton is a positively charged particle having one unit positive charge and has a mass almost equal to mass of one atom of hydrogen. While electron is a negatively charged particle having one unit negative charge and has a mass of 1/1837 times the mass of one atom of hydrogen. 2. The charge carried by a glass rod rubbed with silk and charge carried by ebonite rod rubbed with fur. Ans. When a glass rod is rubbed with silk, a positive charge develops on the glass rod and a negative charge is developed on silk. When an ebonite rod is rubbed with fur, a negative charge develops on ebonite rod and a positive charge develops on fur. 3. Like charges and unlike charges. Ans. Like charges repel each other while unlike charges attract each other. 4. Static electricity and current electricity. Ans. When an electric charge is at rest, it is known as static electricity when an electric charge is in motion, it is referred to as a current electricity. 5. Coductors and insulators. Ans. The substances which have a large number of free electrons such 6
that they start drifting from one end of a substance to the other end, when it is connected to some source of electricity are called conductors. While the substances which have a few free electrons, such that they do not easily drift from one end to the other end, when connected to some source of electricity are called insulators. VIII. Answer the following questions. 1. Describe an experiment to show: (a) Like charges repel each other. (b) Unlike charges attract each other. Ans. (a) Experiment to show that like charges repel each other: Take a glass rod and rub it with silk and suspend it freely by a silk thread. Near this Suspended rod, bring another glass rod which is rubbed with silk. It is observed that suspended glass rod gets repelled. (b) Experiment to show that unlike charges attract each other: Take a glass rod and rub it with silk and suspend it freely by a silk thread. Bring near it an ebonite rod which is rubbed with cat s skin. It is observed that glass rod is attracted by ebonite rod. 2. Briefly describe Rutherford s structure of atom. Ans. Rutherford s structure of atom: (i) (ii) An atom consists of three subatomic particles neutrons, protons and electrons. Neutron has no charge, proton has one unit postive charge and electron has one unit negative charge. 7
(iii) (iv) (v) Protons and neutrons form the central core of atom which is commonly called nucleus. The electrons revolve around the nucleus in fixed orbits. As the atom of a normal element is electrically neutral therefore, it is believed that the number of protons in an atom is equal to the number of electrons. 3. Why does a glass rod get positively charged, when rubbed with silk? Explain on the basis of electron transfer. Ans. The glass rod transfers its electrons to the silk when it is rubbed with silk. Thus, glass rod has less number of electrons than the number of protons in the nucleus. Thus, on the whole, the glass rod gets positively charged. 4. Why does an ebonite rod get negatively charged, when rubbed with fur? Explain on the basis of electron transfer. Ans. Ebonite rod gains electrons from the fur when it is rubbed with fur. Thus ebonite has more negative charges as compared to positive charges in the nucleus. Thus, on the whole, ebonite rod gets negatively charged. 5. state the mass and charge on (a) electron (b) proton (c) neutron Ans. Charge Mass (a) Electron 1 unit negative charge 1/1837 times that of mass of one hydrogen atom. (b) Proton 1 unit positive charge Almost equal to hydrogen atom. (c) Neutron No electric charge Almost equal to hydrogen atom. 6. What are conductors? Define on the basis of structure of atom. Give four examples of different classes of conductors. Ans. A substance, which has a large number of free electrons, such that they start drifting from one end of a substance to the other end, when it is connected to some source of electricity, is called conductor. Four examples of different classes of conductors are: 8
(i) (ii) (iii) (iv) Metals: Iron, copper, gold, silver, etc. Solution of acids in water: Dil hydrochloric acid, dil nitric acid, dil sulphuric acid and carbonic acid, etc. Solution of alkalies in water: Sodium hydroxide, ammonium hydroxide, calcium hydroxide, potassium hydroxide, etc. Solution of soluble salts in water: Sodium chloride, potassium nitrate, etc. 7. What are insulators? Define on the basis of structure of atom. Name six insulators. Ans. A substance, which has a few free electrons, such that they do not easily drift from one end of the substance to the other end, when connected to some source of electricity is called insulator. For example, alcohol, ether, sugar, starch, wool, fur etc. 8. How can you use gold leaf electroscope to detect the charge on a body? Ans. Gold leaf electroscope (G.L.E) is used to detect the charge on a body. Take a glass rod and rub it with silk. Touch the rubbed end of the glass rod with the disc of G.L.E. It is seen that leaf of G.L.E. diverges outward. When the glass rod touches the brass disc of G.L.E, then free electrons from it flow to the glass rod. This causes a deficiency of electrons on the brass disc. To make up for this deficiency the electrons from leaves flow to wards the disc. This, in turn, causes electron deficiency on the leaves. As leaves are positively charged, they repel each other, and hence diverge. Thus, we can say that if a body, on touching disc of gold leaf electroscope makes its leaves diverge, then that body is charged. 9. How can you use gold leaf electroscope to detect the nature of charge on a body? 9
Ans. We take a G.L.E. and charge it positively or negatively by touching its disc with a charged rod. Suppose we have a negatively charged G.L.E. Now touch the given charged body to the disc of negatively charged G.L.E. If the leaves of G.L.E. diverge further, then the body is negatively charged. It is because, similar charges repel each other. If the leaves of G.L.E. collapse, then the body is positively charged. It is because some charges from negatively charged G.L.E. will flow to the positively charged body. 10. (a) State the charge present on the glass rod and silk when rubbed with each other (b) State the charge present on the ebonite rod and cat s skin when rubbed with each other. Ans. (a) Glass rod gets positively charged and silk gains electrons so gets negatively charged after rubbing. (b) Ebonite rod gets negatively charged and cat s skin gets positively charged when rubbed with each other. 11. What is an electroscope? Draw a neat diagram of gold leaf electroscope. Ans. An electroscope is a device used for detecting electric charges, and finding the nature of electric charges. Gold leaf electroscope 10
12. What causes thunder and lightning? Ans. When two clouds carrying opposite charges approach each other, a charge flows from one to the other through the air. This electrical discharge which can be seen as a dazzling flash of light is called lightning. When lightning occurs the air suddenly gets very hot and expands. It subsequently cools and contracts. This sudden and tremendous expansion and contraction of air produces an explosion of loud sound which is called thunder. 13. Explain how a lightning conductor works? Ans. A lightening conductor is made of copper rod, on the one end of which are provided sharp copper points. It is installed on the highest point of the building. Its lower end is connected to a thick copper strip which is attached to flat copper plate and buried deep inside the earth. When lightning strikes the sharp points of lightning conducto, all the electric discharge flows into the earth through the copper rod wire, without damaging the building. 11