Concepts and principles of electricity

Transcription

1 Unit 5 Uses Concepts and Principles of Electricity Physics Chapter 5 Concepts and principles of electricity Competency Uses concepts and principles of electricity in daily life Competency level 5' 1' Produces electrical charges and stores it. Subject Content ² Static electricity ² Charging bodies by rubbing ² Positive charges ² Negative charges ² Detection of charges ² Gold leaf electroscope ² Capacitors (as a charge storage device) 5' 2' Uses the relationship between the potential difference and the current in daily activities. 5' 3' nvestigates how resistance affects the current. 5' 4' Constructs simple electrical circuits to suit the situation. ² Electric current ^& ² Units of measurement of current (ampere - (A) ² Uses of ammeter ² Potential difference (V) ² Meaurement of potential difference (Volt - V & ² Uses of voltmeter ² Ohm s Law (V = R) ² Resistance ² Unit of Resistance ^Ohm - Ω & ² Value of resistor ² Colour code ² The equivalent resistance of series resistors ² R = R 1 + R 2 ² The equivalent resistance of parallel resistors ² 1/R = 1/R 1 + 1/R 2 ² Circuit components ² electric cells ² switches ² resistors ² electric bulbs For free distribution 77

2 Physics 5.1 Generating of electric charges and storing them Static Electricity Electric charge on an object is often called static electricity. Activity 1 Rub a dry drinking straw with a cotton cloth and bring it closer to small pieces of paper (or small pieces of rigifoam). You ll observe that these pieces are attracted to the straw. Now do the same activity using a dry straw which is not rubbed with a cloth. You ll observe that small pieces of paper are not attracted to the straw now. Can you explain these observations? Straw Fig : 5.1 Piece of cotton cloth Activity 2 Take a perspex rod and rub it with a cotton cloth. Bring it closer to small pieces of paper. They are attracted to it. Perspex rod Piece of cotton cloth Fig : 5.2 Activity 3 Rub a plastic rod (or comb) in your hair and bring it closer to small pieces of paper. They are attracted to the comb. Fig : 5.3 What did you notice in all these instances? Small pieces of paper were attracted by the objects (plastic rod, perspex rod, comb, etc.) only if they were rubbed with piece of cloth or hair. 78 For free distribution

3 Unit 5 Uses Concepts and Principles of Electricity Actually what has happened to these objects when they were rubbed? These objects are commposed of atoms. As a result of rubbing against each other, they acquire static charges. i.e. they acquire suface chages by gaining or loosing electrons. They become negatively charged and of they lise electrons. They become postibely charged there these objects can attract small pieces of paper and other light particles. Acceptance or loss of electrons from surfaces of objects leads to static electricity. Activity 4 Take two a and glass rods charge them by rubbing with dry silk cloth and hang them closer to each other. You ll see that they are repelled. (Fig : 5.4) From this activity it is very clear that both have the same charge. (Like charges) Fig : 5.4 Two rods repel each other Now take a glass rod rubbed with silk cloth and a P.V.C. tube rubbed with silk and hang them closer to each other. You ll see that they are attracted. Why? They have unlike charges i.e. the charges present on the surface of glass rod and P.V.C. are not the same. Fig : 5.5 Unlike charges repal each other All substances are made of atoms. Atoms are made of electrons, protons and neutrons. Protons are positively charged particles. Electrons are negatively charged particles. Neutrons have no any charge. They are neutral particles. Protons and neutrons are found in the nucleus of the atom. Electrons orbit around the nucleus. Only the electrons can be removed from atoms. When electrons are removed from a surface it becomes positive. When electrons are added to a surface it becomes negatively charged. For free distribution 79

4 Physics ^ & DO YOU KNOW? Electric Series Fur Flannel Glass Perspex Cotton Silk Leather Wood Wax Resins (Amber) Polythene Plastic P.V.C ^-& There is a series called Electric series. This enables us to know approximately which becomes positive and which becomes negative when rubbed against with eachother. Look at the series given here. When an object above in the series is rubbed with a substance (an object) below in the series, the substance upper in the series becomes positive and the lower one becomes negative. e.g : Glass is found above silk in this series. Therefore if glass is rubbed with silk, glass becomes positively charged and silk become negatively charged. Now let us see how the objects in the activities 1 to 4 are charged. n the activity 1; a straw was rubbed with cotton cloth. Therefore straw (made of plastic) becames negatively charged and the cotton cloth became positively charged. This happens because electrons are removed from the surface of cotton cloth and they are added to the surface of the straw. n the activity 2 ; perspex rod was rubbed with cotton cloth. Therefore perspex becomes positive and cotton cloth becomes negative. n the activity 3 ; comb (plastic) was rubbed with hair. Here electrons were removed from the hair and were added to the comb (plastic). Therefore comb became negative and hair became positive. n the activity 4 ; glass rod was rubbed with silk. Therefore glass rod becomes positive and silk becomes negative. P.V.C. tube was rubbed with silk. Therefore P.V.C. tube became regative while silk become positive. Like charges repel each other Unlike charges attract each other When a neutral object (body) is brought closer to small pieces of paper, they are not attracted to the neutral object (body). 80 For free distribution

5 Unit 5 Uses Concepts and Principles of Electricity When a charged body is brought closer to small pieces of paper or dust, they are attracted to the charged body. Why? Let us conduct a small activity to find out. Bring a charged body (positive or negative) closer to a rigifoam ball or a ball made pith of manioc (pith ball) which is hung from a thread. Observe what happens. Charged rod Pith ball or rigifoam ball 1) Rigifoam ball is attracted to the charged body 2) They come in contact 3) t is repelled Fig : 5.6 Let us assume the rod is a negatively charged one. When the charged rod is brought closer to the pith ball (or rigifoam ball) electric charges are induced in the pith ball as shown in the Fig : 5.6 (a), and therefore the pith ball gets attracted to the charged body. Fig : 5.6 (a) When they come in contact, positive charges on the pith ball are neutralized by the electrons which are coming from the charged body. Now as both the rod and the pithball are negative, repulsion takes place. Metal disc nsulator Metal rod Gold leaf electroscope You have learned about this instrument in grade 8. Gold leaf Fig : 5.7 Gold leaf electroscope Charging a gold leaf electroscope by contact method When the metal disc of the electroscope is made in contact with a positively charged body the electroscope is charged positively. For free distribution 81

6 Physics Bringing closer a positively charged body to an electroscope Positively charged body made in contact + When the charged body is removed Fig : 5.8 Fig : 5.9 dentification of charges on a body using a gold leaf electroscope Charge an electroscope positively. Now bring another positively charged body closer to the metal disc of the electroscope. (Do not touch.) Gold leaves are deflected. The reason for this is the attraction of electrons from the gold leaves towards the metal disc because the charged body is a positive one. The same thing happens when a negatively charged body is brought closer to an electroscope which is also charged negatively. What happens when a negatively charged body is brought closer to an electroscope which is charged positively? Here what happens is electrons are repelled from the metal disc and as they reach the gold leaves the electro positiveness is decreased. Therefore deflected gold leaves collapse. When a neutral body is brought near a charged electroscope also, the gold leaves shrink. Fig : 5.10 A capacitor Applications of electro-static charges Capacitors The function of capacitors is to store electric charges. They are used in radio and TV sets and many electronic circuits. They are also called condensers. A common capacitor is made of two strips of metal foil seperated by a strip of insulator. Or, two metal plates are kept parallely and the area between these two plates is filled with an insulator or dielectric substance. 82 For free distribution

7 Fig : 5.11 e Fig : 5.12 dielectric substance Metal Plate e e Charging condenser e metal plate e Discharging condenser Unit 5 Uses Concepts and Principles of Electricity Some dielectric substances are given below 1. Air 2. Paper 3. Wax 4. Polystyrene 5. Mica 6. Al 2 O 3 (Alumina) When the two plates of a parallel plate condenser are connected to the two terminals of a battery, it gets charged. t happens as follows. Electrons flow from the negative terminal of the battery to one metal plate. Therefore this plate gets negatively charged while the other plate is charged positively. Electrons flow from the positively charged plate to the the positive terminal of the battery. When the two plates of a charged capacitor is connected by a wire, electrons flow from the negatively charged plate to the positively charged plate. Therefore the plates are discharged. When a capacitor is discharged do not forget to connect a suitable resistor to prevent destroying the capacitor. Fig : 5.13 nside a parallel plate condenser Fig : 5.14 Different types of capacitors The unit used to measure the capacity of a capacitor is FARAD (F). 1 F (= 10-3 F) is one milli Farad. (i.e F = 1 mf) 1000 One millionth of a Farad is one micro Farad. (i.e. 1 / F = 10-6 F = 1 µf) One trillionth of a Farad is one pico Farad. (pico is pronounced as peeco ) (i.e F = 1 pf). For free distribution 83

8 Physics ² Factors affecting the capacity of a capacitor 1. Area of the plates 2. The distance between the two plates 3. Nature of the dielectric substance When the area of plates is increased, the capacity is also increase. When the distance between the plates increases the capacity decreases. As the insulating property of the dielectric substance increases, the capacity increases. Some uses of capacitors These are used in electronic devices such as radios, TVs and computers. These are used to prevent electric sparks in electronic circuits. These are used to prevent variations of direct current. Some other instances they are used are given below. n lightning conductors. n photocopying machines. n electro static precipitators. Electro static ink spraying machines. Electric current Electric current is the rate of flow of electric charges. The unit used to measure the charge is Coulomb (C). One coulomb of negative charge is the charge of electrons. Therefore when the rate of flow of electrons through a conductor becomes electrons in one second, this current is called one ampere (1 A). i.e. When the rate of flow is one coulomb per second then current is said to be one ampere. (1 C s -1 = 1 A) Therefore current of 5 A means that the rate of flow of electric charge is 5 coulomb per second. i.e. 5 A = 5 C s -1. The charge carriers found in solid conductors are free electrons. Charge carriers in the molten state and in solutions are positively charged ions and negatively charged ions. Free electrons are the electrons which become free from nuclear attraction. nner energy levels Nucleus Fig : 5.15 Free electrons Flow of current through a solid conductor is due to these free electrons. When the two ends of a conductor are connected to the two terminals of an electric supply (battery), electrons are coming from the negative terminal and these electrons force the free electrons of the 84 For free distribution

9 Unit 5 Uses Concepts and Principles of Electricity conductor to move forwards, that is towards the positive terminal of the battery. But the conventional current is considered to be a positive current which flows from the positive terminal to the negative terminal (against the direction of free electrons). e A e B e B Fig : 5.16 Fig : 5.17 Conventional current is represented by the letter and the electron current by e A i e B When we say that a current is flowing from A to B, actually what happens is electrons flow from B to A. Fig : 5.18 The instrument which is used to measure the current is the ammeter. Bulb Ammeter Fig : 5.19 The ammeter Fig : 5.20 The ammeter is always connected in series. Potential difference Positive potential is there at the positive terminal of a battery and a negative potential is found at the negative terminal. The difference in these potentials is called the potential difference. The unit used to measure the potential difference is volt. The instrument used to measure the potential difference is called volt meter. For free distribution 85

10 Physics Fig : 5.21 Volt meter Fig : 5.22 A voltmeter is always connected in parallel. f one joule (J) of work is done when a charge of 1 coulomb (1 C) flows from one point to another in a circuit, the potential difference between these two points is said to be 1 Volt (1 V). Activity 5 + X - When you want to measure both the current () and the potential difference (V) at the same time, this is how the circuit is set up. Fig : 5.23 This represents how volt meter and ammeter are connected. A piece of nichrome wire is shown here by Fig : 5.24 X. Connect a volt meter in parallel and an ammeter is series. The battery E is X connected to give the electric supply. Switch is shown here by S and Y is a variable resistor (Rheostat). Close the switch to get one reading. i.e. the current () through the nichrome coil and the potential difference S E Y (V) across it. Switch off the circuit as soon as possible to prevent heating the resister X. Now adjust the rheostat a bit and get another reading (V and ). Thus get five readings by adjusting the rheostat. Now divide the volt meter reading by the ammeter reading. i.e. divide potential difference (V) across the resister X by the current () flowing through it. You ll see that the value obtained here to be a constant. i.e, When the temperature of the resister is kept constant, Volmeter reading Ammeter reading = constant 86 For free distribution

11 Unit 5 That is, when the temperature of a conductor is kept constant Potential difference(v) Current () Uses Concepts and Principles of Electricity = constant This relationship was first discovered by a scientist called George Simon Ohm. Therefore this relationship is called Ohm s law. Ohm s Law When the temperature and all other factors are kept constant, the current () through a conductor is directly proportional to the potential difference (V) across it (i.e. V á ). Potential difference i.e = Constant Current (i.e.) V = = Constant This constant is known as the electrical resistance (R) of the conductor. Fig : 5.25 George Simon Ohm i.e. R = V The resistance (R) of a conductor is the potential difference (V) that has to be supplied for 1 ampere (1 A) current to flow through it. f a potential difference of 1 V should be supplied for 1 A current to flow through a conductor, the resistance of that conductor is 1Ω. f a potential difference of 6 V should be supplied for 1 A current to flo through a conductor, the resistance of that conductor is 6 Ω. Solved example The following readings were obtained in an experiment which has conducted to prove the Ohm s law. Potential difference V 3.0 V 4.5 V 6.0 V Current A 1.0 A 1.5 A 2.0 A i) What has proved from the above readings? ii) Calculate the resistance of the conductor. For free distribution 87

12 Physics Potential difference (V) Current () = = = = = 3 The constant obtained is 3. This shows that V = 3. i.e. The resistance of the given connector = 3 Ω. V =R V = R = V R Fig : 5.26 Some resistors Resistors Resistance is the barrier force to the flow of current. Resistors are used to give a resistance to a circuit. Current or voltage (potential difference) can be changed by connecting resistors. Some main types of resistors are, 1. Carbon resistors 2. Metal oxide resistors 3. Wire wound resistors Out of these, mostly used type is the carbon resistors. nk coating is found around resistors, to get them protected by humidity, because the resistance is changed according to it. The external appearance of the carbon resistors is more similar to the metal oxide resistors, but the internal structure is different. Black 0 Green 5 Brown 1 Blue 6 Red 2 Purple 7 Orange 3 Grey 8 Yellow 4 White 9 88 For free distribution Colour bands and relevant numbers Wire wound resistors are made by winding resistance wire arround porcelain core. The resistance of a resistor can be found out by using the colour bands found around the resistor. Colours of the bands and numbers relevant to them are given here.

13 Unit 5 Uses Concepts and Principles of Electricity Find out the first two digits of the resistance value by the first two colour bands. Now see the value for the third colour band. Find out the ten to the power of this value.now multiply the number made from first two digits by the value of the ten to the power of this number. The resistance of any resistor could be calculated like that. The fourth band represents the tolerance value of a resistor. Colour of the 4 th band Tolerance Red ± 2 % Gold ± 5 % Silver ± 10 % No band ± 20 % f the fourth band is absent, it shows that the tolerance value is ± 20%. The colours of the 4 th band and the relevant tolerance values are given below. Tolerance is the percentage of change occurs due to temperature, humidity etc. Worked Example Calculate the resistance of the following resistor. Blue - 6 Green - 5 Red Gold - ±5% Blue = 6 Green = 5 No.obtained from the first two digits = 65 Resistance = = 6500 Ω Tolerance = ± 5 % R 1 R 2 Connecting Resistors / Electronic components in series R 1 and R 2 resistors are connected in series. When the resistors are connected in this way, the current is not divided. The same current flows through both resistors. i Fig : 5.27 i 1 i 2 Fig : 5.28 Resistors in series R 1 i 1 i 2 R 2 Resistors in parallel i Connecting resistors in parallel When the resistors are connected so as to divide the current through the resistors, then they are said to be connected in parallel. Here, the two resistors R 1 and R 2 are connected in parallel. Then the potential difference applied for both is the same. Current gets divided to two resistors. Here the current is divided as i 1 and i 2. f the resistance are equal than the current divided equally. For free distribution 89

14 Physics f, and, R 1 = 12 Ω R 2 = 6 Ω then ; 1 A will flow through 12 Ω resistor while 2 A will flow through 6 Ω resistor. A B C D R 1 R 2 R 3 When resistors are connected in series, same current flows through all the resistors. This current is gained according to the total resistance. Let us study the following circuit. Fig : 5.29 Equivalent resistance of resistors connected in series. n the above circuit all the resistors R 1, R 2 and R 3 connected in series. The total resistance of these is called the equivalent resistance. Then what should be the potential difference? Potential differance between A and D ( V AD ) V AD = Current () Equivalent resistance (R) V AD = R Potential difference between A and B = V AB = R 1 Potential difference between B and C = V BC = R 2 Potential difference between C and D = V CD = R 3 Potensial differance between A and D = PD between A and B + PD between B and C + PD between C and D i.e. V AD = V AB + V BC + V CD As V = R R = R 1 + R 2 + R 3 When the equation is divided by you ll get, R = R 1 + R 2 + R 3 This shows that the equivalent resistance (or the total resistance) can be find out by adding resistances of all the resistors which are connected in series. eg. : The equivalent resistance or the total resistance of a 12 Ω resistor and 6 Ω resistor is, = 18 Ω 90 For free distribution

15 Unit 5 Connecting resistors in parallel The following resistors R 1, R 2 and R 3 are connected in parallel. R 1 Uses Concepts and Principles of Electricity R i 1 i 2 R 2 i 3 R 3 V or Fig : 5.30 Equivalant resistance when the resistors are connected in parallel V Here the potential difference is not divided. Same is applied for all the resistors. Current gets divided here according to their resistance. f the resistances of all the parallel resistors are equal then current gets divided equally. Let the total resistance of the three resistors R 1, R 2 and R 3 to be R. Current gained from the electric supply is. Therefore, = V R Also, Because i 1 = V R 1 i 2 = V R 2 i 3 = V R 3 = i 1 + i 2 + i 3 V = R V + R 1 V + R 2 V R 3 When this is divided by V, 1 = R R 1 R 2 R 3 The receprocal of the total resistance (equivalent resistance) is equal to the sum of the reciprocals of resistance of all the resistors which are connected in parallel. When resistors are connected in series the total resistance is the sum of resistances of all the resistors. When the resistors are connected in parallel, the reciprocal of the total resistance is equal to the sum of reciprocals of all the parallel resistors. For free distribution 91

16 Physics 5.4 Construction of simple electric circuits to suit the needs An electric circuit is a path through which electric charges flow.the following components are used to make electric circuits. Electric cells Switches Electric bulbs Connecting wires Resistors Activity 6 To make a circuit to light a bulb from a dry cell. When the two terminals of the dry cell are connected to the two terminals of the electric bulb, it lights up. But a switch also has to be connect for enable to control it to light or to prevent lighting. Some symbols of some components which are used to make circuits. Electric cell (Off& Switch Resistor Bulb A Electric cell Bulb B Switch + - Circuit A - Without using a switch Circuit B - With using a switch Fig : 5.31 Fig : 5.32 The following diagram shows how to connect two bulbs in series to a dry cell. The following diagram shows how they are connected in parallel. 92 For free distribution Fig : 5.33 Fig : 5.34

17 Unit 5 Uses Concepts and Principles of Electricity Summary Objects acquire static charges as a result of rubbing against each other. When electrons are removed from a surface, it becomes positively charged. When electrons are added to a surface it becomes negatively charged. Charges can be detected by using an electroscope. eg. : Gold leaf electroscope. Capacitors are used to store charges. Electric current is the rate of flow of electric charges. Potential difference is the difference of potensials between two points of a circuit. When the temperature and all other factors are kept constant, Potential differance = Constant. Current There are two main methods to connect resistors. 1. Series method 2. Parallel method When the resistors are connected in series, R = R 1 + R 2 + R 3 When the resistors are connected in parallel, 1 R = 1 R R R 3... Excaercises (1). A plastic rod was rubbed with a dry cloth. What is the charge resent on the plastic rod.. What charge is gained by a perspex rod when it is rubbed with a dry cloth.. Draw a gold - leaf - electroscope and name its parts. V. How is a gold leaf electroscope charged negatively. V. How do you detect charges present on an object using a charged gold leaf electroscope. (2) Three resistors 6 Ω, 12 Ω and 4 Ω are 6 Ω 12 Ω 4 Ω connected in series to an electric supply of 12V.. Calculate the total resistance of these three resistors. 12 V. What is the current gained from the electric supply. For free distribution 93

18 Physics (3) Three resistors 4 Ω, 12 Ω and 6 Ω are connected to a supply of 12 V as shown below.. Calculate the total resistance of these three resistors.. What current is gained from the electric supply?. Calculate the current flowing through 6 Ω. V. What is the current flowing through 12 Ω? V. What is the current flowing through 4 Ω? i 3 i 2 i 1 4 Ω 12Ω 6 Ω 12 v (4). Find out resistance and tolerence of the following resistor. Brown Blue Red Gold.. Draw a diagram to show how a bulb is connected to two electric cells which are connected in series. Draw a circuit diagram to show how two electric bulb are connected parellely to two cells which are also connected in parallel. (5) Three bulls B 1, B 2 and B 3 are connected to 12 V, supply as shown in the following diagram. B 2. Calculate the total resistance of the two bulls B 2 and B 3 B 1 6 Ω. x y z B What is the total resistance 3 6 Ω between the two points X and Z. 3 Ω (between Y and Z).. What is the current gained from the electric supply. V. Calculate the potential difference 12V between X and Y. V. Calculate the potential difference between Y and Z. V. Calculate the current flowing through B 2 bulb. V. Calculate the current through the bulb B 3. V. f the bulb B 3 is removed, then what would be the current gain from the electric supply. 94 For free distribution