ISLAMABAD ACADEMY PHYSICS FOR 10TH CLASS (UNIT # 15)

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1 PHYSICS FOR 10TH CLASS (UNIT # 15) SHORT QUESTIONS Define the term If in the presence of a charged body, an insulated Electrostatic induction? conductor has like charges at one end and unlike charges at other end, then this is called electrostatic induction. In this phenomenon all the charges remain static that is why is known as electrostatic induction. Give any practical application of electrostatic phenomenon? Electrostatic phenomenon is used for separation of smoke and other particles form gases coming out of the chimneys of various factories. Define & explain the Coulomb s Law? The force of attraction or repulsion between two charged bodies is directly proportional to the product of the quantity of charges on them and inversely proportional to the square of the distance between them Explanation If the distance between two-point charges q1 and q2 is r, Coulomb s law can be mathematically written as: - q1 q2 F a r2 q1 q2 F = k r2 Where k is a constant whose value depends upon the medium between the two charges and the system of units in which F, q and r are measured. Why a metallic chain behind the oil tanker is allowed to rolls on the road? Due to friction of air, body of tanker gets charged and tinny spark can cause a havoc. This charge is continuously being transferred to ground through the metallic chain. Thus the danger of spark is eliminated. Give the equivalent number of electrons of 1 columb charge? 6.25 x 1018 electrons. How danger of sparking eliminated in landing plane? During flight, body of aeroplane gets charged. Its tyres are of some conducting material. When plane lands, the charge can easily transferred to ground and the danger of sparking is eliminated. Define Electric Potential. The potential energy of a unit positive charge placed at that point which is equal to the amount of work done in bringing a unit positive charge from infinity to that point Define Electric Field The electric field of a charge is a region of space surrounding it, in which a charge can feel its electrostatic effect in the form of a force Define Electric Intensity The strength of the field at any point is known as electric intensity If F is the force, then electric intensity E would be : - 1

2 E = F/q or F = qe Thus electric intensity can also be defined as the force acting on a unit positive charge place at that point. Who invented the Chemical Battery? In 1776, an Italian scientist Alexendro Volta invented the chemical battery. Define the term capacitance? When we charge a conductor, some work has to be done during process of transferring the charge to the conductor. This rises the potential of the conductor. Experiments show that the charge Q on the conductor is directly proportional to its potential V, i.e. Q a V or Q = CV Where C is a constant whose value depends upon the size of the conductor. It is known as capacitance of the conductor. Define Capacitor In order to store the charge for longer periods, a device is used which is known as capacitor. Differentiate between electric potential and electric potential energy. Potential is a scalar quantity. Its SI unit is known as volt. It is defined as: If the potential energy of one coloulmb positive charge at a point is one joule, the electric potential of that point out be one volt. Electric potential is scalar quantity while electric potential energy is vector quantity. what is an electric field? How is it similar to a gravitational field? The electric field of a charge is a region of space surrounding it, in which a charge can feel its electrostatic effect in the form of a force. It is similar to gravitational field because gravitational field is also the region in which other objects can feel the pulling force. How is coulomb s law similar to Newton s law of gravitation? How are they different? Similarity: Proportionality of both the laws looks like same. Difference: In Coulomb s law product of charges is directly proportional to Force while in gravitational law product of masses of two bodies are directly proportional to the Force. Electric force between a pair of charged bodies diminish when their separation is doubled? Tripled? Electric force depends upon the distance between two charged bodies. Therefore it is diminish when we increase the distance between the charged bodies. What is charge polarization? Charges are of two types. Positive and Negative. Place of conductor carrying positive charges is called positive pole of conductor while other is called negative pole. This phenomenon is called charge polarization. Distinguish between a positively and negatively charged object A positively charged object attract the negatively charged body and repel the positively charged body and vice versa. Briefly explain how an electrically neutral object can be attracted to a charged body? 2

3 When number of positive charges equal to negative charges, object is said to be neutral. So it can be attracted any of positively or negatively charged body near it. Would you feel an electrically neutral object can be attracted to a charged body. Yes. When number of positive charges equal to negative charges, object is said to be neutral. So it can be attracted any of positively or negatively charged body near it. LONG QUESTION Question: Show with the help of experiment that electric charges are of two kinds? When a plastic comb is moved into dry hairs, it starts to attract small pieces of paper. We can say that these substances have developed the property of attraction because they have acquired electric charge. The following experiments show that electric charges are of two kinds: - i) Rub two glass rods with silk and suspend them close to each other. We will see that they repel each other. ii) Now rub two ebonite rods with woolen cloth and suspend them close to each other. We will see that they repel each other again. In both experiments both rods have same kind of charges, and they repel each. So from these experiments, we can concludes that same kind of charges repel each other. iii) Now suspend the glass rod rubbed with silk near an ebonite rod rubbed with wool. This time the two rods attract each other. This shows that the charges on the two rods are not of the same kind and their nature is opposite. Therefore in order to differentiate between the two kinds, we call the charges on glass rod positive and on ebonite rod, negative, From this part of experiment we conclude that unlike charges attract each other. The charge on glass and ebonite rod, is in fact, due to the presence of positive and negative charged particles. The glass rod rubbed with silk has a large number of positive particles, whereas an ebonite rod rubbed with wool has a large number of negative particles. Question: Describe the structure and function of Electroscope and its use? It is an instrument used for detecting and testing the nature of charge on a body. Structure: i) It consists of a brass rod which has a brass disc connected to its upper end. ii) Two very thin gold leaves are attached to its lower end. iii) By the help of cork this assembly is mounted inside a glass jar with a brass disc projected outside. iv) A thin foil of aluminium is pasted in the inner surface of jar. Use/Working of Electroscope: i) First of all electroscope will have to be charged with +ev or eve charge. ii) In order to charge the electroscope positively, touch the disc with positively charged body and vice versa. iii) In order to detect the presence of charge on a body bring it near the disc. iv) If the divergence of leaves increases then the body has the same kind of charge as the electroscope carrying. 3

4 v) If the divergence of leaves decreases then the body has the opposite charge as the electroscope carrying. Question: What are electric lines of force? i) The direction of electric intensity in an electric field can also be indicated by drawing lines. These lines are known as electric lines of force. ii) They originate from a positive charge and terminates at negative charge. iii) The number of lines of force is also related with the strength of the field. iv) The lines are close together in a strong field region and far apart in the weaker field regions. v) So, by seeing the lines of force, we can get information about the magnitude and direction of electric intensity. Question: Define and explain the Electric Potential? The potential at any point in the field is defined as: - The potential energy of a unit positive charge placed at that point which is equal to the amount of work done in bringing a unit positive charge from infinity to that point. Explanation: If W is the amount of work done in brining a test charge +q from infinity to a certain point in the field, the potential V at this point would be given by: V = W/q Potential is a scalar quantity. Its SI unit is known as volt. It is defined as: If the potential energy of one coloulmb positive charge at a point is one joule, the potential of that point out be one volt. When a charge q is left for free motion in an electric field, it moves form a point of higher potential say A to a point of lower potential say B. We have the following expression for the energy supplied by a charge. E = q(va-vb) Question: What is a capacitor? Also describe its structure? Charge cannot be stored on a conductor for a long time because the stored charges mutually repel each other due to which they spread on the whole surface of the conductor and also tend to leak out from there. In order to store the charge for longer periods, a device is used which is known as capacitor. Structure: - It consists of two thin metal plates, parallel to each other with a very small distance between them. The medium between the two plates is air or a sheet of some insulator. This medium is known as dielectric. Question: Why the Unit of capacitance named as Farad? It was named after the scientist Michael Faraday who discovered the law of electromagnetic induction. Question: What are the rules for combination of capacitors? i) In parallel combination. ii) In Series combination. Parallel Combination of Capacitors: 4

5 In this method left plates of all the capacitors are connected to one point and the right ones to another point. This type of combination has the following characteristics: - i) The potential difference across each of the capacitors would be equal to potential of battery. ii) The battery supplied charge to each capacitor, the quantity of which depends upon the capacitance of the capacitor. If Q is the total charge supplied by the battery, then Q = Q1 + Q2 + Q3 = C1V + C2V + C3V = (C1 + C2 + C3)V or Q/V = C1 + C2 + C3 iii) The parallel combination of capacitors, in a circuit can be replaced by its equivalent capacitor of capacitance Ce such that Ce = C1 + C2 + C3 If n capacitors are combined in parallel, the equivalent capacitance of the combination is given by Ce = C1 + C2 + C3+ Cn. iv) Generally capacitors are connected in parallel to increase the value of capacitance. Series Combination of Capacitors: i) In this type of connection charge on each capacitor would be same. ii) The voltage of battery is divided among the capacitors. Hence V = V1 + V2 + V3 = Q/C1 + Q/C2 + Q/C3 = Q(1/C1 + 1/C2 + 1/C3) iii) The series combination of capacitors in a circuit can be replaced by its equivalent capacitance Ce, the value of which is given by: - 1/Ce = (1/C1 + 1/C2 + 1/C3) If n capacitors are connected in series, the equivalent capacitance of the combination is given by: 1/Ce = 1/C1 + 1/C2 + 1/C /Cn Question: Describe the different types of capacitors? On the basis of their construction and the nature of dielectric used capacitors are classified into following types: - Fixed Capacitor: If a capacitor is such that its plates are immovable, it is known as fixed capacitor. Paper Capacitor: It is cylindrical in shape. Generally an oiled or greased paper or a thin plastic sheet is used as a dielectric between two aluminum foils which is firmly rolled in the form of a cylinder and is then enclosed into a plastic case. Mica Capacitor: It is another example of fixed capacitor. In these capacitors mica is used as dielectric between two metal plates. Variable capacitor: 5

6 In parallel plate capacitor some arrangement is made so that the area of the plates facing each other can be changed then it is called variable capacitor. It consists of two sets of plates. One set remains static while the second one can rotate so the distance between the plates remains the same, as they do not touch each other. These capacitors are generally used in radio sets. 6