10th Physics Chapter 13 Long Questions PDF

# Unit 13: Electrostatics ## Long Questions ### 13.1 Production of electric charges: **Q.1** What is electrostatic? How electric charges be produced? Explain it with activities. **Electrostatics:** "Study of charges at rest is called electrostatics or static electricity." **Production of electri...

# Unit 13: Electrostatics ## Long Questions ### 13.1 Production of electric charges: **Q.1** What is electrostatic? How electric charges be produced? Explain it with activities. **Electrostatics:** "Study of charges at rest is called electrostatics or static electricity." **Production of electric charges:** We can produce electric charge by rubbing a neutral body with another neutral body. The following activities show that we can produce two types of electric charges through the process of rubbing. **Activity 1:** Take a plastic rod. Rub it with fur and suspend it horizontally by a silk thread (as shown Fig. 1). Now take another plastic rod and rub it with fur and bring near to the suspended rod. **Observation:** We will observe that both the rods will repel each other. It means during the rubbing both the rods were charged both rods have same charge. **Activity 2:** Now take a glass rod and rub it with slide and suspend it horizontally. When we bring the plastic rod rubbed with fur near to the suspended glass rod as shown fig. 2. **Observation:** We observe that both the rods attract each other. **Conclusions** - In the above activity, rods are unlike and their attraction imply that charge on the two rods are not of the same kind but of opposite nature. - These opposite charges are conventionally called positive and negative charge. During the process of rubbing negative charge is transferred from one object to another object from these activities we conclude that: **Conclusion:** 1. Charge is a basic property of a material body due to which it attracts or repels another object. 2. Friction produces two different types of charge on different material (such as glass and plastic). 3. Like charges always repel each other. 4. Unlike charges always attract each other. 5. Repulsion is the sure test of charge on a body. ### 13.2 Electrostatic Induction: **Q.2** What is meant by electrostatic induction? Explain it with the help of experiments or activities. **Definition:** In the presence of a charged body, an insulated conductor develops positive charge at one end and negative charge at the other end. This process is called the electrostatic induction. **Explanation with the help of the activities** **Activity 1:** If we bring charged plastic rod near neutral aluminium rod, both rods attract each other as shown in Fig. 13.4. This attraction between the charged and uncharged rods shows as if both rods have different charged. But this is not true. Charged plastic rod produces displacement of positive and negative charges on the neutral aluminium rod which is the cause of attraction between them. But total charge on aluminium rod is still zero. It implies that attraction is not the sure test of charge on a body. It also shows that electrostatic induction is another method of charging a body and is described below. **Method of Charging a bodies by electrostatic induction** Consider a metallic sphere placed on an insulated stand Fig. (13.5a). The sphere is neutral as it carries equal number of positive and negative charges. Now bring a negatively charged rubber rod near the conducting sphere. As shown in Fig. (13.5.b) left part of the sphere that is close to the rod becomes positively charged while the right part that is away from the rod becomes negatively charged. Negative charge in the rod repels the negative charge of the sphere and shifts it to the opposite region of the sphere that is away from the rod. Thus, there is excess of positive charge in the region of sphere close to the rod while there is excess of negative charge in the region of the sphere away from the rod. But as a whole the sphere is still neutral, since no charge has been added or subtracted. Now if we remove the rod away from the sphere, the charged again will spread uniformly on the whole surface of the sphere. Now earth the sphere through a conducting wire in the presence of the rod (Fig. 13.6-a) The negative charge will flow to the earth an leaves the sphere with net positive charge Now if we first break the earth connection and then remove the rod form the sphere it will get positive charge permanently (Fig. 13.6-b). ### 13.3 Electroscope: **Q.3** What is gold leaf electroscope? Discuss its working principle with a label diagram. **OR** Describe a gold leaf electroscope. By using an electroscope, how can we find the (i) Presence of charge on a body (ii) The nature of the charge on a body body (iii) Whether a body is insulator or a conductor **Delimitation** 1. The gold leaf electroscope is sensitive instrument for detecting charges. **Construction:** It consists of a brass rod with a brass disk at the top and two thin leaves of gold foil hanging at the bottom (Fig. 1). The rod passes through an insulator that keeps the rod in place and also retains the charges. Charges can move freely form the disk to the leaves through the rod. A thin aluminium foil is attached on the lower portion of the inside of the jar. Usually, the aluminium foil is grounded by connecting a copper wire. This protects the leaves form the external electrical disturbances. **Detecting the presence of charge:** In order to detect the presence of charge on anybody, bring the body near the disk of an uncharged electroscope. If the body is neutral there will be no deflection of the leaves (Fig. 2-a) But if the body is positively or negatively charged, the leaves of the electroscope diverge **For example:** If the body is negatively charged then due to electrostatic induction Fig (2-a, 2-b) positive charge will appear on the disk while negative charge will appear on the leaves (Fig. 2-b). The leaves of electroscope repel each other rand diverge because each leave gets similar charge. The divergence of leaves will depend on the amount of charge. **Charging the Electroscope by Electrostatic induction:** Electroscope can be charged by the process of electrostatic induction. In order to produce positive charge on the electroscope, bring a negatively charged body near the disk of the electroscope (Fig. 3-a). Positive charge will appear on the disk of the electroscope while negative charges will shift to the leaves. Now connect the disk of electroscope to the earthed aluminium foil by a conducting wire (Fig. 3-b). Charge of the leaves will flow to the earth through the wire. Now if we first beak the earth connection and then remove the rod the electroscope will be left with positive charge. Similarly, electroscope can be charged negatively with the help of a positively charged rod. **By Conduction:** Electroscope can also be charged by the process of conduction. Touch a negatively charged rod with the disk of a neutral electroscope. Negative charge from the rod will transfer to the electroscope and will cause its leaves to diverge. **Detecting the Type of Charge:** For the detection of type of charge on a body, electroscope is first charged either positively or negatively. Suppose the electroscope is positively charged as explained before. Now in order to detect the type of charge on a body. Bring the charged body near the disk of the positively charged electroscope. If the divergence of the leaves increases, the body carries positive charge (Fig. 4-a). On the other hand, if the divergence decreases, the body has negative charge (Fig 4-b) **Identifying conductors and insulators:** Electroscope can also be used to distinguish between insulators and conductors. Touch the disk of a charged electroscope with material under test. If the leaves collapse from their diverged position the body would be a good conductor. If there is no change in the divergence of the leaves. It will show that the body under test is an insulator. ### 13.4 Coulomb's Law **Q.4** State and explain Coulomb's law and write its mathematical form. **Introduction:** We know that a force of attraction or repulsion acts between two charged bodies. How is this force affected when the quantity of the charge on the two bodies or the distance between them is changed? In order to find the answers of these question, a French scientist charges coulomb (1736-1806) in 1785 experimentally established the fundamental law of electric force between two stationary charged particles. **Coulomb's Law** **Statement:** "The force of attraction or repulsion between two-point charges is directly proportional to the product of the quantity of charges and inversely proportional to the square of the distance between them." **Mathematically:** $F \propto q_1 q_2 $ $F \propto \frac{1}{r^2} $ Combining Eqs. (i) and (ii) we get $F = K\frac{q_1 q_2}{r^2}$ Equation (iii) is known as coulomb's law Where F is the force between the two charges and is called the coulomb force, $q_1$ and $q_2$ are the quantities of two charges and r is the distance between the center of two charges (Fig. 1-a-b) K is the constant of proportionality given by $K = \frac{1}{4 \pi \epsilon_0}$ **Dependence of value of K** The value of K depends upon the medium between the two charges and the system of units in which F, q, and r, are measured, $ε_0$ is the permittivity of free space. Now if the medium between the two charges is air then the value of K in SI units will be $9 x 10^9 Nm^2/C^2$ Final form of Coulomb's force is: $F = \frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2}$ **Point Charges:** Coulomb's law is true only for point charges whose sizes are very smallest compared to the distance between them. Like other forces, electric forces also obey Newton's third law. ### 13.5 Electric Field And Electric Field Intensity **Q.5** Define electric field and electric intensity also explain electric intensity? **OR** What is meant by electric field and electric intensity? Find the electric intensity due to point charge Electric field **Definition:** "The region of space surrounding the charge q in which it exerts a force on the charge $q_0$ is known as electric field of the charge q." **OR** "The electric field is a region around a charge in which it exerts electrostatic force on another charges." **Explanation:** According to Coulomb's law if a unit positive charge $q_0$ (call it the test charge) is brought near a charge q (call it the field charge) placed in space, the charge $q_0$ will experience a force. The value of this force would depend upon the distance between the two charges. If the charge $q_0$ is moved away from q this force would decrease till at a certain distance the force would practically reduce to zero. Now the charge $q_0$ is out of the influence of charge q. **Electric Field Intensity** "The strength of electric field at any point in space is known as electric field intensity" **Explanation:** In order to find the value of electric intensity at a point in the field, of charge $+q$ we place a test charge $q_0$ at that point Fig. 13.13. If F is the force acting on the test charge $q_0$ the electric field intensity would be given by $E = \frac{F}{q_0}$ Thus, the electric field intensity at any point is defined as the force acting on a unit positive charge place at that point. **SI Unit:** SI unit of electric intensity is $NC^-1$ If the electric field due to a given arrangement of charges is known at some point, the force on any particle with charge q placed that point can be calculated by using the formula $F = qE$ **Quantity:** Electric intensity being a force is a vector quantity. It direction is the same as that of the force acting on the positive test charge. If the test charge is free to move, it will always move in the direction of electric intensity. ### Q.6 What is meant by electric lines of force? Explain **Electric Field Lines:** **Definition:** The direction of electric field intensity in an electric field can also be represented by drawing lines. These lines are known as electric lines of force. **Explanation:** These lines were introduced by Michael Faraday. The field lines are imaginary line around a field charge with an arrow head indicating the direction of force. Field lines always move away from positive charge but toward negative charge. The spacing between the field lines shows the strength of electric field (Fig 1) **Relation of field line with the electric field intensity** Field lines are related to the electric field intensity in any region of space in the following way 1. The electric field intensity is tangent to the electric field lines at each point (Fig. 2a). 2. The number of lines per unit area through a surface perpendicular to the lines is proportional to the electric field strength in a given region (Fig. 2-b) 3. Electric field is strong when the field lines are close together and weak when the lines are for apart. 4. No, two field lines cross each other Electric field lines for an isolated positive and negative point charges are shown below ### 13.6 Electrostatic Potential **Q.7** What is meant by electric potential? Explain **Electric potential:** "Electric potential at a point in an electric field is equal to the amount of work done in bringing a unit positive charge form infinity to that point." **Mathematically:** If W is the work done in moving a unit positive charge from infinity to a certain point in the field, the electric potential V at this point would be given by $V = \frac{W}{q}$ It implies that electric potential is measured relative to some reference point and like potential energy we can measure only the change in potential between two points. **Quantity:** Electric potential is a scalar quantity. **Unit:** It SI unit is volt which is equal to $JC^-1$ **Definition of volt:** If one joule of work is done against the electric field in bringing one coulomb positive charge form infinity to a point in the electric field then the potential at that point will be one volt. Or if the potential energy of one coulomb of charge at a point in the electric field is one joule, the potential of the point will be one volt. **Potential Difference:** A body in gravitational field always tends to move form a point of higher potential energy to a point of lower potential energy. Similarly, when a charge is released in a electric field, it moves form a point of higher potential say A to appoint a lower potential say B (Fig. 13.16). If the potential of point A is $V_a$ and that of point B is $V_b$ the potential energy of the charge at these points will be $qV_a$ and $qV_b$ respectively. The change in potential energy of the charge when it moves from point A to B will be equal to $qV_a - qV_b$. This energy is utilized in doing some useful work. Thus, Energy supplied by the charge = $q (V_a - V_b)$ If q is equal to one unit, then the potential difference between two points becomes equal to the energy supplied by the charge. Thus, we define potential difference between two points as **Potential difference:** "The energy supplied by a unit charge as it moves form one point to the other in the direction of the field." - If a positive charge is transferred form a point of lower potential to a point of higher potential i.e. against the field direction, energy would have to be supplied to it. - When we release a negative charge in an electric field, its behaviour will be opposite to that of positive charge. A more useful unit for the electrical energy is electron volt (eV). ### 13.7 Capacitors and Capacitance **Q.8** What is meant by capacitance of the conductor? Explain. **Capacitance:** "The ability of a capacitor to store charge is known as capacitance. It is the ratio of charge and electric potential". **Explanation:** When we charge a conductor, some work has to be done during the process of transferring the charge to the conductor. This raises the potential of the conductor. Experiments show that the charge Q on the conductor is directly proportional to its potential V $Q \propto V$ $Q = CV$ $C = Q/V$ Where C is a constant whose value depends upon the size of the conductor. It is known as capacitance of the conductor. In Equation [1], if V= 1 volt, then Q = C i.e., the capacitance of a conductor is equal to the amount of charge which raises the potential of the conductor by one volt. **Unit** The unit of capacitance is known as farad. It can be defined as: **Farad** "It is the capacity of that conductor the potential of which rises by one volt when one coulomb charge is given to it." **Q.9** OR Describe a capacitor. How does it store charge? Describe capacitor as an energy storing device? Ans: "The device that is used to store electric charge is known as Capacitor" **Construction** 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. A plane parallel plate capacitor has been shown in Figure. **Storage of charge on metals** Charge cannot be stored on a conductor for a long period of 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. **Process of storing charges on capacitor** If +Q amount of charge is transferred to its plate A, due to electrostatic induction it would induce -Q charge on the inner surface of the plate B and +Q charge on its outer plate as have been show in the figure. The lines offered between the plates have been show. There exists a force of attraction between the charges +Q stored on the plate and the charge -Q induced on the inner surface of plate B. Due to this force of attraction, the charges are bound with the plate and remain stored for long periods. **Mathematical form** Due to presence of the charges on the plate, a potential difference V is created between them which is directly proportional to the charge Q given to the plate 'A'. $Q \propto V$ $Q = CV$ Here C is the constant of proportionality, known as the capacitance of the capacitor. **Combination of Capacitors** **Q.10** How are the capacitors are connected in parallel? Describe the characteristics features of this combination. **OR** Derive the formula for the effective capacitance for a parallel combination of a number of capacitors **Ans:** In this method the left plate of each capacitor is connected to the positive terminal of battery by a connecting wire. In the same way the right plate of each capacitor is connected to negative terminal of the battery. **Characteristics of parallel combination** 1. Each capacitor connected to a battery of voltage V has the same potential difference V across it. i.e., $V_1 = V_2 = V_3 =V$ 2. The charge developed across the plates of each capacitor. it will be different due to different value of capacitances. 3. The total charge Q supplied by the battery is divided among the various capacitors. Hence $Q = Q_1 + Q_2 + Q_3$ or $Q = C_1V + C_2V+C_3V$ or $Q = (C_1 + C_2+C_3)V$ 4. Thus, we can replace the parallel combination of capacitors with one equivalent capacitor having capacitance $C_{eq}$ (Fig. 13.19), such that $C_{eq} = C_1+C_2 + C_3$ In the case of n capacitors connected in parallel, the equivalent capacitance is given by $C_{eq} = C_1+C_2 + C_3+ .....+ C_n$ 5. The equivalent capacitance of a parallel combination of capacitors is greater than any of the individual capacitances. **Q.11** How are the capacitors are connected in series? Describe the characteristics features of this combination. **OR** Derive the formula for the effective capacitance for a series combination of a number of capacitors. **Ans:** In this method the capacitors are connected side by side. The right plate of one capacitor is connected to the left plate the next capacitor as shown in figure. **Characteristics of series combination** 1. Each capacitor has the same charge across it. If the battery supplies + Q charge to the left plate of the capacitor $C_1$ due to induction -Q charge is induced on the right plate of the capacitor and + Q charge on the left plate of the capacitor $C_2$ i.e., $Q_1 = Q_2 = Q_3 = Q$ 2. The potential difference across each capacitor is different due to different values of capacitances. 3. The voltage of the battery has been divided among the various capacitors. Hence $V = V_1 + V_2 + V_3$ $V = \frac{Q}{C_1}+\frac{Q}{C_2}+\frac{Q}{C_3}$ $\frac{1}{V} = (\frac{1}{Q})(\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3})$ $\frac{1}{V} = \frac{1}{Q}(\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3})$ $\frac{1}{C_{eq}} = \frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}$ 4. Thus, we can replace series combination of capacitors with one equivalent capacitor having capacitance $C_{eq}$ i.e., $\frac{1}{C_{eq}} = \frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}$ 5. In the case of n capacitors connected in series. We have $\frac{1}{C_{eq}} = \frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}+ .......+\frac{1}{C_n}$ ### 13.8 Different Types of Capacitors: **Q.12** Discuss the different types of capacitors. Parallel plate capacitors are not commonly used in most devices because in order to store enough charge their size must be large which is not desirable. A parallel plate capacitor has a dielectric between its plates and is made of a flexible material that can be rolled into the shape of a cylinder. In this way, we can increase the area of each plate while the capacitor can fit into a small space. Some other types of capacitors use chemical reactions to store charge, like tiny batteries. **Types of capacitors:** Capacitors have different types depending upon their construction and the nature of dielectric used in them. Capacitors are either variable or fixed. In variable capacitors, **Definition:** **Fixed capacitor** If the capacitor is such that its plates are immovable, it is known as a fixed capacitor. Its value does not change. **TYPES OF FIXED CAPACITOR:** - Paper capacitor - Mica capacitor **Paper capacitors:** The paper capacitor has a cylindrical shape. Usually, an oiled or greased paper or a thin plastic sheet is used as a dielectric between two aluminium foils. The paper or plastic sheet firmly rolled in the form of a cylinder and is then enclosed into a plastic case. **Mica capacitor:** Capacitor another example of fixed capacitors. In these capacitors, mica is used as dielectric between the two metal plates (Fig. 13.23-a). For convenience and safety purposes it is enclosed in a plastic case or in a case of some insulator. Wires attached to plates project out of the case for making connections (Fig. 13.23-b). If the capacitance is to be increased, large number of plates is piled up, one over the other with layers of dielectric in between and alternative plates are connected with each other. **Variable capacitor:** In variable type of capacitors some arrangement is made to change the area of the plates facing each other (Fig. 13:24). It is generally a combination of many capacitors with air as electric. It consists of two sets of plates. One set remains fixed while the other set can rotate so the distance between the plates does not change and they do not touch each other. The common area of the plates of the two sets which faces each other determines the value of capacitance. Thus, the capacitance of the capacitor can be increased or decreased by turning the rotatable plates in or out of the space between the static plates such capacitors are usually utilized for tuning in radio sets. **AN ELECTROLYTIC** An electrolytic capacitor is often used to store large amounts of charge at relatively low voltages (Fig. 13.25). It consists of a metal foil in contact with an electrolyte -a solution that conducts charge by virtue of the motion of the ions contained in it. When a voltage is applied between the foil and the electrolyte, a thin layer of metal oxide (an insulator) is formed on the foil, and this layer serves as the dielectric. Enormous capacitances can be attained because the dielectric layer is very thin. **Q.13** Write down few uses of capacitors. **Uses of Capacitors:** Capacitors have wide range of applications in different electrical and electronic circuits. **For Tuning some appliance:** They are used for turning transmitters, receivers and transistor radios. **For Home appliance:** They are also used for table fans, celling fans, exhaust fans, fan motors in air conditioners, coolers, motors washing machines, air conditioners and many other appliances for their smooth working. **In electronic circuses:** Capacitors are also used in electronic circuits of computers etc. **To differentiate between Low and high frequency:** Capacitors can be used to differentiate between high frequency and low frequency signal which make them useful in electronic circuits. **A so filter circuit:** Capacitors are used in the resonant circuits that tune radios to particular frequencies. Such circuits are called filter circuit. **Ceramic:** Capacitors are generally superior to other types and therefore can be used in vast ranges of application. ### 13.9 Applications of Electrostatics **Q.14** Discuss in detail important application of electrostatic. **OR** Write a note on the following (a) Electrostatic air cleaner (b) Spray Painting **Static electricity has an important place in our everyday lives which include photocopying, car painting, extracting dust from dirty carpets and from chimneys of industrial machinery.** **Electrostatic Air Cleaner:** An electrostatic air cleaner is used in homes to relieve that discomfort of allergy sufferers. **Working of electric static Air cleaner:** Air mixed with dust and pollen enters the device across a positively charged mesh screen. The airborne particles become positively charged when they make contact with the screen. Then they pass through a second, negatively charged mesh screen. The electrostatic force of attraction between the positively charged particles in the air and the negatively charged screen cases the particles to precipitate out on the surface of the screen. Through this process we can removes a very high percentage of contaminants form the air stream. **Spray painting:** Automobile manufacturers use static electricity to paint new cars. **Working of electrostatic spray painting:** The body of car is charged and then the paint is given the opposite charge by charging the nozzle of the sprayer (Fig. 1.3.26) Due to mutual repulsion charge particles coming out of the nozzle form a Fine mist and are evenly distributed on the surface of the object. The charged paint particles are attracted to the car and stick to the body, just like a charged balloon sticks to a wall. Once the paint dries, sticks much better to the car and is smoother because is the uniformly distributed. This is a very effective, efficient and economical way of painting automobiles on large scale. ### 13.10 Some Hazards of Static Electricity Lightening: **Q.15** What are the hazards of static electricity? Explain them. There are so many hazards of static electricity. We are discussing only two of them. **(i) Lightening** **(ii) Fires or Explosions** **(i) Lightening** The phenomenon of lightening occurs due to a large quantity of electric charge which builds up in the heavy thunderclouds. The thunderclouds are charged by friction between the water molecules in the thunderclouds and the air molecules. When the charge on the thunderclouds is sufficiently high, it can produce positive and negative charges in the air. The amount of negative charge is discharged to the highest object on the ground and can harm them. This explains why it is very dangerous to swim in the open sea, play in an open field or hide under a tree during a thunderstorm. **Precaution or prevention:** To prevent lightening from damaging tall buildings, lightening conductors are used. The purpose of the lightening conductor is to provide a steady discharge path for the large amount of negative charge in the air to flow form the top of the building to the Earth. In this way the chances of lightening damage due to sudden discharge can be minimized. **(ii) Fires or Explosions:** Static electricity is a major cause of fires and explosions at many places. A fire or an explosion may occur due to excessive build-up of electric charges produced by friction. **Production of static electricity:** Static electricity can be generated by the friction of the gasoline being pumped into a vehicle or container. It can also be produced when we get out of the car or remove an article of clothing. Static charge are dangerous. If static charges are allowed to discharge though the areas where there is petrol vapour a fire can occur. The results are frightening and may be devastating. Portable oil containers can build up a static electric charge during transport. Consequently, when the container is not placed on the ground for filling, its static electricity could be discharged and result in a fire when filling begins. **Precaution or prevention** Precaution or prevention containers should be placed on the ground during filling and the nozzle should be kept in contact with the container. Containers should not to be filled while inside a vehicle.