10th Class Chapter 4 Physics Long Questions

Given Data:

Q1= 10 uc

= 10*10-6c

Q2 5 uc

= 5*10-6c

R = 150cm =1.5m

K = 9109 Nm2C2

F=?

Solution:

As we know that:

F= k q1 q2/r

Putting values we get

F = (9*10/9) *510/6*10*10-

(1.5)2

= 9*5*10/9-6 +1-6

2.25

= 45 *10-2

2.25

F= 0.2 N

Electric field:

The electric field of a charge is a region around a charge in which it exert electrostatic force on an other charge.

Explanation:

Consider a positive carge q and bring a test carge q/0 will experience a force .According to coulomb’s law the value of this force will depend upon the distance between two charges.

If charge q/0 is moved away from q this force will decrease till a certain distance the force would practically reduce to zero.

Electric field intensity:

The strength of electric field at any point in space is called electric field intensity .

Explanation:

If f is the force acting on test charge q/0 the electric field intensity would be,

F = f/q0

If the electric field due to a given arrangement of charge is known at same point, the force on any particle with charge q placed at that point can be calculated by using the formula,

F= qe …… This I force acting on a unit positive charge placed at that point.

Unit:

SI unit of electric field intensity is NC-1

Quantity:

Electric intensity being a force is a vector.

Given Data:

C1 = 6uf

= 6*10-6f

C2 = 12UF

= 12*10-6F

V= 12 volt

Ce =?

V1 = ?

V2 = ?

Q1 = ?

Q2 = ?

Solution:

We know that,

1/Ce = I/C1+I/C2

I/Ce = I/6+ 1/12

= 2+1

12

= 3/12

1/Ce = 1/4

Ce =4uf

Ce = 4*10-6 f

Q = Cev

= 4*10-6*12

= 48*10-6

Q = 48uc

Zn series combination charge across each capacity is same so,

Q =Q1=Q2+V1 = Q1/C1

= 48*10-6/6*10-6

V1=8 volts

V2 = Q2/C2

= 48 *10-6/12*10-6

V2 = 4 volts.

Given Data:

C1 = 6uf

= 6*10-6f

C2 = 12UF

= 12*10-6F

V= 12 volt

Ce =?

V1 = ?

V2 = ?

Q1 = ?

Q2 = ?

Solution:

We know that,

1/Ce = I/C1+I/C2

I/Ce = I/6+ 1/12

= 2+1

12

= 3/12

1/Ce = 1/4

Ce =4uf

Ce = 4*10-6 f

Q = Cev

= 4*10-6*12

= 48*10-6

Q = 48uc

Zn series combination charge across each capacity is same so,

Q =Q1=Q2+V1 = Q1/C1

= 48*10-6/6*10-6

V1=8 volts

V2 = Q2/C2

= 48 *10-6/12*10-6

V2 = 4 volts.

In this combination capacitors are connected side by side i.e the right plate of one capacitors is connected to left plate of next capacitors.

Characteristics:

Each capacitor has the charge it battery supplies to charge to the left plate of the capacitor q., due to induction – Q charge is induced on its right plate i.e,

Q1=Q2=Q3=Q

The potential difference across each capacitor is different due to different value of capacitances.

The battery voltage has been divided among the various capacitors .Henc

= V1+V2+V3

Q=CV

V=Q/C

Q/C1+Q/C2+Q/C3

=Q ( I/C1 +I/C2+I/C 3)

Q= CV

V/Q =I/C

I/C= I/C1+I/C2+I/C3

Series combination of capacitors can be replaced with one equivalent capacitors having capacitance

= I/C1+I/C2+I/C3

If N capacitors are connected on series then

= I/C1 +I/C2+I/C3+-------+I/Cn

Equivalent capacitance of a series combination of capacitors is smaller than any individual capacitance ,capacitors connected in series decrease the value of capacitance.

Given Data:

Q1= 500 uc

=500*10-6 C

Q2 = 100 uc

= 100*10-6 C

R =0.5m

K= 9*10-9 Nm2C-2

F= ?

Calculations:

According to coulomb’s law,

F= k q1 q2/r

Putting values we get

= 9*10-9/500*10-6*100*10-6

(0.5)2

= 9*5*10-910-210-610-2*10-6

(0.5) (0.5)

= 45*10/9+2+6+2+6

0.25

= 45*10

0.25

F = 1800 N

In the presence of a charged body an insulated conductor develops positive charges at one end and negative charges at their other end this process is called electrostatic induction.

The gold leaf electroscope is a sensitive instruments used for detecting and testing the nature of charges on a body.

A device used for storing electric charge is called a capacitors the air or insulator placed between the plates of a capacitors is called dielectric.

Charge is the basic property of the material body due to which it attracts or repels another body and there are two types of the charges one is positive and second I negative.

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.

F= k q1 q2/r2

A region around a charge in which it exerts electrostatic force on another charge.

Electric field intensity:

The force acting on a unit positive charge placed at a point its SI unit is NC-1.

If the distance between two charge is much greater as compared to their size then these bodies are considered as point charge.

· Paper capacitors

· mica capacitors

· Parallel plate capacitor

· Electrolytic capacitors

The direction of electric field of lines intensity in an electric field can also be represented by drawing lines these lines are known as electric field lines.

The energy supplied by the unit charge as it moves from one point to the other in the direction of the field the unit of potential difference is volt.

Electric potential at a point in an electric field is equal to the amount of work done in bringing a unit positive charge from infinity to that point its unit is volt or JC-1

It consist of two thin metal plates,parallel to each other separated by a very small distance the medium between the two plates is air or a sheet of some insulator this medium is known as dielectric.

Each capacitor has the same charge across each capacitor in series combination if battery supplies +Q charge to the left plate of the capacitors CL,due to induction –‘Q’ cargeharge is induced on its right plate and +Q charge on the left plate of the capacitors i.e on each plate of capacitors charge is same.

They are used for table fans, ceiling fans, coolers motors and washing machines they are used in electronic circuits of computers.

The gold leaf electroscope is a sensitive instruments used for detecting and testing the nature of charges on a body.

Fixed capacitors;

If the plates of the capacitors are immoveable then such capacitors are called fixed capacitors the capacitance of the such capacitor can not be changed.

It is the ability of the capacitors to store charge when one coulomb of charge given to the plates of the capacitor produces a potential difference of one volt between the plates of the capacitor.

The unit of capacitance is fared if one coulomb of charge given to the plates of a capacitor produces a potential difference of one volt between the plates of the capacitor then its capacitance would be one farad.

The phenomenon of lightning occurs due to a large quantity of electric charge which build up in the heavy thunderclouds the thunder clouds are charged by friction between them water molecules in the thunderclouds and the air molecules.

I/Ceq = I/C1 + I/C2 +I/C3

C1 C2 C3

K + V

If a capacitor is connected to a battery of V, then the battery transfers a charge +Q from plate A to plate B so that –Q charge appears on plate B and +Q charge appears on plate A. The charge on each plate attract each other and thus remained bound with io the plate , in this way charge is stored in a capacitors for long time.

The formula of parallel combination of capacitor is Ceq = C1+2+C3 +----Cn

Mica capacitor is an example of fixed capacitors Zn mica capacitor , mica is used as dielectric between the two metal plates mica is enclosed in a plastic case.

The relation between electric potential and potential energy is given by V = W/q.

We have learnt already that an inherent property of an object is its mass. An object with smaller mass will have less inertia as compared to an object with larger mass. Along with mass, another inherent property of an object is its electrical charge. Charge is measured in units of coulomb ©

There are two kinds of charges:

A simple experiment can be performed to show that there are two types of charges, one type is called positive and the other negative.

There are two types of charges and similar charges repel and different charges attract.

Interestingly it is seen that after rubbing the silk cloth and animal fur also acquired the charge in opposite sense to glass rod and rubber rod respectively. `Structure of Atom’ we learnt that all matter is made up of atoms. Atoms have two kinds of charges: protons contained in the nucleus of atom have positive charge, whereas electrons clouding around the nucleus carry negative charge, neutrons inside the nucleus have no charge. The charge on an electron or proton is said to be an elementary or fundamental charge because all known charges are made up of electrons and protons and so all charges are integer multiples of the fundamental charge.

As all the matter is made of atoms, thus every material object has charges in it. We often do not notice the effects of electrical charges because most objects have the same number of electrons and protons in them (therefore we have equal and opposite charges) and as a result the net effect is zero and so these objects are electrically neutral

Electrification:

Electric charge is not created in the process of charging object, charges are only transferred between the objects. In electrification experiments It is seen that silk cloth/animal fur also attained charge. Thus, in those experiments, charge was not produced rather it was only transferred and we can say that objects can be charged by removal or addition of charges (specifically electrons) called electrification.

A change in distribution of electrical charge in an object, caused by the influence of nearby charges is called electrostatic induction. This effect may be shown by bringing a negatively charged rubber strip near to an insulated metal sphere X which is touching a similar sphere Y. Electrons in the spheres are repelled to the far side of Y.

Electroscope is a device used for detecting and testing the nature of charge on a body. It works on the principle that similar charges repel each other.

A simple form of electroscope consists of a metal bar which has a metallic sphere (ball) at its upper end. Thin flexible metal leaf ( made of gold, silver, copper or any other metal ) is attached to the lower end of metal bar.

The lower part is enclosed in an insulated housing.

Working:

In order to detect charge on a body we touch the metal ball with it. For example, if we touch it with negatively charged rubber rod some of its excess electrons will be transferred to the ball and then they will spread throughout the metal rod and the metal foil. The flexible metal leaf will be repelled by the similar change on the metal rod and will move away from the rod by rising higher , As more electrons are transferred to the electroscope, the metal leaf will rise higher, Alternatively if the rod id positively charged it will attract electrons from the electrons from the electroscope, leaving a net positive charge on it. Once again the foil will rise.

However to test the nature of charge on the body we charge the electroscope with some known charge first. Now if the same charge is added to the metal ball, it will increase divergence of flexible metal leaf and opposite charge will decrease the divergence allowing us to identify the unknown charge on the body

Coulomb's lawstates that: The magnitude of the electrostatic force of attraction or repulsion between two point chargesisdirectly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them. The forceisalong the straight line joining them

Statement: The electric force between two stationary point charges is directly proportional to the product of the charges; inversely proportional to the square of the distance between them and is directed along the line joining these charges.

The attraction of an uncharged object by a charged object near it is due to electrostatic induction. For example, it is the reason for the opening question `why after running a plastic comb through our hair on a dry day does the comb attract small pieces of paper?’. The influence of the charge on the comb redistributes the charges in the paper due to electrostatic induction.

Charges in the pieces of paper with the same sign (polarity) are pushed away while charges with opposite sign are pulled closer. Thus change in distance means the attractive force due to the closer opposite sign charges is greater than the repulsion due to the same sign charges which have been pushed further away. As coulomb forces decreases as distance between similar charges is increased, on the other hand it increases for dissimilar charges, as a result it overcomes inertia and lifts small pieces of paper. This creates a net attractive force which pulls the paper towards the comb.

Electric Field and Its Intensity:

The region around a charge in which an electric test charge would experience an electric force is called electric field, And the strength a of the field (equal to the force experience by a (+1C) test charge) at any point is called electric field intensity.

An electric field exist in the region of space around a charged object in three dimensions. When another charged object enters this electric field, an electric forcer acts on it even without any physical contact between the charges.

Consider a small positive test charge q_o near the greater magnitude charge q Now the electric field E at any point in space is defined as the force F acting on unit positive charges q_o , divided by the magnitude of the test charge q_o , mathematically

E =F_E/q_o equation (1)

Equation (1) gives the mathematical form of electric field intensity. The SI unit of electric field intensity is newton per coulomb (NC). It is a vector quantity having direction in which a positive test charge would move under the influence of force.

An easy way to visualize an electric field is to draw lines that follow the same direction as the electric field intensity vector E at any point. These lines are called electric field lines.

Electric field intensity vectors help to visualize the electric field. Electric field lines are a kind of “map” That gives the direction and strength of the field at various places. The direction of the lines is radially outward for a positive charge and radially inward for a negative charge.

Electric Potential:

The electric potential energy `U’ per unit charge `q’ in an electric field is called electric potential V

V= U/q= W/q Equation (1)

The electric potential units of joules per coulomb (J C), or volt. (V) in honor of Italian scientist Alessandro Volta. The potential at a point is one bolt, when it requires one joule of work to move a positive charge of one coulomb form a point of ZERO potential to that point.

1volt = 1joule/1coulomb

The concept of electric potential is closely related to electric field. Electric field is the force per unit charge, whereas the electric potential is the energy per unit charge. However electric potential is a scalar quantity, since, it is easier to solve problems with scalars, therefore, and it is simpler to solve problems with electric potential rather than electric field.

Let us consider a positive charge +q is placed in an electric field at point B. If the charge is allowed to move freely, it will acquire kinetic energy and will move from B to A. Conversely, we can say that an external force is required to keep the charge at rest or to move with uniform velocity from A to B Thus

V_B – V_A = W_AB/q equation (2)

Often point A is taken to be at infinity , meaning a large distance from the charges that produce the electric field, and the electric potential at A is taken to be zero.

Note that the choice of zero potential at infinity is taken arbitrarily and for simplicity, such that

V = W/q equation (3)

In equation (3) V (by definition) the Work that must be done against the Electric Field to bring a test charge q from infinity to a specific location.

Capacitor is a device used for storing charge, it consists of two conductors separated form (without touching) each other, carrying charges of equal magnitude but opposite sign.

The basic elements of any capacitor two isolated conductors (having charge +Q and - Q) of any shape. No matter what their geometry, flat or not, we call these conductors plats, each capacitor plate carries a charge of the same magnitude, one positive and the other negative. Due to the charge the electric potential of the positive plate exceeds that of the negative plate by an amount V.

The insulating medium that separates the plates of capacitor (air or some other insulating material) is referred to as dielectric

Capacitance of capacitor:

The capacitance C of a capacitor, is the ratio of the magnitude of the charge on either conductor, is the ratio of the magnitude of the charge on either conductor to the magnitude of the potential difference between them.

When charge Q is increased on the plates of the capacitor the potential difference V also increases and vice versa

Q V or Q =CV

Where C is the constant of proportionality and is called the capacitance of a capacitor

Therefore C= Q/V eq (1)

The capacitance of capacitor is the amount of charge the capacitor can store per unit of potential difference . The capacitance of a capacitor depends upon the size and shape of the plates. Its also depends upon the separation and nature of insulating material in between the plates.

Unit of Capacitance:

The SI Unit of capacitance is coulombs per volt or the farad (F), named in honor of Micheal Fraday, such that

1F = 1C/C

The farad is a very large unit of capacitance. In practice, typical devices have capacitances ranging from microfarads 10^-6 F). For practical purposes, capacitors often are labeled “µF” for microfarads and “pF” for Picofarads.