Expression for the electric field intensity at a point on the axis of the charged ring –
Question may be asked as ; A charge is distributed uniformly over a ring of radius ‘a’ . Obtain an expression for the electric field intensity E at a point on the axis of the ring. Hence show that for points at large distance from the ring behaves like a point charge.
Suppose a uniform circular ring of radius ‘a’ charged uniformly ‘Q’ which is distributed uniformly over the ring.
Suppose a small element ‘dl’ on the ring then, charge on the element is given by – dQ = (Q/2∏a) dl
The electric field at point p due to this element is given by –
*****At the place of ∑ students can use integration with limit 0 to 2∏a ( in both case answer will be same ) *****
The field E is directed along the axis OP of the charged ring .
If r >> a , then the above expression may be written as ‘
E = (Q/4∏ϵ0 r2 ) . This shows that for far points at long distance from the ring , it behaves like a point charge .
To view the video of this topic electric field at a pint on the axis of uniformly charged circular link click on the link given below-
In this topic we will discuss about the , Expression for torque and potential energy stored in dipole placed in uniform electric field . and what will be Expression for torque and potential energy stored in dipole placed in non-uniform electric field ?
Suppose an electric dipole AB having charges -q and +q are placed at distance 2a , and dipole moment ‘p’ is placed in an uniform electric field ‘E’ .
Then Electrostatic force experienced on both the charges are F = q E , but the direction of forces on each charges are opposite to each other . since the electric field E is constant then net force on the dipole will be zero ( since forces are in opposite directions ) .
Since the forces are equal , unlike and parallel acting at different points so , they form a couple . The couple tends to align the dipole axis along the direction of field ‘E’ .
Let’s draw AC perpendicular on electric field ‘E’ .
But Questions may be asked by the students that ,what will happen if electric field is not uniform ?
then its answer will be that the net force on the dipole will not be zero (0) . It means there will be translational and rotational motion in the dipole . And net force will be equal to F1-F2 .
To view the video based on this topic torque and potential energy on the dipole placed in uniform electric field go to the link given below-
This topic contain numerical for the practice of the chapter electric field
Before to solve the numerical based on electric field students has to study ( learn ) , and all the notes based on electric field , electric dipole ( axial and equatorial point)
Two point charges +16μc and -9μc are placed 8cm apart in air . Determine the position of the point at which the resultant electric field is zero . [ Ans- 24 cm]
A particle of mass m and charge q is thrown at speed u against a uniform electric field E . How much distance will it travel before coming to momentary rest . ? [Ans- mu2/2qE]
A particle of mass m and charge q is released from rest in uniform electric field of intensity E . Calculate the kinetic energy it attains after moving a distance a distance x between the plates . [Ans- qEx]
Eight identical point charges of q coulomb each are placed at the corners of a cube of each side 0.1 m . calculate the electric field at the centre of the cube . calculate the field at the centre when one of the corner charge is removed . [Ans – 0 ; 1.2 x 1012 .]
Two charges + 30μc and -30μc are placed 1cm apart . calculate electric field at a point on the the axial and equatorial line at a distance of 20 cm from the centre of the dipole . [ Ans- 6.25 x 105 N/C and 3,25 x 105‑ N/C .]
Four particles each of charge q are be placed on the vertices A ,B, C, D of a regular pentagon ABCDE . the distance of each corner from the centre is ‘a’ . Calculate the electric field at the centre of the pentagon . [ Ans- q/4∏ϵ0 a2 along OE .]
A small sphere of mass 1g carries a charge of +6μc . The sphere is suspended by a string in an electric field of 400N/C acting downwards. Calculate the tension in the string . What will be the tension if the charge on the sphere be -6μc . [Ans- 1.22x 10-2 N . ; 74 x 10-4 N .]
An electron falls through a distance of 1.5 cm in uniform electric field of value 2 x104N/C . when the direction of electric field is reversed , a proton falls through the same distance . compare the time of fall in each case .
A particle of mass ‘m’ and charge -q enters the region between two charged plates initially moving along x-axis with speed vx ( as shown in the fig) . The length of the plate is L and an uniform electric field E is maintained between the plates . Show that the vertical deflection of the particle at the far edge of the plate is QelL2 / (2m vx2) .
As shown in the figure three charged particles in a uniform electrostatic field . Give the sign of three charges . Which particle has the highest charge to mass ratio .
Electric dipole – An electric dipole consist of a pair of equal and opposite point charges separated by small distance .
Let two charges of equal magnitude but opposite sign( q and -q) are separated by distance 2a , then the arrangement is known as electric dipole .
Dipole moment (p) – Dipole moment of an electric dipole is the measurement of the strength of the electric dipole its magnitude is the product of magnitude of either charge and separation between them .
i.e. Magnetic moment p = q ( 2a )
Direction of magnetic field is given as negative to positive charge i.e. -q to +q ;
Its SI unit is C-m ( coulomb meter ) and dimension is [M0 L1 T1 A1 ] :
** Dipole field – It is the space around the dipole in which electric effect of the dipole can be experienced . For electric dipole electric field E α 1/r3 but for a point charge E α 1/r2
In the link given below , students can find the notes related to electric field at a point on axial and equatorial line due to a dipole.
To see the video of electric field at axial point due to a dipole click on the link given below
To view the video related to the topic electric field at equatorial point of a dipole click on link given below
Electric field intensity at any point due to a short electric dipole , it explain the electric field due to a dipole at any point and using this derivation we can find the electric field at axial and equatorial point of a dipole .
In this topic we will discuss about what is electric field and electric field lines , also electric field intensity at a point due to a point charge .
Electric field – Due to given electric charge it is the space around a charge particle in which electrostatic force can be experienced .
Electric field intensity – Electric field intensity at a point is the force experienced at unit charge placed at that point , it is strength of electric field at that point .
Electric field = F/q0
Electric field intensity due to a point charge – Suppose a charge Q is placed at point O , we have to find the electric field intensity at point p which is r distance away from charge Q .
Suppose a test charge q is placed at point p , then force acting at charge q due to Q is given as F = KQq/r2 ,
therefore electric field E = F/q
so, E =( kQq/r2 )/q
E = kQ/r2
If we have to find the electric field at a point due to group of charges then we can write
Net electric field, E = E1 + E2+E3 ..………..
Electric field lines – It is the imaginary line the tangent at any point on this lines gives the direction of force at that point .
To watch the video on electric field and electric field lines visit on given link below-
Properties of Electric field lines –
Electric field lines are continuous curve . They start from positive charge and end into negative charge . this is why electric field lines not form a closed loop . If there is a single charge then field lines start from the charge and goes into infinity .
2. Tangent to the electric field lines at any point gives the direction of electric field intensity at that point .
3. No two field lines intersect each other . This is because at the point of intersection , there will be two direction of force at the same point which is not possible .
4. The electric field lines are always normal to the surface of a conductor .
5. Where field lines are denser the electric filed intensity is more and vice- versa .
To watch the video of properties of electric field lines click on given link below-
***So in the form of electric field lines electric field intensity is equal to the number of field lines crossing normally a unit area around that point .
The Syllabus class 12 (physics) providing here is totally based on 2019-2020 pattern , if there will be any change in the syllabus , it will be improved and informed to the students through the website – www.physicsclasses.online , to get any improved information remain in contact with this site .
Topic removed from cbse class 12th physics. What are the benefits for the students or what are the drawbacks see in the video given below-
electrostatics –
Chapter 1: electric charges and fields –
Electric charges , properties of charges (Conservation of charge , quantisation of charge etc. ), coulombs law ( between two point charges and due to multiple charges also known as superposition of force due to multiple charges ) , permittivity of a medium , continuous charge distribution ( linear ,surface ,volume charge density) .
Electric field , electric field due to a point charge , electric field lines ( electric lines of force ) , electric dipole , electric field due to a dipole ( axial and equatorial) , torque on a dipole placed in uniform electric field . Electric flux , Gauss’s theorem and its applications (i.e. electric field at a point due to long straight charged conductor, charged infinite plane sheet , charged spherical shell) .
Chapter 2: Electrostatic potential and capacitance –
Electric potential and potential difference , electric potential due to a point charge and system of charges also due to electric dipole ( axial, equatorial and at any point) , equipotential surfaces , electric potential energy of a system of two point charges and of a dipole in an electric field . Conductors and insulators , electric polarization and dielectric constant ( relative permittivity ) , capacitor and its capacitance ; combination of capacitor ( series and parallel ) , capacitance of parallel plate capacitor with dielectric slab /conducting slab , energy stored in capacitor , common potential , lass in energy when two charged capacitors connected with each other .
current electricity-
Chapter 3: current electricity-
Electric current, drift velocity , mobility , relation between drift velocity and current , proof of Ohm’s law on the basis of drift velocity , current density , resistance , conductance , resistivity, conductivity , relation between current density and drift velocity, effect of temperature on resistance and resistivity , graph of resistance(resistivity) Vs temperature for conductors/alloy /semiconductors , Ohmic and non ohmic conductors ( graph) , carbon resister , combination of resistors ( series and parallel ) . Internal resistance (r) , terminal potential (V) and emf (E) of the cell and relation between them . also graph between ( V,E, r and R) . Combination of cells (series, parallel ,and mixed grouping) , Kirchhoff’s law (junction and loop rules) , wheat -stone bridge , meter bridge , potentiometer and its applications ( effects on null points on different -different conditions ). Electrical energy and power .
Magnetic effect of current and magnetism-
Chapter 4: Moving charges and magnetism-
Oersted’s experiment , concepts of magnetic field, Biot-Savart’s law and its application ( magnetic field at a point using Biot -savart’s law due to current carrying wire finite( only formulae ) and infinite , at the centre and on the axis of a circular current carrying coil . Ampere circuital law and its application to infinite long straight current carrying conducting wire ( thin and thick ) , magnetic field inside solenoid / toroid . Force on a moving charge in a magnetic field and direction of force . force on a current carrying wire placed in uniform magnetic field. Force between two parallel current carrying wire and definition of one Ampere . Torque acting on a current carrying coil , and expression for potential energy stored in the coil placed in uniform magnetic field . Moving coil galvanometer its current sensitivity and conversion of galvanometer into ammeter and voltmeter .
Chapter 5: Magnetism and matter-
Earth behaves like magnet , pole strength, magnetic dipole and magnetic moment . Expression for torque and potential energy when a magnetic dipole( short magnet) placed in uniform magnetic field . Current loop as magnetic dipole and its magnetic moment , current carrying solenoid behave as short bar magnet , magnetic field intensity at a point due to magnetic dipole (short bar magnet) at axial, equatorial and any point . Magnetic field lines and its properties . magnetic moment due to revolving electrons in an atom Bohr magneton . magnetic elements of earth and relation between them . Di- para-ferro magnetic substance ( with examples) and its properties . Electro magnet, permanent magnets and factor effecting its strength and material suitable for making them .
Electromagnetic induction and alternating current-
Chapter 6: Electromagnetic induction-
Electromagnetic induction, Faraday’s law , induced emf and current , Lenz’s law , Eddy current . self and mutual induction , expression for self- induction of current carrying solenoid , expression for mutual induction due to coaxial solenoids , units and dimensions of self and mutual induction . grouping of inductors (series and parallel ) .
Chapter 7: Alternating current-
Alternating current. Peak, mean and rms( root mean square ) current/emf . Phasor diagram . phase relation and its phasor diagram when an AC source connected to only resistance, only inductor, only capacitor, resistance and inductor connected in series, resistance and capacitor connected in series , and resistance , capacitor and inductor ( LCR) connected in series , also discussion of reactance and impedance . resonance circuit of LCR and discussion of Quality factor ( sharpness of resonance ) .power in AC circuits . power factor . Watt-less current . Ac generator and transformer .
Electromagnetic wave ( EMW)-
Chapter 8: Electromagnetic wave-
Production of Electromagnetic wave their characteristics their transverse nature ( qualitative idea only ) , displacement current, Maxwell’s improvement of Ampere’s law . Electromagnetic spectrum their production ,properties and uses ( elementary facts )
6.Optics-
Chapter 9: Ray optics and optical instruments –
Reflection of light , spherical mirror , mirror formula , refraction of light , total internal reflection and its application , optical fibres , refraction at spherical surface , lenses thin lens formula , lens maker’s formula , magnification , power of a lens , combination of lens , refraction and dispersion of light through the prism . Scattering of light . Optical instruments ( microscope , telescope ( refracting and reflecting)) and expression for magnifying power .
Chapter 10: Wave optics-
Wave-front , Huygen’s principle , reflection and refraction on the basis of wave theory using Huygen’s principle , interference of light , conditions of constructive and destructive interference using YDSE also expression for fringe width . Diffraction of light using single slit experiment . Width of the central maximum , difference between diffraction and interference. Resolving power of microscope and limit of resolution . polarization , plane polarized light, law of malus , Brewster’s law uses of plane polarised light and polaroids .
Dual nature of radiation and matter –
Chapter 11: Dual nature of radiation and matter –
Emission of electron , photoelectric effect, Hertz’s and Lenard’s observation , effect of intensity and frequency on photoelectric current, stopping potential and threshold frequency , Einstein’s photoelectric equation also the possible graph using these equation . Particle nature of light . Matter waves . Wave nature of particles . De-Broglie relation . Davisson- Germer experiment( only conclusion).
Atoms and nuclei –
Chapter 12: Atoms –
Alpha particle scattering experiment , Rutherford’s model of atom , Bohr’s model, expressions for radius , velocity , total energy and time period ( frequency ) , energy levels , hydrogen spectrum .
Chapter 13: Nuclei –
Atomic mass unit . Composition and size of nucleus , Radioactivity, alpha-beta and gamma emission and their properties . radioactive decay law . half life and mean life . Activity of a radioactive substance . mass defect , binding energy , binding energy per nucleon and its variation with mass number . nuclear fission and fusion .
Electronic devices –
Chapter 14: semiconductor devices and simple circuit-
Energy band in conductors, semiconductors and insulators . Semiconductor diode ( both p-type and n-type) . formation of deplation layer , potential barrier . biasing of diode . forward and reverse characteristics . Diode as rectifier ( full wave and half wave ) . application of diode ( Zener , photo,light emitting diode and solar cell )
In this topic we will discuss about the Coulomb’s law of electrostatic force between two charged particle separated by some distance .
Coulomb’s law –
According to Coulomb’s law , the force of attraction or repulsion between two charged body is directly proportional to product of magnitude of their charges and inversely proportional to square of distance between them .
If Q1 and Q2 are two charges placed at distance ‘r’ from each other , then magnitude of force will be given as,
F α Q1.Q2 ………. (I) and
F α 1/ r2 …………(ii)
After combining these two we get
F α Q1.Q2/r2
Or F = K Q1.Q2/r2 where K is a constant given as
K= 1/4∏ϵ0 = 9 x 109 here ϵ0 is called permittivity in vacuum or air .
ϵ0 =8.85 x 10-12 C2‑N-1m-2
Electric permittivity of a medium is a measure of how the molecules of substance align or polarize under an electric field . The substance which has higher electric permittivity then its molecules polarization is maximum , and the more that substance resist the external electric field . It is denoted by ϵ . its unit is C2‑N-1m-2
Dielectric constant or relative permittivity – Dielectric constant of a given medium is the ratio of permittivity of the medium to the permittivity of free space (vacuum /air )
Or
Dielectric constant of a given medium is the ratio of force of interaction between two charges separated by a certain distance in vacuum/air to the force of interaction between two given charges at the same distance in that medium .
Dielectric constant K ( ϵr ) = ϵm / ϵo = F0 /Fm where ;
F0 = force of interaction between two charges separated by a certain distance in vacuum/air
F0 = force of interaction between two charges separated by a certain distance in the given medium
*Dielectric constant is unit less .
To watch the video of relative permittivity and charge density click on the link given below –
What is electric charge ? And what are the properties of charge ?
in this topic we will discuss that, What is electric charge ? And what are the properties of charge ? We will also discuss that what are the processes of charging of an objects ?
This topic will help the students of class 12th and 10th to understand the topic, What is electric charge and what are the properties of charge ?
What is electric charge ? how can we charge a body? And what are the properties of charges ?
Electric charges – Charge is the intrinsic property of the body which is possessed by the body itself , due to which electrostatic force comes into play .
There are two types of charge positive (+ve ) i.e. proton and negative (-ve ) i.e. electron which is given by the Benjamin Franklin .
Processes to charge a body –
Rubbing –
i) – A glass rod rubbed by a piece of silk brought closer to another glass rod rubbed by the silk cloth repel each other . But these glass rods attract the silk cloth with which it was rubbed . In this case the glass rods gets positive charge and silk cloths gets negative charge .
ii)- When an Ebonite / Amber rods are rubbed with a fur then Ebonite rod gets negative charge and fur gets positive charge.
Direct contact ( conduction)
iii)- When a pith ball is kept in contact with the rubbed glass or rubbed Ebonite rod pith balls get same kind of charge with which it is connected .
It can be understand with the help of following figure-
charging of bodies
Induction – when a charge body ‘A’ is kept near an uncharged body ‘B’ then some charge of opposite sign appears on the near face of body ‘B’ , and equal charge of opposite sign is appear on the another face of body B . so we can say the process of charging of a body without actual contact is called induction .
Let us understand how can we charge a body by induction ; Suppose a positively charged glass rod is kept near an uncharged body ‘B’ , then free electrons of B comes on the surface near the body A but positive charge appear on the opposite face ( because sphere on whole remain neutral ) as shown in fig. again when the sphere B is grounded then electrons move from ground towards the sphere which neutralise the positive charge on the other side of the sphere . The negative charge at the near end remain on the sphere , and when rod A is removed the negative charge appears on the complete sphere . And finally body B gets negatively charged .
charging a body due to induction
Similarly we can see in fig. how can we charge two spheres by induction .
To view its video click on the video link given below –
Properties of charges –
When we see the several experiments carefully we will come to know that the following properties shown by the charged bodies –
(A) Like charges repel and unlike charges attracts , which is finally explained by the Coulomb . According to Coulomb’s The force of repulsion or attraction between charged body is directly proportional to the products of magnitude of charges and inversely proportional the square of the distance between the charged body .
(B) Quantisation of charge – It is the property of the charged particle due to which , the charge on a body is integral multiple of basic unit of charge i.e. electron (-e) or proton (+e) . It is given that charge on a body Q = n. e : where n is the integer . and e= 1.6 x 10-19 C .
( C ) Additivity of charge – It is the property of charge by virtue of which total charge of a system is obtained simply by adding algebraically all the charges present in the system . If q1 , q2 , q3 …… are the charges on the particle of the system then total charge on the system q= q1 + q2 + q3………. We should use proper sign of the charge during the addition of charges .
(D) Conservation of charge – According to this property total charge of the isolated system always remain constant . i.e. charge neither be created nor be destroyed but it can transfer from one body to another . When we rub a body then equal number of charges of opposite sign appear on the surfaces of these bodies .
To view its video click on video link given below –
This is the third sample paper physics class 12 published by physics classes by Nayan Jha . This question paper is totally based on new pattern (2020) applied by the CBSE , it will help the students to practice well before the examination . Physics classes by nayan jha already launched two sample paper previously .
Sample paper physics class12 (Third sample paper)-
1.If ɸ E.ds =0, a surface, that means:- (a)There is net charge present inside the surface (b)Uniform electric field inside the surface (c)Discontinuous field inside the surface (d)Charge present inside the surface
2.A parallel plate capacitor is charged by a battery. Once it is charged battery is removed now a dielectric material is inserted between the plates of the capacitor, which of the following does not change? (a)Electric field between the plats(b)Potential difference across the plates(c)Charge on the plates(d)Energy stored in the capacitor.
3.In a meter bridge experiment, resistance box(with R=2 Ω) is connected in the left gap and the unknown resistance S in the right gap. If balancing length be 40cm, calculate the value of S. (a)2 Ω (b)3 Ω (c)4 Ω (d)5 Ω
4.For a cell of emf 2V, a balance is obtained for 50cm of the potentiometer wire. If the cell is shunted by a 2 Ω resistor and the balance is obtained across 40cm of the wire, then the internal resistance of the cell is ; (a)1Ω (b)5 Ω (c)2 Ω (d)5 Ω
This assignment contains all important questions for class 11th physics.It will help the students to revise all questions in a short period of time .
If (p+ a/V2) (V-b)=RT where the symbols has their usual meanings, then what is the dimension of a/b .
If F= a/ b+√d , where f=force ,d= density , then find the dimension of a and b.
The factors effecting the time period (T) of the simple pendulum depends on mass , length and acceleration due to gravity . Deduce the relation for time period of the simple pendulum.
To determine acceleration due to gravity ,the time of 20 oscillations of a simple pendulum of length 100 cm was observed to be 40 s . calculate the value of ‘g’ and maximum percentage error in the measured value of ‘g’ .
If power P=a-x2/b where x represents displacement find the dimension of a and b.
A physical quantity Qis given by Q= A2.B3/2/C4D1/2 percentage error in A,B,C,D are 1%, 2%,4%,2% respectively . Find the percentage error in Q.
Write the expression for distance covered in nth second by uniformly accelerated body.
A ball is thrown vertically upwards. Draw its:
Velocity-time curve
Acceleration-time curve.
Draw velocity -time graph of uniformly accelerated motion in one dimension .From the velocity time graph of uniform Accelerated motion ,deduce the equation of motion in distance and time.
The displacement (in metre) of a particle moving along x-axis is given by
x=18 t + 5 t2.Calculate :
The instantaneous velocity at t=2s,
Average velocity between t=2s and t=3s, and
Instantaneous acceleration.
Derive an equation for the distance covered by a uniformly accelerated body in nth second of its motion. A body travels half its total path in the last second of its fall from rest . Calculate the time of its fall.
12.A body moving with a uniform acceleration describes 12 m in 3rd second of its motion and 20 m in the 5th second . find the velocity after 10 seconds.
A car travels first half of a length S with velocity u1. The second half is covered with velocities u2 and u3 for equal time intervals .Find the average velocity of the motion.
A body start from rest and accelerates uniformly .find the ratio of the displacement in, (1)one, two and three seconds, (2) first ,seconds and third second.
