12th Class Physics Chapter 3 Electromagnetism Short Question Answers

When the plane of the loop is held perpandicular to the direction of the magnetic field the maximum flux will pass through it As we know:
φ_e =B^→ΔA^→=BΔA………………….. (1)
Because the field is directed along the normal to the surface area so θ= 0 and cos0ᵒ = 1 Thus the flux will  be maximum (Eq.(1)
When the plane of the loop is parallel to the direction of magnetic field the minimum flux will pass through it(as show in fig b.)
As φ_e = B^→.ΔA^→
= BΔAcos90ᵒ
or φ_e = BΔA×0=0
φ_e = 0…………………..(2)
Because the along between the field and normal to the surface area is 90ᵒ i.e. θ =90ᵒ =0 so the flux through the loop will be minimum (equ.2)

A stationary charge cannot produce any magnetic field but it produces only the electric field.In case of static charge the rate of flow of charge is zero so there will be no magnetic field. As moving charges produced around the path of its motion similar to the magnetic field produced around a current carrying conductor.

When current is passed through the coil of a solenoid a magnetic field is produced by it as shown in fig a.The field inside the solenoid is very strong and uniform because a the line of force are continuous at its center.Whereas the field outside the solenoid is negligible weak but not equal to zero outside the solenoid because the lines of force are opposite to each other along the outer surface.Such as the field set up the upper part of the solenoid tends to cancel the field set up by the lower in the opposite direction.

As the magnetic field is directed into the plane of paper that is negative z – direction then the proton moves along the positive x- direction as shown in the fig.Therefore the magnetic force on proton will be given by
F^→=q(^→v×B^→)
According  to R H.rule this Force is directed along y – axis.
No the proton will not continue to move in the positive x-direction. Since the magnetic force is at right angle to the motion of the proton therefore it will move along a circular path in x-y plane.Thus the magnetic force (F = Bqv)provides the centripetal force.

When two charged particles are projected in a magnetic field at right angle to the direction of their velocities then their path will become curved or circular due to the magnetic force acting on them.If the charged particles are deflected in opposite direction then the particles are oppositely charged.Positively charged particle will be deflected in the opposite direction.It means if one particle is proton then the other one is electron.

The magnetic force acting on charged particle is given by
F^→ = q (v^→×B^→)
This force  F^→ is always perpendicular to the direction of motion of the charged particle.Therefore the angle between force F^→ and displacement d^→ is 90ᵒ i.e. θ= 90ᵒ
As   work = F^→d^→=Fd cos θ
Or    work = Fd cos90ᵒ
= Fd×0
or     W = 0
It means that magnetic force is only a deflecting force but it cannot do any work.

If the charged particle is moving in a certain region but its  direction does not change then it is possible that magnetic field is not present i.e.F^→ = 0 Then there may be another possibility that charged particle is moving either parallel or anti- parallel to the magnetic field.In both these cases θ = 0ᵒ
Magnitude of magnetic force = F =qv Bsin θ
Or    F = qv Bsin0 =0
F=0
In anti- parallel case θ = 180ᵒ
Thus     F = qvBsin 180ᵒ
qvB×0
or F= 0
Hence the charged particle does not change its path as B= 0  or  θ =0ᵒ or 180ᵒ.

The picture on a TV screen is formed by the help of beam of electrons.When a magnet is brought near the screen the beam of electrons will be deflected due to the magnetic field.Thus the target of the electron beam will be disturbed.As a result the picture on a TV screen becomes distorted.

When a current  carrying loop is placed in a uniform magnetic field a torque is produced in a loop due to the current.If the loop is deflected in a given region of space then it confirms the presence  of magnetic field otherwise not.If magnetic field B^→ makes an angle α with the plane of the loop the magnitude of the torque is given by
τ= BIANsos α.

When a current carrying loop is placed in a magnetic field it  experiences a torque which is given by
τ = BIN Acos α
where α is the angle between B^→ and plane of the loop.
When plane of the  loop  is placed at right  angle to magetic  field B^→ then angle α = 90ᵒ
or     τ = BIAN cos90ᵒ
τ  = BIAN×0=0
τ  =0
As the value of torque is zero in uniform magnet field so the loop will not tend to rotate.

When a current carrying coil is placed in a uniform magnetic field a torque is produced.If the plane of coil makes an angle α with teh magnetic field B then torque  is given by
τ = BIAN cos α…………….. (1)
If plane of coil is parallel to magnetic field α =0 and torque will be maximum  because cos0ᵒ = 1 that is
τ = BINA cos α = BIAN cos0
or τ =BINA…………….(2)
Equ (2) shows that torque on coil will be maximum.
Torque will be  minimum if the plane of the coil is held perpendicular to the magnetic field i.e. α = 90ᵒ so cos90ᵒ =0
From equ (1)
τ = BIAN cos 90ᵒ
= BIAN × 0=0
τ=0…………………..(3)
Equ (3) shows that torque on coil will be minimum.

In this case there are two positions.As shown in fig A if the current flows in anti- clockwise direction then the magnetic field produced by coil and that of the magnet are both in the same direction and so the coil flows in opposite direction then magnetic field produced by the coil will oppose that of the magnet.In that case the coil will rotate and take up the position of fig A. In case of alternating current the coil may start rotating with the frequency of alternating current(50 cycles in Pakistan).

The resistance of an ammeter should be very low become it is a always connected in series with the circuit.An ammeter must have very low resistance so that it may not change the total value of current passing through the circuit.Thus an accurate value of the current through the circuit is measured.

The voltmeter is always connected in parallel with the point across which potential difference is to be measured.The resistance of voltmeter should be very high so that it may draw a very small value of current from the circuit otherwise it will load the circuit and will change the potential difference which is required to be measured.

A line of magnetic force is the path along which a tiny north pole would move in a magnetic field.Each line force indicates the direction of the magnetic field.If two lines of force cross each other then the field will have two directions at the point of inter- section along the two lines which is impossible.Hence two lines of force can never cross each other.

It is possible to magnetise by suspending it in the field of earth if a piece of magnetic material is supended parallel to the earth’s field and tapped gently it will be magnetized but its magnetism will be very weak.

According to modern theory the magnetism in material bodies is due to the motion of the electrons present in their atoms.These revolving electrons constitute circular currents and the north pole is merely one side of such current loops.The other side of the loop will always be a south pole and thus we can not obtain an isolated north pole.

A moving proton constitutes a current these fore it must produce a magnetic field.But the motion of a proton is highly restricted in the nucleus.So the magnetic field produced by it is very weak.

The flux density of a field is denfined  as the number  of lines of magnetic force passing through it area of a surface  held perpendicular to the field.The flux density is also known as magnetic induction and it is denoted by B.
Flux:-The total number of lines of force passing through a certain  area are know collectively as flux.
Magnetic flux is denoted by φ_B
Relation between B and φ_B
Consider a magnetic field having flux density B.if we want to find out the flux  through a certain vector area ΔA it will be equal to the dot  product of B and ΔA i.e.
φ_B = B. ΔA = B ΔAcos θ
where θ is the angle between  the direction of B and ΔA.

The magnetic lines of force will point anti clock – wise around the current carrying wire according to Right Hand Rule.

No  as a charge at rest  experiences no force in a magnetic field because F = q v Bsinθ if v = 0 F=0.

No we cannot say that the magnetic field in that region is zero because a charged particle can move in a straight line through some region of magnetic field if the direction of motion of the charged particle is parallel to the direction of the magnetic field.

When direct current passes through a loop a magnetic force acts on the two side in the opposite direction.These forces form a torque of couple due to which the loop is deflected from its mean position.On the other hand if A.C.is passed through the loop it reverses the detection of the torque of the couple after half cycle.So it will oscillate about its mean position.

The current  carrying wire should be placed paralled to the magnetic field to make θ =0 in the formula for magnetic force
F= BIL sin θ
F= BILsin0=0
Force will be zero
The current carrying wire should be placed perpendicular to the magnetic field to make θ =90
The formula for magnetic force.
F= BIL sin θ
F= BIL sin90
F = BIL
So the force  on it will be maximum.

Since the two wires have equal currents in magnitude but opposite in directions so the direction of the magnetic field produced around one wire be opposite to the magnetic field produced around the other. Hence there will be no resultant magnetic field from a pair of twisted wires because stary magnetic field produced by them cancel each other’s effect.

The direction of current at upper and lower end is opposite to each other.The direction of both the magnetic fields are along the same direction inside the loop thus the field increases.In addition to it all the lines of magnetic field are confined to a small area.But magnetic lines of force outside the loop scatter in vast area so the strength of magnetic field becomes weak.

Flux through a rectangular coil:- The flux passing through any small vector  area ΔA when it is held in a magnetic field of flux density B is given by
φ_B =B.ΔA = BΔA cosθ
When B and ΔA are  the magnitudes of B and ΔA and θ is the angle between the direction  of B and normal  to the surface of ΔA.
Let the area of the coil be ΔA and it is held in a magnetic field of flux density B.
i) If the coil is held  parallel  to the magnetic  field the angle between the direction of B and normal to the plane of coil will be 90ᵒ i.e. θ = 90ᵒ Therefor  the flux through the coil is
φ_B = BΔAcos 90=0
ii) If the coil is held perpendicular to the magnetic  field then θ=0ᵒ and therefore flux through the coil is.
φ_B = BΔAcosθ = BΔA
i.e the flux through the coil will be maximum.
iii) If the coil is held at an angle  of 45ᵒ then θ= 45ᵒ and the flux through the coil is
φ_B = BΔAcos45 = BΔA ×.707= .707BΔA

Magnetic field due to a current – carrying straight wire.The magnetic field produced by a straight wire carrying a current will consist of circular lines of magnetic force.These lines of force are concentric with their center at the wire.The direction of these line of force i.e.the magnetic field can be found out by the right hand rule.The field is always stronger close to the wire and becomes weaker and weaker as the distance from the wire increases.

Magnet attracts magnet- When a magnet is placed near another magnet the magnetic lines of force due to the two magnets attract each other and the magnets are said to be attracting each other. As non- magnetic substances do not have lines of magnetic force they are not attracted by the magnets.The magnetic substances such as iron which are attracted by the magnets are magnetized by the arrangement of their domains in the presence of any magnet.

Deflection of an electron and proton in a magnetic field:When an election or proton is made to move at right angles to a given magnetic field the force acting on it is given by F= B ve Where B= strength of the magnetic field, e = charged on the particle and, v = speed of the particle, since this force supplies the necessary centripetal force therefore B ve Or r= mv/Be Keeping v constant for both the particle, r α m This shows that the radius of the circular path for the proton will be larger because it is nearly 1800 times heavier that an electron.Thus the electron being lighter will be deflected more because r will be smaller for it as compared to a proton.

When electric current flows through a solenoid its one end becomes north pole while the others as south pole.This can be verified by using a compass needle or suspended bar magnet.Thus the magnetic field due to current flowing through a solenoid is similar ti the magnetic field due to bar magnet.

The polarity of the end of a solenoid can be determined by the following method.On looking at the end of the solenoid if the current appears to flow in the anticlockwise direction then ties ends is north pole and if it is flowing in clockwise direction then the end is south pole.

Lamp and scale Arrangement:- In a sensitive galvanometer the angle of deflection is observed by means of a small mirror attached to the coil along with a lamp and scale arrangement as shown in the fig.A beam of light from the lamp is directed towards the mirror of the galvanometer.After reflection from mirror it produces a spot on a translucent scale placed at a distance of one meter from the galvanometer.When a current is passed through the coil it is rotated and it produces a twist in the suspension wire.The twisting wire rotates the mirror and the spot on the scale is proportional to the angle of deflection provided θ is small.

A small resistance placed in parallel to any instrument say a galvanometer is called shunt.It provides an additional path for the flow of current.Thus a galvanometer is used to convert a galvanometer into an ammeter.

If a current is flowing through a wire a magnetic field is produced around it.Place a compass needle on the carpet.The compass needle will show some deflection only if the current is flowing through the wire otherwise not.

If the suspension wire is thin it will have a small value of c when the current passes through the circuit the pointer of the sensitive galvanometer vibrates about its mean position due to small value of c and large value of B,A and N. It comes to rest after vibrating for a longer time.Thus the galvanometer is not stable due to small value of c and large moment of inertia of the pointer.

A voltmeter is connected in parallel so that it may be able to draw only a small current from the circuit sufficient to produce deflection and the potential difference to be measured should not change to large value.If it is connected in series with the circuit it chokes the current due to its high resistance.

An ammeter is a very low resistance galvanometer and it is connected in series with a circuit to measure the current passing through it.It is connected in series so that the total current passing through the circuit should pass through it because the same current passes through all the resistors which are connected in series.If however it is connected in parallel with the circuit a part of the total current will pass through it and thus it will not measure the current through the circuit accurately.

To measure the current and voltage of an electric lamp an ammeter should be connected in series and a voltmeter in parallel with the filament of the lamp.If however the meters are inter – changed that is the ammeter is connected in parallel and voltmeter in series with the filament the lam will not glow or its light will be very very dim because most of the potential of the battery will fall across the voltmeter will give only the value of the voltage drop across it and not that of the lamp where the ammeter will not give any reading because negligible current will pass through it.

The hair springs in the Weston galvanometer keeps the coil in zero position.The hair springs also provide path for the current to enter and leave the coil.

The characteristic of a good voltmeter is that it draws very small current.This is possible only if the resistance of the voltmeter is very high.If the resistance of voltmeter is small then a large current flows through it.Therefore the potential difference to be measured is reduce.

As an ordinary voltmeter draws some current from the circuit so it cannot measure the potential difference across a resistor correctly.In this case the actual value of P.D. across the resistor will decrease. For accurate measurement of P.D the voltmeter should not draw any current from the circuit which is possible only if voltmeter has infinite resistance.

Following are the main parts of a CRO. 1) Filament ‘F’ which is used for the emission of electrons. 2) Cathode ‘C’ which is heated directly by ‘F’. 3) Grid ‘G’ which is used for brightness control. 4) Anodes A1, A2, A3, which are used for focusing and accelerating electrons. 5) XX = Horizontal deflecting plates. 6) YY = Vertical; deflecting plates. 7) Fluorescent screen.

The electrons emitted by the heated filament of CRO are first accelerated and are attracted towards positive anode.These accelerated electrons pass through the pair of horizontal and vertical deflection plates (XX and YY) and hit the fluorescent screen held perpendicular to the path of electrons.Here the kinetic energy of electrons is converted into the light energy on the fluorescent screen.

The working principle of a galvanometer is based upon the torque acting on the coil of a galvanometer placed under the effect of uniform magnetic field.

A dead beat galvanometer is the most sensitive galvanometer to be used to detect current in part o f a micro – ampere.Such a galvanometer in which the coil comes to rest quickly after the current passes through it or the current is stopped from flowing through it is called stable or dead beat galvanometer.

It indicates that an AVO meter cam measre. i) A means Amperes that is for the measurement of current. ii) V means Volts that is for the measurement of voltage. iii) O means Ohm that is for the measurement of resistance. Thus an AVO meter can be used to measure current voltage and resistance of any circuit.It is also called as multi meter.

DMM means digital multi meter.It is an advance form of an AVO meter and can effectively measure current voltage and resistance points with relevant unit of each measured quantity.These metres are easier to use because they eleminate the human error.