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Chapter 14 Electromagnetism Physics 12th Class Punjab Short Questions

Question No. 1 - Define ammeter and voltmeter.

Answer No. 1 - An ammeter is an electrical instrument which is used to measure current in amperes.
A voltmeter is an electrical device which measure the potential difference in volts between two points.

Question No. 2 - Define Tesla and write its formula.

Answer No. 2 - If a magnetic field exerts a force of 1N on 1m length of the conductor placed at right angles to the magnetic field carrying a current of 1A then the strength of magnetic field is said to be one Tesla.
1 T = 1 NA-1 m-1

Question No. 3 - Distinguished between magnetic flux and magnetic flux density.

Answer No. 3 - Magnetic Flux: The number of magnetic lines of force passing through certain elements of area is called magnetic flux. Magnetic flux is a scalar quantity.

Question No. 4 - What is Cathode ray oscilloscope?

Answer No. 4 - Cathode ray oscilloscope is a high speed graph plotting device. It is called Cathode ray oscilloscope because it trace the desired waveform with a beam of electrons which are also called cathode rays.
It mainly consists of:
1. Electron Gun
2. Vertical Deflection Plate
3. Horizontal Deflection Plates
4. Fluorescent Screen

Question No. 5 - What is the function of grid in cathode ray oscilloscope?

Answer No. 5 - Grid is a negative potential relative to cathode. It controls the number of electrons reaching the screen and thus controls the brightness of spot on the screen.

Question No. 6 - State the right hand palm rule.

Answer No. 6 - If the middle finger of the right hand points in the direction of the magnetic field the thumb in direction of current, the force on the conductor will be normal to the palm towards the reader.

Question No. 7 - What modification is required to convert a galvanometer into ammeter?

Answer No. 7 - A shunt is connected in parallel with galvanometer to convert it into ammeter.

Question No. 8 - State amperes law.

Answer No. 8 - Amperes law states that the sum of the quantity for all path elements into which the complete loop has been divided equal times the total current enclosed by the loop.

Question No. 9 - On what factors the induced currents due to motional emf depend.

Answer No. 9 - The current can be increased by the following factors :
1. Using a stronger magnetic field.
2. Moving the loop faster.
3. Replacing the loop with a coil of many turns.

Question No. 10 - Define electromagnetism and give the name of one device in which electromagnetism is used.

Answer No. 10 - The branch of physics which deals with electricity and magnetism and the interaction between them is known as electromagnetism.
Electromagnetism is used in doorbells, electric motors such as electric fan etc.

Question No. 11 - Why the voltmeter should have the very high resistance?

Answer No. 11 - A voltmeter is connected in parallel to the resistor to measure potential difference across it. It should have very high resistance so that practically a very little current should pass through it and the current of the circuit should almost remains constant,so that it might measure the potential difference across a resistor accurately.

Question No. 12 - Two charged particle are projected into a region where there is a magnetic field perpendicular to their velocities. If the charge is deflected in opposite directions, what you can say about them?

Answer No. 12 - When a charged particle is projected in a magnetic field, it will experience the magnetic force. The magnetic force is a deflecting force. Thus if the charged particle are deflected in opposite direction, then particle are oppositely charged i.e. one particle is positively charged and the other is negatively charged.

Question No. 13 - A current in a conductor produce a magnetic field, which can be calculated by using Amperes law. Since current is defined as the rate of flow of charge, what can you conclude about the magnetic field due to stationary charges? What about moving charges?

Answer No. 13 - In case of stationary charges, the rate of flow of charges is zero. So there will be no magnetic field. But the moving charges produce current so the magnetic field produced around a current carrying conductor.

Question No. 14 - What is digital multi-meter? Give its two advantage over AVO meter.

Answer No. 14 - It is a digital version of AVO meter. It is used to measure resistance, voltage and current.
Advantage:
1. The digital values are displayed automatically with
decimal point, polarity and the unit for voltage,
current and resistance.
2. These meters are generally easier to use because they
eliminate the human error that often occur in reading
the dial of an ordinary AVO meter.

Question No. 15 - Write the formula used to convert a galvanometer into voltmeter. Why the resistance of voltmeter should be high?

Answer No. 15 - Galvanometer can be converted into ammeter using this relation/Rh=V/Ig-Rg
A voltmeter is connected in parallel to the resistor to measure potential difference across it. It should have very high resistance so that practically a very little current should pass through it and the current of the circuit should almost remain constant, so that it might measure the potential difference across a resistor accurately.

Question No. 16 - What is dead beat galvanometer?

Answer No. 16 - The galvanometer in which the coil comes to rest quickly after current passed through it or the current is stopped from flowing through it. This is called stable or a dead beat galvanometer.

Question No. 17 - Define energy.

Answer No. 17 - The magnetic energy stored in the conductor per unit is referred as energy density.

Question No. 18 - Define galvanometer and cathode ray oscilloscope.

Answer No. 18 - Galvanometer: Galvanometer is a device used to detect the passage of current.
Cathode ray oscilloscope: Cathode ray oscilloscope is a high speed graph plotting device.

Question No. 19 - What should be orientation of a current carrying coil in magnetic field so that the torque acting upon the coil is: a Maximum b Minimum.

Answer No. 19 - The torque experienced by a current carrying loop when placed in magnetic field is t=NIBA cos alpha.
When plane of the coil makes and angle of 0 degree with magnetic field, the torque on the coil will be maximum.t=NIBA.
When plane of the coil makes and angle of 90 degree with magnetic field the torque on the coil will be zero or minimum.
t=NIBA cos 90 degree
t=0.

Question No. 20 - What is lamp and scale arrangement in galvanometer?

Answer No. 20 - In sensitive galvanometer,the angle of deflection is observed by means of a small mirror attached to the coil along with the lamp and scale. A beam of light directed towards the mirror of galvanometer. After reflection it produce a spot on the screen. The scale provides the small angle of deflection.

Question No. 21 - Discuss briefly digital multi meter.

Answer No. 21 - It is an digital version of an AVO meter. It is used to measure resistance voltage and current. It has become very popular testing device because the digital values are displayed automatically with decimal point, polarity and the unit for voltage current and resistance.

Question No. 22 - How can you use a magnetic field to separate isotopes of chemical elements?

Answer No. 22 - Sinci isotopes of an elements have same charge number but different mass number. As we known that the radius of a charged particle inside a magnetic field is given as,
r=mv/qB
It shows that the isotopes projected from the same point at right angle to the magnetic field B will follow circular path of different radii due to their different masses, So they can be distinguished easily.

Question No. 23 - Distinguished between sensitive and dead beat galvanometers.

Answer No. 23 - A galvanometer which shows large deflection for a small value of unit current is sensitive one whereas a galvanometer in which the coil suddenly comes to rest after the current passed through it or the current is stopped from flowing through it is called dead beat out stable galvanometer.

Question No. 24 - What is Time base generator?

Answer No. 24 - A voltage that is applied across X plates is usually provided by a circuit that is build in CRO and is called sweep or time base generator. Its output waveform is a saw tooth voltage of period T.

Question No. 25 - Name the main part of CRO.

Answer No. 25 - A filament, cathode grid, anodes, horizontal deflections plates, vertical deflection plates and fluorescent screen.

Question No. 26 - Why soft iron cylinder is placed inside the coil of galvanometer?

Answer No. 26 - The soft iron cylinder makes the magnetic field stronger and radial such that into whatever position the coil rotates, the magnetic field is always parallel to its plane.

Question No. 27 - Define magnetic flux and solenoid.

Answer No. 27 - The number of magnetic field lines passing through certain elements of area is known as magnetic flux through that surface.
A solenoid is a long tightly wound, cylindrical coil of wire, when current passes through such a coil, it behaves like a bar magnet.

Question No. 28 - Define magnetic flux and write its unit.

Answer No. 28 - The number of magnetic lines of force passing through certain area element is called magnetic flux. Magnetic flux is a scalar quantity and its SI unit is NmA-1which also called weber Wb.

Question No. 29 - How can galvanometer be made more sensitive?

Answer No. 29 - Since I=c/BAN theta
A galvanometer can be made more sensitive if c/BAN is made small.
Thus, to increase the sensitivity of galvanometer, c may be decreased or B,A and N may be increased.

Question No. 30 - Define Lorentz Force.

Answer No. 30 - The combined effect of electric force and magnetic force exerted on charged particle is called Lorentz force.

Question No. 31 - How the beam of electron is focused on the screen of CRO?

Answer No. 31 - CRO works by deflecting beam of electrons as they pass through uniform electric field between the two sets of parallel plates. The deflecting beam then falls on a fluorescent screen where it makes a visible spot.

Question No. 32 - What is Lorentz force?

Answer No. 32 - The vector sum of electric force and magnetic force is called Lorentz force.

Question No. 33 - How can a current loop be used to determine the pressure of a magnetic field in a given region of space?

Answer No. 33 - The torque is experienced by a current carrying loop when placed in magnetic field is t=NIBA cos alpha.
If the loop is deflected in a given region, then it confirms the presence of magnetic field otherwise not.

Question No. 34 - Why the resistance of an ammeter should be very low?

Answer No. 34 - An ammeter is connected in a series with a circuit to measure the current. It is connected in series so that total current passing through the circuit should pass through it. If the resistance of the ammeter will be large, it will alter the current of the circuit to great extent and the measurement of current will not be accurate.

Question No. 35 - What is current sensitive of a galvanometer?

Answer No. 35 - A galvanometer which gives large deflection for the small current is called a sensitive galvanometer. The degree of deflection of galvanometer by unit current passing through it is referred as it sensitivity. It can be made more sensitive.

Question No. 36 - Is it possible to orient a current loop in a uniform magnetic field such that the loop will not tend to rotate? Explain

Answer No. 36 - The torque experienced by a current carrying loop when placed in magnetic field is t=NIBA cos alpha.
Clearly when plane off the coil makes and angle of 90 degree will magnetic field, the torque on the coil will be zero. In this condition the coil will not tend to rotate.

Question No. 37 - How can you explain the waveform of various voltage formed in CRO?

Answer No. 37 - We can easily find the instantaneous value and peak value of the voltage with the help of calibration of y axis in volts. The time period can also be determined by using the time calibration of x axis information about the phase difference between two voltage can be obtained by simultaneously their wave forms.

Question No. 38 - Can a charge at rest be set into motion by bringing a magnet close to it?

Answer No. 38 - No a charge at rest can not be set into motion by bringing a magnet close to it.

Question No. 39 - Define stable galvanometer and Ohmmeter.

Answer No. 39 - A galvanometer in which the coil comes to rest quickly after the current passed through it or the current is stopped from flowing through it is called Stable or Dead Beat galvanometer.
Ohmmeter is a useful device for rapid measurement of resistance. It measure resistance in Ohm.

Question No. 40 - What is B non zero outside a solenoid?

Answer No. 40 - The magnetic field outside a solenoid is not zero. This is only true for infinitely long solenoid. Infinitely long solenoid can not be found in nature. The magnetic field outside a real solenoid is less dense that inside the solenoid and often one is only concerned with the field inside which is approximately constant.

Question No. 41 - What is AVO-meter? Explain.

Answer No. 41 - It is an instrument which can measure current in amperes potential difference in volts and resistance in ohms. It basically consists of sensitive moving coil galvanometer which is converted into multi range ammeter, voltmeter or ohmmeter accordingly as a circuit.

Question No. 42 - Is it possible to obtain an isolated north pole?Give reason.

Answer No. 42 - No,the source of magnetism of an atom is the electrons. Accepting this view of magnetism it is concluded that it is possible to obtain an isolated north pole. The north pole is merely one side of a current loop. The other side will always be present as south pole and these can not be separated. This is an experimental reality.

Question No. 43 - Briefly give the function of Filament, Cathode, Grid and plates in C.R.O.

Answer No. 43 - Filament: It heats the cathode.
Cathode: It emits the electrons.
Grid: It controls the number of electrons.
Plates: The two sets of plates are used to deflect the beam of electrons along X axis and Y axis.

Question No. 44 - If a charge particle moves in a straight line through some region of space can you say the magnetic field in the region is zero?

Answer No. 44 - The magnitude of a magnetic force on a charge particle is F=qVB sin theta
Magnetic field will be zero due to the following reasons.
1. Magnetic field strength B in the region is zero.
2. Magnetic field is parallel or anti parallel to the
direction of motion.

Question No. 45 - Write any two use of CRO.

Answer No. 45 - 1. The CRO is used for displaying the waveform of given
voltage.
2. Once the waveform is displaying, we can measured the
voltage, its frequency and phase.

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