12th Class Physics Chapter 2 Current Electricity Short Question Answers

Physics short QA

12th Class Physics Chapter 2 Current Electricity Short Question Answers Below

1.A potential difference is applied across the ends of a copper wire.What is the effect on the drift velocity of free electrons by i) increasing the potential difference. ii) Decreasing the length and the temperature of the wire.
Drift velocity:A small uniform velocity that an electron gains in the presence of an electric field of the battery is called the drift velocity in the direction of – E.
i)The drift velocity of free electrons increases with the increase of potential difference.This is because by increasing the value of applied potential difference the value of electric field increases which causes the increase in the value of drift velocity of the electrons.
ii) When the length of the wire is decreased then the value of resistance of wire will also decrease.Thus the value of flowing current will increase it means that the value of drift velocity will also increase.Similarly the resistance of wire will decrease with the decrease of temperature.Thus the value of the current will increase due to which the drift velocity will also increase.
2.Do bends in wire affect its electrical resistance?
3.What are the resistances of the resistors given in the figures A and B?What is the tolerance of each?
Figure A: As we know first three bands on the left show the values of resistance.Thus
First band for Brown = 1
Second band for Green = 5
Third band for Red = 00
This means that given resistance is
R = 1500 Ω
Fourth band is gold which shows tolerance = ±5%
Thus the actual of resistance R = 1500±5%
For Figure B:-
First band for Yellow = 4
Second band for white = 9
Third band for orange = 000
since the value of yellow is 3 therefore it has three zeros (000)
Resistance = 49000
Tolerance value in fourth band of slicer = ±10%
Thus the actual value of resistance R = 49000 ±10%
Tolerance:- Tolerance means the possible variation from the marked value.For example a 1000Ω resistance with a tolerance of ±10% will have  an actual resistance anywhere between 900Ω and 1100Ω.
4.Why does the resistance of a conductor rise with temperature?

Increase in resistance with temperature:- We know that the resistance offered by a conductor to the electric current is due to the collidions of free electrons with the atoms present in the latices of conductor.With the rise of temperature these atoms vibrate with larger amplitude and the chances of their collision with free electrons increases.It shows that with rise of temperature the collisions of free electrons with the atoms become more frequent and hence the resistance increases.The following relation expresses the change of resistance with temperature.
Rt = Ro (1+ ἆt)
Where Ro is the resistance of conductor at OᵒC and Rt is the resistance at t ᵒC and ἆ is the temp.Co – efficient of resistance t is the rise in temperature.

5.What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm’s law?
Ohm’s law states that the potential difference is proportional to the current provided there is no change in the physical state of the conductor.The physical state of a conductor means its density elasticity and temperature etc.in case of a lighted bulb as the temperature increases resistance of the filament goes on increasing according to the relation
Rt = Ro (1+ ἆt)
Where ἆ is the temperature  co – efficient and t is the rise of temperature.It means that the temperature of a lighted bulb does not remain constant so its filament does not obey Ohm’s law.Thus the main difficulty in testing whether the filament of lighted  bulb obeys Ohm’s law is the change in its temperature with the flow of current and electrical resistance.
6.Describe a circuit which will give a continuously varying potential?
7.Explain why the terminal potential of a battery decreases when the current drawn from it is increased?
As we already know a relation for terminal potntial difference of a battery that is,
IR = E – Ir
Or Vt = E – Ir …………….. (1)
Here E= emf of battery
r = internal resistance of battery
Ir  = P.D across internal resistance
Equation (1) shows that when 1 is increase the factor Ir becomes  large and Vt becomes small.Thus terminal potential difference of a battery decreases when the current drown from it is increase.
8.What is wheatstone bridge?
9.Verify that in series resistor the equivalent resistance is always larger than largest resistance in series 3 and for parallel resistors the equivalent resistance is always smaller than the smallest resistance in the combination?
10.Why is it dangerous to touch a live wire while standing on earth bare – footed?
A man standing on the earth bare- footed is at zero potential.When he touches the live wire at high potential (the current will flow through his body due to high potential difference that may cause his death.
11.If the positive and negative connections to the terminals of an automobile battery were revered would you expect the automobile light to function?
Yes the light will function because the light is produced by heating of the filament in automobile light and heating does not depend upon the direction of flow of current.
12.Is the principle of energy conservation always applicable to electrical circuits?
The principle of energy conservation is always applicable to all forms of energy.As electrical energy is involved in electrical circuits.Therefore the principle of conservation of energy is applicable to electrical circuits.
13.What is short circuit and open circuit mean to you?
A short circuit is a closed circuit in which no resistor is included in the external circuit (R=0) of if the terminals of a battery are directly connected by a copper wire we can speak of short – circuit.The circuit is said to be open if a resistor connected across the battery has infinite resistance or a circuit which does not have a continuous path for flow of current (circuit is broken).
14.Is it possible to have a situation in which the terminal voltage will be greater than the emf of the battery?
Yes it is possible in that situation when the battery is being charged.
15.Why a thin region of a light bulb filament has more possibility to burn than the thicker one?
We know that the temperature of a conductor depends upon the resistance.If the same current passes through the light bulb filament the temperature of the region will be more than that of thick region because the resistance of a constructor is inversely proportional to its cross – sectional area A’ i.e.R 1/A but the area of thick region is greater than tat of thin region.Therefore the resistance of thin region will be greater than that of thin region.Thus more heat will be produced in the thin region which will make its temperature higher,Hence this higher temperature has more possibility to burn this region than thicker one.
16.Why resistance of a conductor is inversely proportional to the area of cross- section of a conductor?
17.Difference between emf Terminal P.D?
Difference between emf and Terminal P.D.
E.M.F:-The emf of a battery is the potential difference between its terminals when no current is being drown from at to an external circuit i,e.when the circuit is open.
Terminal Potential Difference:-The voltage across the terminals of a battery when the current is being dream from it to an external circuit is called terminal potential difference.In other word’s it means that f a battery is not giving current to any external circuit the P.D between its known as emf (E) and if it is giving current to some external circuit the terminal potential difference V is given by
V= E – Ir
Where Ir potential drop across the internal resistance r’of battery.Thus whenever current is drown form a battery it terminal potential and emf become different.
The terminal P.D of battery (source ) is always less than emf.
18.Why is heat produced in a conductor due to flow of electric current?
Electric current produces heat:- When electric current is passed thrugh a conductor heat is produced.We know that free electrons moving through a conductor constitute the electric current and they lose potential energy which is equal to VQ where V is the potential difference between the ends of the ends of the conductor and Q is the total charge transferred by the electrons through it.The loss of potential energy is die to bumping of free electrons wit the atoms in the lattice.During collision the free electrons transfer their energy to the atoms and they vibrate more vigorously.This increase in the average kinetic energy of the atoms appears as heat energy of the conductor.
19.Give some examples of flow of current in which Ohm’s law in not valid?
Examples for which ohms law is not valid:- i) Current passing through the filament of a bulb.The resistance (R) of the filament goes on increasing with the rise of temperature and thus it does not obey ohm’s law. ii) Current through electronic tube does not obey the Ohm’s law. iii) Current through arcs and ionic conductor does not obey the ohm’s law.
20.Resistance of 10 Ohm’s and 40 Ohms are connected in series.If the current flowing through one of them is 1.0 A,What is the current through the other resistors?
Current through resistors in series;- When a number of resistors are connected in series the same current passes through all of them but potential drop may be different across each resistor.Thus same current of 1.0 A will flow through all the resistors given in the question.
21.Ten resistors of different values are connected in parallel.If the potential drop across one of them is 5 volts what is the potential drop across the remaining nine resistors?
Potential drop across resistors in parallel:- When the resistors are connected in parallel their ends are connected to two common terminals and thus the potential drop across each resistor is equal to the potential difference between the terminals.Hence the potential drop across each of the nine resistors will be 5 volts.
22.An ordinary bulb is marked 60 watt 200 volt what is its resistance?
23.Why the switch is always placed in series with the live wire in house circuits and not in series with the neutral wire?
We know that the live wire is always at higher potential and the neutral wire is at zero potential therefore switch is always placed in series with the live wire to On and Off the current in the circuit.If we place the switch in series with the neutral wire it will not touching the circuit even the switch is off.
24.Why the current flows in conductor is not due to the flow of positive changes?
In conductors there is a large number of free electrons.The positive charges are bound inside the nuclei of atoms.Thus the current in conductors will be due to flow of free electrons and not due to positive charges.
25.Why the resistance of a wire is directly proportional to its length?
If the length of conductor is increased then the number of collisions of free electrons with atoms increase due to increase in number of atoms and vice versa. Thus we can say that resistance of a conductor is directly proportional to it length i,e. R α L.
26.Why some of the electrons are free in conductors?
In conductor the valence electrons of atoms are loosely bound to the nucleus therefore small energy given to the atoms can make them free.Some of the electrons are free in conductor because it is the movement of free electrons that is responsible for electrical induction through a substance.
27.When a Wheatstone bridge is balanced then no current flows through the galvanometer why?
Galvanometer has zero deflection when the bridge is balanced.This is so because both the terminals of the galvanometer are at the same potential or the terminal have zero potential difference so no current flows through the galvanometer.
28.What is slide wire bridge?Why are the lengths of the wire taken instead of its resistance while determining unknown resistance?
Definition:-It is another form of the Wheatstone bridge which is used to find out unknown resistance. According to the relation the resistance of a wire is given by R = ṗ I/A If ṗ and A are constant then R α / i.e.the length of the wire is directly proportional to its resistance.So ratio of two resistances is equal to the ratio of the corresponding two lengths of a wire.That is why lengths are use instead of their resistance.
29.A voltmeter cannot read the exact emf of a cell.Why?
If we want to measure the exact e.m.f, potential measuring instrument must not draw any current from the circuit.When a voltmeter is connected across a cell then it will draw some current from the cell.So due to current flowing in the internal resistance of the cell a small potential

drop takes place.As a result the actual e.m.f.of the cell decreases.That is why voltmeter cannot read exact value of a cell.However e.m.f of a cell can be exactly measured by a potentiate meter as it draws no current from the circuit.

30.Why should the potentiometer wire be long and uniform in diameter?
According to principle of potentiometer if the diameter of potentiometer wire is uniform the potential difference is directly proportional to the length of the wire for the constant current i.e.
V α I
So for high value of potential difference a long wire of uniform diameter is required.On the other hand a long wire provides a wide range of potential difference which can be used to balance the P.D.across a circuit.
31.Why we prefer potentiometer in place of voltmeter for measuring potential difference?
We always prefer potentiometer in place of voltmeter for measuring the exact value of potential difference.This is because the voltmeter will draw some current from the circuit so a small potential drop will take place.As a result the actual value of the P.D.will decrease.That is why voltmeter cannot measure the exact value of P.D.However the P.D. of a resistor can be exactly measured by a potentiometer because it draws no current from the circuit.
32.What is potentiometer?Describe its principle?
Definition:-A very simple instrument which can measure and compare potentials without drawing any current from the circuit is called potentiometer.Principle:- This potential difference across any length of wire of uniform area of cross- section is directly proportional to its length when a constant current flows through it.
33.What is an avometer?
Definition:- An avometer is an electrical device which can be used for the measurement of i) Current in the circuit. ii)Potential difference between any two points. iii) The resistance of any conductor. Although an avometer is basically used to measure direct current and voltages yet it can be used to measure A.C. voltages. But for this purpose the A.C voltage is first rectified and then fed to the avometer as a D.C voltage.
34.What do you mean by electroplating?
The process of coating a thin layer of one metal on the surface of another metal is called elector- plating.In electroplating the pure metal like gold is made anode and the object being coated is made cathode like iron.This is the practical application of the phenomenon of electrolysis.The plating of expensive metals like copper gold silver etc is done on cheap articles or metals.
35.What do you understand by ohmic and non-ohmic substances?
Ohmic substance:- The substance which obeys ohm’s law is called ohmic substance.For an ohmic substance the graph between V’and I is a straight lines which shows that an ohmic substance strictly obeys ohm’s law. Non-Ohmic substance:-A substance which does not obey the Ohm’s law strictly is called a non – Ohmic substance.Tungsten filament and semi- conductor diodes are examples of non-Ohmic substandce.The graph between V and I is not a straight line for a non- Ohmic substance.It is basically due to the increase in the resistance of the filament which gradually increases due to rise in temperature of the substance.
36.Can you describe some substances whose resistance decreases with increase in temperature?
Yes the resistance of Germanium and silicon will decrease with increase in temperature.Such materials have negative temperature co-efficient.When the temperature of these substances is increased they conduct more current due to fall in their resistance.
37.Explain the term Tolerance resistor?
Tolerance means The possible variation in the worked value of resistance of a resistor.For example a 1000Ω resistor with a tolerance of ± 10% will have an actual  resistance anywhere  between 900Ω and 1100Ω.
38.What is thermistor?Describe its main uses?
Thermistor is a heat sensitive resistor.They are made from semi-conductor ceramic made from mixtures of metallic oxides of manganese nicle cobalt and copper etc. USES: i) Thermistors can be used to measure very low temperatures. ii)They can be used to convert temperature variations into electrical voltage.
39.Find the heat dissipated through a bulb of 100 w in an hour?
As we know the formula for heat dissipated Heat = p×t 100×1 hour 100× 3600 = 36×104 j
40.State Kirchhoff’s rules?
Kirchhoff’s first rule:-It states that the sum of all the currents meeting at a point in the circuit is zero i.e. ∑I = 0 It observes the law of conservation of charge. Second rule:- It states that the algebraic sum of voltage changes in a closed circuit or loop is zero. It verifies the law of conservation of energy.These rules are applied to understand the complex networks of electrical circuits.
41.Discuss briefly the heating and magnetic effects of current?
Heat Effect:- When current flows through the metallic wire the electrons collide with the atoms of the wire.At each collision they lose  some of their  kinetic  energy and give this energy  to the atoms in the wire and make them vibrate more fast .Due to fast vibrations the wire becomes hotter.The geat produced by a current I in the wire of resistance R during a time interval t is given by
H= I2 Rt
Magnetic Effect:-
The magnetic  field around  a moving charge was first found by prof.H Oersted in 1819.He showed  that an electric current in a wire deflects a nearby compass needle.It means that an electric current in a wire produces a magnetic field around it.
42.Describe the uses of heating effect and Magnetic effect?
Uses of Heating Effect:- The geating effect of current is used in electric heater electric kettles toaster and electric iron etc. Uses of Magnetic Effect:-Magnetic effect is used in the detection and measurement of current. All the machines like electric motors ammeter voltmeter and electric fans also use the magnetic effect of current.
43.What is electric current?
44.What do you understand by resistivity?
45.What are thermistors?What is their importance or use?
A thermistor is a heat sensitive resistor.Most of the thermistors have negative temperature co-efficient of resistance i.e. resistance of such temperatures decreases when their temperature is increased.Thermistors with positive temperature co-efficient are also available. USES:- (1) Thermistors have wide applications as temperature sensors i.e.they covert changes of temperature into electrical voltage. (2) The thermistors have different shapes in the form of beeds rods or washers which can be used according to their requirement.
46.What is the difference between conventional current and electronic current?
Conventional current:- It is defined as that current which asses from a point at a point at higher potential to a point at a lower potential as if its represented movement of positive charges. Electronic current:-The current due to motion of negative charges that flows from the negative terminal of the battery to the positive terminal in the electrical circuit is called electronic current.
47.Describe the various types of sources of current?
Following are some types of sources of current: 1) There are two types of cells (i) Primary cell (ii)Secondary cell Both of them convert chemical energy into electrical energy. 2) Electric generators:- They convert mechanical energy into electrical energy. (3)Thermo- couples:- These convert heat energy into electrical energy.| 4) Solar cells:- The solar cells convert sunlight directly into electrical energy.
48.State Ohm’s law.Express it mathematically?
49.Define electrical power write down its three equations for the power dissipation in a resistor?
The rate at which  the battery is supplying electrical energy is called the power output  or electrical power of the  battery.
The SI unit of power is Joule/Seconds or watt.
Three equations:- Three relations for the power dissipation are given below.
i) P = VI
ii) P = I2R
iii) P = V2/R
50.What materials are used for making common resistors?
The common resistors are mad up by carbon material.The resistive material of carbon is deposited in the form of thin coating on a high coating on a high grade cermic rod or cone called the substrate.Carbon resistors are most common in electronic equipment.

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