Download current electricity

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CURRENT ELECTRICITY
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Define resistivity and conductivity of a material. Does it depend on temperature ? Write down
the units.
When a battery of e.m.f. E and internal resistance r is connected to a resistance R, a current I
flows through it. Derive a relation between E, I, r and R.
(a)
State and explain Faraday’s laws of electrolysis. Define electro-chemical
equivalent. Define Faraday’s.
(b)
State Faraday’s laws of electrolysis. Draw a labelled circuit diagram to illustrate
deposition of copper on a given metallic object.
(c)
State Faraday’s laws of electrolysis. Derive the relation connecting chemical equivalent
and electrochemical equivalent of an element.
(a)
Explain how you will convert a galvanometer into (i) a voltmeter (ii) an ammeter of a
given range.
(b)
Explain briefly how you will convert a galvanometer into a millivoltmeter of
range 0 – 10 mV.
(a)
What are the Kirchoff’s laws for electric circuit. Apply Kirchoff’s laws to obtain the
condition of balance in a Wheatstone’s bridge.
(b)
Explain with the help of a circuit diagram how the value of an unknown resistance can
be determined using a Wheatstone bridge. Give the formula used.
State and explain Peltier effect. Define Peltier coefficient.
Three resistances P, Q and R are connected in parallel. Derive an expression for their equivalent
resistance.
Explain Seebeck effect. Discuss the thermoelectric series.
State one main difference between primary and secondary cells.
A dry cell of e.m.f. 1.6 V and internal resistance 0.10 ohm is connected to a resistor of resistance
R ohm. If the current drawn from the cell is 2A
(i)
what is the voltage drop across R ?
(ii)
what is the energy dissipation in the resistor ?
With the help of circuit diagram, explain how would you compare the resistances of two given
resistors using a potentiometer.
Three indentical cells, each of e.m.f. 2V and internal resistance 0.2 ohm are connected in series
to an external resistor of 7.4 ohm. Calculate the current in the circuit.
To deposit 0.5 kg of silver per hour on the cathode of a silver voltmeter of resistance 0.70 milli
ohm, how much p.d. must be maintained between the plates ?
Write the expression connecting the thermoelectric e.m.f. for a thermocouple, with temperature
difference of the junctions.
Write two special characteristics of the wire of a electric heater.
A metallic wire of length 1 m is stretched to double its lengh. Calculate the ratio of its initial and
final resistance assuming that there is no change in its density on stretching.
Out of an ammeter and a voltmeter, which of the two has higher resistance and why ?
Einstein Classes,
Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road,
New Delhi – 110 018, Ph. : 9312629035, 8527112111
2
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Write two special characteristics of magnanin due to which it is used in making standard resistances.
A carbon resistor of 74 k is to be marked with rings of different colours for its identification.
Write the sequence of colours.
You are given a primary and a secondary cell of the same e.m.f. From which cell will you be able
to draw larger current and why ?
A current of 5.0 A flows through an electric press of resistance 44. Calculate the energy
consumed by the press in 5 minutes.
Draw a circuit diagram and explain how the e.m.f. of primary cells can be compared by using a
potentiometer.
How is electrical conductivity of an electrolyte affected by increase temperature ?
A battery of e.m.f 3 volt and internal resistance r is connected in series with a resistor of 55
through an ammeter of resistance 1. The ammeter reads 50 mA. Draw the circuit diagram and
calculate the value of r.
With the help of suitable diagram explain the construction and working of a dry cell.
A wire of resistivity  is stretched to double its length. What will be its new resistivity ?
Draw a circuit diagram of a meter bridge arranged to compare two resistance. Explain the
principle of the experiment, and give the formula used.
Two cells of e.m.f. 6V and 12V and internal resistances 1 and 2 respectively are connected
in parallel so as to send current in the same direction through an external resistance of 15.
(i)
Draw the circuit diagram.
(ii)
Using Kirchoff’s laws calculate
(a)
current through each branch of the circuit
(b)
p.d. across the 15 resistance.
Name the device used for measuring the internal resistance of a secondary cell.
A set of n identical resistors, each of resistace R ohm,when connected in series have an effective
resistance X ohm and when the resistors are connected in parallel, their effective resistance is Y
ohm. Find the relation between R, X and Y.
Give reasons why the electrical conductance of electrolysis is less than that of metals.
Calculate the electrical conductivity of the material of a conductor of length 3 m, area of cross
section 0.02 mm2 having a resistance of 2 ohm.
With the help of a circuit diagram explain how the internal resistance of a cell can be determined
using a potentiometer. Write the formula used.
Give any two factors on which thermo-electric emf produced in thermo-couple depends.
Name any one material having a small value of temperature coefficient of resistance. Write one
use of this material.
Write two advantages and two disadvantages of a secondary cell over a primary cell.
A wire of uniform cross-section and length l has a resistance of 16. It is cut into four equal
parts. Each part is stretched uniformly to length l and all the four stretched parts are connected
in parallel. Calculate the total resistance of the combination so formed. Assume that stretching of
wire does not cause any change in the density of its material.
A student obtains resistances of 3, 4, 12 and 16 ohms using only two metallic resistance wires
either separately or joined together. What is the value of resistance of each of these wires ?
Einstein Classes,
Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road,
New Delhi – 110 018, Ph. : 9312629035, 8527112111
3
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The sequence of bands marked on a carbon resistor are yellow, Red, Orange and Silver. What
is its (i) resistance and (ii) tolerance ?
In the V-I graph of a given resistance for temperature T1 and T2, identify higher temperature.
Give reason.
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Fig shows three cylindrical copper conductors with their face area and length. Which has highest
Resistance ?
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Though electric current has magnitude and direction but it is scaler. Explain.
Fig. shows an electric current that passes through a copper wire of two different cross sections.
Find the ratio of the drift velocity of electrons in them.
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A steady current flows in a metallic conductor of non-uniform cross section as shown in fig.
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Out of current density, electric field and drift speed, which remain constant along the conductor?
Give the condition under which the Wheatstone bridge is said to be most sensitive ?
If the temperature of a conductor increases, how does the relaxation time of electrons change ?
Define potential Gradient. Give its S.I. Unit.
Why do we prefer potentiometer to measure e.m.f. of a cell than a voltmeter ?
Einstein Classes,
Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road,
New Delhi – 110 018, Ph. : 9312629035, 8527112111
4
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Can an ohmic conductor behaves as non ohmic ? Explain.
How can a given 4 wire potentiometer be made more sensitive ?
In the Fig. What is the potential difference between point A and B.
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Two wires of same diameter but different material are joined together to carry the same current.
In first wire the density of charge carries is twice that in the second wire. Compare the drift
velocity of electrons in the wires.
Two bulbs are rated 60W, 200V and 100W, 200V. Which bulb has greater resistance ?
Explain why we prefer to connect all the electric bulbs in parallel combination with the mains
circuit of the house.
How does the internal resistance of cell change on increasing the concentration of electrolyte.
Name the depolariser in Daniel cell.
What happens to the power dissipation if the value of electric current passing through a conductor
of constant resistance is halved ?
What is a thermistor ? Give one application of it.
Give two differences between Joule’s heating and heating due to thermoelectric effect.
Write two applications of thermoelectricity.
If the temp of cold junction in a thermocouple is lowered then what will be the effect on neutral
temp and temp of inversion ?
Two wires, one of manganin and the other of copper have equal lengths and equal resistances,
which one of two wires will be thicker ?
The current through a wire depends on time as I = io + At where io = 5A and A = 2As–1. Find the
charge crossing through a section of the wire in 15 seconds.
Kirchhoff’s Laws have been formulated in accordance with the conservation of some physical
quantity. Write the name of the physical quantities on which they have been formulated.
Define temperature coefficient of Resistance ‘’. Why do we prefer manganin or constanton
wire for making standard Resistance coils ?
Three resistors of 3 each are given. What are the maximum and minimum values of resistance
that can be obtained using them ?
Explain the phenomenon of super conductivity. Give one application of it.
Two wires of Cu and Fe of same diameter are put in parallel across the same source of e.m.f.
Which will dissipate more heat ?
If the temperature of cold junction is 200c and thermo e.m.f. change sign at 850 K, Find the
temperature at which thermo e.m.f. will be maximum.
Write two characteristics each of electric heater wire and fuse wire.
On what factors does Neutral temperature and temperature of inversion depend.
Draw a labelled diagram of simple lithium solid state cell.
What is the defect of polarisation in a primary cell ? How is the use of CuSO4 in Daniel cell able
to overcome the defect of polerisation ?
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Einstein Classes,
Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road,
New Delhi – 110 018, Ph. : 9312629035, 8527112111
5
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Two manganin wires whose lengths are in the ratio 1 : 2 and whose resistance are in the ratio
1 : 2 are connected in series with a battery. What will be the ratio of drift velocities of free
electrons in the two wires.
Establish the relation between drift velocity Vd of the electrons in a conductor of cross section
‘A’ carrying current ‘I’ and concentration of free electrons per unit volume of the conductor
being ‘n’.
What is a button cell ? Name any two types of button cells and explain the working of any one
of them.
What is a secondary cell ? When a fully charged secondary cell is discharged, then what changes
occur at electrodes and electrolyte. Support your answer with chemical equations.
Explain the principle of potentiometer, with the help of a circuit diagram, explain how a
potentiometer can be used to compare e.m.f. of two primary cells.
State the principle of meter bridge. Describe its use to determine the specific resistance of a
given wire.
A silver wire has a resistance of 2.1 at 27.50C and a resistance of 2.7 at 1000C. Determine
the temperature coefficient of resistance ?
Thermo e.m.f. of Cu-Fe thermocouple varies with temperature  of hot junction (cold junction
at o0c) as E (µV) = 0.350 – 0.00352. Calculate (i) Neutral temperature (ii) Temp of inversion.
Two voltameters A and B are connected in series. The mass of substance deposited at the
cathode in voltameter A is 4g. Find the mass deposited on cathode of voltmeter B. Chemical
Equivalents of A and B are 32 and 48 respectively.
A fuse wire of length 16cm, radius 0.2 mm melts for a current of 2A. Calculate the current for
which another fuse wire of length 20cm and radius 1.8 mm will melt.
A 10m long wire of resistance 40 is connected in series with a battery of e.m.f. 5V (negligible
internal resistance) and a resistance of 10. Find the potential gradient along the wire.
An electric bulb is marked 100W 230V. If the supply voltage drops to 115V, what is the heat
and light energy produced by the bulb in 20 minute.
Whan a resistance of 2 is connected across the terminals of the battery, the current is 0.5 A.
When the resistance across the terminal is 5, the current is 0.25A. Determine the e.m.f. of the
battery. What will be the current drawn from the cell when it is short circuited ?
A storage battery of e.m.f. 8V and internal resistance 0.5 is being charged by a 120V d.c.
supply. How much resistance is to be put in series for charging the battery safely, by maintaining
a constant current of 7A in the battery. Also estimate the terminal voltage of the battery while
charging.
Find the value of unknown resistance X in the following circuit for which current flows through
section AO is zero. Also calculate the current drawn by the circuit from the battery of e.m.f. 6V
and negligible internal resistance.
Einstein Classes,
Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road,
New Delhi – 110 018, Ph. : 9312629035, 8527112111
6
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The terminal potential difference across the terminal of a battery of e.m.f. 12V and internal
resistance 2 drop to 10V, when it is connected to a silver voltmeter. Calculate the silver
deposited at the cathode in half an hour. Atomic mass of silver is 108g/mol.
With a certain unknown resistance X in the left gap and a resistance of 8 in the right gap, null
point is obtained on the meter bridge wire. On putting another 8 in parallel with 8 resistance
in the right gap, the null point is find to shift by 15 cm. Find the value of X from the observations.
Fig. shows four situations in which positive and negative charges move horizontally through a
region and gives the rate at which each charge moves. Rank the situations according to the
effective current through the regions in ascending order.
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A D.C. supply of 120V is connected to a large resistance X. A voltmeter of resistance 10 k
placed in series in the circuit reads 4V. What is the value of X ? What do you think is the purpose
of using a voltmeter instead of an ammeter, to determine the large resistance X ?
Einstein Classes,
Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road,
New Delhi – 110 018, Ph. : 9312629035, 8527112111