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CBSE Guess > Papers > Important Questions > Class XII > 2011 > Physics > Physics By Mr.Ashis kumar satapathy
CBSE CLASS XII
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Current, Resistance & Resistivity
A: Following questions are very short answer type. They are to be answered in one or two sentences. They may carry one mark each.
- What are the factors on which resistance of a conductor depends? Give the corresponding relation.
- What are Super Conductors?
- Define electrical resistivity of a material. Does it depend on temperature?
- Two wires of equal length, one of copper and the other of manganin, have the same resistance. Which wire is thicker?
- State Ohm's Law.
- How does conductivity of a semi conductor vary with temperature?
- On a given resistor the colour bands are in the sequence Green, Violet and Red. What is its resistance?
- A carbon resistor of 47K ohms is to be marked with rings of different colours for its identification, Write the sequence of colours.
- Manganin is used for making standard resistors. Why?
- A wire of resistivity ρ is stretched to double its length. What will be its resistivity?
- Of metals and alloys, which has greater value of temperature co efficient of resistivity?
- A copper wire of resistivity ρ is stretched to reduce its diameter to half of its previous value. What will be its new resistivity?
- Why is a voltmeter always connected in parallel with a circuit element across which voltage is to be measured?
- Two wires are of same metal, have the same cross sectional area and their lengths are in the ratio 2:1. What will be the ratio of currents flowing through them respectively when the same potential difference is applied across length of each of them?
- A carbon resistor is marked in green, red, and orange bands. What is the approximate resistance of the resistor?
- The pd across a given copper wire is increased. What happens to the drift velocity of the charge carriers?
- If the temperature of a good conductor increases, how does the relaxation time of electrons in the conductor change?
- State the condition in which the terminal voltage across a secondary cell is equal to its emf?
- If pd V applied across a conductor is incrased to 2 V, how will the drift velocity of the free electrons change?
- A carbon resistor is marked in coloured bands of red, black, orange and silver. What is the resistance and tolerance value of the resistor?
- The given graph shows the variation of resistance of mercury in the temperature range 0 < T < 4 K. Name the phenomenon shown by the graph.
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- What will be the change in the resistance of a Eureka wire, when its radius is halved and
length is reduced to one-fourth of the original length?
B: The following questions are of short answer type. Each of them may carry 2 or 3 marks.
- Explain the difference in the temperature variation of resistivity of metals and semi conductors?
- . The sequence of bands marked on a carbon resistor is Brown, Black, Brown and Gold. What is the resistance? Also give the tolerance.
- State Ohm's Law. A carbon resistor has the coloured ring in the sequence Yellow, Violet, Brown and Gold. What is its resistance? Also give the tolerance.
- Assuming that electrons are free inside a solid, sketch graphically the distribution n(v) of electron with velocity v.
- The sequence of marked rings on a carbon resistor is Red, Red, Red and Silver. What is the resistance? Also give the tolerance.
- When a battery of emf E and internal resistance r is connected to a resistance R, a current I flows through it. Derive the relation between E, I, r and R.
- Define Conductivity of material. Give its SI unit.
- State the conditions under which Ohm's Law is not obeyed in a conductor?
- Three resistances P, Q and R are connected in parallel. Derive their equivalent resistance.
- Define Conductance of a material. Give its SI unit.
- A given copper wire is stretched to reduce its diameter to half of its previous value. What will be its new resistance?
- What do you understand by the resistivity of a substance? Discuss its temperature dependence for a
(i) conductor, (ii) semi conductor and (iii) electrolyte.
- . What are Ohmic and Non ohmic circuits? Give one example of each.
- Show that one Ampere is equivalent to a flow of 6.25 x 1018 elementary charges per second.
- Define resistivity of a conductor. How does it vary with temperature? Give an experimental method for determining the resistivity of metallic wire.
- The sequence of bands marked on a carbon resistor is Yellow, Red, Orange and Silver. What is its
(i) resistance and
(ii) tolerance?
- A 10 ohms thick wire is stretched so that it length becomes three times. Assuming that there is no change in its density on stretching, calculate the resistance of the new wire.
- A metallic wire of length 1m is stretched to double its length. Calculate the ratio of initial and final resistance assuming that there is no change in its density on stretching.
- A voltage of 30 V is applied across a colour coded carbon resistor with first, second & third rings of Blue, Black and Yellow colours. What is the current flowing through the resistance?
- Define EMF of a cell. Show that the voltage drop across a resistor connected in parallel with a cell is different from the EMF of the cell.
- Calculate the electrical conductivity of the material of a conductor of length 3m, area of cross section 0.02 mm2 having a resistance of 2 ohms.
- A potential difference V is applied across a conductor of length L and diameter D. How are the electric field E and the resistance R of the conductor affected when in turn
(I) V is halved, (ii) L is halved and (iii) D is doubled? Justify your answer in each case.
- Why does the resistivity of a semiconductor decrease with increase in temperature?
- The resistivity of a conductor increases or decreases with increase in temperature? Explain.
- Name any two materials having a small value of temperature co-efficient of resistance. Write one use of this material.
- A wire has a resistance of 32 ohms. It is melted and drawn into a wire of half of its original length. Calculate the resistance of the new wire. What is the percentage change in resistance?
- Define resistivity of a substance. Give its unit. How does it vary with temperature in
(I) good conductors, (ii) ionic conductors, and (iii) semi conductors?
- V I graph for a metallic wire at two different temp. T1 and
T2 is as shown in the fig. Whichof the two temp. is higher and why
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- A wire of 10 Ohms resistance is stretched to thrice of its original length. What will be its
(i) resistivity and (ii) new resistance?
- A cell if emf, E and internal resistance r gives a current of 0.8 A with an external resistance 24 ohms and a current of 0.5 A with an external resistance of 40 ohms. Calculate
(i) emf E and (ii) internal resistance r of the cell.
- A conductor of length l is connected to a dc source of potential V. If the length of the conductor is tripled, by stretching it, keeping V constant, explain how do the following factors vary in the conductor:
(i) Drift speed of electrons, (ii) Resistance and (iii) Resistivity?
- Define resistivity and state its S.I. unit. State and explain how the resistivity of a conductor varies with temperature?
- Define the term resistivity of a conductor. Give its S.I. unit. Show that the resistance R of conductor is given by ml/(ne2ԏ A), where the symbols have their usual meanings.
- Are the paths of electrons straight lines between successive collisions (with the positive ions of the metal) in the
(i) absence of electric field and (ii) presence of electric field?
- Establish a relation between drift velocity vd of an electron in a conductor of cross-section A, carrying current I and concentration n of free electrons per unit volume of the conductor. Hence obtain the relation between current density and drift velocity.
- Define resistivity. Write an expression for the resistivity of a conductor in terms of relaxation time.
- Explain how does the resistivity of a conductor change with
(I) number density ‘n’ of free electrons and (ii) the relaxation time λ ?
- Explain, with the help of graph, the variation of conductivity with temperature for a metallic conductor.
- What is superconductivity? State two applications of the phenomenon of superconductivity.
- Give the nature of V-I graph for
(i) ohmic and (ii) non-ohmic circuit elements. Give one example of each type.
- The voltage-current variation of two metallic wires X and Y at constant temperature are as shown.Assuming that the wires have the same length and the same diameter, explain which of the two wires will have larger resistance.
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- The voltage-current graphs for two resisters of the same material and same radii with lengths L1 and L2 are shown in the figure. If L1 > L2, state with reason, which of these graphs represents voltage-current change for L1 .
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- Breifly explain, how the drift velocity of electrons in a metallic conductor varies, when
(I) the temperature of the conductor is increases, and (ii) applied potential difference is decreases,keeping temperature constant.
- The diagram shows a piece of pure semiconductor, S in series with a variable resistor R, and a source of constant voltage V. Would you increase or decrease the value of R to keep the reading of ammeter (A) constant, when the semiconductor S is heated? Give reason.
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- Draw the graphs to show the variation of resistivity with temperature for
(I) nichrome and (ii)silicon.
- The circuit shown in the diagram contains a battery B, a rheostat Rh and identical lamps P and Q. What will happen to brightness of the lamps, if the resistance through the rheostat is increased? Give reasons.
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- A voltage of 30V is applied across a carbon resistor with first, second and third rings of blue black and yellow colours respectively. Calculate the value of current in mA, through the resistor.
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Submitted By Mr.Ashis kumar satapathy
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