Question Bank : Chapter - 3
Current Electricity
Class 12 Physics chapter-3 Current Electricity important questions topic wise
Questions on current-
Q.1 What is the current flowing through a conductor if 1 million electrons are crossing in 1 ms through a cross-section of it. Ans 1.6 x 10^-10 A
Q.2 If 3.2 x 10^17 electrons pass through a wire in 0.5s, calculate the current through it. Charge on each electron is 1.6x 10^-19 C. Ans. 0.1024A
Q.3 In the Bohar model of hydrogen atom, the electron circulates around the nucleus in a path of radius 5.1 x 10^-11m at a frequency of 6.8×10^15 revolution per second. Calculate the equivalent current. Ans. 1.088 x 10^-3 A
Q.4 The charge flowing in a conductor varies with time as q= at - 1/2 bt² + 1/6 ct^3 where a,b,c are positive constants. Then, find i) the initial current ii) the time after which the value of current reaches a maximum value iii) the maximum or minimum value of current. Ans i) i=a, ii) t= b/c, iii) i = a-b²/2c.
Q.5 A current of 4 amperes flows for 4 minutes in a 5 W resistance. Calculate the number of electrons flowing across any cross-section of the conducting wire. Ans. 6 x10^21
Q.6 A current of 6.4 A is maintained in a conductor of cross-section of 10^ -4 m². If the number density of free electrons is 8×10^28 / m^3, calculate the drift velocity of the free electrons. Given that the charge on electron is 1.6×10^-19 C. Ans. 5.0 x 10^-6 m/s
Q.7 Calculate the average drift velocity of conduction electrons in a copper wire of cross-section 0.1 mm² carrying a current of 1 A. Assume that each copper atom contributes one conduction electron. Given that the density of copper is equal to 9×10^ 3 kg per metre cube and its atomic weight is equal to 63.5. Avogadro’s number is 6.023 x 10^26 per kg atom. Ans. 7.33 x 10^-4 m/s
Q.8 A potential difference of 12 V is applied across a conductor of length 0.2 m. If the drift velocity of electrons is 3×10^ -4 m/s, calculate the electron mobility. Ans. 5 x10^-6 m²/Vs.
Q.9 Find the current flowing through a copper wire of length 0.2 m, area of cross-section 1 mm² , when connected to a battery of 4 V. Given that electron mobility = 4.5×10^ -6 m²/Vs and charge on electron = 1.6×10^-19 C. The number density of electron in copper is 8.5×10^ 28 per metre cube. Ans. 1.2 A.
Q.10 A potential difference of 3 V is applied across a conductor of resistance 1.5 ohm. Calculate the number of electrons flowing through it in one second, charge on electron ,e =1.6×10^ -19 C. Ans. n = 1.25 x 10^19.
Questions on Resistivity, Ohms law & conductivity
Q.11 A potential difference of 200 V is maintained across a conductor of resistance hundred ohm calculate the number of electrons flowing through it in one second. Ans. 1.25 x 10^19.
Q.12 A wire having a mass of 0.45kg possesses a resistance of 0.014 ohm. If the resistivity of the material of wire is 1.78 into 10^ -7 Ohm meter, calculate its length and radius. Given that the density of the material of wire is 8.93×10^3 Kg per metre cube. Ans. L= 1.99m & r = 2.84mm
Q.13 A wire of resistance 5 ohm is drawn out so that its length is increased by twice its original length. Calculate its new resistance. Ans. 45 Ohm.
Q.14 Calculate the electrical conductivity of the material of a conductor of lent two meter, area of cross-section 0.2 mm² having a resistance of 2 ohm. Ans. 7.5 x 10^7 S/m
Q.15 Find the relaxation time for free electrons in copper, if the density of mobile electrons is 8.4×10^28 per metre cube. The resistivity of copper at room temperature is 1.7×10^-80 m. Given mass of electron = 9.1x 10^-31 Kg and the charge on electron = 1.6×10^ -19 C. Ans. 2.49 x 10^-14s.
Questions on Temperature dependence & coefficient
Q. 16 The temperature coefficient of resistance of wire is 12.5 x 10^-4/C°. At 300K the resistance of the wire is 1 Ω. What will be the temperature at which resistance of wire will be 2 Ω. Ans. 1100K
Q.17 Draw the graph showing the variation of resistivity with temperature of Nichrome. Which property of nichrome is used to make standard resistance coils?
Q.18 The resistance of the platinum wire of platinum resistance thermometer at the ice point is 5 Ω and at steam point is 5.23 Ω. When the thermometer is inserted into a hot bath, the resistance of the platinum wire is 5.795 Ω. Calculate the temperature of the bath. Ans. 345.65°C
Questions on Kirchhoff’s law
Q.19Use Krichhoff’s rule to determine the potential difference between the points A and D. When no current flows in the arm BE of the electric circuit shown in the figure below-
Ans. Potential difference across A and D is 9V. I = 2A)
Q.20 Use krichhoff’s rules to determine the value of the current I1 flowing in the circuit shown in the figure-
Ans. I 1 = 2/5 = 0.4 A
Q.21 State Krichhoff’s rules. Use this rule rules to write the expressions for the currents I1, I 2 and I3 in the circuit diagram shown in the figure below- ( mistake in direction of i3 take direction of i3 opposite to as shown)
Q.22 Apply Krichhoff’s rules to the loops ACBPA and ACBQA to write the expressions for the currents in the network-
Ans. I1 = -84/37 A, I2 = 106/ 37 A, I3 = 22/37 A.
Q.23 Calculate the value of the resistance are R in the circuit shown in the figure, so that the current in the circuit is 0.2A. What would be the potential difference between A and B.
Ans. Potential across A and B , VAB = -1V.
Questions on power and energy
Q.24 Two equal resistances when connected in series to a battery consume electric power of 60 W. If these resistances are now connected in parallel combination to the same battery, the electric power consumed will be? Ans. 240 W
Q.25 An electric bulb is marked at 100w, 220 V. If the supply voltage drops to 110 V. What is the new power dissipated in bulb? Ans. 25W
Q.26 A car has a fresh storage battery of emf 12V and internal resistance 5.0 x 10^-2 Ohm. If the starter motor draws a current of 90A, what is the terminal voltage of the battery when the starter is on? Ans 7.5 V
Questions on grouping of cells
Q.27 If n identical cells each of EMF E and internal resistance r are connected in series across an external resistance R. Find equivalent EMF, internal resistance and current in R.
Q.28 If n identical cells each of EMF E and internal resistance r are connected in series. If polarity of m cells are reversed find equivalent EMF and internal resistance.
Q.29 Two identical cells, whether join together in series or in parallel give the same current, when connected to external resistance of one ohm. Find the internal resistance of each cell. r = 1ohm.
Q.30 Eight cells, each of internal resistance 0.5 ohm and EMF 1.5 V are used to send a current through an external resistance of (a) 200 ohm (b) 0.002 ohm (c) 1.0 ohm. How would you arrange them to get the maximum current in each case find the value of current in each case.
Q.31 n Identical cells of EMF 6 V, connected in series with an external resistor of 5 Ohm, carry a current of 10 A. If polarity of one cell is reversed, the current is 8 A . Find n and internal resistance of each cell.
Q.32 If n identical cells each of EMF E and internal resistance r are connected in parallel across an external resistance R. Find equivalent EMF, internal resistance and current in external resistance R.
Q.33 Three cells of EMF 3 V, 4 V, and 6 V are connected in parallel, if there are internal resistances are 1 Ohm, 2 Ohm, 1 Ohm. Find the equivalent EMF, internal resistance and the current in the external load R= 1.6 ohm. Ans. I = 1.4 A , Eeff= 2.8, reff= 0.4
Questions on Wheatstone & MeterBridge
Q.34 In metre bridge, null point is found at a distance of 60 cm from A. If now a resistance of 5 ohm is connected in series with S, then null point occurs at 50 cm. Determine the values of R and S. Ans. S= 10Ohm ,R= 15Ohm
Q.35 The resistances in the two arms of the metre bridge are R= 5Ohm and S respectively. When the resistance S is shunted with an equal resistance, the new balance length is found to be 1.5 L1, where L1 is the initial balancing length. Calculate the value of S. Ans. S = 10 Ohm
Q.36 In a metre bridge experiment, Null point is found at a length of 20 cm. In the same experiment when the known resistance R is shunted by 10 ohm resistance, Null point is found to be shifted by 10 cm. Find the unknown resistance X. Ans. X= 50Ohm
Questions on potentiometer
Q.37 A potentiometer wire is 10 m long and a potential difference of 6 V is maintained between its ends. Find the EMF of the cell which balances against a length of 180 cm of the potentiometer wire. Ans. 1.08V
Q.38 A 10 m long wire of uniform cross-section of 20 ohm resistance is used as a potentiometer wire. This wire is connected in series with a battery of 5 V, along with an external resistance of 480 ohm. If an unknown resistance EMF E is balanced at 600 cm of the wire, calculate the potential gradient of the potentiometer wire and the value of unknown EMF E. Ans. K = 0.02V/m , E = 0.12V
Q.39 A cell can be balanced against 110 cm and hundred centimetre of potential meter wire respectively when in open circuit and when short-circuit it through a resistance of 10ohm. Find the internal resistance of the cell. Ans. 1 ohm
Q.40 The length of a potentiometer wire is 1200 cm and it carries a current of 80 mA. For a cell of EMF 4.0 V and internal resistance 20 ohm, the Null point is found to be at 1000 cm. If a voltmeter is connected across the cell, the balancing length is decreased by 20 cm. Find i) the resistance of the whole wire ii) reading of the voltmeter, iii) resistance of voltmeter. Ans. i) 60ohm , ii) 3.92 V, iii) 980 ohm
Q.41 In a potentiometer arrangement, a cell of EMF 1.20 V gives a balance point at 30 cm length of the wire. This cell is now replaced by another cell of unknown EMF. If the ratio of EMF of two cells is 1.5, calculate the difference in the balancing length of the potential meter wire in the two cases.
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