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TOPIC – ELECTRIC CURRENT(ASSIGNMENT)

1. The Earth's Surface has a negative surface charge density of 10-9 C/m2 . The potential difference of 400kV between the top of the atmosphere and the surface , results in a current of only 1800A over the entire globe. If there is no mechanism of sustaining atmospheric electric field ; how much time (roughly) would be required to neutralize the earth's surface ?

2. In a discharge tube, the number of hydrogen ions (i.e. , protons) drifting across a cross section per second is 1.0 × 1018 while the number of electron drifting in the opposite direction across another cross section is 27 × 1018 per second . If the supply voltage is 230V , what is the effective resistance of the tube ? Ans: 389 ohm,

3. A negligibly small current is passed through a were of length 15m and of uniform cross-section 6.0 × 10-7 m2 and its resistance is measured to be 5.0 ohm . What is the resistivity of the material at the temperature of the equipment? Ans: 2.0 X 10-7 ohm-m.

4. Two wires of equal length one of copper and the other of aluminum, have the same resistance. Which of the two wires as lighter? Hence , explain why aluminum wires are preferred for overhead power cables. ( pal= 2.63 × 10-8 ohm-m, p cu= 1.72× 10-8 ohm-m , relative density of Al = 2.7 , Cu = 8.9 )

5. A cylindrical aluminum shell is made of two coaxial cylinders of inner radius 1.0cm 2.0 cm and length 3.0m . Estimate the resistance of the conductor ( between the cylinders ) , given that resistivity of aluminum is 2.63 × 10 -8ohm-m at room temperature. (Ans.: 9.7× 10 -10 ohm. )

6. A cylindrical aluminum shell is made of two coaxial cylinders of inner radius 1.0cm and outer radius 2.0 cm and length 3.0m . Estimate the resistance of the conductor (between the cylinders) , given the resistivity of aluminum is 2.63 × 10-8 ohm-m at room temperature.

7. A carbon resistor has coloured stripes as shown in fig. What is its resistance?

8. The sequence of bands marked on a carbon resistor is brown, black, brown , gold . what is the value of the resistance? Also give the tolerance.

9. A conductor with the cross - section of 10-4 m2 carries an electric current of 1.2 A . If the number of free electrons be 5× 1028 per m3 ,calculate the electron drift velocity .

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10. In a circuit given in fig. the cells E1 and E2 have emf of 4V and 8V and internal resistances 0.5ohm and 1ohm respectively. Calculate the current in each resistance.

11. A letter ‘A’ consists of a uniform wire of resistance 1 ohm/cm . Each side of the letter is 20cm long and the cross - piece in the middle is 10cm long while the apex angle is 60°. Find the resistance of the letter between the ends of the legs.

12. Calculate the steady current in the 2 ohm resistor shown in fig. The internal resistance of the battery is negligible and the capacitance C in 0.2 µF .

13. Given n resistors , each of resistance R ; how will you combine them to get : (i) maximum (ii) minimum effective resistance? what is the ratio of maximum to minimum resistance ?

14. A parallel combination of three resistors takes a current of 7.5A from a 30V supply. If the two resistors are 10ohm and 12 ohm , Find the third one .

15. Two coils have a combined resistance of 9ohm when connected in series and 2ohm when connected in parallel . Find the resistance of each coil.

16. In a factory , 220 bulbs are connected in parallel and each bulb takes a current of 0.1 A at 220 V . Calculate the combined resistance of all the bulbs and current in the leads.

17. A battery of E and internal resistance r gives a current of 0.5amp with an external resistance of 12 ohm , and a current of 0.25 amp with an external resistance of 25 ohm . Calculate: (a) internal resistance off the cell and (b) emf of the cell.

18. Two identical cells of emf 1.5V each joined in parallel provide supply to an external circuit consisting of 2 registers of 17 ohm each joined in parallel. A very high resistance voltmeter read the terminal voltage of the cell to be 1.4 V . What is the internal resistance of a cell?

19. 20 cells , each of 2V emf and 0.5 ohm internal resistance are given . How will you arrange them in mixed grouping so as to send maximum current through a 2.5 ohm external resistance? Calculate the value of the current.

20. A battery of emf 2V and internal resistance 0.1 ohm is being charged with a current of 5 amp . What is the Potential difference between the terminals of the battery?

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