Ohms law numericals

OHMS LAW
1.definition
potential applied is directly proportional to current flowing in a conductor
2.V ∝ i
3,v=ir
removing proportinality constant keep a constant r
4.drawing graph between v and i we get a staright line passing through origin
5.straight line is called slope
6.slope of vi graph is called resistance
7 substances which obey ohms law are ohmice conductors
examples many metals
8.substances which do not obey ohms law are non ohmic conductors
example.transistor diode,filament,led,
9.for ohmic conductors graph is straight line
and non ohmic graph wont be straight line
10.least count of ammeter or voltmeter
formula is
least  ount= reading/divisions
11.instument which has less least counts t is best instrument

Ohm’s Law Applications

The main applications of Ohm’s law are:

To determine the voltage, resistance or current of an electric circuit.

Ohm’s law is used to maintain the desired voltage drop across the electronic components.

Ohm’s law is also used in DC ammeter and other DC shunts to divert the current.

Limitations of Ohm’s Law

Following are the limitations of Ohm’s law:

Ohm’s law is not applicable for unilateral electrical elements like diodes and transistors as they allow the current to flow through in one direction only.

For non-linear electrical elements with parameters like capacitance, resistance etc the voltage and current won’t be constant with respect to time making it difficult to use Ohm’s law.

When does Ohm’s law fail?

Ohm’s law fails to explain the behaviour of semiconductors and unilateral devices such as diodes. Ohm’s law may not give the desired results if the physical conditions such as temperature or pressure are not kept constant.

Worksheet for practice

1.find potential when current of 3 mA flows across a resistance of 1000ohms ?
2.find potential when 5 c of charge flows for 2 secs across a resistance of 4 ohms?
3.find resistance when a current of 5 amp flows across a p d of 110v?
4.find potential when 10 power 19 electrons flow for 10 seconds acoss a resistance of 12ohms?
5.find current flowing in a conductor when potential of 220v applied across a resistance of 12ohms?
6.find resistance when slope makes an angle of 30 degrees alon x axis in x y plane?
7.what happens to current if potential applied is reduced to half or doubled?
8.what potential is applied in india ?
9.what is source of potential in circuit?
10.what is use of neutral wire in along with live wire?
11.what is difference between potential and potential difference?
12.what is work done if 10 col of charge flows and 5 milli amp of current flows across 100 ohms resistance?
13.what is work done if 10 power 20 electrons flows across carrying 5 milli ampere a over 1000 ohms resistance?
14.Give an example of non-ohmic devices
Ans is transistors and diodes
15.what is slope of v-i graph gives .What is conductance and its units?
16.what instrument is used to measure resistance and how do you denote it?
17.The values of currents flowing through a given resistor of resistance R for corresponding values of potential differences v across the resistors are given below
0.5v - 1 amp
1v - 0.2 amp
1.5v - 0.3 amp
2v - 0.4 amp
2.5 v - 0 5 amp
3v - 0.6 amp
4 v - 0.8 amp
5v - 1 amp
Plot a graph between v and I and determine the reistance
18.What would you suggest to a student if while performing an experiment if while performing a experiment the pointer needle of ammeter and voltmeter do not coincide with the zero mark on the scale when circuit is open.and there is no extra ammeter and voltmeter is available in the laboratory

19.Calculate the resistance if 5 A of electric current flows through a conductor having potential difference between two points is equal to 15 V.

20.If the potential difference between the terminals of an electric motor is 220 V and an electric current of 5 A is flowing through it what will be the resistance of electric motor

21.An electric current of 15 A is flowing through an electric fan. If the potential difference between two terminals of electric fan is 240 V, what will be resistance?

22.If the resistance of an electric iron is 48 Ω and an electric current of 5 A is flowing through it, what will be the potential difference between two terminals of electric iron.

23.Calculate the potential difference between two points of a terminal, if an electric current of 10 A is flowing through it having resistance of 20Ω.

24.If the resistance between two terminals of an electric heater is 15Ω and an electric current of 15 A is flowing through it then what will be the voltage of electric current?

25.0.6A current flows through a resistor shown in figure. Voltage of two points of resistor is 12V. What is the resistance of the resistor?

26 Resistance of an electric iron 50 Ω.4.2A Current flows through the resistance. Find the voltage between two points.

27.Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key, all connected in series.

28.cell is connected to a bulb which develops a potential difference of 24 volts across it. The current in the circuit is measured to be 4 A. Find the resistance offered by the filament of the bulb to the flow of the current.

29. Calculate the potential difference across the ends of a wire of resistance 1 ohm when a current of 3 A passes through it.

30 Calculate the current flowing through a wire of resistance 15 ohm connected to a battery of potential difference 3 volt.

31.cell of potential difference 12 V is connected to a bulb. The resistance of the filament of the bulb when it glows is 24 ohm. Find the current drawn from the cell.

32. A bulb draws current 1.5 A at 6 V. Find the resistance of filament of bulb while glowing?

33. A current 0.2 A flows in a wire of resistance 15 ohm. Find the potential difference across the ends of the wire.

34.When a 24 V battery is connected to a resistor, the current in it is 0.4 A. What is the resistance of the resistor? What would be the current through it when it is connected to a battery of 6 V?

35.R1 = 10 ohm and R2 = 5 ohm in the diagram. Cell voltage is 7.5 V

Find out (a)the equivalent resistance (b) the electric current (c) the potential difference across each resistor in the circuit shown in figure (1).

36. Two resistances of 6 ohm and 12 ohm are connected in parallel. Calculate the equivalent resistance.

37.. On 1 ohm resistor is connected in parallel to a 11 ohm resistor. What is equivalent resistance?

38.Two resistors of resistances R1 =10 ohm and R2= 20 ohm are connected in parallel. A battery supplies 6 A of current to this resistance combination as shown in figure 2. Calculate the current in each resistor.

39.. A uniform wire of resistance R is cut into two equal pieces, and these pieces are joined in parallel. What is the resistance of the combination?

40. How will you join the resistors of resistances 3 ohm, 6 ohm and 8 ohm to get an equivalent resistance of 10 ohm?

41. 3 resistors, each of resistance 12 ohm, are connected in parallel. What is the equivalent resistance?

42..In the following circuit, R1 = 15 ohm and R2=30 ohm.(a) The equivalent resistance (b)The current passing the cell and, (c) the current passing through the 30 ohm resistor in the circuit shown above.

43. R1 = 20 ohm and R2 = 15 ohm in the circuit shown in figure 4 below.
The battery cell provides 7 V.
Find the current through the circuit. Also find the potential difference across the 20 ohm resistor.

44. A metal wire of resistance 6 ohm is stretched so that its length is increased to twice its original length. Calculate its new resistance.

45. How is the resistance of a wire affected if its (a) length is doubled. (b) radius is doubled? (c) length and radius both are doubled

46. What is the current in milliamperes produced by the solar cells of a pocket calculator through which 5.00 C of charge passes in 5.00 h?

47. A total of 1200 C of charge passes through a flashlight in 1 h. What is the average current?

48.What is the current when a typical static charge of 0.125 μC moves from your finger to a metal doorknob in 0.5 μs ?

49.Find the current when 2.00 nC jumps between your comb and hair over a 0.500 – μs time interval.

50. A large lightning bolt had a 20,000-A current and moved 30.0 C of charge. What was its duration?

51.The 200-A current through a spark plug moves 0.300 mC of charge. How long does the spark last?

52.the resistance of an electric iron is 50 Ω and a current of 3.2 A flows through the resistance. Find the voltage between two points.

53.An EMF source of 8.0 V is connected to a purely resistive electrical appliance (a light bulb). An electric current of 2.0 A flows through it. Consider the conducting wires to be resistance-free. Calculate the resistance offered by the electrical appliance.

54.

light numericals

Q.1. A convex lens of focal length 10 cm is placed at a distance of 12 cm from a wall. How far from the lens should an object be placed so as to form its real image on the wall?

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Q.2. If an object of 7 cm height is placed at a distance of 12 cm from a convex lens of focal length 8 cm, find the position, nature and height of the image.

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Q.3. An object 4 cm high is placed at a distance of 10cm from a convex lens of focal length 20cm. Find the position, nature and size of the image.

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Q.4. A small object is so placed in front of a convex lens of 5 cm focal length that a virtual image is formed at a distance of 25 cm. Find the magnification.

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Q.5. Find the position and nature of the image of an object 5 cm high and 10 cm in front of a convex lens of focal length 6 cm.

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Q.6. Calculate the focal length of a convex lens, which produces a virtual image at a distance of 50 cm of an object placed 20 cm in front of it.

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Q.7. An object is placed at a distance of 100 cm from a converging lens of focal length 40 cm. What is the nature and position of the image?

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Q.8. A convex lens produces an inverted image magnified three times of an object at a distance of 15 cm from it. Calculate focal length of the lens.

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Q.9. An object placed 4 cm in front of a converging lens produces a real image 12 cm from the lens. What is the magnification of the image? What is the focal length of the lens? Also draw the ray diagram to show the formation of the image.

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Q.10. A lens of focal length 20 cm is used to produce a ten times magnified image of a film slide on a screen. How far must the slide be placed from the lens?

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Q.11. Determine how far an object must be placed in front of a converging lens of focal length 10 cm in order to produce an erect image of linear magnification 4.

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Q.12. A convex lens of focal length 6 cm is held 4 cm from a newspaper, which has print 0.5 cm high. By calculation, determine the size and nature of the image produced.

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Q.13. A convex lens of focal length 0.10 m is used to form a magnified image of an object of height 5 mm placed at a distance of 0.08 m from the lens. Find the position, nature and size of the image.

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Q.14. An erect image 2 cm high is formed 12 cm from a lens, the object being 0.5 cm high. Find the focal length of the lens.

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Q.15. The filament of a lamp is 80 cm from a screen and a converging lens forms an image of it on a screen, magnified three times. Find the distance of the lens from the filament and the focal length of the lens.

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Q.16. An object 2 cm tall is placed on the axis of a convex lens of focal length 5 cm at a distance of 10 cm from the optical centre of the lens. Find the nature, position and size of the image formed. Which case of image formation by convex lenses is illustrated by this example?

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Q.17. A converging lens of focal length 5 cm is placed at a distance of 20 cm from a screen. How far from the lens should an object be placed so as to form its real image on the screen?

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Q.18. An object 5 cm high is held 25 cm away from a converging lens of focal length 10 cm. Find the position, size and nature of the image formed. Also draw the ray diagram.

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Q.19. At what distance should an object be placed from a convex lens of focal length 18 cm to obtain an image at 24 cm from it on the other side? What will be the magnification produced in this case?

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Q.20. The magnification produced by a spherical lens is +2.5. What is the nature of image and lens?

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Q.21. What is the nature of the image formed by a convex lens if the magnification produced by a convex lens is +3?

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Q.22. What is the nature of the image formed by a convex lens if the magnification produced by a convex lens is –0.5?

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Q.23. What is the position of image when an object is placed at a distance of 10 cm from a convex lens of focal length 10 cm?

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Q.24. Describe the nature of the image formed when an object is placed at a distance of 30 cm from a convex lens of focal length 15 cm.

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Q.25. At what distance from a converging lens of focal length 12 cm must an object be placed in order that an image of magnification 1 will be produced?

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Problems on concave lens:

Q.26. A concave lens produces an image 20 cm from the lens of an object placed 30 cm from the lens. Calculate the focal length of the lens.

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Q.27. The magnification of a spherical lens is +0.5. What is the nature of lens and image?

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Q.28. If an object is placed at a distance of 50 cm from a concave lens of focal length 20 cm, find the position, nature and height of the image.

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Q.29. An object is placed at a distance of 4 cm from a concave lens of focal length 12 cm. Find the position and nature of the image.

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Q.30. An object is placed at a distance of 50 cm from a concave lens produces a virtual image at a distance of 10 cm in front of the lens. Draw a diagram to show the formation of image. Calculate focal length of the lens and magnification produced.

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Q.31. A 50 cm tall object is at a very large distance from a diverging lens. A virtual, erect and diminished image of the object is formed at a distance of 20 cm in front of the lens. How much is the focal length of the lens?

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Q.32. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

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Q.33. An object 60 cm from a lens gives a virtual image at a distance of 20 cm in front of the lens. What is the focal length of the lens? Is the lens converging or diverging? Give reasons for your answer.

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Q.34. A concave lens of 20 cm focal length forms an image 15 cm from the lens. Compute the object distance.

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Q.35. A concave lens has focal length 15 cm. At what distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? Also find the magnification produced by the lens.

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Q.36. Calculate the image distance for an object of height 12 mm at a distance of 0.20 m from a concave lens of focal length 0.30 m and state the nature and size of the image.

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Q.37. A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed.

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Q.38. An object is placed 20 cm from (a) a converging lens and (b) a diverging lens of focal length 15 cm. Calculate the image position and magnification in each case.

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Q.39. A 2.0 cm tall object is placed 40 cm from a diverging lens of focal length 15 cm. Find the position and size of the image.

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Q.40. Find the position and size of the virtual image formed when an object 2 cm tall is placed 20 cm from (a) diverging lens of focal length 40 cm and (b) converging lens of focal length 40 cm.

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Q.41. The magnification produced by a spherical lens is +0.75. What is the nature of image and lens?

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Q.42. The magnification produced by a spherical lens and a spherical mirror is +0.8. What is the nature of lens and mirror?

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Q.43. The magnification produced by a spherical lens and a spherical mirror is +2.0. What is the nature of lens and mirror?

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Q.44. The lens A produces a magnification of –0.6 whereas lens B produces magnification of +0.6. What is the nature of lens A and B.

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Q.45. An object is 2 m from a lens which forms an erect image one-fourth (exactly) the size of the object. Determine the focal length of the lens. What type of the lens is this?


Determine the speed of light in diamond if the refractive index of diamond with respect to vacuum is 2.42. Speed of light in vacuum is 3×1083×108m/s.
(ii) Refractive indices of glass, water and carbon disulphide are 1.5, 1.33 and 1.62 respectively. If a ray of light is incident in these media at the same angle (say θ�), then write the increasing order of the angle of refraction in these media.
(iii) (A) The speed of light in glass is 2×1082×108 m/s and in water is 2.25×1082.25×108 m/s.
(a) Which one of the two is optically denser and why ?
(b) A ray of light is incident normally at the water-glass interface when it enters a thick glass container filled with water. What will happen to the path of the ray after entering the glass ? Give reason.
(iv) (B) The absolute refractive indices of water and glass are 4/3 and 3/2 respectively. If the speed of light in glass is 2×1082×108 m/s, find the speed of light in (i) vacuum and (ii) water.


. A concave mirror has a focal length of 20 cm. At what distance from the mirror should a 4 cm tall object be placed so that it forms an image at a distance of 30 cm from the mirror? Also calculate the size of the image formed.

Q.2. (a) Water has refractive index 1.33 and alcohol has refractive index 1.36. Which of the two medium is optically denser? Give reason for your answer.

(b) Draw a ray diagram to show the path of a ray of light passing obliquely from water to alcohol.

(c) State the relationship between angle of incidence and angle of refraction in the above case. 

Q.3. Three mirrors, one plane, one concave and one convex are lying on the table. identify them without touching them or using any other apparatus or device? 

Q.4. A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in closed contact with each other. Calculate the lens power of the combination.

Q.5. An object is placed at a distance of 12 cm in front of a concave mirror of radius ofcurvature 30 cm. List four characteristics of the image formed by the mirror

Q.6. A convex lens of focal length 2.0 m can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?

Q.7. “The magnification produced by a spherical mirror is -3”. List all information you obtain from this statement about the mirror/ image.

LONG ANSWER TYPE QUESTIONS (4 MARKS)

Q.1. (i) A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

(ii) A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Q.2. (a) A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find the position, nature and size of the image formed.

(b) Draw a labelled ray diagram showing object distance, image distance and focal length in the above case.






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