Download complete note of Science
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1. What is magnetic field? Ans: The space around a magnet where a force due to the magnet is observed is called the magnetic field.
2. What is magnetic line of force? Ans: The curved path followed by the North Pole in a magnetic field is called the magnetic line of force.
3. Draw the diagrams of the magnetic lines of force of a bar magnet in its magnetic field with its north pole facing towards the geographical North Pole.
Ans: If the lines of force are drawn by pointing the North Pole of bar magnet facing towards the geographical north, we get magnetic field pattern as shown in the figure below.
From the figure, it is seen that the horizontal component of the earth's magnetic field and the magnetic field due to the bar magnet are in opposite direction. The magnetic field due to earth remains constant while the magnetic field due to the bar magnet is strong near the pole and it weakens as the distance from the magnet increases.
4. What does it mean that the angle of dip in Kathmandu is 42° ?
Ans: It means that the magnetic needle makes an angle of 42° with the horizontal in Kathmandu.
5. What is a generator or dynamo?
Ans: A generator or dynamo is a device that converts mechanical energy into electrical energy.
6. What is magnetism?
Ans: Magnetism is the property of the magnet which affects another magnet or magnetic material within a field around it.
7. What is meant by the angle of declination? Ans: The angle between true north and the direction in which the compass needle points is called angle of declination.
8. What is meant by the angle of dip?
Ans: The angle made by the dip needle with the horizontal is called angle of dip.
9. State any three evidences which prove the presence of the magnetic property on the earth.
Ans: The following evidences prove the presence of the magnetic property on the earth: A freely suspended magnet always points toward N-S direction by the influence of terrestrial magnetism on it. Some iron ores possess magnetism which may be due to the effect of the earth's magnetism. Neutral are obtained on the magnetic field of a magnet which is due to the combined effects of its magnetic field and earth's magnetism.
10. Why the needle shows different values at different places of the earth?
Ans: Magnetic poles may be the region of the earth where the deep needle is shown, because angle of dip is 90ᵒ at poles. In the figure dip needle make an angle of 90ᵒ with the horizontal. Angle of dip is the angle made by a freely suspended dip needle with horizontal. It is measured with a device called dip needle. The angle of dip is not same everywhere on the surface of the earth because the earth's magnetic field varies from place to place on the surface of the earth.
11. Write any two factors which affect resistance. If the power of the bulb used in a car is 50 watt and 5 volt battery is used on it what will be the resistance of the bulb?
Ans: Factors which affect resistance are as followings:
Cross-sectional area of wire
The resistance (R) of a conductor is inversely proportional to the cross-sectional area (A) of the conductor.
R ∝ 1/A Length of conducting wire
Resistance is directly proportional to the length of conducting wire. .
12. State ohm's law. Prove that R=ρ l/A
Ans: Ohm's law states that the electric current passing through a conductor is directly proportional to the potential difference across its two ends at a condition physical condition (temperature, cross-sectional area, length, nature of material etc.)
13. Study the given diagram and answer the following questions.
What is the mistake made in the diagram?
Identify the north and South Pole of the magnet.
The magnetic lines of forces are closer near the poles, why?
Ans: Magnetic lines of force will never intersect. But in figure, two magnetic lines of force are intersecting.
Magnetic lines of force start from the North Pole and reach the South Pole of a magnet externally. So A is the North Pole and B is the South Pole of given magnet.
The magnetic lines of forces are closer near the poles, because magnetic field is strong in the poles.
14. Calculate the resistance of the device x if 0.3A of current flows through it when the potential difference across it is 9 volt.
Solution: Here, Given,
Potential difference (V) = 9 V
Current (I) = 0.3 A
We have,
From Ohm's law,
V = RI
Or, 9 = R х 0.3
R = 30 Ω
Hence the resistance is 30 Ω.
15. Write any three properties of magnetic lines of force.
Ans: Properties of magnetic lines of force are as followings:
#They start from the North Pole and end at the South Pole of a magnet.
#Lines of force never intersect each other.
#They are crowded near the poles where the magnetic field is strong and get further separated where the magnetic field becomes weak.
16. The length of conductor wire is 3.42 meter. If the diametre of the wire is 0.44 mm and the resistance is 10 ohm. What is the resistivity of the wire?
Ans: Do yourself.
17. Define resistance of a conductor.Ans: The resistance of the conductor is defined as the property by the virtue of which the conductor opposes the flow of current through it.
18. Define resistivity of a material.Ans: Resistivity of a material is defined as the resistance offered by a conductor having the unit length and unit cross section area.
19. What causes a body to be negatively charged? Ans: When a body gains electrons from another body, it causes a body to be negatively charged.
20. If 100 Watt is written on an electric bulb .What do you mean by it?
Ans: It means that the bulb can convert 100 Joules electrical energy into heat and light energy in one second.
21. What is current electricity?
Ans: When there is continuous flow of charge from one body to another through a material, then the electricity is called current electricity.
22. What is alternating current (AC)?
Ans: Alternating current (AC) is that current which changes its magnitude continuously and reverses its direction periodically.
23. Study the schematic diagram of an electric circuit and answer the questions given below.
I. Which physical quantity is indicated by 3V?
ii. Name the instrument used for detection of the flow of charges.
iii. If the resistance of the bulb is 2Ω, calculate the magnitude of the current that flows in the circuit.
Ans: i.
The physical quantity indicated by 3V is potential difference. ii.
The instrument used for detection of flow of charges is termed as electrometer. iii.
Here,
Given,
Resistance (R) = 2Ω
Potential difference (V) = 3V
Current (I) =?
We have,
I = V/R = 3/2 = 1.5A
Hence, magnitude of current flowing in the circuit is 1.5 A.
24. What is electricity? Write two defects of a simple cell.
Ans: The form of energy that results from the existence of charged particles is electricity.
The two defects of a simple cell are as follows:
a. Local action
b. Polarization
25. How does a body become charged?
Ans: A body becomes charged by the transfer of electrons from one body to another body.
26. Write three factors affecting the resistance of conductor wire. Mention the relation of the resistance and these factors.
27. What is a potential difference?
Ans: The potential difference between two points of an electric circuit is defined as the amount of work done per unit charge that flows from one point to the other.
28. What do you understand by electromotive force?
Ans: Electromotive force of a cell is defined as the energy supplied by the cell to drive a unit charge round the whole circuit.
29. What is static electricity?
Ans: The electricity due to the collection of charge on a body is called static electricity.
30. What do you mean by hydro-electricity?
Ans: The electricity generated by the energy of water flowing under gravity to drive turbines and hydroelectric generators is called hydroelectricity.
31. What is meant by terrestrial magnetism?
Ans: The magnetism associated with the earth is called terrestrial magnetism
32. What is neutral point?
Ans: A neutral point is the point at which the magnetic field due to a magnet is equal and opposite to the earth’s magnetic field.
33. Why does a freely suspended magnet show north and south poles at rest?
Ans: A freely suspended magnet shows North and South Poles at rest because of the earth’s magnetic field.
34. Why is an atom neutral in normal condition?
Ans: An atom is neutral in normal condition because it possesses equal number of positively and negatively charged particles.
35. What is a closed circuit?
Ans: A circuit in which electricity flows is called closed circuit.
36. What do you mean by electric power?
Ans: The electric energy consumed or work done per unit time is called electric power.
37. What is photocell?
Ans: Photocell is a device that converts light energy into electrical energy.
38. What is meant by electric current?
Ans: The rate of flow of electric charge in an electric circuit is called electric current.
39. What do you understand by electric circuit?Ans: The path of flow of electricity starting from one terminal of the cell and returning to the other is called an electric circuit.
40. What is an open circuit?
Ans: A circuit in which there is a break or disconnection is called open circuit.
41. What is a series combination?
Ans: If the electric loads are joined one after another in the circuit so that same current passes through each of them, then such combination is called a series combination.
42. What is a parallel combination?
Ams: If the electric loads are connected to two common points such that the current is divided through each of them, then the combination is called a parallel combination.
43. Study the given figure and answer the following questions. i. Name the devices shown in A and B
ii. If B reads 0.2 ampere, what will be the resistance?
iii. A connected in parallel in electric circuit, why?
Ans:
i.
A is ammeter and B is Voltmeter ii.
Here,
Potential differece (V) = 6V
Current (I) = 0.2A
Resistance (R) =?
We have,
R = V/I = 6/0.2 =30Ω
Hence, the resistance will be 30 Ω
iii.
A voltmeter is always connected in parallel because if it is connected in series, it will change the value of potential difference to be measured by minimizing the current in the circuit as it has very large resistance and hence will give faulty reading. Connecting it in parallel will not change the value of potential difference being measured as the voltmeter will draw negligible current and hence give correct value of potential difference.
44. What is ohm’s law?
Ans: Ohm’s law states that, “the current flowing through a conductor is directly proportional to the potential difference across its ends, provided the temperature and other physical conditions are constant”.
45. What is 1 ohm?
Ans: The resistance of a conductor is said to be one ohm if the current of 1 ampere flows through it with the supplied potential difference of one volt across it.
46. What is an ammeter?
Ans: The instrument which is used to measure the magnitude of current flowing in a circuit is called an ammeter.
47. What is a voltmeter?
Ans: The instrument which measures the potential difference across any two points in a circuit is called a voltmeter.
48. What is a cell?
Ans: A cell is a device that converts chemical energy into electrical energy.