CBSE Class 10 Science Electricity, Magnetic Effects, Light and the Human Eye Important Questions Set 02

Answer: Ohm's law: at constant temperature, the current through a conductor is directly proportional to the potential difference across its ends, i.e. V ∝ I or V = IR, where R is the resistance.
Experiment: connect a cell, key, ammeter, the resistor and a rheostat in series, and a voltmeter in parallel with the resistor. Close the key and set the rheostat to get a small current. Note the ammeter and voltmeter readings. Change the rheostat to take five or six pairs of readings, and calculate V/I each time. A graph of V (x-axis) against I (y-axis) is a straight line through the origin and V/I is constant, which verifies Ohm's law. The slope of the line gives the resistance R.

Teacher's Note:
1. The voltmeter is always connected in parallel and the ammeter in series.
2. Doubling the potential difference doubles the current for the same conductor.

Answer: (a) Electric charge is the property of matter that causes electric and magnetic effects; its SI unit is the coulomb (C), where Q = ne and e = 1.6 × 10⁻¹⁹ C.
(b) Electric current is the rate of flow of charge through a conductor, I = Q/t; its SI unit is the ampere (A), which is one coulomb per second.
(c) Potential difference between two points is the work done to move a unit positive charge from one point to the other, V = W/Q; its SI unit is the volt (V), which is one joule per coulomb.

Teacher's Note:
1. Current is charge per unit time and voltage is energy per unit charge.
2. Write the formula along with every definition.

Answer: Resistance is the property of a conductor that opposes the flow of current through it. Its SI unit is the ohm (Ω), where 1 Ω = 1 V/A. It arises from collisions of the moving electrons with the ions of the conductor.
For a uniform conductor R = ρl/A, so resistance depends on:
1. Length l (directly proportional).
2. Area of cross-section A (inversely proportional), so thin wires have more resistance than thick ones.
3. Nature of the material, expressed by its resistivity ρ; alloys like nichrome have high resistivity and copper and silver have low.
4. Temperature; the resistance of metals rises with temperature.

Teacher's Note:
1. Resistivity depends on the material and temperature, not on the shape or size.
2. Alloys are used in heating devices because of their high resistivity.

Answer: Electric power is the rate at which electrical energy is consumed, P = W/t = VI = I²R = V²/R. Its SI unit is the watt (W), where 1 W = 1 J/s.
Electric energy is the total work done by the current, E = P × t = VIt. Its SI unit is the joule (J).
Relation: energy = power × time. The commercial unit is the kilowatt-hour (kWh): 1 kWh = 3.6 × 10⁶ J, and one kWh is called one unit of electricity.

Teacher's Note:
1. Check that kilowatt and hour are used together to get kWh.
2. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.

Answer: When current flows through a resistor, electrical energy is converted into heat. The work done in moving charge Q through potential difference V is W = VQ = VIt. Using Ohm's law V = IR, the heat produced is H = I²Rt (Joule's law of heating).
Applications: (1) electric heaters, irons and toasters use high-resistance nichrome coils; (2) the filament of a bulb, made of tungsten, glows by heating; (3) a fuse wire melts when excess current flows and protects appliances.

Teacher's Note:
1. Heat produced depends on the square of the current.
2. It also depends directly on the resistance and the time.

Answer: (a) Series: the same current I flows through each resistor and the total potential difference is the sum: V = V₁ + V₂ + V₃. Using V = IR for each, IR_s = IR₁ + IR₂ + IR₃, so R_s = R₁ + R₂ + R₃.
(b) Parallel: the potential difference V is the same across every resistor and the total current is the sum: I = I₁ + I₂ + I₃. With I = V/R, V/R_p = V/R₁ + V/R₂ + V/R₃, so 1/R_p = 1/R₁ + 1/R₂ + 1/R₃.

Teacher's Note:
1. In series the equivalent resistance is larger than any single resistor.
2. In parallel it is smaller than the smallest one.

Answer: A magnetic field is the region around a magnet or a current-carrying conductor in which its magnetic influence can be felt by another magnet.
Properties of field lines:
1. They emerge from the north pole and merge at the south pole outside the magnet; inside, they run from south to north.
2. They form closed continuous curves.
3. They never cross each other, because the field at a point has only one direction.
4. Closer lines mean a stronger field, so they are crowded near the poles.
5. The tangent to the line at any point gives the direction of the field there.

Teacher's Note:
1. Field lines are imaginary lines used to represent the field.
2. They never intersect since two directions at one point are impossible.

Answer: A solenoid is a long cylindrical coil of many closely wound turns of insulated copper wire. When current flows, the field pattern resembles that of a bar magnet: one end behaves as a north pole and the other as a south pole. Inside the solenoid the field lines are parallel straight lines, so the field is uniform.
Its strength increases with (a) the current through it, (b) the number of turns per unit length and (c) a soft iron core inside it, which makes an electromagnet.

Teacher's Note:
1. The uniform field inside is the special feature of a solenoid.
2. A soft iron core greatly increases the strength.

Answer: Concave mirror: (1) used as reflectors in torches, headlights of vehicles and searchlights to give a parallel beam; (2) used by dentists to see an enlarged image of teeth; also as shaving mirrors and in solar furnaces.
Convex mirror: (1) used as rear-view mirrors in vehicles because they give an erect image and a wider field of view; (2) used in shops and at road bends for surveillance.

Teacher's Note:
1. A concave mirror can give an enlarged image when the object is near.
2. A convex mirror always gives a smaller erect image and a wide field of view.

Answer: 1. The incident ray, the refracted ray and the normal to the interface at the point of incidence all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for light of a given colour and for the given pair of media: sin i / sin r = constant. This is Snell's law, and the constant is the refractive index of the second medium with respect to the first.

Teacher's Note:
1. The ratio is of sines, not of angles.
2. The constant is the refractive index and has no unit.

Answer: The refractive index of medium 2 with respect to medium 1 is the ratio of the speed of light in medium 1 to its speed in medium 2. The absolute refractive index of a medium is n = c/v, where c is the speed of light in vacuum and v is the speed in the medium. It is a pure number and has no unit. It depends on the nature of the medium, the wavelength (colour) of the light and the temperature.

Teacher's Note:
1. A higher refractive index means light travels slower in that medium.
2. Water is about 1.33 and glass about 1.5.

Answer: The power of a lens is the reciprocal of its focal length in metres: P = 1/f. Its unit is the dioptre (D). One dioptre is the power of a lens whose focal length is 1 m. A convex lens has positive power and a concave lens has negative power.
Uses: convex lenses are used in magnifying glasses, cameras, microscopes and spectacles for hypermetropia; concave lenses are used in spectacles for myopia and in peepholes.

Teacher's Note:
1. Always convert the focal length to metres before finding the power.
2. The power of a combination of lenses in contact is the algebraic sum.

Answer: The ability of the eye lens to adjust its focal length, by the action of the ciliary muscles, so as to focus objects at different distances on the retina is called the power of accommodation. The least distance of distinct vision (near point) is the minimum distance at which an object can be seen clearly without strain; for a normal adult eye it is about 25 cm.

Teacher's Note:
1. The far point of a normal eye is at infinity.
2. The near point of a normal eye is about 25 cm.

Answer: Myopia (near-sightedness): the eye sees near objects clearly but not distant ones, because the image forms in front of the retina. Causes: the eyeball is too long, or the eye lens is too curved (focal length too short). Correction: a concave lens of suitable power.
Hypermetropia (far-sightedness): the eye sees distant objects clearly but not near ones, because the image forms behind the retina. Causes: the eyeball is too short, or the focal length of the eye lens is too long. Correction: a convex lens of suitable power.

Teacher's Note:
1. The position of the image relative to the retina tells the defect.
2. Myopia needs a diverging lens and hypermetropia needs a converging lens.

Answer: The Tyndall effect is the scattering of light by the particles of a colloid or by fine suspended particles, which makes the path of a light beam visible, for example sunlight through mist in a forest or a thin beam entering a dusty room. The colour of the scattered light depends on the size of the scattering particles and the wavelength of the light: very fine particles scatter mainly blue light, while larger particles scatter longer wavelengths and look whitish.

Teacher's Note:
1. Scattering is greater for shorter wavelengths.
2. This is why blue light scatters more than red light.

Answer: Sunlight is scattered by air molecules and very fine particles in the atmosphere, which scatter shorter wavelengths (blue) much more than longer wavelengths (red). The scattered blue light reaches our eyes from all directions, so the sky looks blue.
At sunrise and sunset the Sun is near the horizon and its light travels a longer path through the atmosphere. Most of the blue light is scattered away, and the light that reaches us is rich in red and orange, so the Sun appears red. At noon the path is shorter and the Sun looks white.

Teacher's Note:
1. The shorter wavelength scatters most.
2. A longer path through air removes more blue from the direct sunlight.