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Q1. Insulators like rubber and glass have resistivity of the order of
Insulators like rubber and glass have very high resistivity, typically in the range of 10¹² to 10¹⁷ Ω m. This means they strongly resist the flow of electric current. In contrast, conductors like copper have very low resistivity (around 10⁻⁸ Ω m). The high resistivity of insulators makes them suitable for covering wires and preventing electric shocks.
Q2. Glass is an
Glass is an insulator because it has very high resistivity and does not allow electric current to flow through it easily. The electrons in glass are tightly bound to their atoms and are not free to move. This is why glass is used in electric bulbs, insulators on power lines, and as a covering for electrical components.
Q3. Filaments of electric bulbs are made of
Filaments of electric bulbs are made of tungsten. Tungsten has a very high melting point (about 3422°C), which allows it to glow at high temperatures without melting. It also has high resistivity, which makes it suitable for producing heat and light. Tungsten is used in incandescent bulbs, halogen lamps, and some other lighting applications.
Q4. Resistance depends on the
Resistance depends on the nature of the material (its resistivity). Different materials have different resistivities—copper has low resistivity, while nichrome has high resistivity. Resistance also depends on the length and cross-sectional area of the wire, but not on the shape, direction of current, or colour.
Q5. Metals and alloys have resistivity of the order of
Metals and alloys have very low resistivity, typically in the range of 10⁻⁵ to 10⁻⁶ Ω m. This means they allow electric current to flow easily. Copper, aluminium, and silver are good conductors with low resistivity. Alloys like nichrome have slightly higher resistivity but are still good conductors compared to insulators.
Q6. Alloys are preferred in heating devices because they
Alloys are preferred in heating devices because they do not oxidise readily at high temperatures. Unlike pure metals, alloys are more resistant to oxidation and can withstand high temperatures without degrading. Nichrome, for example, forms a protective oxide layer that prevents further oxidation, making it ideal for heating elements.
Q7. For same material and voltage, thinner wire carries
For the same material and voltage, a thinner wire carries less current because it has higher resistance. Resistance is inversely proportional to the cross-sectional area (R ∝ 1/A). A thinner wire has a smaller area, so its resistance is higher, resulting in less current flow according to Ohm’s law (I = V/R).
Q8. A component of same size offering higher resistance is a
A component of the same size offering higher resistance is a poor conductor. Poor conductors have high resistivity and do not allow current to flow easily. Nichrome and other alloys are poor conductors compared to copper and silver. Insulators have extremely high resistance and are not considered conductors at all.
Q9. Resistance of a uniform conductor is proportional to
The resistance of a uniform conductor is directly proportional to its length (R ∝ l). This means if the length of a wire is doubled, its resistance also doubles. This is because electrons have to travel a longer distance and collide with more atoms, increasing the opposition to current flow.
Q10. When the length of wire is doubled, the ammeter reading becomes
When the length of a wire is doubled, its resistance doubles (R ∝ l). Since current is inversely proportional to resistance (I = V/R), if resistance doubles, the current becomes half. So the ammeter reading (which measures current) becomes half of its original value, assuming the voltage remains constant.
Q11. Resistivity is denoted by the symbol
Resistivity is denoted by the Greek letter rho (ρ). It is a material property that indicates how strongly a material opposes the flow of electric current. The SI unit of resistivity is ohm-meter (Ω m). Resistivity is independent of the shape and size of the conductor and depends only on the material and temperature.
Q12. Resistance of a conductor opposes
The resistance of a conductor opposes the flow of electrons (electric current). When electrons flow through a conductor, they collide with atoms and other electrons, losing energy and experiencing opposition. This opposition is called resistance. It converts electrical energy into heat energy, which is why wires get warm when current flows.
Q13. An insulator of the same size offers
An insulator of the same size offers very high resistance. Insulators like rubber, glass, and plastic have very high resistivity, so they strongly oppose the flow of current. This is why they are used to cover wires and prevent electric shocks. They are the opposite of conductors, which offer low resistance.
Q14. Electric irons and toasters use
Electric irons and toasters use alloys like nichrome for their heating elements. Alloys have high resistivity and do not oxidise readily at high temperatures. They can withstand repeated heating and cooling without breaking. Nichrome, an alloy of nickel and chromium, is commonly used because it has high resistance and a high melting point.
Q15. Electrical transmission lines commonly use
Electrical transmission lines commonly use copper and aluminium because they have very low resistivity and high conductivity. This allows electricity to be transmitted over long distances with minimal energy loss. Copper is used for household wiring, while aluminium is often used for overhead power lines because it is lighter and cheaper.
Q16. Electrons in a conductor are restrained due to
Electrons in a conductor are restrained due to the attraction of atoms (ions). As electrons flow through the conductor, they collide with the vibrating atoms of the metal lattice. These collisions oppose the motion of electrons, creating resistance. The stronger the attraction and the more frequent the collisions, the higher the resistance.
Q17. Resistance increases when
Resistance increases when the length of the conductor increases. This is because R ∝ l—the longer the wire, the more collisions electrons have with atoms, increasing opposition to current flow. Resistance decreases when the cross-sectional area increases (R ∝ 1/A). Temperature also affects resistance—in most metals, resistance increases with temperature.
Q18. Resistance of a conductor depends on
The resistance of a conductor depends on its length (l), cross-sectional area (A), and the material (resistivity ρ). The formula is R = ρl/A. This shows that resistance increases with length and resistivity, and decreases with an increase in cross-sectional area. It does not depend directly on voltage or current.
Q19. Current drawn by a 100 Ω heater from 220 V source is
Using Ohm’s law, I = V/R = 220 V / 100 Ω = 2.2 A. So, a 100 Ω heater draws a current of 2.2 amperes from a 220 V source. This is a typical current for a household electric heater.
Q20. When the nichrome wire is replaced by a 10 W bulb, the ammeter reading
When a nichrome wire is replaced by a 10 W bulb, the ammeter reading changes because the bulb has a different resistance. The bulb’s resistance can be calculated from its power rating (P = V²/R), and it will be different from the nichrome wire. Since current depends on resistance (I = V/R), the ammeter reading will change.
Q21. Current flows more easily through
Current flows more easily through a thick wire because thicker wires have a larger cross-sectional area. Resistance is inversely proportional to area (R ∝ 1/A). A larger area means lower resistance, allowing more current to flow for the same voltage. Thick wires are used in power transmission to reduce energy loss.
Q22. Main reason for using alloys in heaters is their
Alloys are used in heaters because they have high melting points and high resistivity. This allows them to generate significant heat without melting. Nichrome, for example, has a high melting point and high resistivity, making it ideal for heating elements in toasters, irons, and hair dryers.
Q23. Which material has the highest resistivity listed?
Glass has the highest resistivity among the options listed. Glass is an insulator with resistivity in the range of 10¹² to 10¹⁷ Ω m. Nichrome, aluminium, and copper are conductors with much lower resistivity. Nichrome has higher resistivity than aluminium and copper but still much lower than glass.
Q24. Best conductor among iron and mercury is
Iron is a better conductor than mercury. Iron has lower resistivity than mercury, which means it allows current to flow more easily. Mercury is a liquid metal at room temperature but has higher resistivity than iron. However, mercury is still a conductor and is used in some special applications like thermometers and switches.
Q25. SI unit of resistivity is
The SI unit of resistivity is ohm-meter (Ω m). Resistivity is defined as the resistance of a conductor of unit length and unit cross-sectional area. The formula for resistivity is ρ = RA/l, where R is resistance, A is area, and l is length. The unit is Ω × m² / m = Ω m.
Q26. If a wire of resistance 4 Ω is doubled, new resistance is
If a wire of resistance 4 Ω is doubled (folded on itself), its length becomes half, but this question likely refers to doubling the length. If the length is doubled, resistance doubles: R₂ = 2 × R₁ = 2 × 4 Ω = 8 Ω. If the wire is folded on itself (halving length and doubling area), resistance becomes one-fourth (1 Ω). The phrasing suggests doubling length, so the answer is 8 Ω.
Q27. Using a thicker wire of same length and material causes current to
Using a thicker wire of the same length and material causes current to increase. Thicker wire has a larger cross-sectional area, which reduces resistance (R ∝ 1/A). With lower resistance and the same voltage, the current increases according to Ohm’s law (I = V/R).
Q28. Current drawn by a 1200 Ω bulb from 220 V source is
Using Ohm’s law, I = V/R = 220 V / 1200 Ω = 0.1833 A ≈ 0.18 A. So, a 1200 Ω bulb draws a current of about 0.18 amperes from a 220 V source. This is a typical current for a low-power bulb.
Q29. Resistivity values are independent of
Resistivity is a material property and is independent of the length and cross-sectional area of the conductor. It depends only on the material and temperature. The formula R = ρl/A shows that resistance depends on length and area, but resistivity (ρ) remains constant for a given material at a given temperature.
Q30. Components that offer an easy path to electric current have
Components that offer an easy path to electric current have low resistance. Conductors like copper and aluminium have low resistance, allowing current to flow easily. Good conductors are used in wires and electrical connections to minimize energy loss.
Q31. Copper is not used for heating elements because it
Copper is not used for heating elements because it has very low resistivity. This means it conducts electricity too well and would not produce enough heat. Heating elements require materials with high resistivity (like nichrome) to convert electrical energy into heat efficiently.
Q32. Poor conductor among metals listed is
Mercury is a poor conductor compared to copper, silver, and aluminium. It has higher resistivity than these metals. While mercury is still a conductor (not an insulator), it is the poorest conductor among the metals listed. Silver is the best conductor, followed by copper and aluminium.
Q33. Resistance and resistivity vary with
Resistance and resistivity vary with temperature. For most metals, resistivity increases with temperature because the atoms vibrate more, causing more collisions with electrons. For semiconductors, resistivity decreases with temperature. Resistance also depends on length and area, but resistivity itself is a material property that changes with temperature.
Q34. Doubling a wire on itself makes its resistance
Doubling a wire on itself (folding it to half its length and doubling its cross-sectional area) makes its resistance one-fourth of the original. Since R = ρl/A, the new length is l/2 and the new area is 2A. So, R’ = ρ(l/2)/(2A) = ρl/(4A) = R/4. So resistance becomes one-fourth.
Q35. Resistance is inversely proportional to
Resistance is inversely proportional to the area of cross-section (R ∝ 1/A). This means a larger cross-sectional area results in lower resistance, allowing more current to flow. This is why thicker wires have less resistance than thinner wires of the same length and material.
Q36. Different components in the same circuit give different currents because they have
Different components in the same circuit give different currents because they have different resistances. Since all components in a series circuit have the same current, this question likely refers to a parallel circuit. In a parallel circuit, the voltage is the same across each component, but the current through each depends on its resistance (I = V/R).
Q37. A conductor of given size with low resistance is called
A conductor of a given size with low resistance is called a good conductor. Good conductors like copper, silver, and aluminium have low resistivity and allow electric current to flow easily. They are used for wiring and electrical connections.
Q38. Replacing nichrome wire with copper wire of same dimensions causes current to
Replacing nichrome wire with copper wire of the same dimensions causes the current to increase because copper has lower resistivity than nichrome. Lower resistance means more current flows for the same voltage (I = V/R). Copper is a better conductor than nichrome.
Q39. A heater draws 4 A at 60 V. At 120 V, current becomes
Using Ohm’s law, first find resistance: R = V/I = 60 V / 4 A = 15 Ω. At 120 V, I = V/R = 120 V / 15 Ω = 8 A. So, when the voltage doubles, the current doubles (assuming resistance remains constant).
Q40. A thicker wire has
A thicker wire has a larger cross-sectional area. Resistance is inversely proportional to area (R ∝ 1/A). So a larger area means lower resistance. This is why thicker wires can carry more current without overheating.
Q41. The formula R = ρl/A shows resistance depends on
The formula R = ρl/A shows that resistance depends on the geometry (length l and cross-sectional area A) and the material (resistivity ρ). It does not depend directly on current or voltage, though temperature affects resistivity.
Q42. Resistance of a wire decreases when
Resistance of a wire decreases when the cross-sectional area increases. According to R = ρl/A, increasing the area decreases resistance. Changing to an alloy (higher resistivity), increasing length, or increasing temperature (for most metals) would all increase resistance.
Q43. Resistivity of a wire calculated as 1.84 × 10⁻⁶ Ω m corresponds to
A resistivity of 1.84 × 10⁻⁶ Ω m corresponds to manganese. Copper has a resistivity of about 1.72 × 10⁻⁸ Ω m, aluminium is about 2.63 × 10⁻⁸ Ω m, and iron is about 9.71 × 10⁻⁸ Ω m. Manganese has higher resistivity and is used in alloys like manganin.
Q44. Resistivity is a
Resistivity is a material property. It depends only on the nature of the material and its temperature, not on the shape or size of the conductor. Copper has low resistivity, while nichrome and glass have high resistivity. This makes resistivity a characteristic of the material itself.
Q45. When potential difference is doubled, current (resistance constant)
When the potential difference is doubled and resistance remains constant, the current doubles according to Ohm’s law (I = V/R). If V doubles, I doubles. This shows the direct proportionality between voltage and current when resistance is fixed.
Q46. When the length of a conductor is doubled, its resistance
When the length of a conductor is doubled, its resistance becomes double. This is because resistance is directly proportional to length (R ∝ l). Doubling the length means electrons travel twice as far and experience twice as many collisions, doubling the opposition to current flow.
Q47. A conductor with appreciable resistance is called
A conductor with appreciable resistance is called a resistor. Resistors are components specifically designed to have a certain resistance and are used in circuits to limit current, divide voltage, and perform other functions. A resistor’s resistance is its key property.
Q48. If potential difference is reduced to half, current becomes
If the potential difference is reduced to half, the current becomes half (assuming resistance remains constant). According to Ohm’s law, I = V/R. If V is halved, I is also halved. This shows the direct proportionality between voltage and current.
Q49. Alloys generally have resistivity
Alloys generally have higher resistivity than pure metals. This is because the presence of different types of atoms in an alloy disrupts the regular arrangement of atoms, creating more obstacles for electron flow. This is why alloys like nichrome are used in heating elements—their higher resistivity helps generate heat.
Q50. Nichrome has high resistivity, so it is used in
Nichrome has high resistivity, so it is used in heating elements. The high resistance of nichrome causes it to generate heat when current flows through it (Joule heating: H = I²Rt). Nichrome also has a high melting point and does not oxidise readily, making it ideal for heaters, toasters, hair dryers, and electric irons.