Physics Test 205

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Physics Test 205

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Question 1
4 / -1

Materials suitable for making electromagnets should have

A
high retentivity and high coercivity

B
high retentivity and low coercivity

C
low retentivity and low coercivity

D
low retentivity and high coercivity

Solution

The materials suitable for making electromagnets should have high retentivity and low coercivity. Coercivity is usually measured in oersted or ampere/meter units. When the coercive field of a ferromagnet is large, the material is said to be a hard or permanent magnet.
Question 2
4 / -1

A parallel plate capacitor is made of two circular plates separated by a distance of 5 mm and with a dielectric of dielectric constant 2.2 between them. When the electric field in the dielectric is 3 × 10⁴ V/m, the charge density of the positive plate will be close to

A
6 × 10⁴ C/m²

B
6 × 10^-7 C/m²

C
3 × 10^-7 C/m²

D
3 × 10⁴ C/m²

Solution

Here, dielectric constant, K = 2.2 and E = 3 × 104 V m^-1

Electric field between the parallel plate capacitor with dielectric, E = σ / Kε₀

So, charge density, σ = Kε₀E = 2.2 × 8.85 × 10^-12 × 3 × 10⁴ = 5.8 × 10^-7 or 6 × 10^-7 C m^-2.
Question 3
4 / -1

A circular loop of radius 0.3 cm lies parallel to a much bigger circular loop of radius 20 cm. The centre of the small loop is on the axis of the bigger loop. The distance between their centres is 15 cm. If a current of 2.0 A flows through the smaller loop, then the flux linked with bigger loop is

A
6.6 x 10^-9 weber

B
9.1 x 10^-11 weber

C
6 x 10^-11 weber

D
3.3 x 10^-11 weber

Solution
Question 4
4 / -1

A thin wire of length L and mass M is bent to form a semicircle. What is the magnitude of gravitational field at the centre?

A
πGM/L along the line joining the ends

B
πGM/L² perpendicular to the line joining the ends

C
2 πGM/L² along the line joining the ends

D
2 πGM/L² perpendicular to the line joining the ends

Solution

Resolve dF into dF cosθ and dF sinθ

Since, a symmetrical element is present on the other side, the vertical components cancel each other. The only component is horizontal.

∴ Field at the centre due to element

So, magnitude of the gravitational field at center is perpendicular to the line joining the ends.
Question 5
4 / -1

Two cars of masses m₁ and m₂ are moving in circles of radii r₁ and r₂, respectively. Their speeds are such that they make complete circles in the same time t. The ratio of their centripetal acceleration is

A
m₁ : m₂

B
r₁ : r₂

C
1 : 1

D
m₁r₁ : m₂r₂

Solution
Question 6
4 / -1

Assume that a drop of liquid evaporates by decrease in its surface energy, so that its temperature remains unchanged. What should be the minimum radius of the drop for this to be possible? (The surface tension is T, density of liquid is ρ and L is its latent heat of vaporisation)

A

B

C

D

Solution

Change in the surface energy will be utilised for latent heat of evaporation, if dA is change in the surface area and dm is the mass of volume element.
Question 7
4 / -1

A wooden wheel of radius R is made of two semicircular parts (see figure).

The two parts are held together by a ring made of a metal strip of cross-sectional area S and length L. L is slightly less than 2πR. To fit the ring on the wheel, it is heated so that its temperature rises by ΔT and it just steps over the wheel. As it cools down to surrounding temperature, it presses the semi-circular parts together. If the coefficient of linear expansion of the metal is ∝ and its Young's modulus is Y, the force that one part of the wheel applies on the other part is

A

B

C

D

Solution

From FBD of one part of the wheel, F = 2T, where F is the force that one part of the wheel applies on the other part.

∴ F =
Question 8
4 / -1

A shell at rest explodes. The centre of mass of the fragments

A
moves along a parabolic curve

B
moves along an elliptical path

C
moves along a straight line

D
remains at rest

Solution

Blasting and fragmentation is something in which no external force is needed. It occurs due to some chemical reaction (not in all cases) going on inside the bomb. Since no external force is there, so net change in momentum for the bomb will be zero. So, all fragments will move in such a way that their net momentum (we know momentum is a vector) should remain the same or you can also say that centre of mass moves with the same velocity as it was moving with before the explosion. In this question, since the bomb is at rest before the explosion, so even after the explosion, its centre of mass must be at rest to conserve the momentum.
Question 9
4 / -1

Resistance of a given wire is obtained by measuring the current flowing in it and the voltage difference applied across it. If the percentage errors in the measurement of the current and the voltage difference are 3% each, then error in the value of resistance of the wire is

A
zero

B
1%

C
3%

D
6%

Solution
Question 10
4 / -1

Directions For Questions

Directions: This question has Statement 1 and Statement 2. Of the four choices given after the statements, choose the one that best describes the two statements

...view full instructions

If two springs S₁ and S₂ of force constants k₁ and k₂, respectively, are stretched by the same force, then it is found that more work is done on spring S₁ than on spring S₂.

Statement 1: If stretched by the same amount, work done on S₁ will be more than that on S₂.

Statement 2: k₁ < k₂

A
Statement 1 is true, but Statement 2 is false.

B
Statement 1 is true, but Statement 2 is true. Statement 2 is the correct explanation of Statement 1.

C
Statement 1 and Statement 2 are true. Statement 2 is not the correct explanation of Statement 1.

D
Statement 1 is false, but Statement 2 is true.

Solution