Physics Self-Test - Page 11
76- When the distance of operation between the two conductors increases, what happens to the force between the two conductors?
- a) Increases
- b) Decreases
- c) Remains the same
- d) Becomes zero
Explanation: Ampere's force law for parallel currents states that the force per unit length is inversely proportional to the separation distance $d$ between them ($\frac{F}{L} = \frac{\mu_0 I_1 I_2}{2\pi d}$). Increasing the operating distance inherently decreases the interactive magnetic force.
77- Magnetic field at a point d distance away from long wire due to electric current i in it is _________
- a) μ₀i/2r
- b) μ₀i/r
- c) μ₀i/2πd
- d) μ₀i/πr
Explanation: According to Ampere's Law, evaluating a closed circular path around a long straight wire yields $\oint B \cdot dl = B(2\pi d) = \mu_0 i$. Rearranging for the field isolation gives $B = \frac{\mu_0 i}{2\pi d}$.
78- If length of current carrying conductor increases, what is the effect on the force?
- a) increases
- b) decreases
- c) remain same
- d) become zero
Explanation: The magnetic force acting on a straight conductor in a uniform field is directly proportional to its length ($F = I L B \sin\theta$). Extending the structural length increases total interactions, magnifying the cumulative force.
79- Which of the following is used to determine the direction of magnetic field in a current carrying conductor?
- a) Left hand thumb rule
- b) Right hand thumb rule
- c) Right hand palm rule
- d) Left hand palm rule
80- A substance whose permeability is less than the permeability of free space is?
- a) Diamagnetic
- b) Paramagnetic
- c) Ferromagnetic
- d) Not a magnetic substance
Explanation: Diamagnetic substances feature a relative magnetic permeability slightly less than 1 ($\mu_r < 1$), meaning their absolute structural permeability ($\mu$) evaluates below the vacuum reference constant ($\mu_0$).
81- Which among the following, is a correct expression for μ₀.
- a) μ₀=BH
- b) μ₀=B/H
- c) μ₀=H/B
- d) μ₀=BH²
Explanation: The mathematical definition linking magnetic flux density $B$ and magnetic field intensity $H$ in free space is given by $B = \mu_0 H$. Isolating the permeability constant yields $\mu_0 = \frac{B}{H}$.
82- Calculate the magnetic flux density if the magnetic field strength is 3A/m.
- a) 4π10-7T
- b) 8 π 10-7T
- c) 10 π 10-7T
- d) 12 π 10-7T
83- Which one of the following, when suspended freely, slowly sets itself parallel to the direction of the magnetic field?
- a) Ferromagnetic materials
- b) Diamagnetic materials
- c) Paramagnetic materials
- d) Ferrimagnetic materials
Explanation: Paramagnetic materials experience weak alignment forces along external field vectors. When suspended freely, they realign *slowly* parallel to the induction lines, unlike ferromagnetic materials which snap rapidly into alignment.