It\'s all True/False Qs 1.( ) When B field is parallel to a plane surface, the f
ID: 1558051 • Letter: I
Question
It's all True/False Qs
1.( ) When B field is parallel to a plane surface, the flux through the plane is zero.
2.( ) When B field is parallel to a plane surface, the flux through the plane is a maximum.
3.( ) When B field is perpendicular to a plane surface, the flux through the plane is zero.
4.( ) When B field is perpendicular to a plane surface, the flux through the plane is a maximum.
5.( ) For a uniform B field, that makes an angle q with the normal direction of the surface with area A, the flux is BAcos q
6.( ) For a uniform B field, that makes an angle q with the normal direction of the surface with area A, the flux is BA sin q
Read Example 30.7, notice that when B is not a constant ( it changes as 1/r) , B cannot be take out from the integral. When B is a function of r, dA needs to be expressed as a function of r or dr, and r needs to be integrated from r=c to r=a+c.
Notice the strip of thin area dA highlighted in Figure 30.21 is a thin rectangle shape, hence, dA= b times dr.
7.( ) Gauss’s law for electric field says that the total net E field flux through a closed surface is proportional to the amount of charges enclosed in the closed surface.
8.( ) The total net magnetic flux through a closed surface is proportional to the amount of current enclosed in the closed surface.
9.( ) The total net magnetic flux through any surface is always zero.
10.( ) The path integral of B vector dot ds vector, along a closed loop is always zero.
11.( ) The path integral of B vector dot ds vector, along a closed loop is proportional to the amount of current enclosed in the closed loop.
12.( ) The total net magnetic flux through any closed surface is always zero, because B field lines form loops and don’t originate or terminate anywhere, hence any B field lines entering the closed surface will always leave the surface, keeping the net flux equal to zero.
13.( ) Any current loop, including the atomic level current loops has a magnetic field, therefore has a magnetic dipole moment (a north pole and a south pole).
14.( ) An electron moving in a circular orbit has an angular momentum and its magnetic momentum is opposite to its angular momentum, because the electron’s charge is negative and the effective current direction is opposite to the motion direction.
15.( ) The electrons have intrinsic property called spin, as if it is spinning around its axis, which also contributes to its magnetic momentum.
16.( ) The spin and orbital moments from pairs of electrons may cancel, so that the magnetic moment of an atom may be zero, such as He and Ne.
17.( ) Protons and neutrons have much larger magnetic moments than electrons, because they have much larger mass.
18.( ) Some substances contain permanent atomic magnetic moments that tend to align parallel to each other even in a weak external B field and remain aligned after the external field is removed, such as iron and nickel, they are called Paramagnetism.
19.( ) Some substances contain permanent atomic magnetic moments that tend to align parallel to each other even in a weak external B field and remain aligned after the external field is removed, such as iron and nickel, they are called Ferromagnetism.
20.( ) At very high temperature, which is above a critical point called Curie temperature, a ferromagnetic substance may lose its residual magnetization due to thermal agitation.
21.( ) Some substances have weak permanent magnetic moments, which orient randomly without external B field and tend to align with an external B field, however, the alignment has to compete with the randomization due to thermal motion. They are called Paramagnetism.
22/( ) Some substances have weak permanent magnetic moments, which orient randomly without external B field and tend to align with an external B field, however, the alignment has to compete with the randomization due to thermal motion. They are called Diamagnetism.
23.( ) Many matters have a little bit diamagnetism effect, which induce a weak magnetic moment OPPOSITE to the external B field. But this effect is much weaker than the effect of Paramagnetism or Ferromagnetism, and can hardly be observed.
24.( ) When a weak magnetic moment OPPOSITE to the external B field is induced in a substance, the substance will be attracted by the external field.
25.( ) When a weak magnetic moment OPPOSITE to the external B field is induced in a substance, the substance will be repelled by the external field.
Explanation / Answer
1. True -Because flux= B.dA=BAcos
here = 90 , therefore flux=0
2. false (same explaination as above)
3. false ( = 0 )
4. true (same explaination as 3rd)
5. true
6.false
7. true
8. true
please post the other answers separately
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