中国科学技术大学：《电磁学》课程教学资源（PPT课件讲稿）Chapter 10 Magnetic Fields：III

Chapter 10 Magnetic Fields: Ill a The lorentz force Example: The Closed-Field Mass Spectrometer Example: The hall Effect Example Magnetic Mirrors

Chapter 10 Magnetic Fields:III ◼ The Lorentz Force ◼ Example: The Closed-Field Mass Spectrometer ◼ Example:The Hall Effect ◼ Example:Magnetic Mirrors

10.1 The lorentz Force Experiments show that a charged particle moving in a magnetic fIeld experiences a force F=Q(v×B), Q-the charge of the particle v-the velocity of the particle(vector B- the magnetic field If there is also an electric field the force acting on the particle is F=Q(E+v×B)

Example: The Closed-Field Mass spectrometer See fig 10-1 The positive ions originate from the source s at a potential V. Passing between the plates, the ion beam splits vertically according to its mass

For the ion going from the source s to the deflect- ing plates, the potential energy is converted into the kinetic energy mv 2 Q Between the plates the electric field e is upward the electric force Qe is upward the magnetic field b is towards us, the magnetic force Qv× B is downward If the net force on the ion is zero F=Q(E+VXB)=0, en v EB

Plu thi E uggIng B into the relation 2mv=QV gIves m=2QV BE A particle with this mass will not be affected by the electromagnetic field between the plates, and will be arrive at the collector C, and the ammeter will show he current When this happens, we can tell the mass m, since the values B, e, V, and Q are known from the experiment

Example: The hall Effect See Fig 10-2(b ) and(d )for an n-type semiconductor the electrons as charge carriers a bar of semiconductor is placed in a transverse mag- netic field B A current I flows through th e bar 2 B Ovx BlE B (b)

When the electrons flows with a velocity v to form a current I, they experience a magnetic force Fm=QvB directing downward So the electrons drift downward, and accumulate at the lower portion of the bar Thus an electric field Ey forms, directing downward and the electrons experience an electric force QEg, directing upward

When Fe= Fm,(balanced), the electrons stop drift- ing We ha ave =-B This electric field is called the hall field The corresponding potential is V= eab- bub

Example: Magnetic Mirrors See Fig 10-4 for a give magnetic field a proton goes toward the left, spiraling around the magnetic line(x-axIS

The magnetic field is approximately K B at the corner (vO, zo of the dashed lines, it is simply K B=roi+rok The velocity of the proton at(0, zo)is given by 0=0J+00 Note that here 100< 0, and v0z <0 The lorentz force acting on the proton is K F=e(v×B)=e(voi+o2k)×(xoi+aok)