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how does a EM wave propagate in a medium?
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That somebody is correct in say that EM wave can travel through vacuum because of its changing E-field and B-field. Before i give you the explanation, it is necessary for you to understand the Lenz's Law and also the Right-Hand Screw Rule (The following section highlighted in pink can be skipped) [Lenz's Law] It states that the direction of the induced emf is sich that it tends to oppose the flux change causing it, and does oppose it if induced currents flows. (You just only need to pay attention to the first part of this law) [Right-Hand Screw Rule] It states if a right-handed screw move forward in the direction of the (conventional) current, then the direction of rotation of the screw gives the direction of the magnetic field. Now, suppose an EM wave is initiated by an oscillating electron in vacuum, the oscillating electron can be seem like a movement of charge-carrier inside a wire owing to the varying E-field, such that it produces a varying circular B-field . The direction of B-field can be found using Right-Hand Screw Rule. When we are looking at just a particular point in the space, there is a fluc change. By Lenz's Law, a B-field of the opposite direction is induced at that point, which can be seem part of another circular B-field (we call this induced circular B-field). Due to the influence of this induced circular B-field, the electrons inside this induced circular B-field will be driven to motion. Up to this point, we may conclude that a primary oscillating electron in vacuum will initiate a secondary oscillating electron in vacuum. This story continues, and we conclude that an EM wave is propagated. Note that all the changes are due to the varying E-field and B-field, which means it doesn't need any medium for propagation, which accounts for its propagation in vacuum. I hope the above information helps By 小儒 2007-07-26 18:19:52 補充： Take it easy guy~ The above explanation is actually elaborated from a tutorial notes by Ken Chan. I think it has its value of reference =)
其他解答:
To explain the propagation of em wave in space, we need to apply Maxwell's equations. (Lenz's Law and Right Hand Screw Rule are not appropriate as they are not fundamental and they deal with electric current which is not found in space. Actually, in vacuum, there is certainly no electrons to be set in motion.) The thrid or fourth Maxwell's equations in vacuum (i.e. without charges or currents) read as curl (E) = - ?B/?t (1) curl (B) = -(ε0/μ0)?E/?t (2) (The curl of vector field tells us about the rotation the field has at any point. To have more information about curl, please visit http://en.wikipedia.org/wiki/Curl ) So, what Maxwell cleary indicated is "the extent of the rotation of E field is proportional to the rate of change of the B field at the same point in space". Now assume the wave propagate in x-direction and the E field is in y-direction (i.e. E = E j) Equation (1) simplifies to ?E/?x = -?B/?t where B = B k So, when E field is at maximum, ?E/?x is zero, hence ?B/?t is zero and B is at maximum, too. When E field is at zero, ?E/?x is maximum, hence ?B/?t is at maximum and B is zero, too. Therefore, E and B field are in phase.
how does a EM wave propagate in a medium?
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i dont understand how a EM wave propagates in space. somebody told me that a Em wave can travel through vacuum because of its chaging E-field and B-field. can anyone give out more details in this aspect or explain it in a simple way? THZ X999! 更新: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35 i found a animation about the propagation of EM wave,which has concluded what u have explained to me. Still i got some problems in the understanding of the changing E-field and B-field. 更新 2: 1.in the right hand grip rule,it state that a moving curent will generate a circular magnetic field.But in the animation the moving electron generate a line E-field(why?) 更新 3: 2.Every text resource or web resource depicts the E and B field components as reaching a maximum and minimum in phase. Yet the theory of Maxwell clearly indicates that the rate of field change in the mutual self-induction cycle is what is important, ( E∝ ￡GB/￡Gt ). 更新 4: I think during the wave cycle when the E field is a maximum, the rate of change temporarily falls to zero and the the B field at that instant would be zero. The opposite would be true for the generation of the E field by a changing B field. 更新 5: Therefore a maximum E and B field can never be produced together in time and space. (this is a question asked by a person in that website,but don't understand too) P.S. i am only a A-Level student, is this question involves sth which i havn't learnt before?最佳解答:
That somebody is correct in say that EM wave can travel through vacuum because of its changing E-field and B-field. Before i give you the explanation, it is necessary for you to understand the Lenz's Law and also the Right-Hand Screw Rule (The following section highlighted in pink can be skipped) [Lenz's Law] It states that the direction of the induced emf is sich that it tends to oppose the flux change causing it, and does oppose it if induced currents flows. (You just only need to pay attention to the first part of this law) [Right-Hand Screw Rule] It states if a right-handed screw move forward in the direction of the (conventional) current, then the direction of rotation of the screw gives the direction of the magnetic field. Now, suppose an EM wave is initiated by an oscillating electron in vacuum, the oscillating electron can be seem like a movement of charge-carrier inside a wire owing to the varying E-field, such that it produces a varying circular B-field . The direction of B-field can be found using Right-Hand Screw Rule. When we are looking at just a particular point in the space, there is a fluc change. By Lenz's Law, a B-field of the opposite direction is induced at that point, which can be seem part of another circular B-field (we call this induced circular B-field). Due to the influence of this induced circular B-field, the electrons inside this induced circular B-field will be driven to motion. Up to this point, we may conclude that a primary oscillating electron in vacuum will initiate a secondary oscillating electron in vacuum. This story continues, and we conclude that an EM wave is propagated. Note that all the changes are due to the varying E-field and B-field, which means it doesn't need any medium for propagation, which accounts for its propagation in vacuum. I hope the above information helps By 小儒 2007-07-26 18:19:52 補充： Take it easy guy~ The above explanation is actually elaborated from a tutorial notes by Ken Chan. I think it has its value of reference =)
其他解答:
To explain the propagation of em wave in space, we need to apply Maxwell's equations. (Lenz's Law and Right Hand Screw Rule are not appropriate as they are not fundamental and they deal with electric current which is not found in space. Actually, in vacuum, there is certainly no electrons to be set in motion.) The thrid or fourth Maxwell's equations in vacuum (i.e. without charges or currents) read as curl (E) = - ?B/?t (1) curl (B) = -(ε0/μ0)?E/?t (2) (The curl of vector field tells us about the rotation the field has at any point. To have more information about curl, please visit http://en.wikipedia.org/wiki/Curl ) So, what Maxwell cleary indicated is "the extent of the rotation of E field is proportional to the rate of change of the B field at the same point in space". Now assume the wave propagate in x-direction and the E field is in y-direction (i.e. E = E j) Equation (1) simplifies to ?E/?x = -?B/?t where B = B k So, when E field is at maximum, ?E/?x is zero, hence ?B/?t is zero and B is at maximum, too. When E field is at zero, ?E/?x is maximum, hence ?B/?t is at maximum and B is zero, too. Therefore, E and B field are in phase.
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