What is the role of velocity in the equation F = qv x B?

In summary, the conversation discusses the concept of velocity in relation to the equation F = qv x B, where a particle with charge q and velocity v experiences a force F in a magnetic field B. The conversation explores whether the velocity used in the equation is the drift velocity or the actual velocity of the particle. It is clarified that the velocity used is the actual velocity of the particle, and various scenarios are discussed where this may be different. The conversation also touches on the relationship between charge and current in the equation and how it may not follow the usual correspondence of variables. It is suggested that the equation F = iL x B may be more fitting for a current carrying wire.
  • #1
Feldoh
1,342
3
I was just wondering, since my textbook doesn't really explain it too well, what velocity is part of the equation F = qv x B? Is it the drift velocity, or...?
 
Physics news on Phys.org
  • #2
A particle of charge q that moving with velocity v through a space where there exists a magnetic field B will experience force F.
 
  • #3
Ok assume there is an electron in a magnetic field, will the force be affected by said electrons drift velocity, or would it be something else?
 
  • #4
There is only a force if the velocity of a particle is perpendicular to the direction of the magnetic field. For a positive charge, hold out your hand and put your thumb in the direction of positive current, then put your straightened fingers in the direction of the magnetic field, and your palm will face the direction of force.
 
  • #5
No I know how to calculate the force, all I want to know is if we put an electron into a magnetic field, so that the electron is moving perpendicular to the field, would the velocity we use be the drift speed of that electron?
 
  • #6
The velocity used in the calculation of Force of magnetic field is not drift velocity but velocity of the particle. Drift velocity is different from the velocity. Drift is called by the combination of magnetic force and other kind of force, and drift velocity is departed from velocity. You can refer to the definition of drift in magnetic field.
 
  • #7
Snazzy said:
There is only a force if the velocity of a particle is perpendicular to the direction of the magnetic field.

No. The magntude of the magnetic force is [itex]qvB \sin \theta[/itex], where [itex]\theta[/itex] is the angle between the velocity of the particle and the direction of the magnetic field. The field is zero only when the velocity is along the direction of the field. The force is maximum when the velocity and the field are perpendicular.
 
  • #8
Whoops, should've said when a component of the velocity is perpendicular to the direction of the magnetic field.
 
  • #9
Feldoh said:
I was just wondering, since my textbook doesn't really explain it too well, what velocity is part of the equation F = qv x B? Is it the drift velocity, or...?
Practicly you would use v relative to the charge that causes B, because you can easily calculate B in that system.
In general you can use any inertial coordinate system and measure the speed of the charge q in that system. You might get different magnetic force F in diferent systems, but this is not a problem, since B and E are also dependent on the system and particle feels both electric and magnetic force. For systems moving relative to each other at nonrelativistic speeds you will get aproximately the same electromagnetic force F. If speeds are relativistic, you get different F in different systems since a relativistic particle has different acceleration in different inertial systems (but those different forces will still describe the same movement).
 
  • #10
Hello dear,

Let me help ypu..

the velocity is the net velocity of CHARGE ,

Lets take some cases here...

1.charge is moving independently with velocity V, so This V will be used in our equation

2.A charge is moving inside a metal with velocity(drift velocity) V1 (e.g free electrons in semi-conductors) and metal is moving with velocity V2,

this means-NET velocity is the VECTOR SUM of V1(drift velocity) n V2(metal velocity) ,

this situation is justified in "ELECTROMAGNETIC INDUCTION" where we use velocity of moving metal(metal velocity) to find magnetic force on CHARGE
 
  • #11
Sorry, but we don't use relative velocity

Lojzek said:
Practicly you would use v relative to the charge that causes B, because you can easily calculate B in that system.
In general you can use any inertial coordinate system and measure the speed of the charge q in that system. You might get different magnetic force F in diferent systems, but this is not a problem, since B and E are also dependent on the system and particle feels both electric and magnetic force. For systems moving relative to each other at nonrelativistic speeds you will get aproximately the same electromagnetic force F. If speeds are relativistic, you get different F in different systems since a relativistic particle has different acceleration in different inertial systems (but those different forces will still describe the same movement).
Sorry, but we don't use relative velocity as you said in this post,we only use net ACTUAL velocity of charge
 
  • #12
mr.survive said:
Sorry, but we don't use relative velocity as you said in this post,we only use net ACTUAL velocity of charge
You did not read my post carefully. I used the phrase "relative velocity" because it may not be clear which system of coordinates we should choose. In this case "actual velocity" has little meaning.
 
  • #13
Feldoh said:
I was just wondering, since my textbook doesn't really explain it too well, what velocity is part of the equation F = qv x B? Is it the drift velocity, or...?

Please correct me if I'm wrong. For a wire, v serves as the drift velocity in equation F = qv x B, even if it isn't an actual velocity. With this assignment, q must be interpreted as the amount of current entering the wire, per unit time.

Importantly, with these assignments, q is not the charge carried by each particle.

I realize that this isn't the usual correspondence of variables, but it seems to serve.

Maybe you were really looking for this equation: F = iL x B, the force on a current carrying wire of length, L within B.
 
Last edited:
  • #14
Phrak said:
Please correct me if I'm wrong. For a wire, v serves as the drift velocity in equation F = qv x B, even if it isn't an actual velocity. With this assignment, q must be interpreted as the amount of current entering the wire, per unit time.

Importantly, with these assignments, q is not the charge carried by each particle.

I realize that this isn't the usual correspondence of variables, but it seems to serve.

Maybe you were really looking for this equation: F = iL x B, the force on a current carrying wire of length, L within B.

Ah thanks, I get it now.
 

1. What is a magnetic field?

A magnetic field is an invisible force field that surrounds a magnet or a moving electric charge. It is created by the movement of electric charges and is responsible for the attraction or repulsion between magnets and other electrically charged objects.

2. How is a magnetic field measured?

The strength of a magnetic field is measured using a unit called tesla (T), which is equivalent to one newton per ampere-meter. Another common unit of measurement is gauss (G), with 1 T equal to 10,000 G.

3. What is the relationship between a magnetic field and electric current?

Electric currents create magnetic fields, and magnetic fields can induce electric currents. This relationship is described by Maxwell's equations and is the basis for many modern technologies such as electric motors and generators.

4. How does the strength of a magnetic field affect its force?

The strength of a magnetic field directly affects the force it exerts on other objects. The stronger the magnetic field, the greater the force between two objects. This force is also dependent on the distance between the objects and the orientation of their magnetic poles.

5. What are some real-world applications of magnetic fields?

Magnetic fields have a wide range of applications, including in electric motors, generators, MRI machines, particle accelerators, and magnetic levitation trains. They are also used in everyday items such as credit cards and speakers.

Similar threads

Replies
8
Views
952
Replies
5
Views
628
Replies
4
Views
1K
  • Electromagnetism
Replies
5
Views
1K
  • Electromagnetism
Replies
3
Views
1K
  • Electromagnetism
Replies
7
Views
891
Replies
3
Views
709
Replies
8
Views
654
Replies
2
Views
665
Replies
0
Views
49
Back
Top