- #1

- 1,341

- 3

## Main Question or Discussion Point

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...?

- Thread starter Feldoh
- Start date

- #1

- 1,341

- 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...?

- #2

- 536

- 2

- #3

- 1,341

- 3

- #4

- 458

- 0

- #5

- 1,341

- 3

- #6

- 3

- 0

- #7

jtbell

Mentor

- 15,407

- 3,198

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.There is only a force if the velocity of a particle is perpendicular to the direction of the magnetic field.

- #8

- 458

- 0

- #9

- 249

- 0

Practicly you would use v relative to the charge that causes B, because you can easily calculate B in that system.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...?

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

- 9

- 0

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

- 9

- 0

Sorry, but we dont use relative velocity as you said in this post,we only use net ACTUAL velocity of chargePracticly 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).

- #12

- 249

- 0

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.Sorry, but we dont use relative velocity as you said in this post,we only use net ACTUAL velocity of charge

- #13

- 4,222

- 1

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

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

- 1,341

- 3

Ah thanks, I get it now.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 isnotthe 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 Post

- Replies
- 3

- Views
- 721

- Replies
- 0

- Views
- 4K

- Replies
- 2

- Views
- 6K

- Replies
- 1

- Views
- 7K

- Replies
- 16

- Views
- 5K

- Replies
- 2

- Views
- 2K

- Replies
- 10

- Views
- 4K

- Last Post

- Replies
- 2

- Views
- 1K