Question on the Lorentz force: Why is the force not F=q(v×B) = F=qv×qB

In summary, the equation of Lorentz force for the force acting on a moving charge in electric and magnetic field is:
  • #1
unplebeian
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1
TL;DR Summary
Why is the charge not multiplied to the cross product
Background:
cb96d860cadff3d60e8ffb90b067b7f2b453c8e1
is the equation of Lorentz force for the force acting on a moving charge in electric and magnetic field.

For the magnetic field only it is : F=qv×B.

Question:
For magnetic field only why is the force not F=q(v×B) = F=qv×qB
 
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  • #2
unplebeian said:
TL;DR Summary: Why is the charge not multiplied to the cross product

Background:
cb96d860cadff3d60e8ffb90b067b7f2b453c8e1
is the equation of Lorentz force for the force acting on a moving charge in electric and magnetic field.

For the magnetic field only it is : F=qv×B.

Question:
For magnetic field only why is the force not F=q(v×B) = F=qv×qB
You are only multiplying by q once, so
##q \textbf{v} \times \textbf{B}##

##= q ( \textbf{v} \times \textbf{B} )##

## = (q \textbf{v} ) \times \textbf{B}##

##= \textbf{v} \times (q \textbf{B})##

-Dan
 
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  • #3
Hi, Dan,
I'm sorry I didn't get it. That is a scalar multiplication so q should be multiplied to both. Generally a(bxc)= abxac.
Why are we multiplying only once?
 
  • #4
unplebeian said:
Generally a(bxc)= abxac.
This is wrong.
$$a(\mathbf b \times \mathbf c) = a\mathbf b \times \mathbf c = \mathbf b \times a\mathbf c$$You must be thinking of:
$$a(\mathbf b + \mathbf c) = a\mathbf b + a\mathbf c$$
 
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  • #5
unplebeian said:
Hi, Dan,
I'm sorry I didn't get it. That is a scalar multiplication so q should be multiplied to both. Generally a(bxc)= abxac.
Why are we multiplying only once?
Is ##2(3 \times 4 ) = (2 \cdot 3) \times (2 \cdot 4)##?

-Dan
 
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  • #6
PeroK said:
This is wrong.
$$a(\mathbf b \times \mathbf c) = a\mathbf b \times \mathbf c = \mathbf b \times a\mathbf c$$You must be thinking of:
$$a(\mathbf b + \mathbf c) = a\mathbf b + a\mathbf c$$
Easy to make mistake if in elementary school you learned the order of operations as "Dot (##\cdot## and ##\colon##) before stroke (##+## and ##-##)", because that's how the basic operators are written in your country.
 
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  • #7
Thank you, Dan. I thought about it graphically and it's evident that the scalar multiplication to both vectors prior to the cross product operation is incorrect. Rather take the cross product and then perform the scalar multiplication or simply any one vector like you suggested.

Thank you.
 
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  • #8
## \vec F = q ( \vec E + \vec v \times \vec B ) ##

## q ( \vec E + \vec v \times \vec B ) = q \vec E + q ( \vec v \times \vec B ) ## – the distributive property of scalar multiplication over the vector addition

## q ( \vec v \times \vec B ) = ( q \vec v ) \times \vec B = \vec v \times ( q \vec B ) ## - the multiplication by a scalar property of the vector product (the multiplication by a scalar is not distributive over the vector product)
 
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  • #9
Gavran said:
## \vec F = q ( \vec E + \vec v \times \vec B ) ##

## q ( \vec E + \vec v \times \vec B ) = q \vec E + q ( \vec v \times \vec B ) ## – the distributive property of scalar multiplication over the vector addition

## q ( \vec v \times \vec B ) = ( q \vec v ) \times \vec B = \vec v \times ( q \vec B ) ## - the multiplication by a scalar property of the vector product (the multiplication by a scalar is not distributive over the vector product)
:welcome:
 
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FAQ: Question on the Lorentz force: Why is the force not F=q(v×B) = F=qv×qB

Question 1: Why is the Lorentz force not F=q(v×B) = F=qv×qB?

The Lorentz force is defined as F = q(v × B), where q is the charge, v is the velocity, and B is the magnetic field. The expression F = qv × qB is incorrect because it suggests multiplying the charge twice, which has no physical meaning in this context. The charge q is a scalar and should only be applied once to the vector cross product of velocity and magnetic field.

Question 2: What is the correct formula for the Lorentz force?

The correct formula for the Lorentz force is F = q(v × B). This represents the force experienced by a charged particle moving with velocity v in a magnetic field B. The cross product v × B gives a vector that is perpendicular to both v and B, and multiplying by q scales this vector by the magnitude of the charge.

Question 3: How does the cross product in the Lorentz force formula work?

The cross product v × B results in a vector that is perpendicular to both the velocity vector v and the magnetic field vector B. The direction of this force vector follows the right-hand rule: if you point your right-hand fingers in the direction of v and curl them towards B, your thumb points in the direction of the force. The magnitude of the force is given by |v||B|sin(θ), where θ is the angle between v and B.

Question 4: Why is the Lorentz force dependent on the charge of the particle?

The Lorentz force depends on the charge q because the interaction between the particle and the magnetic field is proportional to the amount of charge. A larger charge results in a stronger interaction and hence a larger force. This is why the charge appears as a multiplicative factor in the formula F = q(v × B).

Question 5: Can the Lorentz force be zero? If so, under what conditions?

Yes, the Lorentz force can be zero under certain conditions. This occurs if any of the following are true: (1) the charge q is zero, (2) the velocity v is zero, (3) the magnetic field B is zero, or (4) the velocity vector v is parallel or antiparallel to the magnetic field vector B, making the angle θ between them zero or 180 degrees, resulting in sin(θ) being zero.

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