Register to reply

Static Magnetic Field and Work

by I done know
Tags: field, magnetic, static, work
Share this thread:
I done know
#1
Mar30-06, 06:28 PM
P: 5
Why can't a static magnetic field (not changing in time) ever do work? How do I express this formulaically? My only guess is that work is zero for a closed path.
Phys.Org News Partner Science news on Phys.org
New model helps explain how provisions promote or reduce wildlife disease
Stress can make hard-working mongooses less likely to help in the future
Grammatical habits in written English reveal linguistic features of non-native speakers' languages
Physics Monkey
#2
Mar30-06, 06:36 PM
Sci Advisor
HW Helper
Physics Monkey's Avatar
P: 1,332
The force on a charged particle in a magnetic field is [tex] \vec{F} = q \vec{v} \times \vec{B} [/tex], right? Stare at that formula for a bit. Now ask yourself, how is the force related to the velocity? Then ask, how is work related to force?
I done know
#3
Mar30-06, 07:07 PM
P: 5
I see, then dv/dt is 0 when the B field is static, so if a=0 then F=0 then W=0. Sound right?

Unfortunately this was on our last exam, and my answer was that W=qV and induced voltage is only a result of B flux changing in time.

Physics Monkey
#4
Mar30-06, 07:16 PM
Sci Advisor
HW Helper
Physics Monkey's Avatar
P: 1,332
Static Magnetic Field and Work

Wait, how did you conclude that [tex] \frac{d\vec{v}}{dt} = 0 [/tex]? The acceleration certainly isn't zero, there is a force acting.
I done know
#5
Mar30-06, 07:37 PM
P: 5
Is it that the force is perpendicular to the magnetic field and work must be parallel to the displacement?
Physics Monkey
#6
Mar30-06, 07:47 PM
Sci Advisor
HW Helper
Physics Monkey's Avatar
P: 1,332
You are so close! The force is perpendicular to the field, but that's not what matters. What else is the force perpendicular to?
I done know
#7
Mar30-06, 07:54 PM
P: 5
Ah, so because the force is perpendicular to the velocity, the force is perpendicular to the displacement.
Physics Monkey
#8
Mar30-06, 07:59 PM
Sci Advisor
HW Helper
Physics Monkey's Avatar
P: 1,332
Indeed. In simple terms, the power [tex] \vec{F} \cdot \vec{v} [/tex] is identically zero. Hence no work is done.


Register to reply

Related Discussions
Pop quiz - electron magnetic field - dynamic or static Classical Physics 3
The magnetic field and work General Physics 5
Static Magnetic Field Theory Introductory Physics Homework 0
Work done by magnetic field Classical Physics 31
Work done by magnetic field... Classical Physics 17