# Tangent Galvanometer and others

1. Dec 23, 2004

### coldfusion

Why is it that in a tangent galvanometer the aluminium pointer is fixed perpendicular to the magnetic needle and not parallel? Anyways , why is the aluminium pointer used after all cant we just measure the current by the defletion of the needle ?

There is a statement that says "In a stationary wave all the particles in one loop are in the same phase while particles in the succeding loop are in opposite phase? " Can anyone explain ??

why is it that in deriving the expression for pressure exerted by a gas by the kinetic theory , we dont consider the gas molecule making collisions with any other molecule while colliding with the walls of the container back and forth ?

If a solid conducting mass is moved in a uniform magnetic field, with the field always perpendicular to the plane of the conductor then is emf induced considering there is no change of flux thru the conductor.

A Simple question to think about : If a hydrogen baloon is tied to the seat of a car moving in a circular path then what will be the direction of the balooon when windows are closed.
ie will it move 1) inwards toward the centre, 2) Outwards , 3) Remain vertical.

Last edited: Dec 24, 2004
2. Dec 24, 2004

### Andrew Mason

I expect that the aluminum needle provides the east/west reference so you can measure the deflection angle. The magnetic needle shows the deflection. But you have to measure the east/west component of its deflection to find the sine of the angle, the accuracy of which needed to measure small angles.

I think by phase they mean moving in the same direction (up or down). Phase usually refers to angle (ie. 0 => 2pi for one wavelength). Successive loops are always moving in opposite directions at any given time - ie. a phase difference of pi.

'We' do. But the collisions are elastic so the energy is simply passed on to other molecules. Ultimately, collisions between the molecules and the container wall produce the force on that wall.
The Lorentz force law: $F = q\vec E + q\vec v \times \vec B$ applies. This force multiplied by a distance through which it acts determines the induced potential energy / unit charge or voltage (emf). The induced emf ($E_{induced} = \vec v \times \vec B \times \vec{Distance}$) becomes $\vec v \times \vec B \times \vec L$ or vBL where L is the length of the conductor perpendicular to the direction of B and v is perpendicular to both L and v. There is no emf across the conductor width, so thickness of the conductor has no effect on the induced emf. However, no work is done, or energy consumed, by the voltage since no current flows in the conductor.

But this may be a trick question if it asks about its direction of motion. The balloon has to continually move inward toward the centre in order to move in a circle. So although it leans outward relative to the car (so string tension can have a horizontal component to provide the centripetal force), it is continually changing direction (moving) toward the centre as well as tangentially.

AM