# Electromagnetism, finding velocity and acceleration

1. Jun 2, 2013

### physics604

1. The diagram attached shows a 0.010 kg metal rod resting on two long horizontal frictionless rails which remain 0.40 m apart. The circuit has a resistance of 3.0 W and is located in a uniform 0.20 T magnetic field. Find the initial acceleration and maximum velocity for the rod.

a) Initial acceleration: 120 m/s2; maximum velocity: 300 m/s
b) Initial acceleration: 40 m/s2; maximum velocity: 190 m/s
c) Initial acceleration: 120 m/s2; maximum velocity: 190 m/s
d) Initial acceleration: 40 m/s2; maximum velocity: 300 m/s

2. Relevant equations

ε=Blv

3. The attempt at a solution

ε=Blv
v=$\frac{ε}{Bl}$ = $\frac{15}{0.20×0.40}$ = 187.5 m/s

Now I'm not exactly sure how to find acceleration.

I know the initial v was 0 m/s and the final v was 187.5 m/s. I don't have the length of the rods or anything.

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2. Jun 3, 2013

### rude man

Your emf = BLv equation assumes the only source of emf is the velocity of the bar in the B field. Buth there is also a second source of emf, to wit, the 15V battery.

Ponder the following statement: "as long as there's current flowing thru the bar, the bar experiences force and therefore acceleration".

3. Jun 3, 2013

### physics604

Okay, I got it!

I just use F=ma and solve for the acceleration, since both F and m are given.