# Homework Help: Induced EMF in a Triangle

1. Apr 1, 2016

### SataSata

1. The problem statement, all variables and given/known data
Magnetic Field B is going into the plane.
Bar is moving to the right with velocity v.
Neglect resistance of the conducting bar and all contacts.
a.) Determine the induced emf in the circuit.
b.) To maintain the uniform motion of the conductive bar, there must be an external force Fapp to pull the conductive bar. How much should Fapp be?

2. Relevant equations
Let the length of conducting bar in the triangle be $l$
Hall Effect: $emf = Blv$
Faraday: $emf = -d\Phi/dt$
$\Phi = BA$
$A=xl/2$
$l=xtan\theta$
$F_B=IlB$

3. The attempt at a solution
$emf = -d\Phi/dt = -BdA/dt$
$dA/dt = dA/dx \times dx/dt$
$dA/dt = vxtan\theta$
$\therefore emf = -Bvxtan\theta$
Or, Using Hall Effect:
$emf = Blv = Bvxtan\theta$
Next, $F_app = -F_B$
$Current I = emf/R = -Bvxtan\theta/R$
$F_app= IlB = B^2vx^2tan^2\theta/R$

Can anybody check if my attempt is correct? Are all the negative signs correct?
Faraday's Law and Hall Effect give different signs. Which one is correct?
Fapp sign is supposed to be positive or negative?

2. Apr 3, 2016

### Staff: Mentor

Depends on the unspecified direction of your current flow. You don't need that direction, however.
I would choose a positive sign for the force needed to pull the bar, which means the induction leads to a negative force. But that is just an aesthetic choice (have force and velocity with the same sign convention if v is positive), the physical direction of the force is given in the problem statement already.

3. Apr 4, 2016

### SataSata

But is it right to express my answer in terms of x? Since x is a function of time, I can simply write it as x(t) and so emf and Fapp will both be a function of time too. However, the question simply asked for emf induced and Fapp, which caused me to doubt my answers.
In a non-triangle shape like a rectangle that's extending in length, the breadth is a constant and hence I can write my answer with it. But in this case, all sides of the triangle are varying and so I'm not sure how to express my answers.

4. Apr 4, 2016

### Staff: Mentor

Well, both depend on time here. Expressing them as function of x or t should be fine.