# BR physics, section VIII, passage 1, question 6 , pg 143

#### Student1331

##### Full Member
BR physics, section VIII, passage 1, question 6 , pg 143

If the rod in Figure 2 is released with zero velocity, and if the rails on which the rod sits are frictionless, the rod will:

A,B,C,D - long answers, the question makes sense with the passage

I dont get how D is the answer? Doesnt magnetic flux grow bigger with increasing area, thus inc emf (V)?

#### pfaction

##### Full Member
10+ Year Member
a) It's not moving as it is.
b) It's through a constant magnetic field - nothing is changing.

So I would have put D?

EDIT: I read the answer choice for A, I don't even understand why that explanation is real. How is it being pushed to left or right?

#### johnwandering

##### Full Member
10+ Year Member
When a current is running through the loop the bar Will move depending on the direction of the current.

Look at the magnetic field, look at the motion of current, apply to right hand rule to get magnetic force that will move the bar.

#### johnwandering

##### Full Member
10+ Year Member
A is wrong because if a current is running through the circuit the bar will move.
D is correct

#### EnginrTheFuture

##### Full Member
a) It's not moving as it is.
b) It's through a constant magnetic field - nothing is changing.

So I would have put D?

EDIT: I read the answer choice for A, I don't even understand why that explanation is real. How is it being pushed to left or right?

The magnetic field is into and out of the page. The current is along the plane of the page either up or down. Right hand rule says that the current in the rod, if going "up" the rod, will push the rod left. If the current is going through the rod downward, right hand rule says the rod is push to the right.

That is what would justify movement. BUT the fact that when you push the rod to the left or right, the magnetic flux inside the circuit grows..... this causes there to be an equal but opposite force resisting.

----> D

#### pfaction

##### Full Member
10+ Year Member
****ing misunderstood the question as always. I didn't attribute it going through the wire itself. Yeah.

#### hellocubed

##### Full Member
The magnetic field is into and out of the page. The current is along the plane of the page either up or down. Right hand rule says that the current in the rod, if going "up" the rod, will push the rod left. If the current is going through the rod downward, right hand rule says the rod is push to the right.

Considering that Magnetic force of the right hand rule is relative to (+) charges in a field, for a circuit such as this where negative charges dictate the current... the effects of current direction on the bar are opposite.

When current is going UP the rod, the electrons move in the opposite direction of magnetic force, and the bar will move RIGHT.
When current is going DOWN the rod, the bar will move to the LEFT.

It takes some getting used~

#### MrNeuro

##### Full Member
7+ Year Member

Considering that Magnetic force of the right hand rule is relative to (+) charges in a field, for a circuit such as this where negative charges dictate the current... the effects of current direction on the bar are opposite.

When current is going UP the rod, the electrons move in the opposite direction of magnetic force, and the bar will move RIGHT.
When current is going DOWN the rod, the bar will move to the LEFT.

It takes some getting used~

im pretty sure for these questions the term current is referring to the flow of imaginary positive charges

and i think for A

the wording messes w/ how the railgun works

moving the bar induces an emf
thus if you dont move the bar (aka no change in flux b/c Area isn't changing) then you won't induce a current to oppose that change b/c its not changing.

#### ipodtouch

##### Full Member
10+ Year Member
im pretty sure for these questions the term current is referring to the flow of imaginary positive charges

What makes you say that?

The term "current" is almost always associated with negative charge. Especially in the BR questions.

#### hellocubed

##### Full Member
Current direction through a circuit is ALWAYS associated with negative charge. Unless specifically told otherwise.

It is negative here.

#### Dasypus

##### Full Member
Conventional current is always, always in the direction a positive charge would go. This is an artifact of the way electric current was discovered, and we're stuck with it (thank Ben Franklin for this; he thought the current was carried by positively charged particles).

So when using the right-hand rule for magnetic field induced by a current-carrying wire, you stick your thumb in the direction imaginary positive particles would go, which is opposite the way real electrons are going.

#### EnginrTheFuture

##### Full Member
In the world of circuits... current it defined as the flow of positive charge.

But no matter what one defines it as... with respect to the right hand rule, "direction of current" is ALWAYS applied to the direction of positive charge flow! That is why when you are given the direction of negative charge flow, you always use your left hand or switch the answer from what you get using your right hand.

A proton going through a magnetic field will obey the right hand rule, an electron flying through a magnetic field will do the opposite. Revert back to the mass spectrometer problems/lecture for a better explanation.

I used the term current in the positive sense which is what gave me the right/left in my first post. You used the term current as negative sense... you got the opposite directions. Both of these are correct since they don't tell us which way the current is flowing or define current in either sense.

#### hellocubed

##### Full Member
In the world of circuits... current it defined as the flow of positive charge.

But no matter what one defines it as... with respect to the right hand rule, "direction of current" is ALWAYS applied to the direction of positive charge flow! That is why when you are given the direction of negative charge flow, you always use your left hand or switch the answer from what you get using your right hand.

A proton going through a magnetic field will obey the right hand rule, an electron flying through a magnetic field will do the opposite. Revert back to the mass spectrometer problems/lecture for a better explanation.

I used the term current in the positive sense which is what gave me the right/left in my first post. You used the term current as negative sense... you got the opposite directions. Both of these are correct since they don't tell us which way the current is flowing or define current in either sense.