# Two MC questions (1 Viewer)

#### emilios

##### Well-Known Member

^ I got the answer out for this (it was A) but that was before I considered the implication of Lenz's Law. We don't apply Lenz's law here do we? Since the cathode ray tube isn't a conductor.

And for this one:

I've narrowed it down to A and C since they're the only ones that would give me a clockwise rotation: but how do I know what the direction of the magnetic field is?

#### mreditor16

##### Well-Known Member
View attachment 31224
^ I got the answer out for this (it was A) but that was before I considered the implication of Lenz's Law. We don't apply Lenz's law here do we? Since the cathode ray tube isn't a conductor.

And for this one:
View attachment 31225

I've narrowed it down to A and C since they're the only ones that would give me a clockwise rotation: but how do I know what the direction of the magnetic field is?
for the first Q, no need to consider Lenz's Law.

as for the 2nd Q, I'm not sure :/

#### itsalberttrinh

You're right for the first one (A). Using the left hand palm rule our thumb will point into the page since cathode rays are a stream of negatively charged particles (thumb represents conventional current), fingers will point in the direction of the north pole, thus palm will be facing up so it will deflect upwards towards A.

For the second question the answer is B. Using your right hand coil rule on each magnet we can determine the polarity of the magnets. B is the only case that will provide a clockwise rotation, where the left magnet is a south pole and the right magnet is a north pole

#### emilios

##### Well-Known Member
You're right for the first one (A). Using the left hand palm rule our thumb will point into the page since cathode rays are a stream of negatively charged particles (thumb represents conventional current), fingers will point in the direction of the north pole, thus palm will be facing up so it will deflect upwards towards A.

For the second question the answer is B. Using your right hand coil rule on each magnet we can determine the polarity of the magnets. B is the only case that will provide a clockwise rotation, where the left magnet is a south pole and the right magnet is a north pole
That's the right hand curl rule for solenoids right? (fingers curl in direction of current, thumb points to north pole)

I don't get how that works in this case :/

#### Kurosaki

##### True Fail Kid
I'm not too flash at physics myself, but I'll have a go, though I take 0 responsibility for lost ATAR gains.
The answer for the second one is B, because if you just look at the left and right, what you have is essentially a solenoid - you have current running through a coil of wire- so you just have to apply the right hand rule for solenoids to work out the magnetic field that it produces, from which you can deduce the resultant force acting on the sides of the coil, and hence the answer.

Sorry if it's hard to understand :/

#### Lethal Toxin

##### Member
First one i have no idea, second one is A i think, because the positive end is always the north pole, and conventional current runs from + to -, so you can use the right hand rule to figure it out