Magnetic Flux Questions from Japanese University Exam Papers

[BeginnerPhysicsMan]

Hello everyone,

This is my first post here

Firstly, I'd like to start by saying that it's my best intention to follow the rules on this forum. Therefore, if I've posted this in the wrong section please forgive me, I will try and learn the ropes quickly.

One of my students has given me some Japanese university exam paper questions for Physics (NOTE: I'm a maths teacher). I'm finding them very difficult to answer. I think I've got the answer for the first two but I would like to double-check here. I am pretty confused by the third one, which is quite embarrassing.

First One: I got (2) (H, O, H) by using Fleming's second right-hand rule and the equation for Magnetic field strength being directly proportional to current, and inversely to the distance from the wire

Second One: I got (2) By using Faraday's Law and Lenz's law. Looking at the constant change in flux against time, and the graph flipped over the x-axis due to Lenz's law telling us that an induced emf opposes the change which causes it.

Third One: I'm stuck to be honest

Any help will be massively appreciated. Thank you

 

[cnh1995]

Welcome to PF!

Third One: I'm stuck to be honest

This type of questions belong to the homework help forum.
https://www.physicsforums.com/forums/homework-and-coursework-questions.152/.
Since this is your first post, moderators may move it there for you. Your own attempt at a solution is also required.

For the 3rd question, write the force balance equation for the rod. Which forces act on the rod as it descends?

 

[BeginnerPhysicsMan]

Hello

thank you for the reply

3rd question:

At constant velocity I have that mg would equal BIL

Then I'm stuck. The magnetic flux would be constant since it's a uniform field. But I can't see how to get to one of the answers

 

[cnh1995]

At constant velocity I have that mg would equal BIL

Correct, and what is I in terms of the given quantities?

The magnetic flux would be constant since it's a uniform field.

Current flows in the rod because of the motional emf. Look up the equation for motional emf.

Or if you want to use the flux rule, calculate the rate of change of flux w.r.t. the loop. The loop gets smaller as the rod descends and hence, the flux in the loop is changing. This way, you'll derive the motional emf equation.

 

[BeginnerPhysicsMan]

Ah, I got number 3

Thank you very much

 

[cnh1995]

Ah, I got number 3

That is correct.

 

[BeginnerPhysicsMan]

Thank you for the help

This is a great forum

 

[cnh1995]

Thank you for the help

You're welcome!

This is a great forum

Indeed it is..