2 Q's on 2001 and 2002 HSC (1 Viewer)

[pocydto]

Can someone please explain to me about the answers in Q10 in the 2001 HSC exam and Q5 in the 2002 HSC exam?

Q10 in the 2001 exam asks: Which graph best shows how the current through the motor varies with speed? And the answer is A, which shows that as the current decreases, the speed increases....

Now, correct me if I'm wrong, but in the Jacaranda book pg 121, it says that the speed of the motor can be increased by increasing the current.

The graph which shows the correct answer
http://www4.boardofstudies.nsw.edu.au/online_mcq/question_images/physics_q10a.gif

Also for Q5 in the 2002 exam, I'm just wondering how I can come up with a period of 2.47x10^8s? Which formula would I need to use to get that answer?

The data table for Q5
http://www4.boardofstudies.nsw.edu.au/online_mcq/question_images/15330_5.jpg

 

[Lazarus]

Make sure you read the question carefully.

1. An electric motor is connected to a power supply of constant voltage. This means that the person running the motor never increases or decreases the current (hence your Jacaranda quote is inapplicable).

2. This part is the key to the answer: The motor is allowed to run at different speeds by adjusting a brake.

When the brake is applied completely, the motor is not moving (speed = 0), and hence it simply draws a constant current according to V = IR. However, as soon as the brake is loosened slightly, the motor will begin to spin. This movement of a conductor in a magnetic field will give rise to a magnetic field which is the reverse of the original one - this field in turn induces eddy currents in the conductor which combine to oppose the direction of the original current. Hence, the net current flow decreases.

I think that's right. It's been too long for me to attempt an answer for the second one.

 

[Antisocial]

Our school offered a tutorial day for Physics, and we sat around doing the 2001 paper. When asked about M/C Q10, our physics teacher was absolutely convinced that the answer is C (constant current, speed increasing).

 

[Antisocial]

As for the second question, seeing as it's orbiting a distant star, you cannot use r / T = Gm / 4pi because the planets aren't orbiting Earth. But you can still relate Kepler's Law of Periods to the question...

Alif (r / T) = Ba (r / T) and solve for T.

 

[SmokedSalmon]

Thanks antisocial I was doing the past HSC papers too and got stuck on that question. grr stupid me.

About the first query though. I believe that the first answer is correct. Remember when you first turn on a motor and there is a VERY small amount of back emf to oppose the current? Hence the high amount of current to start off with, but as the speed of the motor increases over time the back emf increases therefore lessening the size of the current to a constant number... Well that is what the diagram shows. But I think that is the adequate explanation.

 

[pocydto]

Thanks for your response guys .

Now I get the idea