Dc motor multiple choice 2008 cssa (1 Viewer)

[mtsmahia]

Hi guys,

can someone help with this Q

A DC motor connected to a 12V Power supply accelerates from start up speed to its maximum speed.
Which statement is correct?

A) Its net energy increases during this time
B)The supply voltage is decreased as it reaches maximum speed
C) The sum of electrical, heat loss kinetic energy remains constant
D) A larger current is generated in the motor due to the greater rate of change of flux

I though it was D, but the answers say C

can someone please explain why?

 

[Dx_God]

well, i would have went with C) because of law of conservation of energy and when u actually consider all the energies, there are only overall voltage (supply+back emf), heat due to the resistance of the material used, and finally the kinetic energy which is ur motor effect.

If that wasn't clear enough to persuade u then u can read why i would have to say D) is wrong. If u actually think about it, how can the current from the relative motions of the magnets and the coil be larger than the supply current itself? if this was to be said true then overall, the current would be backwards (or negative). this would cause the motor to move in the opposite direction due to motor effect before the current is reversed again. i don't see a dc motor going forwards and backwards all the time so i have to go with C)

But i'm not a genius at this stuff and my physics' knowledge is getting rusty so i might be wrong. but hopefully this helped

 

[study-freak]

Hi guys,

can someone help with this Q

A DC motor connected to a 12V Power supply accelerates from start up speed to its maximum speed.
Which statement is correct?

A) Its net energy increases during this time
B)The supply voltage is decreased as it reaches maximum speed
C) The sum of electrical, heat loss kinetic energy remains constant
D) A larger current is generated in the motor due to the greater rate of change of flux

I though it was D, but the answers say C

can someone please explain why?

C is the most correct although I don't like it very much.
I would have said that power remains constant.

D is wrong as current actually decreases.
Net emf=supply emf-back emf
and this net emf causes current.
As the motor runs faster, back emf increases as the rate of change of flux through the coducting part of the motor increases. Hence net emf and current decreases.

 

[Dx_God]

I would have said that power remains constant.

well the truth is that power doesn't remain constant or there will no need in finding an alternate solution in reducing power loss during transmission (this is in the syllabus). only in some theoretical situations, the power is said to remains constant. but, the reality is, there is no possible solution to keep power constant, even if ur using superconductors, the power will always reduce as the resistance is never zero or there would be no voltage in the first place.

 

[study-freak]

well the truth is that power doesn't remain constant or there will no need in finding an alternate solution in reducing power loss during transmission (this is in the syllabus). only in some theoretical situations, the power is said to remains constant. but, the reality is, there is no possible solution to keep power constant, even if ur using superconductors, the power will always reduce as the resistance is never zero or there would be no voltage in the first place.

Electrical+heat+kinetic power should be constant.
I just wanted to change 'energy' to 'power.' (which is evident from how I was talking about C)

 

[Aerath]

Yeah, I agree with study-freak on all accounts, C being the most correct, although, still not perfect.

 

[mtsmahia]

Electrical+heat+kinetic power should be constant.
I just wanted to change 'energy' to 'power.' (which is evident from how I was talking about C)

I understand why D is wrong now... but when it says "The sum of electrical, heat loss kinetic energy remains constant"
.....what does it mean by "constant" . It is reffering to the law of conservation of energy?

 

[Aerath]

Yeah, LOCOE - where no energy is gained or lost, but instead changed from one from to another.

 

[mtsmahia]

K cool ... thanks !!

and BEST OF LUCK FOR UNI!!!