# HSC Physics MC Thread (1 Viewer)

#### Crisium

##### Pew Pew
You have to take into account Back EMF

#### JavaScript

##### Member

Why is it B?

i thought it would be an INCREASING straight line.

Because it is a radial magnetic field and THETA is always 0 degrees it must be some form of straiht line, but i thought if it speeds up then the torqu needs to be larger
Because a motor has almost instant torque and the torque doesn't decrease if its radial. The motor effect happens at every angle (in a perfect motor).
That's why its a straight line.

#### Mr_Kap

##### Well-Known Member
wait, so why is the answer a straight line with NEGATIVE gradient.

#### InteGrand

##### Well-Known Member
wait, so why is the answer a straight line with NEGATIVE gradient.
Back EMF.

#### JavaScript

##### Member
wait, so why is the answer a straight line with NEGATIVE gradient.
What???

#### InteGrand

##### Well-Known Member
Because a motor has almost instant torque and the torque doesn't decrease if its radial. The motor effect happens at every angle (in a perfect motor).
That's why its a straight line.

#### JavaScript

##### Member
Dat feel when you're shit lol

How is it A??

#### JavaScript

##### Member
It's (D), as he said in his edit.
Ohh. Yeah it will be a line going down cause back emf will slow it down.

#### InteGrand

##### Well-Known Member

$\bg_white Basically, since the angle is constant, we have \tau = nBIA \Rightarrow \tau \propto I. When the motor speeds up, its back EMF increases by Lenz's Law, and so the net current I flowing through the coil decreases as the motor picks up speed, so torque decreases.$

#### godofindolence

##### Member
Remember Faraday's law which states that relative movement of a conductor in a magnetic field results in induced EMF. Lenz's law further refines this by stating that the induced EMF produces a magnetic field that opposes the initial change in flux.

Thus as a motor is spinning, the coils in the rotor cuts through the magnetic field in the stator. This creates induced EMF by Lenz's law. The magnitude of this opposes the initial change, so by necessity, this results in the reverse of the supplied current. (Just use right hand rule on one side of the coil, and flip the forces). This is back EMF.

Lenz's also states that the induced EMF is proportional to rate of change of flux, (negative change in flux over change in time). Thus, as the motor spins faster and faster, the rate of change of flux increases, and the magnitude of the back EMF increase. Thus the supplied current and the induced current cancels each other out, reducing the total current.

Since torque = nBIA cos thetha and I is the decreasing variable, this represents a linear decrease as rotational speed goes up and back emf increases.

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thx guys

#### Crisium

##### Pew Pew
Remember Faraday's law which states that relative movement of a conductor in a magnetic field results in induced EMF. Lenz's law further refines this by stating that the induced EMF produces a magnetic field that opposes the initial change in flux.

Thus as a motor is spinning, the coils in the rotor cuts through the magnetic field in the stator. This creates induced EMF by Lenz's law. The magnitude of this opposes the initial change, so by necessity, this results in the reverse of the supplied current. (Just use right hand rule on one side of the coil, and flip the forces). This is back EMF.

Lenz's also states that the induced EMF is proportional to rate of change of flux, (negative change in flux over change in time). Thus, as the motor spins faster and faster, the rate of change of flux increases, and the magnitude of the back EMF increase. Thus the supplied current and the induced current cancels each other out, reducing the total current.

Since torque = nBIA cos thetha and I is the decreasing variable, this represents a linear decrease as rotational speed goes up and back emf increases.

LOL don't worry the same thing has happened to Zlatman heaps of times recently

InteGrand's LaTex skills are too OP

#### Mr_Kap

##### Well-Known Member

In this question, shouldn't the current made in the coil generate a magnetic field such to oppose the magnet, hence the magnetic field in the coil will be down (as the bottom of the coil needs to be south to oppose the south end of the magnet).

This means when i use the right hand curl rule, the current is CLOCKWISE. So the answer should be D.

HOWEVER the answer is actually B. Why??? What am i doing wrong??

bump

#### keepLooking

##### Active Member
I need this answered properly too. The moment when I realise I have the 2014 Success One and it doesn't contain the 2014 paper..

#### Mr_Kap

##### Well-Known Member

In this question, shouldn't the current made in the coil generate a magnetic field such to oppose the magnet, hence the magnetic field in the coil will be down (as the bottom of the coil needs to be south to oppose the south end of the magnet).

This means when i use the right hand curl rule, the current is CLOCKWISE. So the answer should be D.

HOWEVER the answer is actually B. Why??? What am i doing wrong??
anyone??

#### DattMyball

##### New Member
In this question, shouldn't the current made in the coil generate a magnetic field such to oppose the magnet, hence the magnetic field in the coil will be down (as the bottom of the coil needs to be south to oppose the south end of the magnet).

This means when i use the right hand curl rule, the current is CLOCKWISE. So the answer should be D.

HOWEVER the answer is actually B. Why??? What am i doing wrong??

You might have the right hand rule the wrong way. I think the fingers point in the north direction of the b-field so the field goes up

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