iGCSE EDEXCEL PHYSICS DISCUSSION 2012

OP

How exactly does a commutator work in a D.C. motor? I know that it keeps the current flowing in the same direction, but how? Could someone explain to me how?

Basically imagine the coil and on the N side of the magnet the current goes in and the S side the current goes out so the motor effect is produced and the coil comes to the vertical position. If the commutator did not have gaps in it then the coil would try to turn to the expected horizontal position in which current would now on the N side of the magnet go out and on the S side the current would go in. This would also produce a motor effect but in the opposite way to what we desire so the coil is not able to complete a turn but is stuck in the middle in its vertical position. BUT with the split ring, when the coil reaches the vertical position, the connection is broken so the momentum from the motor effect allows it to topple over into another horizontal position. This time the sides are also switched so technically the current should go out on the N side and in through the S side but because the commutator broke connection and now remakes it, it resets these positions so the current actually, again, goes in through the N side and out through the S side thus allowing the motor effect to happen in the direction we desire again; this process keeps happening and the motor keeps turning. :smile:

I hope this helped, it's a bit difficult to explain without diagrams :smile:

Reply 161

member963009

OP

Original post by Carmeee

could anyone explain the stuff related to the seatbelt and montum?please :colondollar:

Momentum is a measure of how easily something can be brought to rest and the quicker something's momentum is 'lost' the larger the force exerted according to the equation: Force = Change in Momentum / Time. Crumple zones and air bags in cars are designed to increase the time it takes for the car to come to rest thus decreasing the force acting by increasing the time which allows a better chance of survival for passengers. The seatbelt has a similar purpose, without it passengers would smash into the next neat or the windscreen, stopping their momentum quickly and causing injury. With seatbelts, the momentum of the passengers is lost at the same rate as the car because the seatbelt anchors the passenger to the car body meaning that their momentum is lost over a longer period of time, exerting less force and bettering chances of survival. In addition seat belts stretch a little, further increasing the time taken for momentum to be stopped.

(edited 11 years ago)

Reply 162

member963009

OP

Original post by rtrtrt

So, the coil begins to move up. When it reaches 90 degrees vertical, using Fleming's left hand rule, we can work out that this end of the wire will try to carry on moving up (so not

allowing it to rotate by moving down (clockwise)). The carbon brushes are not in contact with the split ring commutator here - they are in contact with where it splits. Momentum takes the

wire past 90 degrees, allowing the carbon brushes to be in contact with the other half of the split-ring. To enable it to carry on rotating, the split rings switch the direction of the

current, and by doing this the direction of the force changes, allowing the wire to move down.

So the three stages are:

1. Wire moves up.
2. Wire reaches 90 degrees vertical.
3. Wire carries on past vertical due to momentum.
4. Commutator switches direction of current.
5. Wire carries on by moving on downwards, resulting in a continuous rotation.

Sorry I didn't see your post until after i posted :tongue:

Reply 163

Asad14

could someone :please:

post marking scheme for physics from May 2006 to November 2010. I just finished the papers and i cant find the MS anywhere
:ta:

Reply 164

numpty1225

Is the motor effect in tomorrow's paper? What other major topics aren't in it?

Reply 165

Asad14

Okay, i just got the Marking schemes from Jun 07 to Nov 10 on the edexcel site
If anyone has the May & November 2006 MS please post them. Thanks

Reply 166

IWantSomeMushu

17

After doing some past papers, does anyone else feel that the Physics exams do not test our knowledge well enough?

I know I'm only doing the Double Award, but the exams are kinda predictable.

Watch me fail now :laugh:

Reply 167

member963009

OP

Original post by IWantSomeMushu

After doing some past papers, does anyone else feel that the Physics exams do not test our knowledge well enough?

I know I'm only doing the Double Award, but the exams are kinda predictable.

Watch me fail now :laugh:

THANK YOU!!! The papers really don't it's all about experiments -_- To everyone on here saying oh Physics is so easy blah blah blah 'Call me arrogant for calling it easy', yes I agree the concepts are VERY easy but the papers don't test any of that! That's the point no one is willing to accept that's why I'm worried for physics because these people who claim the papers are easy; obviously their physics knowledge is far superior than mine because they can 'evaluate conclusions' and 'improve plans' :tongue:

Reply 168

IWantSomeMushu

17

Original post by sahajkaur

THANK YOU!!! The papers really don't it's all about experiments -_- To everyone on here saying oh Physics is so easy blah blah blah 'Call me arrogant for calling it easy', yes I agree the concepts are VERY easy but the papers don't test any of that! That's the point no one is willing to accept that's why I'm worried for physics because these people who claim the papers are easy; obviously their physics knowledge is far superior than mine because they can 'evaluate conclusions' and 'improve plans' :tongue:

Just study the mark schemes. That's what I'm doing :smile:

Reply 169

numpty1225

anyone going to take physics for as level?

Reply 170

member963009

OP

Original post by numpty1225

anyone going to take physics for as level?

Yes, to A2 :smile:

Hopefully they'll actually test some knowledge instead of this crap

(edited 11 years ago)

Reply 171

member963009

OP

Original post by IWantSomeMushu

Just study the mark schemes. That's what I'm doing :smile:

Same

Reply 172

kinglj81

edexcel set an average june paper, but the jan paper was impossible - just how science works qs. its difficult to learn stuff if u dont know what to learn - actual physics which is easy or stupid evaluate improve question which make no sense whatsoever.

Reply 173

username967389

Original post by numpty1225

anyone going to take physics for as level?

Pre-U.

Reply 174

Pretending2study

Guysss, anyone have the january 2012 marking schemes and question papers?
Link please ;D

Reply 175

kinglj81

http://www.xtremepapers.com/community/threads/2012-igcse-papers-marking-schemes-biology-chemistry-economics-physics-english-maths.13760/

just scroll down to physics

Reply 176

RoyalBlue7

19

Well I think Physics Igcse is the most sane exam. With such an attitude that most people have physics will be definitely meaningless. Actually in the real world no one expects you to recall stuff but to evaluate stuff. That's what the paper is testing as it clearly says in the edexcels website

This was posted from The Student Room's Android App on my Nexus One

(edited 11 years ago)

Reply 177

Big-Daddy

13

Original post by rtrtrt

1. Wire moves up.
2. Wire reaches 90 degrees vertical.
3. Wire carries on past vertical due to momentum.
4. Commutator switches direction of current.
5. Wire carries on by moving on downwards, resulting in a continuous rotation.

This is a very precise explanation. :smile:

I would just add that the coil moves up as a result of the force that acts on it due to the current through it and the magnetic field acting on it, thus creating a force as per Fleming's Left Hand Rule. When the current alternates, it moves in the opposite direction, so the direction of force alternates as well (as the magnetic field remains constant).

Reply 178

member963009

OP

The june paper was much better at testing knowledge and understanding. Asking you to make an experiment to measaure angle of refraction in a glass block does not test my understanding of physics IMO, if that's what you think realphysics is then you're in for a surprise at A -level

Reply 179

RoyalBlue7

19

Original post by sahajkaur

The june paper was much better at testing knowledge and understanding. Asking you to make an experiment to measaure angle of refraction in a glass block does not test my understanding of physics IMO, if that's what you think realphysics is then you're in for a surprise at A -level

Well theoretical physics is damn easy at this stage

This was posted from The Student Room's Android App on my Nexus One

iGCSE EDEXCEL PHYSICS DISCUSSION 2012

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