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# OCR Gateway Physics P4P5P6 - 18/06/13 Watch

1. Let me tell you the things that ****ed me over in the workbook:

Polarization, Momentum and Interference of waves
2. (Original post by IHTWFR)
Let me tell you the things that ****ed me over in the workbook:

Polarization, Momentum and Interference of waves
3. Can someone explain what a commutator is please?
4. (Original post by Scarlett44)
Can someone explain what a commutator is please?
A commutator is used in a DC motor and it ensures that the motor continues to spin. It does this by changing the direction of the current every half turn so the force on the coil of wire is always in the same direction.

You can test this out by using Fleming's left hand rule- e.g. let's say the motion is upwards and when the coil of wire has spun half way, then if the current direction remained the same, Fleming's left hand rule would mean the motion was now downwards so it would turn in the opposite direction. So, if you add a commutator, then the direction of the current reverses every half-turn so it continues to spin in the correct direction.

(I think this is correct but if it isn't then please feel free to correct me )
5. (Original post by BP_Tranquility)
A commutator is used in a DC motor and it ensures that the motor continues to spin. It does this by changing the direction of the current every half turn so the force on the coil of wire is always in the same direction.

You can test this out by using Fleming's left hand rule- e.g. let's say the motion is upwards and when the coil of wire has spun half way, then if the current direction remained the same, Fleming's left hand rule would mean the motion was now downwards so it would turn in the opposite direction. So, if you add a commutator, then the direction of the current reverses every half-turn so it continues to spin in the correct direction.

(I think this is correct but if it isn't then please feel free to correct me )

Thank you, that makes a bit more sense now :P
6. Optics is so damn confusing
7. (Original post by benwalters1996)
I meant to see what kind of topics they like to ask about

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Well, I've been doing the old past papers for practice and these have been the most common topics -
P4-
Sparks/ electrostatics (has come up almost every time)
Fission and Fusion (although fission has come up more than fusion)

P5-
Action and Reaction and Momentum (has come up almost every time)
Vectors and Equations of Motion Calculations
Satellites and Orbits
Optics
Constructive/ destructive interference

P6-
Ohmic/non-Ohmic graphs
Relay circuits (fairly frequent topic)
Motors/ Generators (come up almost every time)
Transformers (usually calculations and identifying a stepup/stepdown transformer)
Diodes and Capacitors
And there's usually a question or two on working out the output voltage in a potential divider circuit
8. (Original post by BP_Tranquility)
P6-
Ohmic/non-Ohmic graphs
Relay circuits (fairly frequent topic)
Motors/ Generators (come up almost every time)
Transformers (usually calculations and identifying a stepup/stepdown transformer)
Diodes and Capacitors
And there's usually a question or two on working out the output voltage in a potential divider circuit
whats ohmic and non ohmic graphs!?
9. (Original post by Scarlett44)
whats ohmic and non ohmic graphs!?
Graphs attached- the top one is an Ohmic graph/conductor as it follows Ohm's Law. The second one is a non-Ohmic graph/conductor as it doesn't follow Ohm's law.

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10. (Original post by BP_Tranquility)
Graphs attached- the top one is an Ohmic graph/conductor as it follows Ohm's Law. The second one is a non-Ohmic graph/conductor as it doesn't follow Ohm's law.

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thanks
11. (Original post by BP_Tranquility)
Graphs attached- the top one is an Ohmic graph/conductor as it follows Ohm's Law. The second one is a non-Ohmic graph/conductor as it doesn't follow Ohm's law.

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the caption for the Non-ohmic one seems wrong?
12. (Original post by BP_Tranquility)
Well, I've been doing the old past papers for practice and these have been the most common topics -
P4-
Sparks/ electrostatics (has come up almost every time)
Fission and Fusion (although fission has come up more than fusion)

P5-
Action and Reaction and Momentum (has come up almost every time)
Vectors and Equations of Motion Calculations
Satellites and Orbits
Optics
Constructive/ destructive interference

P6-
Ohmic/non-Ohmic graphs
Relay circuits (fairly frequent topic)
Motors/ Generators (come up almost every time)
Transformers (usually calculations and identifying a stepup/stepdown transformer)
Diodes and Capacitors
And there's usually a question or two on working out the output voltage in a potential divider circuit

confident with P4 and 5....p6 is by far my weakest one.
13. But we can hardly judge what will come up as this is the first paper of the spec. I am so screwed for this exam though, I had the crappest teacher who somehow has a degree is Physics but she probably doesn't even know the syllabus herself!

P.S. P5 is a *****
14. (Original post by benwalters1996)
the caption for the Non-ohmic one seems wrong?
Yeah, it's not the first one in my book believe it or not...
15. (Original post by benwalters1996)
confident with P4 and 5....p6 is by far my weakest one.
Same here
16. Does anyone know the answer for question15 p6 in cgp revision guide. Answer should be on pg 113 but I can't find it.
thanks
17. any takers?

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18. any ideas?

No mark scheme

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19. (Original post by benwalters1996)
any ideas?

No mark scheme

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The graph is not straight since the bulb is a non-ohmic conductor. When electrons travel through the wire, they collide against each other as well as the atoms in the material. The collisions results in greater resistance as it becomes more difficult for the electrons to flow easily due to the thin wire. So when the current increases, the number of collisions between the electrons and the material also increases, which increases the temperature as a result.
20. (Original post by benwalters1996)
any takers?

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Well, these are some things I would include:
-parabolic trajectory
-horizontal velocity constant
-vertical velocity changes (due to gravity)
-Newton's prediction on what would happen if the ball was kicked hard enough
-same principle that keeps satellites in orbit
-the critical angle would be 45 degrees if he wanted to kick it as far as possible

Is that from the specimen by the way?

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