# GCSE OCR 21st Century Triple Science (CBP1-7) ThreadWatch

4 years ago
#1421
(Original post by romaiseb)
Hi, thanks for this! The only thing that I can't get my head around is that if resistance is lower, then it should be easier for current to flow to the light bulb (???) - so in light, when resistance is low, the current should flow easier to the light bulb? (and vice versa for LDRs in the dark...)
OMG. So weird! Just sent this question to my teacher who replied -

The answer is that the light bulb is not in the same circuit as the LDR. The LDR is in a potential divider circuit, with the voltage across it measured by a sensor. As the light intensity decreases, the resistance will increase. This will cause the voltage across it to increase. The sensor will detect this and turn on the light (in a separate circuit) when the voltage reaches a certain value.
0
#1422
You're right! And ssh, you'll come out of the exam tomorrow grinning and getting everything right as we all chat about it on here lol.
I've never 'aced' P456 I'll need to when it comes to P7! TSR is great because I'm procrastinating by being on here yet revising and learning from it! And I'm sure you will too .
2
4 years ago
#1423
Think about it as ratio. All maths!

So. They usually say ''a charger is plugged into a 230V mains supply which is changed to a 12V supply''. Therefore, 230V was the INPUT voltage at the beginning and stepped-down to 12V which is therefore the OUTPUT voltage. So now we know one half of the equation -

230 over 12.

Then it will give you values eg. there are two different pairs of coils which could be used in the transformer:

Turns on primary coil - Turns on secondary
3450 - y
x - 600

Find x and y.

So now it's all ratio. Let's start with x. To find turns on the primary coil, we always use the equation Voltage(primary) over Voltage (secondary) = Turns (primary) over turns (secondary).

So we use:
230/12 = x/600

then we do 230/12 TIMES 600 = x
x = 11500
Check you've got a whole number? Yes. Let's continue

That's one half of the question done! Now let's find y. To find the turns on the secondary we do the opposite of the first equation so it's voltage(secondary) over voltage (primary) = turns (secondary) over turns (primary).

So..
230/12 = 3450 / x

230/12 TIMES 3450 = 180

DONE! Check the numbers are logical and boo-yah. Easy marks.
woaahh wait a sec.... 230/12=3450/x .... surely then its 3450/(230/12)
0
4 years ago
#1424
Is this how motors work?
When you introduce direct current (flow of electrons, a charge) into a magnetic field, the force causes movement? That's all I know about it... please can somebody explain motors and generators?
0
4 years ago
#1425
In a parallel circuit, why does the current through each component depend on the resistance of each component? Is it because the current is split between each branch and reaches different components which have different resistances, unlike series circuits?
0
4 years ago
#1426
(Original post by lyricalvibe)
in the cgp book says ( i'm paraphrasing here) " when a magnet does a half turn, the direction of the magnetic field through the coil reverses.meaning voltage reverses.meaning current flows in the opposite direction around the coil of wire... if the magnet keeps turning in the same direction the voltage ( and hence the current) will keep reversing every half turn-> AC"
you seem to be ready though, i'm no way near still gotta do majority of p4 and whole of p5 again

i hate physics -_-
Yes. The current and voltage technically reverse to produce the current. This does not mean that when you put ac through a wire it will go one direction for a bit and then suddenly turn around and go back and forth.
The current is produced as an alternating current due to the reversal of the current. In a wire..the current moves in one direction. You're basically over complicating things
(Original post by romaiseb)
Hi, thanks for this! The only thing that I can't get my head around is that if resistance is lower, then it should be easier for current to flow to the light bulb (???) - so in light, when resistance is low, the current should flow easier to the light bulb? (and vice versa for LDRs in the dark...)
Yes, resistance is low and current does flow easier. however whether or not it is easy for current to flow is slightly irrelevant. The more resistance, the harder it is for the current to flow and so the current has to work more on the wire. Think about it this way. You have a swimming lane, it's completely free of any obstacles. It is easy to swim through, you aren't exerting much energy or doing much work. The resistance is low.

Then picture this, there are obstacles in the water. You're working harder to swim through, you're exerting more energy, doing more work. More energy is being put into the pool as heat energy.

In this way, increased resistance is like water with obstacles. The current (swimmer) has to work harder and so does more work, more energy is transferred and the light will switch on.

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0
4 years ago
#1427
(Original post by shivani210598)
woaahh wait a sec.... 230/12=3450/x .... surely then its 3450/(230/12)
Ah, thanks for reading through it. I explained it in the sentence above but I did it wrong haha. Yup it should be

12/230 times 3450 = 180
0
4 years ago
#1428
(Original post by KJC79)
Is this how motors work?
When you introduce direct current (flow of electrons, a charge) into a magnetic field, the force causes movement? That's all I know about it... please can somebody explain motors and generators?
I've described this before a few times. Check back through my posts on this thread.

Posted from TSR Mobile
0
#1429
(Original post by physicalgraffiti)
In a parallel circuit, why does the current through each component depend on the resistance of each component? Is it because the current is split between each branch and reaches different components which have different resistances, unlike series circuits?

In a series circuit, the total resistance of each component is higher. This is because there are less paths for the current to travel through.
In a parallel circuit, the total resistance of each component is less. This is because there are more​ paths for the current to travel through.

'More scientifically' - for a series circuit, less paths mean more collisions between the free ions so the resistance increases. For a parallel circuit, more paths mean less collisions between the free ions so the resistance decreases.
0
4 years ago
#1430
Ah, thanks for reading through it. I explained it in the sentence above but I did it wrong haha. Yup it should be

12/230 times 3450 = 180
thanks anyway, i somehow get the maths bit now!
0
4 years ago
#1431
(Original post by superdarklord)
I know
I used to hate it but I don't know what it is, seriously love it now. I've read over it a billion times and I find that I understand it really well now. Don't give up you'll get there and eventually enjoy it

Posted from TSR Mobile
Not gonna happen
0
4 years ago
#1432
(Original post by shivani210598)
thanks anyway, i somehow get the maths bit now!

Haha yay
0
4 years ago
#1433
(Original post by Sulfur)

In a series circuit, the total resistance of each component is higher. This is because there are less paths for the current to travel through.
In a parallel circuit, the total resistance of each component is less. This is because there are more​ paths for the current to travel through.
Yay thank you for the simplification I hate circuits so much...
0
#1434
(Original post by physicalgraffiti)
Yay thank you for the simplification I hate circuits so much...
Ha, I've just edited my post to go more in-depth with the resistance so maybe it won't be so simple now! So do I - and drawing them is even worse.
0
4 years ago
#1435
(Original post by superdarklord)
Yes, resistance is low and current does flow easier. however whether or not it is easy for current to flow is slightly irrelevant. The more resistance, the harder it is for the current to flow and so the current has to work more on the wire. Think about it this way. You have a swimming lane, it's completely free of any obstacles. It is easy to swim through, you aren't exerting much energy or doing much work. The resistance is low.

Then picture this, there are obstacles in the water. You're working harder to swim through, you're exerting more energy, doing more work. More energy is being put into the pool as heat energy.

In this way, increased resistance is like water with obstacles. The current (swimmer) has to work harder and so does more work, more energy is transferred and the light will switch on.

Posted from TSR Mobile
Thank you so much this really helps!
0
4 years ago
#1436
A nuclear power station has to release 4.5x10^7 J of energy to provide one person with their
daily electricity needs. Use the formula m= e/c^2t o calculate the mass of fuel which must be
lost to provide this energy
c= 3.0 x10^8 m/s

How do i work this out?
Thanks
0
#1437
Is anyone doing AQA Science next year btw? I'm doing Biology and Chemistry on AQA but Physics on OCR.
0
4 years ago
#1438
Theyre doing aqa next year for biology and chem in my school
0
4 years ago
#1439
If the voltage is shared in a series circuit, and say there are 2 components and the voltage f the battery is 3 would each component get 1.5v
0
#1440
(Original post by student4879489)
A nuclear power station has to release 4.5x10^7 J of energy to provide one person with their
daily electricity needs. Use the formula m= e/c^2t o calculate the mass of fuel which must be
lost to provide this energy
c= 3.0 x10^8 m/s

How do i work this out?
Thanks
E = 4.5x107
C = 3x108
M = unknown

(In a calculator):

4.5x107 / (3x108)2 = mass
(C2 = 9x1016 if you want to speed the calculation up).

Spoiler:
Show
5x10-5 kg
0
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