You are Here: Home >< Physics

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

1. (Original post by Knowing)
I think this is it:

You replace R2 with the LDR. When it's in bright light, the resistance is lower, so R1's share of resistance increases. Therefore, more voltage is used up by R1, so there is lower voltage when it gets to R2/the LDR.

I haven't reached logic gates yet
(Original post by BP_Tranquility)
There are two main concepts we have to keep in mind:
When R1 is greater than R2 in a potential divider circuit, then output is 0 volts. If R1 is less than R2, then output voltage is nearly the same as the input voltage.

Essentially, in a potential divider circuit, you replace R2 with a thermistor so when there is a high temperature, then the resistance of the thermistor falls. Therefore, because the resistance of the thermistor is low, the resistance of R1 is higher so there is no output voltage. Similarly, if there is a low temperature, the resistance of the thermistor is high and because of this, the resistance of R1 is lower which means there is an output voltage (roughly equal to the input voltage).

I'll use an example for the second question. This is very similar to the above example where you have R2 replaced with an LDR for instance. There is an AND gate where there is an input of a normal current but for the second input, a potential divider circuit in connected. This potential divider circuit has had its R2 replaced with an LDR so there is only a 'high' signal when the light intensity is low (because a low light intensity means that resistance is high so like before, the resistance of R1 is lower than that of the LDR, so there's a current at the second input of the AND gate). Hence, because there's a current at the first input as well as the second input, the AND gate also has an output and switches on whatever it's connected to.

Posted from TSR Mobile
First attempt at multi quoting let's hope I don't screw up lol, but thank you so much guys! I think I finally get it now, I'm just gonna learn both of your explanations, really helpful.
You're right but if the LDR was the first resistor then the opposite effect happens (when it is light the Vout decreases) because the first resistor's resistance is large (relative to the second one), meaning there is a large voltage drop.

Edit: I just confused myself. Forget what I said.
LOL don't worry about it, thanks for trying anyhoo!
2. In the exam, are we allowed to use a pencil to draw diagrams, because I don't think it says so on the front?
3. (Original post by BP_Tranquility)
In the exam, are we allowed to use a pencil to draw diagrams, because I don't think it says so on the front?
Yes - and graphs
4. For the relay circuit symbol, which one do we have to learn? There is loads of different symbols for a relay
5. Does anyone understand Optics?
6. I'm finding 'images' really hard to get my head around
7. Could someone explain interference please? The bit where you have two coherent wave sources and you need to know where there is a destructive or a constructive interference pattern.

8. I think I understand the equations from points one to four, but I don't understand how they relate to points five and six?

Where is the force coming form? I'm really confused?
9. (Original post by cameron262)

I think I understand the equations from points one to four, but I don't understand how they relate to points five and six?

Where is the force coming form? I'm really confused?
When the gas particles collide with the walls or the container, their momentum changes because they slow down. The momentum is conserved because a bit of the particle's momentum goes into pushing the wall a bit, so a force is exerted on the wall. This happens loads of times and in loads of different places. This is pressure.

The equations are just proving why the gas particles cause there to be pressure.
10. (Original post by Knowing)
Could someone explain interference please? The bit where you have two coherent wave sources and you need to know where there is a destructive or a constructive interference pattern.

You have a constructive interference with areas of bright light (using light source) or loud areas (using sound waves) and destructive interference pattern are areas of no light or quiet areas
11. (Original post by cameron262)

I think I understand the equations from points one to four, but I don't understand how they relate to points five and six?

Where is the force coming form? I'm really confused?
When particles are moving they have momentum, so when they hit the walls of the container, that momentum changes, as the particles rebound off of it. When the particles collide with the wall, this creates a force on the walls, because the particles have hit the walls (just like how if you hit a tennis ball with a racket, you're creating a force on the ball so it moves).

The force created by the particles against the walls, also generates pressure (force is measured in Newtons whilst pressure is measured in Newtons per metre squared, so they are very similar but pressure is more specific as it tells you how much force is acting per a certain amount of area whilst force is 'general' and not as specific).

The pressure on the walls can be changed, which is what point 6 is referring to. By increasing the temperature, the particles inside the container have more kinetic energy so they collide more frequently and more violently against the walls of the container, which in turn creates a larger pressure being exerted on the walls. You can also increase the pressure on the walls by reducing the amount of volume inside the container- e.g. if you halve the amount of volume/space inside the container, then the particles are closer together so they are more likely to collide against the walls because there's less space for them to move about in.
I'm finding 'images' really hard to get my head around
What do you struggle with?
If you mean the virtual/real images, then all we have to know is the difference between them- virtual cannot be projected and is the right way up whilst real images can be projected and the image is inverted...
13. (Original post by Knowing)
Could someone explain interference please? The bit where you have two coherent wave sources and you need to know where there is a destructive or a constructive interference pattern.
Think of it in terms of path difference (basically the distance the two waves have traveled relative to each other):

If the waves have a path difference of a whole number of wavelengths (e.g. 1λ, 2λs, 5λs or 47247392λs) then the crests of the wave line up when they meet each other. They're both trying to cause a disturbance in the same direction so the wavelengths join and form one wave with an increased amplitude (this is constructive interference).

If the waves have a path difference with half a wavelength (e.g. 1.5λs, 3.5λs, 43.5λs) the crests and the troughs line up. The waves are tying to cause a disturbance in opposite directions so when the wavelengths meet there is no disturbance (there's just a line). This is destructive interference.

With light, constructive interference shows a light spot. Destructive interference shows a dark spot (because the waves have cancelled each other out).

Hope that helps! (It helped me trying to explain it)
14. (Original post by BP_Tranquility)
What do you struggle with?
If you mean the virtual/real images, then all we have to know is the difference between them- virtual cannot be projected and is the right way up whilst real images can be projected and the image is inverted...
Yeah that makes sense, cheers. It's just GCSE over-simplification of quite an abstract topic, so I was trying to understand it fully when we only need to know it on a really simple level
Yeah that makes sense, cheers. It's just GCSE over-simplification of quite an abstract topic, so I was trying to understand it fully when we only need to know it on a really simple level
GCSE oversimplification sometimes makes things more complicated, since you don't fully understand it (just like how I found metallic bonding confusing at first because we weren't, and still haven't, been taught why it happens), so you're not alone
16. (Original post by NatashaG)
You have a constructive interference with areas of bright light (using light source) or loud areas (using sound waves) and destructive interference pattern are areas of no light or quiet areas
Think of it in terms of path difference (basically the distance the two waves have traveled relative to each other):

If the waves have a path difference of a whole number of wavelengths (e.g. 1λ, 2λs, 5λs or 47247392λs) then the crests of the wave line up when they meet each other. They're both trying to cause a disturbance in the same direction so the wavelengths join and form one wave with an increased amplitude (this is constructive interference).

If the waves have a path difference with half a wavelength (e.g. 1.5λs, 3.5λs, 43.5λs) the crests and the troughs line up. The waves are tying to cause a disturbance in opposite directions so when the wavelengths meet there is no disturbance (there's just a line). This is destructive interference.

With light, constructive interference shows a light spot. Destructive interference shows a dark spot (because the waves have cancelled each other out).

Hope that helps! (It helped me trying to explain it)
Thanks you two got it now
17. (Original post by BP_Tranquility)
GCSE oversimplification sometimes makes things more complicated, since you don't fully understand it (just like how I found metallic bonding confusing at first because we weren't, and still haven't, been taught why it happens), so you're not alone
Yup! And sometimes in the exam I have to stop myself from going into too much detail because I remind myself that it's just GCSE
18. (Original post by Knowing)
Thanks you two got it now
But mainly thanks to me

Yup! And sometimes in the exam I have to stop myself from going into too much detail because I remind myself that it's just GCSE
Yeah, same here! Still can't believe that the atom model we're taught in schools for years is "obsolete" though lol ...Hopefully A Levels won't have be like this
20. Can someone please explain to me transistors I don't get them I have read the revision guide a million times and I dont undersatnd why they are useful please help me

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: June 18, 2014
Today on TSR

### Is this person a genius?

...with these A Level results?

### I think I'm transgender AMA

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

Can you help? Study help unanswered threadsStudy Help rules and posting guidelinesLaTex guide for writing equations on TSR

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.