Circuits help? AS Physics OCR

WilliamSlim

3

Does anyone have any good resources or websites to help with the 3 modules on circuits? I'm really struggling with them

Reply 1

lerjj

13

Original post by WilliamSlim

Does anyone have any good resources or websites to help with the 3 modules on circuits? I'm really struggling with them

Try this site:
http://www.physicsandmathstutor.com/physics-revision/

I've also got some notes which are more AQA oriented. If you've got a copy of your specification, could you find out what you need to know and post it here? People can then help you with specific questions.

Reply 2

WilliamSlim

OP

3

Original post by lerjj

Try this site:
http://www.physicsandmathstutor.com/physics-revision/

I've also got some notes which are more AQA oriented. If you've got a copy of your specification, could you find out what you need to know and post it here? People can then help you with specific questions.

Off the top of my head I'm having difficulties with the different IV graphs and explaining them and also the circuits to test the emf and pd. For the iv graphs I'm not sure what it would be like for a test circuit for emf, and I don't really understand how a test circuit work, and struggle with iv graphs and explaining them in general.

Is it okay if I update with more stuff I struggle with?

Reply 3

WilliamSlim

OP

3

Also, is there any website where I can make a circuit and add resistors, voltmeters, ammeters, resistors and other components to check what the current, resistance and voltage would be like? I think messing around with something like that would be a great help.

Reply 4

lerjj

13

Original post by WilliamSlim

Also, is there any website where I can make a circuit and add resistors, voltmeters, ammeters, resistors and other components to check what the current, resistance and voltage would be like? I think messing around with something like that would be a great help.

There's probably something out there that's the right level, if you just google 'circuit simulators' most are pretty complicated... this one isn't but is probably too simplistic:http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

You can probably find something more the right level if you look hard enough for a little bit, that phet simulation is the best I could find in 5 minutes.

Reply 5

lerjj

13

Original post by WilliamSlim

Off the top of my head I'm having difficulties with the different IV graphs and explaining them and also the circuits to test the emf and pd. For the iv graphs I'm not sure what it would be like for a test circuit for emf, and I don't really understand how a test circuit work, and struggle with iv graphs and explaining them in general.

Is it okay if I update with more stuff I struggle with?

In an ideal world you'd have some clear problem that you don't understand, and you would make a thread regarding the issue. (Well, in an ideal world you wouldn't have problems... but that's irrelevant). If the questions are distinct, make separate threads. If they follow on from one another, ask them in the same one. I don't know what you mean by test circuit at the moment, so if you have a picture/diagram that you could post that would be helpful.

For I/V characteristics, the three main graphs that you have to know are: a filament lamp, a fixed resistor, a diode. The fist thing to note is that all three components have 0 current when there is no applied voltage, so they all go through the origin. The next thing is that lamps and resistors don't notice sign changes, so their graphs are the same for positive and negative voltages (technically it's rotational symmetry about (0,0) but whatever)

I don't have pictures to hand right now, this is what you meant by I/V graphs I assume? Things to remember: gradient is resistance, so constant for fixed resistor and variable for filament lamp. Diodes are weird graphs, because diodes are themselves weird.

Reply 6

WilliamSlim

OP

3

Here is a test circuit IV graph question
http://i.imgur.com/puMZKlUh.jpg
I don't really understand how the test circuit works though

Here's another IV graph question that I don't really get, it's c and d I'm struggling with
http://i.imgur.com/5SusuJ8h.jpg http://i.imgur.com/372DZblh.jpg

Reply 7

uberteknik

21

Original post by WilliamSlim

Here is a test circuit IV graph question
http://i.imgur.com/puMZKlUh.jpg
I don't really understand how the test circuit works though

Here's another IV graph question that I don't really get, it's c and d I'm struggling with
http://i.imgur.com/5SusuJ8h.jpg http://i.imgur.com/372DZblh.jpg

Have a look at these websites, the first gives you most of what you need in a nutshell.

To get the best out of it, start from 'electricity concepts and units 1' and work forwards from there.

http://www.a-levelphysicstutor.com/elect-emf-int-res.php

This guy's quite reasonable (if a little dry) and produced a series of videos which are on his youtube page. Well worth going through his series of lectures by organising yourself to view them in a similar order to the first website.

http://youtu.be/I3mcgfRiV6Y

This last website gives a more in depth resource for when you want to improve your basic understanding and proficiency.

http://www.allaboutcircuits.com/vol_1/index.html

Reply 8

DanyaI

12

CGP Books all the way, lol they are the sole reason I got decent grades!

Reply 9

lerjj

13

Original post by WilliamSlim

Here is a test circuit IV graph question
http://i.imgur.com/puMZKlUh.jpg
I don't really understand how the test circuit works though

Here's another IV graph question that I don't really get, it's c and d I'm struggling with
http://i.imgur.com/5SusuJ8h.jpg http://i.imgur.com/372DZblh.jpg

With the first one, I'm not sure whether you can use that circuit to be honest. If you're only measuring the voltage across one fixed resistor, and the current through it then you've got variable emf, voltage and current. Uberteknik's link include an easier setup, with the corresponding I/V characteristics.

With the second one... your teacher's comments kind of sum this up. In part C, you seem confused about the nature of current. It's a flow, and doesn't get 'used up' by any individual component, what happens is when you add an extra component you introduce more resistance and the total current flow 'slows down'. For part D, you need to think about how you would conduct the experiment i.e. how you actually vary the voltage for each diagram.

Reply 10

WilliamSlim

OP

3

Okay this is confusing me.What affects what?

if the resistance is increased does that mean there would be less current flowing through and less voltage or the current stays the same and instead more voltage is required?
In a series circuit will the current always be the same across the components even if they have different resistances?
I'm just wondering how current, voltage, resistance and power all affect each other in a circuit as i've tried just doing it from looking at the equations (e.g. P=VI) but that doesn't seem to be getting me the right answers in the past papers.

Reply 11

NotYourType

2

the basic formula to understand is the R=V/I formula.This says that current is directly proportional to the voltage and inversely proportional to the resistance.

(edited 9 years ago)

Reply 12

WilliamSlim

OP

3

Original post by NotYourType

the basic formula to understand is the R=V/I formula.This says that current is directly proportional to the voltage and inversely proportional to the resistance.

So if i increase the resistance that will result in less current going through, but then won't that mean that there will be less voltage as well. But i thought if it was ins series the current would stay the same and if resistance is decreased then the voltage would increase to get the current through the higher resistance?

Reply 13

NotYourType

2

now you've got me confused ..Lol..
check out drphysicsA's videos on YouTube, they'll be able to help you out.

Reply 14

uberteknik

21

Original post by WilliamSlim

Okay this is confusing me.What affects what?

if the resistance is increased does that mean there would be less current flowing through and less voltage or the current stays the same and instead more voltage is required?
In a series circuit will the current always be the same across the components even if they have different resistances?
I'm just wondering how current, voltage, resistance and power all affect each other in a circuit as i've tried just doing it from looking at the equations (e.g. P=VI) but that doesn't seem to be getting me the right answers in the past papers.

Go back to the basics of what each term means.

Charge: electrons and protons are charged particles. i.e. they exhibit the property of exerting a force between themselves and other charged particles. Like gravity and magnetism for instance. Like charges repel. Unlike charges attract. The difference is noted by a sign -ve = electrons, +ve = protons. Hence a battery produces an excess of -ve charge (electrons) at one electrode and an excess of +ve charge (fewer electrons) at the other. The net result is a 'pressure' between the electrodes measured in volts. (Joules per coulomb of charge).

Voltage is a measure of the collective force exerted by lots of charge of one type with another collection of opposite charges. i.e. between electrons and protons.

Current is a net flow of charge. In a conductor where atoms have fixed positions in relation to each other, only electrons are able to move. hence in electric circuits, current is the rate at which electrons flow.

Resistance is a measure of how difficult it is for current to flow through a conductor. As resistance increases, the flow of charge is reduced.

All of this means that a voltage is needed in order to provide the necessary pressure to enable current to flow between two points.

The relationship between the rate of flow of current and that voltage pressure is given by ohms law

R = \frac{V}{I}

Think of it like the flow of water in a pipe:

There needs to be a source of water (charge) which can be provided by a volume of water in a tank (coulombs of charge).

The tank is raised above ground to produce a pressure in the pipe. (charges are separated creating a voltage pressure).

Water can now flow from the tank via the pipe. The water in the tank has potential energy (mass x height x g). More water and the higher it is off ground provides a greater potential to do work. the same for charge, the greater the voltage (height) and the more charge in the tank (coulombs) the more potential to do work.

Constrictions in the water pipe creates resistance to the flow of water. restrictions in the conductor creates resistance to the flow of charge.

Can you see how this relates to the definition of power in both?

Circuits help? AS Physics OCR

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