23 November Internal Resistance Questions

The topic of Internal Resistance builds on and uses many of the concepts and equations we've learned earlier – it’s a good summative topic.

All you really need for internal resistance questions are a good understanding of:

• Ohm's law V = IR
• The fact that potential differences add up in series
• To remember that p.d.s or voltages are measuring the energy of the charge, in joules per coulomb
• In addition, that the 'lost volts' is the p.d. 'across' the internal resistance.  Lost volts = Ir where r is the internal resistance
• It might also be useful in some questions to know that Power = VI = I2R
• Power dissipated in the cell or other source = I2r
• Also, power from the source is e.m.f. x current = Iε

By doing questions on internal resistance you will be revisiting these topics – this will consolidate your understanding and recall.

Remember that the internal resistance equation is in the formula book:

 V_load = ε – Ir

Also, remember that V_load = IR where R is the load resistance, Ir = the lost volts and ε is the e.m.f. of the source

Questions from the Internal Resistance booklet:  Internal resistance of power supplies.


Exam Questions Q 1.  Remember that power = VI =  I2R


Exam Questions Q 2.  Involves some algebra!


Exam Questions Q 3.  This is quite a challenging question requiring you to think about uncertainties carefully.

11 November Part 1 of Internal Resistance (AKA "the mystery of the lost volts")

Internal resistance - the mystery of the lost volts.

Extract from the revision guide (slightly edited) : 

The energy provided by a source (e.g. a cell or battery) is delivered to the components of the circuit by charge flowing round the circuit.  However for most sources some of the energy is dissipated inside the source due to the source's internal resistance. When a current is drawn from the source this internal resistance causes the potential difference across the terminals of the source to be less than the emf of the source. 

The lost p.d. in the source is the energy dissipated per unit charge (i.e. per coulomb) inside the source due to its internal resistance. The lost p.d. depends on the current and on the internal resistance of the source.    Lost p.d. = current x internal resistance
For a source of emf ε with internal resistance r connected to a load of resistance R, as shown in the circuit below  ε = I R + I r  where IR is the potential difference across the load resistance and Ir is the lost p.d. 

The external p.d. V = I R = ε – Ir. The graph below shows how the external p.d. V varies with the current drawn. This graph has a gradient of  –r and a y-intercept equal to ε.   The intercept on the x-axis is the short circuit current, with a terminal p.d. of zero volts.

Note that the p.d. V falls as the current increases. This is why the output potential difference of an electrical source of energy (including a laboratory power supply unit) falls if more current is drawn from the source. The headlights of a car often dim for a moment as you operate the starter motor. 

The videos

Here are a number of introductory videos - watch them and add to  your  notes.

Also make sure you go over the topic in the Chapter 2 revision guide and in the textbook chapter 2.


Useful introduction from www.Fizzics.org.uk



Further introduction and explanation of the graph of V against I -



Quite a good lecture on the theory



Demonstration of a potato cell.  Different metal plates are inserted into a potato to make a cell that produces about one volt.  However it has a high resistance and can only provide very small current - in micro amps.  The current is varied by switching a resistance box to different values.  No sound track - just watch the meters.




This final video shows some practical work - the image is bit fuzzy but the physics explanations are clear.  Note the results plotted on the graph of V against I

11 November Part 2 of Internal Resistance (AKA "the mystery of the lost volts")

These videos are from Steve4physics (not me) and give a really good, detailed explanation of internal resistance.

Part 1: Introduction - the basics



Part 2: Calculations - the basics



Part 3: Calculations - more complex



Part 4: Calculations - how to measure internal resistance N.B. the graphical method here plots Current on the y axis against terminal potential difference on the x axis.  Our exam board normally show the graphs the other way round, so the section after 5 minutes 19 seconds is less useful and might be confusing unless you are a very confident mathematician.

3 November, second section of revision question answers and explanations.

Make sure you've attempted the questions yourself first, and marked them using the mark scheme provided at the end of the booklet.

Then watch the videos below - even for questions you got completely correct, to reinforce your learning.

Questions on the test will be similar, and some will be the same as these!!

Questions 7 and 8



Questions 9 to 11 - note that there is a new Question 11 as the previous one was a duplicate of Q 2!  A copy of the new question is here

Questions 12 and 13

3 November. Revision Questions 1 to 6

Make sure you've attempted the questions yourself first, and marked them using the mark scheme provided at the end of the booklet.

Then watch the videos below - even for questions you got completely correct, to reinforce your learning.

Questions on the test will be similar, and some will be the same as these!!

This first video is just a reminder about the key equations that are in the formula booklet - I've extracted the key ones and put them onto one page here.


This video covers Q 1 to 3


This one explains Q 4 to 6

22 October Potential Dividers (also known as Voltage dividers in the USA) part 2

Part 2 of potential dividers
This video shows a potato used as a potentiometer - fun but don't try it at home! (If you ask nicely we might try it using a safe p.d. like 24 V and a high resistance voltmeter)  http://youtu.be/1l2kgi4DOOk




The website below might be useful, particularly for the potential divider calculator.  Also students aiming at high grades could make sure that they understand the approximations section - understand, not learn!!.   (Please note, if only one connection is shown for Vout, the other one is the negative of the supply, usually labelled 0 volts in electronics circuits.)  https://learn.sparkfun.com/tutorials/voltage-dividers/ideal-voltage-divider



Further work, more examples to practice on.   http://www.youtube.com/watch?v=rIEnMpgIaU
However this video makes the calculations look more complicated than they need to be as he leaves the current value as a calculation of V/R instead of working out the actual value.  For example in this screenshot 5V/(2+3)kilo_ohms is of course equal to 0.0010 A (1.0 mA) .  This approach can be useful as it shows directly how the ratio of the resistors is the key factor in working out the output p.d.  See further note and screenshot below the embedded video (at the bottom of this blog)




Note:  Instead of labelling V1, V2 as the voltages across the resistors he labels them V2kohm instead, as you can see in this screenshot.  The final answer for V2kohm is clearly 5V x (2/5) = 2 Volts!

22 October Potential Dividers (also known as Voltage dividers in the USA) part 1

On 25 October and in a later lesson we will be working on Potential Dividers. It is essential that you get a good understanding of these because:

• passive sensors are normally connected in a potential divider
• the resistance of the sensor changes in response to the environment (e.g. an LDR has a high resistance in the dark and a low resistance in the light)
• the change in resistance produces a change in the output voltage of the LDR

1)  The screenshot  below is from http://www.bbc.co.uk/schools/gcsebitesize/design/electronics/componentsrev4.shtml  which explains how to use a potential divider in an electronic circuit.  Please note that in electronics it's common practice not to show the power supply or battery - instead the diagram just shows a 'top rail' which would have the positive of the supply connected to it (V in) and a 'bottom rail' which is connected to 0 volts, the negative of the battery or supply.

2) The video below is a very good introduction to potential dividers - watch it and make notes for yourself.
http://www.youtube.com/watch?v=Mn2i3DPI-a4
 b



The video below includes some clear explanations - watch and add examples to your notes.  He mentions the concept of 'loading' the potential divider but the examples given only cover 'no load' situations - which are what you can expect to come across in AS physics ('loading' is putting a resistor across the output of the potential divider.   http://www.youtube.com/watch?v=u8pAGROJ5N4

16 October - resistors in series and parallel, to prepare for lesson on Friday 18 Oct.

On Friday we will do some practical work on combining conductors and resistors in series and parallel. This post provides you with the key information you need and some videos that explain it.  I suggest you write down summaries of the key points and copy or work through the calculations while you watch the videos.

The key information you need to learn is given here and in the text and image immediately below here. The videos are linked further down this post.

Basically: in series the more resistors the current has to go through, the greater the total resistance.  i.e. in series resistances add up.

In parallel, additional resistors provide additional paths for the current, so the conductance gets bigger the more resistors there are.  i.e. in parallel the conductances add up.

In the diagrams below,

• ·         G is a single conductor that is equivalent to G₁ and G₂ in parallel
• 
  
• ·         R is a single resistor that is equivalent to R₁ and R₂ in parallel

Here is a good summary - the section on combining resistors starts after 6 mins 50 seconds.  (the first section is a good revision about Electrical Power and includes a short section GCSE work on paying for electrical energy).  This video explains what is going on really well including some good animations using the Phet circuit modelling software.

In fact, I can't find any videos on the web that show how to combine conductors in parallel - they all use the parallel resistors rule 1/Rtotal = 1/R1 + 1/R2.  As you will see if you look at the summary above, this is just the same as Gtotal = G1 + G2, but harder to remember!  

Other videos for reinforcement:

I've left these as links instead of embedding - these should let you open the video if you click on teh link.  Please let me know what works best.

Series https://www.youtube.com/watch?v=UJa8dzAyFvo&feature=c4-overview-vl&list=PL88B748FA3DA87FD0  

parallel  https://www.youtube.com/watch?v=SfijTQ18x80&list=PL88B748FA3DA87FD0 

Extension work for those aiming at the highest grades: 

https://www.youtube.com/watch?v=CZgqGTxL9cA

How to solve a complex combination of resistors : https://www.youtube.com/watch?v=9Yv9vrxDvg4&list=PL88B748FA3DA87FD0 

Here are the first two above embedded, in case the links don't work

Characteristics of conductors, V = IR, I = GV and ohm's law.

On Friday 11 October we took measurements to enable us to plot the characteristic graphs of tow resistors and a lamp bulb and work out their conductance and resistance.

Read the extract below from the revision guide for Chapter 2 Sensing on Conductance and Resistance which defines these terms and gives a brief explanation:

Additional point on 'ohmic' conductors:
The above definitions of  resistance R = V/I and conductance G = I/V can always be applied to any component.  However for some special conductors the conductance or resistance is constant - if so, the current will always be proportional to potential difference and a graph of I against V will be a straight line.    Ohm's law states that 'Current is proportional to potential difference for a metallic conductor at constant temperature'  - so we call a component where this is true an 'ohmic' conductor.  The resistors we used are 'ohmic' and your graphs should be straight lines through the origin.

The video below explains this a bit further and asks you to annotate your graphs and the last page of the experiment instruction sheet on Characteristics of Conductors (apologies for it being a bit rough and the sound of running water in the background - it was raining heavily and there was a lot of water running down the conservatory gutter)



Further videos to watch.
Because resistance and ohm's law are so fundamental to lots of physics and electronics there are loads of resources on the internet.  I've selected some of these below - hopefully you'll manage to watch most of them.  This is a good chance to follow some expert teachers on this topic and makes sure you have a really good grasp of what's going on.

Several of these are from a YouTube channel by Steve4physics but he's not me!!  He teaches a similar specification but does not cover conductance.

Resistance part 1 the basics   This video is rather slow but very helpful.  It would be a good idea for you to write notes including the definition of resistance and write up all the calculations (you'll get most out of it if you try the questions for yourself first - pause the video before the answer is revealed).





Resistance part 2  including graphs - 

Remember that you need to be able to interpret graphs of both

• V against I – i.e. p.d. on the y axis, current on the x axis
• I against V – i.e. current on the y axis, p.d. on the x axis.

It would be good to write your own summary (for your notes) of the material in this video




The video linked below goes into ohmic conductors in more detail; the first 8 minutes are very helpful. Again, it would be good to write your own summary (for your notes) of the material in this video.

After 8 minutes he tries to make it more mathematical but complicates matters by introducing new terms K and k to describe the straight line graphs (Y proportional to X).  In fact k is resistance and K is conductance, but he doesn't explain this.  Probably best not to watch this section unless you are a keen mathematician!


STeve4Physics' has two more useful videos for this topic:

1. IV characteristics of  resistors and lamps
2. IV characteristic curve for diodes.  This second one includes a really good explanation from 8 minutes onwards on why a potentiometer used for making measurements of p.d. and current - this has come up on some exam questions in the past.

Use these videos to reinforce your learning - you might add a few extra points to the notes you've already made:

I-V characteristics of resistors and lamp bulbs:

Diodes (and in the last 3 minutes why we use a potentiometer circuit)



Here are a few more - if you want some more examples, try these:

The very basics again (embedding not available) http://www.youtube.com/watch?v=QwNSa_8ro_Y&feature=share&list=TLjBES-ykOa1BRvarq-fJp8HfvQHY6tf-P

This one goes through some examples - good for improving how you write down your calculations (he uses i instead of I for current)



And some more examples to follow:

October 4: Why does the resistance of a metal increase with increasing temperature?

On 04 October I set you a study task to find out why the resistance of a metal wire changes with temperature, and how it changes.

I found that when I searched for information on this on the internet there was too much information in most articles and videos - for example many of them mention resistivity which we haven't covered yet.

The title of this post tells you the answer to how - the resistance of metals increases with increasing temperature. To explain why the resistance changes needs you to get a picture of what is going on inside the wire, and why the wire resists the flow of electrons in the first place.

N.B. I'd originally told you to type this up but I'd prefer you to write it by hand, with hand-drawn diagrams.  You need to explain:

• why metals have resistance to the flow of electrons
• why the resistance increases when the temperature of the metal increases.

Page 4 of this Bitesize GCSE revision site gives a quick explanation.  (It's worth going through all 6 pages)

Extracts from an old science film.
I also found some extracts from a very old (1945) American film called 'Principles of Electricity' that might help - read my comments and watch these three short extracts linked below.

The first extract is a reminder of what electric current actually is.  The narrator uses the term 'pressure' for potential difference or voltage - this makes sense, as it the the potential difference which 'pushes' the current through a circuit, but it is not a term that we use in A level physics in the UK.

N.B. At about 40 seconds he makes a mistake when he says that "the current is the number of electrons that pass a given point in a given time" when it should have said "the current depends the number of electrons that pass a given point in a given time"



This second extract explains the difference between insulators and conductors, and how collisions with the lattice of atoms causes resistance in metals and why a resistor gets hot when a current flows through it:

This third section should improve your understanding of resistance in metals - it's to do with how often the conduction electrons collide with the metal atoms.

The first 2 minutes and 25 seconds of this video are useful (after this it goes into some equations that are no longer included in the A level physics specification - so don't watch the rest!).




This video shows balls (the electrons) moving through a lattice of rods (the atoms in the wire).  You can see how the collisions with the rods (atoms) takes energy from the balls (electrons) and resists their movement through the model (the wire).  I think you can also image that if the rods vibrated about at random then the balls would collide with them more often, increasing the resistance.


The first section of this webpage might also help you.

However I found that the best illustration is by using the Phet java simulation of a Battery-resistor circuit  (download from here and try it yourself).

I recorded myself using it and added an explanation - see my video below.



Extension:  we'll soon have to explain why the resistance of a thermistor decreases with increasing temperature - so get an early idea of this here.

September 27 Homework - check your answers from this video.

This homework is about applying the definitions of current and potential difference to pulses of current in an X-ray tube.  It also gives you practice with handling calculations involving the charge on one electron (e = -1.6 x 10^-19 coulombs) and the large number of electrons that flow in electrical currents.

The equation for the energy transformed E = QVapplies to both the pulse of current and to each individual electron - for one electron the energy transformed is simply:

E = (the charge on one electron) x V, i.e. for one electron,  E = eV.

The worksheet is also on Moodle - here:

http://moodle.qeliz.ac.uk/file.php/127/Chapter_2_Sensing/2_8_Ions_in_X_ray_machines_-_large_and_small_numbers.pdf

 

Watch the video, check your understanding and mark your work.  (I had a problem playing this via Internet Explorer in college - if this happens, try using Chrome instead.  Let me know by email (shunnisett at qeliz dot ac dot uk) if you can't watch the video.

http://youtu.be/Hgpk07VqLLY

Make sure you have written down your calculations clearly - add extra if you haven't.

Hand in your work - make sure you've marked it yourself, clearly - by Thursday 3 October.

Thanks, Steve

Downloading and using the PHET construction kit

The DC circuits construction kit is hosted by the University of Colorado in the city of Boulder on the eastern edge of the Rockies.  See http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc 

Watch this short video to see how to download the Java app.  

 http://youtu.be/xDieC5MDQuQ

100%

The next objective is to find out how Potential Divider circuits work.

Watch the video linked below to see how to use the circuit construction kit to make series circuits.  When you have two components in series the potential difference is divided between the two components.  This type of circuit is called a Potential Divider. Series circuits are the basis of many sensor circuits - we'll try making some sensor circuits in a future lesson.

Video:  http://youtu.be/rR4UPSKgffA

Homework 20 September

Some people have said they couldn't find the link for some of the videos last week.

They are all linked from the document on moodle (the document is in AS physics on moodle - go to https://moodle.qeliz.ac.uk to log in, then click here: 

https://moodle.qeliz.ac.uk/mod/resource/view.php?id=29185)

 

Here are the links for the videos -I've just copied them from the document, hope they work

1. Potential difference explanation (ski slope analogy) https://www.youtube.com/watch?v=F1p3fgbDnkY
2. What is an amp? https://www.youtube.com/watch?v=8gvJzrjwjds 
3. Recap on Series and parallel circuits: https://www.youtube.com/watch?v=x2EuYqj_0Uk
4. It's not the volts that kill you, it's the amps [video explains why this isn't exactly true - you need both amps and volts: http://www.youtube.com/watch?v=8xONZcBJh5A