mrmackenzie

parallel circuits

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Here is a short test to let you find out how much you’ve learned about parallel circuits so far.  Click on the circuit diagram below to open the question page.  You will be asked to enter values for V1, V2, A4 & A5.

image courtesy of MATTER project

If you need some help to find all four values, click on the check answers button to view the working.

Are you ready for some more challenging questions on series and parallel circuits?  Try this page.

capacitors – charge, energy and graphs

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We’ve just completed the topic on capacitors in dc circuits, finishing off with a detailed study of the graphs obtained for current & voltage against time when a capacitor is charged or discharged through a series resistor.  There are some additional notes and practice questions at the end of this post but please watch the clips first.

This introduction to capacitors from the nice people at Make Magazine is a good starting point.

The S-cool revision site has some helpful notes and illustrations on capacitor behaviour; try page 1 (how capacitors work) and page 2 (charging and discharging).

Here is a video that covers some of the areas we discussed in class. Ignore the maths at the end of each section of the film, you won’t need it.  Notice how the man in the film uses a lightbulb, rather than an ammeter, to show when the current is large or small.  Clever, eh?

One use of capacitors you should know about is the flashing lamp.  We’ll cover this application next week.

Blinking Neon Bulb (5F30.60A) from Ricardo Alarcon on Vimeo.

I compared normal electrolytic capacitors to a 10F supercapacitor, and we observed its superior performance in terms of energy storage.  This video goes one step further and shows the fun you could have with an ultracapacitor. Do not try this at home!

Of course, you can always make your own capacitor with paper and electrically conductive paint.

Now download the pdf below. It contains notes to help with your prelim revision and some extra capacitor problems.

Thanks to Fife Science for the original pdf from Martin Cunningham.

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beautiful equations

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Those of you not out celebrating New Year might have spotted a programme called Beautiful Equations on the BBC schedule.  The programme follows an artist as he asks about five famous physics equations.

One of the featured equations should be familiar to you from unit 1 of the AH Physics course;

F = \displaystyle {{G m_1 m_2} \over r^2}

Hopefully you recognise this as Newton’s equation for the gravitational force between two bodies.  I have extracted the nine minutes or so relating to Newton’s work and embedded it below.

The link below will download the entire programme, which also looks at E=mc^2, time dilation in special relativity, the Dirac equation and Stephen Hawking’s work on black holes.

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Wheatstone bridge circuits

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We’ve just completed the section of Higher unit 2 that investigates the behaviour of a Wheatstone Bridge. The bridge circuit is really just a pair of voltage dividers connected in parallel.  A voltmeter, ammeter or galvanometer (very sensitive ammeter) connects the two voltage divider chains together, as shown below.

When the voltage (or current) displayed on the meter is zero, we say that the Wheatstone bridge is balanced.  For a balanced bridge, it is possible to show that

\displaystyle {R_1 \over R_2} = \displaystyle {R_3 \over R_4}

[you have this proof in your notes folder]

For the circuit shown above, the voltmeter will display the difference in electrical potential between points B and D.  We can calculate this potential difference by finding the voltages at points B and D using the voltage divider equation you used for Standard Grade/Intermediate 2 Physics.

V_2  = \displaystyle { R_2 \over {R_1+R_2}} \times V_s

So in this example,

V_D = \displaystyle {R_2 \over {R_1+R_2}} \times V_s

and

V_B  = \displaystyle {R_X \over {R_3+R_X}} \times V_s

The voltmeter displays the potential difference between these two points, i.e.

V_G  = V_D-V_B

Here is a short video that provides a recap of the Wheatstone Bridge.

and a worked example from an old SQA past paper

Now click on the picture below to try an interactive Wheatstone Bridge problem (you will need to have Java installed).

Instructions:

  • Press the Reset button to change the value of all the resistors in the circuit.
  • Use the slider to balance the bridge. The circuit uses a centre-zero meter, so aim to get the indicator dead centre.
  • Find the unknown resistance (R4) using the value of the other 3 resistors when the circuit is balanced.

You can repeat this simulation as many times as you like by pressing Reset to change the resistor values…..it’s great practice!

Here is an example of an application of the Wheatstone Bridge, called the metre bridge.

When a Wheatstone Bridge is slightly out of balance, it will provide a linear response.  In other words, small changes in resistance will produce proportionally small changes in voltage or current.  When these small changes are plotted, we obtain a straight line through the origin, like this:

We tried to use this property of a Wheatstone Bridge to find the temperature of the physics classroom.  We used some of the snow outside for a low temperature and boiling water for a high temperature.

As we discussed today, this was not a particularly successful experiment due to the non-linear response of the thermistor to changes in temperature – you might remember this from Standard Grade or Int 2 Physics.

For temperature ranges much smaller than the 100°C we attempted, it is possible to obtain an accurate estimate of room temperature.

Click on the download link below to try some Wheatstone Bridge questions.

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Power!

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We have been looking at electrical power this week.  The man I mentioned in class today is James Watt.  Here is a short biography by the BBC.  I also spoke about how he calculated the power output of working horses and compared them to his steam machines.  You can read more about his horsepower experiments here.
I’d say he’s a pretty famous scientist – not many people get their name on every lightbulb in the world!  😉

how to measure internal resistance

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Warning: I am expecting you to do more than just read this text.  Please plot the graph and find the properties of the cell.

You’ve just completed an experiment in class (it is listed as “Method 2″ on page 8 your printed notes) where you built a simple series circuit using a cell, a resistance box and an ammeter.  A voltmeter was connected across the resistance box and you recorded the voltage across (TPD) & current through the resistor as you changed the resistance from 0.5? to 1.5? in steps of 0.1?.

The video below shows the same type of experiment, but uses a potato and two different metals in place of normal cell.  Watch the video and note the values of I and V each time the resistance is changed – remember you can pause the video or go back if you miss any.

Now plot a graph with current along the x-axis and TPD along the y-axis.  If you don’t have any sheets of graph paper handy, there is a sheet available to download using the button at the end of this post.  Or you could try printing out a sheet from a graph paper site, use Excel or download the free LibreOffice.org Calc spreadsheet.

Draw a best-fit straight line for the points on your graph and find the gradient of the line.  When calculating gradient, remember to convert the current units from microamps (uA) to amps (A).

The gradient of your straight line will be a negative number. The gradient is equal to -r, where r is the internal resistance of the potato cell used in the video.

You can obtain other important information from this graph;

  • Extend your best fit line so that it touches the y-axis.  The value of the TPD where the line touches the y-axis is equal to the EMF of the cell. (Explanation: on the y-axis, I is zero so TPD = EMF)
  • Now extend the best-fit line so that it touches the x-axis, the current at that point is the short-circuit current – this is the maximum current that the potato cell can provide when the variable resistor is removed from the circuit altogether and replaced with just a wire.
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