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By mrmackenzie, January 21, 2012 7:12 pm

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.

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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By mrmackenzie, January 4, 2012 12:43 pm

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;

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 , time dilation in special relativity, the Dirac equation and Stephen Hawking’s work on black holes.

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By mrmackenzie, December 16, 2011 8:30 am

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

[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.

So in this example,

and

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

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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By mrmackenzie, December 13, 2011 9:52 pm

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! 

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By mrmackenzie, November 25, 2011 8:06 pm

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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By mrmackenzie, November 15, 2011 9:39 pm

image courtesy of sonrisaelectrica

The section of the electromagnetic spectrum with wavelengths ranging from 10nm to 400nm is called ultraviolet radiation (uv for short).  Sunlight contains uv rays and it’s those uv rays that are responsible for the suntan you get during the summer holidays.  This Australian animation shows how the ultraviolet in sunlight causes our skin to tan and explains why too much uv will damage our skin.  The SunSmart page has loads of information on staying safe in the sun.

The damage that uv can do to cells is put to good use in some sterilisation equipment, such as this bottle for safe drinking water and the toothbrush sanitiser shown below.

The Nobel Prize for Medicine was awarded to Niels Rydberg Finsen in 1903 for his research into the effects of ultraviolet on the bacteria that cause tuberculosis.

There is a Bank of England leaflet (pdf) with further information on the security features in our banknotes.

Remember that whenever something glows under a uv light, we’re not seeing the uv radiation itself because our eyes can’t detect ultraviolet.  Instead, we see the fluoresence; visible light given out in response to the uv falling on the material.

Kuba demonstrated fluorescence in class with some uv hair gel.  Here he is with some of the gel in his hair.

but we don’t see anything until we turn on the uv light.

Cool, eh?

You can even buy genetically modified tropical fish that glow under uv light.

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By mrmackenzie, November 1, 2011 10:45 pm

X-rays are a form of electromagnetic radiation.  They have a much higher frequency than visible light or ultraviolet.  The diagram below, taken from Wikipedia, shows where x-rays fit into the electromagnetic spectrum.

image by Materialscientist

Wilhelm Röntgen discovered x-rays and the image below is the first x-ray image ever taken.  It shows Mrs. Röntgen’s hand and wedding ring.  The x-ray source used by Röntgen was quite weak, so his wife had to hold her hand still for about 15 minutes to expose the film.  Can you imagine waiting that long nowadays?

This was the first time anyone had seen inside a human body without cutting it open.  Poor Mrs. Röntgen was so alarmed by the sight of the image made by her husband that she cried out “I have seen my death!” Or, since she was in Germany, it might have been “Ich habe meinen Tod gesehen!” that she actually said.

Röntgen continued to work on x-rays until he was able to produce better images. The x-ray below was taken about a year after the first x-ray and you can see the improvements in quality.

Notice that these early x-rays are the opposite of what we would expect to see today. They show dark bones on a lighter background while we are used to seeing white bones on a dark background, such as the x-ray shown below.  The difference is due to the processing the film has received after being exposed to x-rays.

In hospitals, x-rays expose a film which is then developed and viewed with bright light.  X-rays are able to travel through soft body tissue and the film behind receives a large exposure.  The x-rays darken the film. More dense structures such as bone, metal fillings in teeth, artificial hip/knee joints, etc. block the path of x-rays and prevent them from reaching the film.  Unexposed regions of the film remain light in colour.

Röntgen’s x-ray films would have involved additional processing steps.  The exposed films were developed and used to create a positive.  In creating a positive, light areas become dark and dark areas become light.  So the light and dark areas in Röntgen’s x-rays are the opposite of what we see today.  Our modern method makes it easier to detect issues in the bones as they are the lighter areas.

Röntgen was awarded the first ever Nobel Prize for Physics in 1901 for his pioneering work in this field of physics.

Medical imaging has come a long way since Röntgen’s discovery of x-rays.  This promotional video from German company Siemens outlines the advances that have been made since the early 20th century.

I have attached a recording of a short BBC radio programme about the first x-ray and what people in the Victorian era thought of these new images.  Click on the player at the end of this post or listen to it in iTunes.

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By mrmackenzie, October 25, 2011 8:32 pm

Here is the answer to the wave equation question.

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By mrmackenzie, October 25, 2011 8:30 pm

Here is the answer to the amplitude question.

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By mrmackenzie, October 25, 2011 8:27 pm

This is the solution to the frequency question.

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