Higher

how satellites rule our world

by

BBC2 showed a really good programme about satellites last night.  This screenshot showing a satellite passing over the Highlands is taken from about 17 minutes into the show.  Click on the picture to visit the BBC’s own page about the documentary.

It was quite eye-opening to see just how much modern society relies on satellite technology.

You can download the entire programme using the link below.

Podcast Video Download (2188)

capacitors – charge, energy and graphs

by

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.

eBook Download (11800)

Wheatstone bridge circuits

by

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.

eBook Download (3457)