Higher

higher – photoelectric effect

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We looked at the photoelectric effect earlier this week.  This video has a similar demonstration to the gold leaf electroscope experiment I showed you in class and includes an explanation of the process.

Click on the picture below to download the simulation we used to investigate the effect of irradiance on frequency on photocurrent.  You’ll be prompted to install Java if you don’t have it already.

Once the animation is running, you can;

  • change the metal under investigation (we used zinc in class)
  • vary the wavelength of the incident light
  • vary the irradiance of the incident light.

Notice that below the theshold frequency you can’t get any photoelectrons, even if you set the light to its brightest setting.

Compare your results to the graphs provided in your notes.

I have attached some notes & questions on the photoelectric effect. Click on the link below to download a copy.

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diffraction gratings

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We have used the grating equation

n \lambda = d \sin \theta

to measure the wavelength of light produced by a laser.  This equation is also useful to predict the location of bright fringes produced by a diffraction grating.  Remember that d in this equation is the distance between adjacent lines in the grating and not the number of lines per metre/millimetre. More about d on this A-level revision page.

We’ll look at applications of this equation a little more this week, e.g. using a spectrometer to measure the angle so we can calculate the wavelength of the light used.

spectrometer [photo by mrmackenzie]

In the meantime, get some practice at using the grating equation with the simulation site shown below.  You can select how many lines you would like per millimeter of grating and alter the wavelength.  Try calculating the angle for the first or second order spots and then use the simulated protractor to see if you are correct.

Click on the image below to get started.

You need to have Java installed to run the simulation.

I have attached a pdf with some notes on diffraction gratings.  Read through them and make sure you are happy with the theory so you are able to complete the experiments in class.

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answers to higher unit 2 practice NAB

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Here are my solutions to the practice NAB for unit 2.

Check your own answers carefully.  Did you;

  • mix up the equation for charged particles with those for capacitors?
  • remember to calculate the period of an ac signal in seconds?
  • make the correct substitutions for V1 and V2 in the differential amplifier question?
  • use “it” instead of nouns in your explanations?
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Capacitor revision

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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).

There is information on charging and discharging capacitors on BBC Bitesize.

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?

You must be explain how a flashing neon bulb can be controlled using a capacitor & resistor arranged in series.  We shall cover this in class but here is a short video introduction.

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

A few lessons back, I compared energy storage using 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.

We also looked at capacitors in ac circuits.  You need to know that a capacitor will allow an ac current to flow. The current in such a circuit will increase as the current increases.  Mr Mallon’s site has a revision activity about capacitors in ac circuits.

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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review of Wheatstone Bridge circuits

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With your prelim scheduled for the end of this month, I thought you might find a repost of this review of the Wheatstone Bridge useful for revision.  There are practise questions in the attached pdf document.

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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transit of Venus

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I set my alarm clock for 4am and was disappointed to find a sky full of clouds that would prevent us from viewing the transit of Venus from Thurso and the surrounding area. 

The BBC Horizon programme broadcast last night was very good.  You can still catch it on iPlayer for the next week or download it using the link below.

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