Transcript

from a think aloud activity

Context: A sixth form (c.17 year old) student works through some software designed to support independent learning.

© Keith S Taber: this material is not to be copied or retained after use, and should not be passed on to any other person.

Okay so we’ve started up the first screen and we’ve gone to introduction. We’ve got, erm, (reading from the screen) Introduction, Wayne’s new camera. Wayne has got a new camera, click on the areas to find out more [see transcription conventions]

so it’s numbered 1, 2, 3, so,

click on number 1, oh Wayne is excited about his his new digital camera, and he’s keen to take photographs of his friends.

So I’m guessing we’re gonna have – something to do with photography.

Wayne has read the information leaflet, and graphic – talking about photographic imaging. Wayne asks his physics teacher about charge coupled devices, and his physics teacher tells him they work by making use of photoelectric effect.

So it’s put photoelectric effect in a context with digital cameras. I’m hoping they explain a bit more, because I’m lost, not lost, but it doesn’t mean anything to me yet.

Okay.

Aims. Wayne wants to find out more about how his camera works. In this unit you will learn, more about, you will learn about the photoelectric effect and how it is used in charge coupled devices. This subject can be part of your AS or A2 syllabus depending upon the examining board you are using. Before beginning this unit you should already be familiar with the properties of electromagnetic radiation.

And it’s got it highlighted, so you can click on it, and there’s probably a link. Which I’ll do once I finished reading this.

You will need a pen and paper handy, in order to work out the answers to some of the questions in the unit.

So, clicked on electromagnetic radiation, and it’s gone to the glossary, and it’s given me definition: radiation consisting of alternating electric and magnetic field oscillating at right angles to each other, and the direction that the wave travels, so it’s talking about how electric fields, and erm, magnetic fields work at right angles to each other, between two objects

Okay so, First thoughts. First thoughts. What do you know about electromagnetic radiation. Which of the following statements are true for all forms of electromagnetic radiation? Select one or more.

Statements are true.

So it’s given me five things and I’ve got to read them and decide, so they can all travel through a vacuum – I would think that’s right.

Yeah.

They travel at the speed of… I don’t know, I think that changes

They all have the same wavelength and frequencies – no they can all have different wavelengths and frequencies

They all consist of a combination of oscillating electric and magnetic fields that oscillate at right angles to each other, and the direction of travel to the wave – sounds possible. That was a bit of a guess. {Note: he’s just been told this by the glossary}

they all have characteristic wavelengths and hence frequencie – yeah.

So, I’ve selected 3 out of the 5, click okay.

That’s right, all forms of electromagnetic radiation travel at the speed of 3 times 10 to the 8, full stop.

330 metres per second is the speed of sound. On the next page you will see the effect of electromagnetic radiation has on some materials.

So

told to go through to the next page..

Presentation. What is the photoelectric effect? It’s explaining the key term of this sort of topic.

The photoelectric effect is the emission of electrons caused by photoelectrons, from the surface of certain materials when illuminated by electromagnetic radiation of particular wavelengths.

Ah. Okay, so. Electrons - okay.

So now it’s got a diagram with light wavelengths, hitting a piece of metal and it’s showing the erm, the electrons being repelled from the metals. I/m guessing it’s got something to do with the angle which they’re being repelled from, I don’t know.

On the next page you will see an experiment set up that can be used to detect photoelectrons. A piece of metal is illuminated by light and photoelectrons are emitted from the surface of the metal. The photoelectrons are detected by the presence of an ammeter, which registers a current when photoelectrons are emitted. This I called the photocurrent.

So it’s sort of explained what we’re gonna do, and it’s used ammeter, I recognise what that is, from the work I’ve done with electronics, and it’s obviously how it’s going to work.

The experiment can be used to investigate three different scenarios. The frequency of the light is constant, but the intensity can be varied. The intensity of the light is constant, but the frequency can be varied. The frequency of the light is a minimum by the intensity can be varied. So it’s talking about, erm, it’s putting the ideas, laying down the ideas of frequency, intensity, and sort of, it’s obviously going to show you what effect one has on the other. Move to the next page to investigate the scenario. Next page.

What factors govern the emission of photoelectrons? The animation below shows the metal surface illuminated by a light source. You can vary the intensity and frequency of the light by moving the slider. Erm, the slider in appropriate direction. Photoelectrons are detected by the presence of the ammeter which registers a current. Investigating the three scenarios below, what happens to the photocurrent measured by the ammeter?

So, it’s asking me to – use the mouse to – alter the erm intensity or the frequency - so then – start

so, click on frequency of light constant, so you can alter the intensity of the light, select it, increase the intensity of the light – we should see a change n the ammeter – try move it up a bit more – I’ve moved it up a little bit and it hasn’t made any noticeable change, I’ve moved it up some more and the reading on the ammeter has gone up.

So, I’m guessing that that means that there are more electrons emitted, and being detected by the metal plate in the circuit with the ammeter, erm when I increase the intensity of the light.

So now I’m going to click on intensity of the light is constant, and now I’m going to erm increase the frequency of the light, so at the moment the ammeter’s reading zero. – It’s moving the frequency up.

Right, it’s made, yeah. Erm, for the first sort of section there wasn’t any change. And then there was a little bit of change once I got over sort of half way between the minimum and maximum frequency of the light.

Go all the way up to maximum, it hasn’t changed a great deal more. So, I’m thinking that erm, - ’cause when I increased, when the frequency was constant, the intensity was altered the increase in the reading on the ammeter was greater over a shorter period of time, or over a shorter difference in intensity, than it has been when the frequency was increased.

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I know what I want to say, but I can’t really say it, basically I saw a greater change in the reading on the ammeter, when I increased the intensity by less, when I increased the frequency by more there was less of a change.

Just my sort of what I’ve seen. Okay, now the frequency of the light is a minimum. It’s the last of the three, erm factors that they said you could change. So I’m gonna alter the intensity again.

There’s been no change in, no change in the reading on the ammeter.

Check that.

Yeah.

Okay so, erm, there’s obviously a relationship between intensity and frequency, although if you have frequency at a very small amount, there is no relationship, or not that’s being read on the ammeter that’s been calibrated in pico, picoamps. So, I’ll move on to the next page.

Investigation – what happened to the photocurrent. The following paragraph describes what happened to the photocurrent for each of the scenarios in the experiment, but some of the words are missing. Select the correct options from the drop list.

So, first, sort of bullet point, when the intensity of the light was increased, the photocurrent measured by the ammter, I’m expecting there to be an option of increased here. I’m hoping. Yeah, so increased, {unclear], and the second bullet point, oh, second bullet point, when the intensity of the light was kept constant, the frequency was at its minimum, the photocurrent measured by the ammeter,

when the intensity of the light was kept constant, and the frequency was at its minimum value, so I think this should be zero. Where the, it’s where the ammeter started. - zero.

Third bullet point, when the frequency of the light was at a minimum, okay, and the intensity was increased the photocurrent – didn’t change. Or is that zero?

Yeah.

Does that make sense? When the intensity of the light is constant at its minimum value. Ammeter, zero. Yeah. So click okay.

That’s right. The reading on the ammeter increased as the illumination increased. When the frequency is reduced below a particular value, the photocurrent decreased to zero. Once this minimum frequency had been reached, investigating the intensity of the illumination of the light had no effect, and the current remained at zero.

So, erm, this sort of page was just going back and sort of, making sure you actually understood what you’d been looking at in the last diagram.

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It’s amazing, for a Friday afternoon anyway. Erm. So yeah, erm. I guess it is just making sure what you are able to understand what you are looking at in the diagram and what it is showing. That you’re not getting lost, or behind.

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Index of topics