133.Answering Essay Questions in Physics

Physics Factsheet www.curriculum-press.co.uk

Number 133

Answering Essay questions in Physics Possible answer B The patient lies under the X-ray machine and X-rays are sent through them. There is photographic paper behind the patient. Bone stops the X-rays. A shadow of the bone is produced on the photographic paper. The rest of the paper is black because X-rays can reach it. Xrays are waves in the electromagnetic spectrum. Bone stops the Xrays because it has a high density. X-rays can reach the rest of the paper because they can pass through soft tissue.

For many of you, the thought of writing an essay during an examination is quite a frightening one. The good news is that A Level Physics essays are rarely worth more than 7 – 9 marks. They are really extended answers where you need to make sure you have your facts in a logical order, use correct English and, of course, answer the question that was asked! Answering the question that was asked: What does the question ask you to do: • Describe something ? • Explain something ?

Possible Answer C: Because X-rays are high energy electromagnetic waves they are able to pass easily through substances which have a low density but are attenuated by substances which have a higher density. Generally, the higher the density, the greater the attenuation. Bone has a much higher density than soft tissue, therefore, X-rays are attenuated much more by bone than by soft tissue. When X-rays are sent into the part of the body being investigated, they pass through the soft tissue and expose photographic paper that has been placed below the patient. This causes a chemical reaction on the photographic paper which causes it to blacken. Because the X-rays cannot pass through the higher density bone, the area of photographic paper underneath the bone remains unexposed {white}.

Does it ask you to say: • What happens? • How something happens? • Why something happens? In order to access all the available marks for a question you need to make sure you are covering what the question is asking for. Most essay questions, because of the number of marks available, will be asking you to explain. This means you will nearly always need to use the words “therefore” or “because” in your answer.

Possible answer D The patient lies on a table. There is photographic film under them. X-rays are emitted from the anode of an X-ray machine. The anode produces X-rays after being hit by electrons which have been emitted from the cathode of the X-ray machine. Lead casing makes sure the X-rays only come out where they are supposed to as Xrays can’t pass through lead. This is to keep the radiographer and the patient safe because the X-rays only reach the part of the patient being photographed. When the X-ray is taken the photographic film shows a good contrast between bone and soft tissue.

Example Question 1 State what X-rays are, and explain how X-rays are able to produce an image of bones on photographic paper. [7marks] [This includes two marks for quality of communication.]

Where are the marks? The markscheme would probably be looking for the following physics points: • That X-rays are high energy/high frequency electromagnetic waves • The link between density and attenuation • The density difference between bone and soft tissue. • Which parts of the photographic paper are exposed. • The effect on the photographic film being exposed. The quality of communication marks would probably be looking for: • Correct use and spelling of technical words • Answer written in a logical order.

Possible answer A X-rays are high frequency electromagnetic waves. They can travel easily through soft tissue but they are stopped by bone. This means a shadow of the bone is formed on the photographic paper underneath the patient.

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Physics Factsheet

133.Answering Essay questions in Physics So how well has each example done?

Answer C: Because X-rays are high energy electromagnetic waves ✓ they are able to pass easily through substances which have a low density but are attenuated by substances which have a higher ✓ . Generally, the higher the density, the greater the density✓ attenuation. Bone has a much higher density than soft tissue, therefore, X-rays are attenuated much more by bone than by soft ✓ . When X-rays are sent into the part of the body being tissue✓ investigated, they pass through the soft tissue and expose photographic paper that has been placed below the patient. This causes a chemical reaction on the photographic paper which ✓ . Because the X-rays cannot pass through causes it to blacken✓ the higher density bone, the area of photographic paper ✓. underneath the bone remains unexposed {white}✓ Communication marks :2 Total 7 marks

✓ correct, 1 mark. ▲ not wrong but not enough to gain the mark. ✓bod borderline answer - benefit of doubt given. Answer A ✓ X-rays are high frequency electromagnetic waves.✓ They can travel easily through soft tissue but they are stopped by bone ▲ This means a shadow of the bone is formed on the photographic paper underneath the patient. ✓ bod Communication marks: 1 Total 3 marks. Answer B The patient lies under the X-ray machine and X-rays are sent through them. There is photographic paper behind the patient ▲ . Bone stops the X-rays ▲ . A shadow of the bone is produced on the photographic paper. The ✓ rest of the paper is black because X-rays can reach it.✓ X-rays are waves in the electromagnetic spectrum ▲ . Bone has a high density ▲ . X-rays can reach the rest of the paper because they can pass through soft tissue ▲ .

Answer D The patient lies on a table. There is photographic film under them. X-rays are emitted from the anode of an X-ray machine. The anode produces X-rays after being hit by electrons which have been emitted from the cathode of the X-ray machine. Lead casing makes sure the X-ray only come out where they are supposed to as X-rays can’t pass through lead. This is to keep the radiographer and the patient safe because the X-rays only reach the part of the patient being photographed. When the X-ray is taken the photographic film shows a good contrast between bone and soft tissue.

Oh dear – there are ‘bits’ of marks all over the place in this answer but it is not written in a logical way. This makes it hard for the examiner to find and put together the ‘bits of marks’ to make whole marks……and means at least one communication mark has been lost! There aren’t many technical terms used here so the other communication mark would also probably not be given. Communication marks: 0 Total 1 mark

Sorry – no marks here! The candidate obviously knows quite a bit about X-rays but what they’ve written has nothing to do with the question asked ! Take care – read the question! It doesn’t say “ Tell me what you know about X-rays.”

Exam Hint: • Try writing brief notes/bullet points before starting to write your answer. You can then number these so that you write them in a logical order. • Note down any technical words you may want to use in your answer & think about their spelling. • If the question asks you to explain, write down “because” and/or “therefore” as a reminder to your self to use these words.

Let’s re-organise this answer: X-rays are waves in the electromagnetic spectrum. ▲ The patient lies under the X-ray machine and X-rays are sent through them. There is photographic paper behind the patient. ▲ Bone has a high density. ▲ Bone stops the X-rays ▲ . X-rays can reach the rest of the paper because they can pass through soft tissue ▲ . ✓ bod A shadow of the bone is produced on the photographic paper. The ✓ rest of the paper is black because X-rays can reach it.✓

• • • • • • • • •

There will now be a communication mark for the answer being in a logical order. There is also more chance of getting the borderline ✓ bod mark because all the relevant parts that would be considered for this mark are next to each other and therefore easier for the examiner to see. Communication marks: 1 Total 3 marks Now let’s see what adding a few more words would do to this answer: X-rays are high frequency waves in the electromagnetic spectrumv. The patient lies under the X-ray machine and X-rays are sent through them. There is photographic paper behind the patient ✓ . ✓. Bone has a high density therefore bone stops the X-rays✓ X-rays can reach the rest of the paper because they can pass through soft tissue ✓ bod A shadow of the bone is produced on the photographic paper. The ✓ rest of the paper is black because X-rays can reach it.✓ Communication marks: 1{logical order but there’s an obvious technical word missing which means it may not get 2} Total 5 marks

High freq. electromagnetic waves [1] X-rays blacken paper [5] Bone absorbs – go through soft tissue – density [2] Reach/don’t reach photo paper [4] Density – bone high [3] Frequency Attenuate Density Because NOW START WRITING YOUR ANSWER!

So the things to watch out for are: • Answer the question asked – Describe….? Explain….? • Make your points in a logical order • Use correct technical terms – spelt correctly! • Avoid “it” and “they” • If asked to explain use “because” and/or “therefore”

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Physics Factsheet

133.Answering Essay questions in Physics

Exam Hint : don’t use the words “it” or “they” in an essay type question if you can help it. It is sometimes hard to tell what “it” refers to in a sentence. For example: When the X-ray hits bone it causes it to be absorbed so it doesn’t reach it and the photographic paper isn’t exposed. HELP! What’s being absorbed? The X-ray or the bone? What doesn’t reach what? Sorry – no marks here!

Practice Question Radioisotopes are frequently used as medical tracers. A small amount of the radioisotope may be swallowed by the patient or injected into their bloodstream and its path through the body followed by a radiation detector positioned outside the body. The radioisotope Technetium-99m is often used as a medical tracer. Technetium-99m has a half-life of 6 hours and emits only gamma radiation. It can also be easily and cheaply manufactured on site. Use this information to explain the following: • Why Technetium-99m is suitable for use as a medical tracer • Why Technetium-99m is usually made in the hospital where it is to be used (rather than bought in from a supplier and stored until it is needed). [9 marks, including 2 marks for quality of communication]

o Why it would not be appropriate to buy it in and store it until needed. o Why type of radiation emitted makes it a good tracer o Why half-life makes it a good tracer •

Answer written in a logical order – there are three separate issues which need to be clearly organised:

The quality of communication marks would probably be looking for: • Correct use and spelling of technical words – decay, penetration, ionising, half-life. Dangers of storage/cost of unnecessary storage or wastage of sample which decays before it can be used.

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Short half-life means Technetium-99m would decay substantially before being used if it was stored so may be too weak to trace.

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Alpha and beta are more ionising/ gamma less damaging as non-ionising

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No alpha or beta emitted so less danger of damage to surrounding tissue.

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Gamma radiation sufficiently penetrating to be detected outside the body.

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Half life of this radioisotope is long enough for procedure to be carried out.

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Short half-life means less danger to patient

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Answer: The markscheme would probably be looking for the following physics points: Acknowledgements: This Physics Factsheet was researched and written by Beverly Rickwood The Curriculum Press,Bank House, 105 King Street,Wellington, Shropshire, TF1 1NU Physics Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136

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