How Science works : Errors

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Teachers Notes This sequence of slides is
designed to introduce, and explain, the idea of
errors in practical work, as explained on pages
8 and 362 in New Physics for You, 2006 2011
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How Science works Errors
New Physics for You, pages 8, 362
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Learning Objectives You should learn

• About different types of errors,
• How to reduce them when you are doing your
  practical work.

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What is an error?
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What is an error?
For example, by not using the equipment correctly
Lets look at some examples.
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Human error
Example 1
Professor Messer is trying to measure the
length of a piece of wood
Discuss what he is doing wrong. How many mistakes
can you find? Six?
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Human error
Answers

• Measuring from 100 end
• 95.4 is the wrong number
• mm is wrong unit (cm)
• Hand-held object, wobbling
• Gap between object the rule
• End of object not at the end of the rule
• Eye is not at the end of the object (parallax)
• He is on wrong side of the rule to see scale.

How many did you find?
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Human error
Example 2
youreye
Reading a scale
Discuss the best position to put your eye.
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Human error
youreye
2 is best. 1 and 3 give the wrong readings. This
is called a parallax error.
It is due to the gap here, between the pointer
and the scale.
Should the gap be wide or narrow?
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Anomalous results
When you are doing your practical work, you may
get an odd or inconsistent or anomalous
reading. This may be due to a simple mistake in
reading a scale. The best way to identify an
anomalous result is to draw a graph. For example
. . .
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Anomalous results
Look at this graph
Which result do you think may be anomalous?
A result like this should be taken again, to
check it.
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Types of errors
When reading scales, there are 2 main types of
error

• Random errors
• Systematic errors.

Lets look at some examples . . .
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Random errors
These may be due to human error, a faulty
technique, or faulty equipment.
To reduce the error, take a lot of readings, and
then calculate the average (mean).
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Systematic errors
These errors cause readings to be shifted one
way (or the other) from the true reading.
Your results will be systematically wrong.
Lets look at some examples . . .
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Systematic errors
Example 1 Suppose you are measuring with a ruler
If the ruler is wrongly calibrated, or if it
expands,
then all the readings will be too low (or all too
high)
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Systematic errors
Example 2 If you have a parallax error
with your eye always too high
then you will get a systematic error
All your readings will be too high.
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Systematic errors
A particular type of systematic error is called a
zero error.
Here are some examples . . .
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Zero errors
Example 3 A spring balance
Over a period of time, the spring may weaken,
and so the pointer does not point to zero
What effect does this have on all the readings?
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Zero errors
Example 4 Look at this top-pan balance
It has a zero error.
There is nothing on it, but it is not reading
zero.
What effect do you think this will have on all
the readings?
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Zero errors
Example 5 Look at this ammeter
If you used it like this, what effect would it
have on your results?
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Zero errors
Example 6 Look at this voltmeter
What is the first thing to do?
Use a screwdriver here to adjust the pointer.
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Zero errors
Example 7 Look at this ammeter
What can you say?
Is it a zero error? Or is it parallax?
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Zero error, Parallax error
Example 8 Look at this ammeter
It has a mirror behind the pointer, near the
scale.
What is it for? How can you use it to stop
parallax error?
When the image of the pointer in the mirror is
hidden by the pointer itself, then you are
looking at 90o, with no parallax.
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In summary

• Human errors can be due to faulty technique.

• Parallax errors can be avoided.

• Anomalous results can be seen on a graph.

• Random errors can be reduced by taking many
  readings, and then calculating the average (mean).

• Systematic errors, including zero errors, will
  cause all your results to be wrong.

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Learning Outcomes You should now

• Understand the effects of
• - Human error, including parallax error,
• - Random errors,
• - Systematic errors, including zero errors
• Be able to reduce these errors when doing your
  practical work
• Be able to identify anomalous results.

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• For more details, see
• New Physics for You, page 362
• For more free PowerPoints, visit
• the web-site at www.physics4u.co.uk

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