Physics practical
Unit page → 339 (from the book)
3
1
Instrument you should know
1.
2.
3.
4.
5.
6.
7.
8.
Metre rules
Balances
Protractors
Stopwatches
Ammeters
Volt metres
Micrometre screw Gauge
Calipers
a.
Dial caliper
b.
Vernier caliper
2
4
You need to know how
how to use them
5&6
7
8/a
8/b
prongs
Calipers
What are calipers?
Jaws
- They are designed to grip an object
w/ 2 jaws
- they are used to measure the
internal diameter
How?
The lower jaws (jaws[from the dial])
are placed into the
Gathering evidence
Worked example from the book
Things you need to consider:
Q1. You are given resistors of the
following; #50, 100, 150, 200, 250,
300, 350, 400, 450, 500.
You are asked to take measurements
w/ just 6 resistors which would you
choose? & why?
-
The range of results that you
are going to choose
Make sure you take values that
are widely spread from each
other to consider what happens
when it is in different
conditions
50, 150, 250, 350, 450, 500
Because then we can see the
different values in different
conditions
Precision, Accuracy , Error & uncertainties
~ There will always be uncertainties in your measurements.
What causes this?[think about on a practical exam]
1.
The equipment (it is imperfect)
2.
The technique (may need some improvement)
[how to improve{in(1.)&(2.)}to result in less uncertainties]
[how to present your uncertainties{in(1.)&(2.)}]
Precision
Accuracy
What is it?
When you make several measurements
and the result becomes the same or
similar.
What is it?
It is when the value obtained is
the same or similar to the true
value.
-
-
-
More times you do the
experiment and the results
become more precise.
Difficulties & Making judgments
will limit the precision
It is reflected on how the
results are recorded
-
‘15m’ suggests that it is
measured to the nearest metre.
‘15.0m’ suggests that it was
measured to the nearest 0.1m.
Even if the result is precise(
produces the same value each
time), it can still be
inaccurate because, there may
be the same error
Ex:
-
When measuring with a gauge and
the result is the same, but
there me an error if it does
have a 0 error
Example questions from the book
Q2.a. draw a target that has holes
that are both precise and accurate
Q2.b. draw a target where the holes
that is not precise or correct
Q3.the positions of the holes on
the targets shows the attempts for
measuring the position of the
centre of the circle, which shows a
more systematic error and which
shows a more random error.
1
2
Systematic error: (1)
Random error: (2)
Types of errors
Systematic Errors:
A Spring on a force metre:
It might become weaker over
time which can cause the
force metre to be continually
high.
On a magnet in an amp meter:
It becomes weaker over time
which causes the needle to
not go as far down across the
scale
Parallax error:
When the person reads the
measurements at a different
angle rather than reading it
directly from above
How can it be corrected?
By changing the instrument or
improving the technique
Zero Error:
~ When the 0 of the ruler
is no exactly at the
beginning of the ruler,
which causes a fixed error
(unless it is allowed
for[when the ruler has a
one sided 0 error])
~ considered as a
systematic error
~ Can have an actual
value[we can find the
uncertainty which can give
us an estimate]
Random Error:
When a judgments being
made by the observer,
the result can be above
or below the true value
How can we prevent
this?
By getting multiple
values then averaging
the results.
Error & uncertainty
Uncertainty:
characteristics/meaning
~ is an estimate of the difference between the
reading and the true value
~ lays in between them
~ It is a number w/ a unit
~ it is ok if the uncertainty is half the value
bc/ its given to ONE significant figure
~ can never be 0
The difference between error and uncertainty:
It is a problem that causes the result to be
different from the true value.
Which method of finding the uncertainty should we
use?
It is better to repeat the reading and then use
the second method, but they are all the same then
we use both
EX:(when we know the true value) if we knew that a
length is 21.0cm but an error occurred and made it
21.5cm. bc/ the true value is 21.0cm ∴ makes the
uncertainty ±0.5
Types of uncertainty
using the division scale
For looking at smallest division on the scale.
What that means is for ex:
If we measure an object on a ruler, & it shows
that it is at 50, it will never be at exactly 50,
depending on the ruler you are using you can find
the uncertainty of the ruler on the object.
EX:when an amp meter shows the result 0.35A the
we know that the uncertainty is ±0.01
Why?
Because the smallest division on the result 0.05,
but bc/ it it is not exactly 5 we can saye that
it is ± 0.01
Repeating readings:
Which is when we have multiple values(results).
How can we find the estimate uncertainty?:
By taking the smallest value(result) &
subtracting it from largest value(result),That
value is then halved.
Examples of difficulties & uncertainties
Using a stopwatch:
Uncertainties:
You may not be able to repeat the
measurements, the reaction time will be at
least (1)sec, & when we press the button it
does not go off at exactly that moment but
more likely at (0.1)sec. Even though the
smallest division on it is around (0.01)sec,
we still have errors that are much larger
than it that we have to take in for
consideration
Measuring the displacement of a pendulum
Very hard to see where the pendulum stops
exactly because it is moving constantly.
Even if we consider putting it in slow-motion
it will be very difficult to where the
pendulum stops exactly
Answering questions from the book
Q.4.
1. The person that got the mass might be looking at an angle
which can cause an error in the result
2. A 0 error might also occur bc/ the scale needle might not
be exactly at 0 in the beginning
Q5. ±0.020mm
Q6. ±0.50/1.0oc
Q7. ±2.0mm
Q8. ±0.s
Q9:
a.
±0.4 →
b.
±28 → 28.6
c.
±0.9 →
d.
±0.1 → 0