Transcript Practise_ISA_review

Practise ISA review
Precision Of An Instrument
• The precision of an instrument is
equivalent to the smallest division of
measurement of which it is capable
On a digital instrument, it is equivalent to the
lowest reading that can be “resolved”
On this voltmeter you could
have readings of
5.45V
7.89V
4.34V etc.
We say its precision is 0.01V
because this is the lowest
resolution of the instrument.
Non digital instruments
How long is this rod?
We can only claim to measure to the
nearest millimetre.
So the precision of the ruler is 1mm
The precision of this voltmeter is
1V
The resolution of this milliameter is 2mA
UNCERTAINTY
Because the precision of you instrument is
always limited
you always have an uncertainty
in your reading.
The uncertainty in the reading on this
voltmeter is ± 0.01V
( that is ± the precision
of the instrument)
Percentage Uncertainty Of A Single
Reading
The Precision of the Instrument
The reading you have taken
Here the percentage uncertainty is
0.01
100
7.24
0.14%
X 100 = percentage
uncertainty
7.24
Percentage Uncertainty Of A Single
Reading
The Precision of the Instrument
The reading you have taken
X 100 = percentage
uncertainty
Here the percentage uncertainty is
0.01
100 9.1%
0.11
So a low reading on the meter involves a much
greater percentage uncertainty.
0.11
Non digital instruments
The same applies to non digital instruments.
What is the percentage uncertainty In the length of this wire?
A rule should only be used to measure lengths in excess of
10cm which leads to a percentage uncertainty of less than 1%
mA
R
In this circuit would a large resistance (R) lead to a
reading with greater or lower uncertainty than a small
resistance (R)?
Reliability
• Reliable results are results that can be
repeated.
• An experiment will be reliable if the points
drawn on a graph lie on or close to the line
of best fit.
Reliable
Unreliable
In this experiment we measure current and pd for different values of R.
. V = IR.
V
r
mA
R
The terminal p.d. over the cell is the same as the p.d. measured directly
over the resistor. V = IR.
In this experiment we measure current and pd for different values of R.
V
we know that
V =ε - Ir
r
mA
R
Changing the value of this resistor changes the terminal pd and the current
V=IR
The form of the graph is a straight line
because V=
εx
V/V
ε-Ir
Can be written
x
V=-rI +ε
y= mx + c
x
x
The gradient =-r or – the
internal resistance of the cell
x
x
I/A
mA
R