1. Practise ISA review

2. Precision Of An Instrument
The precision of an instrument is equivalent to the smallest division of measurement of which it is capable

3. 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.

4. 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

5. The precision of this voltmeter is 1V

6. The resolution of this milliameter is 2mA

7. 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)

8. Percentage Uncertainty Of A Single Reading
The Precision of the Instrument
The reading you have taken
X 100 = percentage uncertainty
7.24
Here the percentage uncertainty is

9. Percentage Uncertainty Of A Single Reading
The Precision of the Instrument
The reading you have taken
X 100 = percentage uncertainty
0.11
Here the percentage uncertainty is
So a low reading on the meter involves a much greater percentage uncertainty.

10. 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%

11. mA R
In this circuit would a large resistance (R) lead to a reading with greater or lower uncertainty than a small resistance (R)?

12. 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.

13. Reliable
Unreliable

14. In this experiment we measure current and pd for different values of R.
. V = IR. r mA R
The terminal p.d. over the cell is the same as the p.d. measured directly over the resistor. V = IR.

15. In this experiment we measure current and pd for different values of R.
we know that
V =ε - Ir r mA R
Changing the value of this resistor changes the terminal pd and the current
V=IR

16. ε y= mx + c The form of the graph is a straight line because V= ε-Ir
Can be written V=-rI +ε ε x x
V/V
y= mx + c x x
The gradient =-r or – the internal resistance of the cell x x
I/A

17. mA R