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 instrumentsHow 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 takenX 100 = percentage

uncertainty

7.24Here the percentage uncertainty is

0.01100 0.14%

7.24

Percentage Uncertainty Of A Single Reading

The Precision of the Instrument

The reading you have takenX 100 = percentage

uncertainty

0.11Here the percentage uncertainty is

0.01100 9.1%

0.11

So a low reading on the meter involves a much greater percentage uncertainty.

Non digital instrumentsThe 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%

R

mA

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

R

mA

r

In this experiment we measure current and pd for different values of R.

V

The terminal p.d. over the cell is the same as the p.d. measured directly over the resistor. V = IR.

. V = IR.

R

mA

r

In this experiment we measure current and pd for different values of R.

V

Changing the value of this resistor changes the terminal pd and the currentV=IR

we know that

V =ε - Ir

V/V

I/A

x

x

x

x

x

x

The form of the graph is a straight line

because V= ε-Ir

Can be written V=-rI +ε

y= mx + c

ε

The gradient =-r or – the internal resistance of the cell

R

mA