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Uncertainty & Errors in MeasurementPowerPoint Presentation

Uncertainty & Errors in Measurement

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Keywords

- Uncertainty
- Precision
- Accuracy
- Systematic errors
- Random errors
- Repeatable
- Reproducible
- Outliers

Measurements = Errors

- Measurements are done directly by humans or with the help of
- Humans are behind the development of instruments, thus there will always be
associated with all

instrumentation, no matter how

precise that instrument is.

Uncertainty

When a physical quantity is taken, the uncertainty should be stated.

Example

If the balance is accurate to +/- 0.001g, the measurement is 45.310g

If the balance is accurate to +/- 0.01g, the

measurement is 45.31g

Exercise

A reward is given for a missing diamond, which has a reported mass of 9.92 +/- 0.05g. You find a diamond and measure its mass as 10.1 +/- 0.2g. Could this be the missing diamond?

Significant Figures

- ____ significant figures in 62cm3
- ____ significant figures in 100.00 g.

The 0s are significant in (2) What is the uncertainty range?

Exercise

- Express in scientific notation.
- 0.04g
- 222 cm3
- 0.030g

Random (Precision) Errors

- An error that can based on
- individual inter__________.
- Often, the error is the result of
mistakes or errors.

- Random error is not ______ and can fluctuate up or down. The smaller your random error is, the greater your ___________ is.

Random Errors are caused by

- The readability of the measuring instrument.
- The effects of changes in the surroundings such as temperature variations and air currents.
- Insufficient data.
- The observer misinterpreting the reading.

Minimizing Random Errors

- By repeating measurements.
- If the same person duplicates the experiment with the same results, the results are repeatable.
- If several persons duplicate the results, they are reproducible.

10 readings of room temperature

19.9 , 20.2 , 20.0, 20.0, 20.1, 19.9, 20.3, 19.9, 20.2, 22.3

- What is the mean temperature?
The temperature is reported as

as it has a range of

Read example in the notes.

Systematic Errors

- An error that has a fixed margin, thus producing a result that differs from the true value by a fixed amount.
- These errors occur as a result of poor experimental design or procedure.
- They cannot be reduced by repeating the experiment.

10 readings of room temperature

20.0 , 20.3 , 20.1, 20.1, 20.2, 20.0, 20.4, 20.0, 20.3

All the values are ____________.

- What is the mean temperature?
The temperature is reported as

19.9 , 20.2 , 20.0, 20.0, 20.1, 19.9, 20.3, 19.9, 20.2

Examples of Systematic Errors

- Measuring the volume of water from the top of the meniscus rather than the bottom will lead to volumes which are too ________.
- Heat losses in an exothermic reaction will lead to ______temperature changes.
- Overshooting the volume of a liquid delivered in a titration will lead to volumes which are too ______ .

Minimizing Systematic Errors

- Control the variables in your lab.
- Design a “perfect” procedure ( not ever realistic)

If all the temperature reading is 200C but the true reading is 190C .

This means, is gives us a precise but inaccurate reading.

If you have consistently obtained a reading of 200C in five trials. This could mean that your thermometer has a large systematic error.

systematic error accuracy

random error precision

random error precision

Putting it together

Example

The accurate pH for pure water is 7.00 at 250C.

Scenario I

You consistently obtain a pH reading of 6.45 +/- 0.05

Accuracy:

Precision:

What can be done realistically to improve the investigation?

Scenario 2

Choose a better instrument or method of obtaining

the pH so that your random error is lower.

The more precise your reading , the more likely you can

reproduce the same results among your partners.

Scenario 1

Recalibrate pH prole. Do not use a better probe

because the instrument is very precise

Absolute & Percentage Uncertainty

Consider measuring 25.0cm3 with a

pipette that measures to +/- 0.1 cm3.

We write

Absolute Uncertainty

Percentage Uncertainty

Presenting Uncertainties in your reading

When adding and subtracting, the number of decimal places is important.

Example:

Suppose we need to find the total mass of two pieces of zinc of mass 1.21g and 0,56g.

The total mass is

Report the total mass of solution prepared by adding 50g of water to 1.00g of sugar.

Would the use of a more precise balance for the mass of sugar result in a more precise total mass?

The precision of the total is limited by the precision

of the mass of the water. Using a more precise

balance for the mass of sugar would not improve the

precision.

- When +/- measurement , add the absolute uncertainties
Example

(15.5 +/- 0.5 mL) + (12.2 +/- 0.2 mL)

(13.5 +/- 0.2 g) + (14.55 +/- 0.05 g)

When using a burette ( +/- 0.02 cm3) , you subtract the initial volume from the final volume. The volume delivered is

Final vol = 38.46 +/- 0.02 cm3

Initial vol = 12.15 +/- 0.02 cm3

Total volume delivered =

Multiplying & Dividing

When multiplying or dividing, it is the number of significant figures that is important.

The number with the fewest significant figures used in the calculation determines how many significant figures should be used when quoting the answer.

Example

When the temperature of 0.125kg of water is increased by 7.20C.

Heat required

= mass of water x specific heat capacity x temperature rise

= 0.125 kg x 4.18 kJ kg-1 0C-1 x 7.20C

=

Since the temperature recorded only has 2 sig fig,

the answer should be written as ____________

With the absolute uncertainties included, the percentage uncertainties should be used and then convert it back to absolute uncertainties when the final results is presented.

Example

Consider a sample of sodium chloride with a mass of 5.00+/- 0.01g and a volume of 2.3 +/-cm3.What is the density?

The concentration of a solution of hydrochloric acid = moldm3 and the volume = cm3 .

Calculate the number of moles and give the absolute uncertainty.

Factory made thermometers

Assume that the liquid in the thermometer is calibrated by taking the melting point at 00C and boiling point at 1000C (1.01kPa).

If the factory made a mistake, the reading will be biased.

Instrument s have measuring scale identified and also the tolerance.

Manufacturers claim that the thermometer reads from -100C to 1100C with uncertainty +/- 0.20C.

Upon trust, we can reasonably state the room temperature is

20.10C +/- 0.20C.

Graphical technique

- Graphs can be useful to us in predicting values.
- Interpolation – determining an unknown value within the limits of the values already measured.
- Extrapolation – requires extending the graph to determine an unknown value that lies outside the range of the values measured.

Plotting Graphs

- Give the graph a title.
- Label the axes with both quantities and units.
- Use sensible linear scales – no uneven jumps.
- Plot all the points correctly.
- A line of best fit should be drawn clearly. It does not have to pass all the points but should show the general trend.
- Identify the points which do not agree with the general trend.

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