Uncertainty & error
- An error is the difference between a value or quantity obtained in an experiment and an accepted or literature value for an experiment
- There are two types of errors in experiments, random errors and systematic errors
- Uncertainties are the same as random errors
- Uncertainties express the confidence to which the measurement can be taken
Random Errors
- When you are reading an instrument and estimate the final digit, there is an equal chance that you may read it slightly too high or slightly too low
- This is a random error
- Random errors are can be affected by:
- How easily the instrument or scale is to read
- The person reading the scale poorly
- Changes in the environment, for example
- fluctuations in the temperature of the lab
- air currents in the room
- Random errors will pull a result away from an accepted value in either direction (either too high or too low)
Systematic Errors
- Systematic errors are errors that occur as a result of a faulty or poorly designed experimental procedure
- Systematic errors will always pull the result away from the accepted value in the same direction (always too high or always too low)
- For example,
- If you forget to zero an electronic balance (using the tare button) the mass weighings will always be higher than they should be
- If you don’t read the volume in a burette at eye level, the volumes will always be smaller than they should be due
- If you fail to keep a cap on a spirit burner in a calorimetry experiment, the alcohol will evaporate and give you a larger mass loss
Systematic errors always pull the result away from the accepted value in the same direction: either too high or too low