Practical Skills COPY Flashcards
When is there a parallax error?
When your reading is wrong because your eye is not at the same level as the point you are measuring
How can you avoid parallax error?
By using a set square to correctly line your eye up or by using a mirror
How do you use a mirror to reduce parallax error?
Place your scale behind the pin you are trying to measure, place your mirror next to the scale and behind the pin, move your eye until the pin hides its own reflection.
What can you use callipers to measure?
Measurements of distances up to 20 cm
When you use a micrometer for measurements?
When the distance is below 3cm
Why are micrometers better than callipers?
They have a better resolution
How do you use a micrometer correctly?
Check its 0 reading and note the 0 error if there is one, Take a reading and note the measurement after allowing if necessary for the 0 error
What type of error is a 0 error?
A systematic error
What kind or error is a variation in diameter?
A random error
When should you do repeat readings?
When measuring something such as diameter of a wire or the thickness of a slide
Why is it good to repeat readings?
It reduces the percentage error
What is the error in a stopwatch?
+/- 0.01
Why is there some error when using stopwatches? What kind or error is this?
Random errors caused by starting and stopping too early or too late due to human reaction times / difficulty judging
What is the error in a range of readings?
Half the range
What is the uncertainty if half of the range of the readings < 0.01?
It is the precision of the instrument measuring
Why should you do multiple oscillations instead of one when timing oscillations?
Reduces the percentage error
Where should a fiducial marker be placed when measuring oscillations?
At the centre of the oscillation as this is where the speed is greatest
What do fiducial markers reduce when timing oscillations?
Absolute errors
What must you always say when a balance is used?
State that you zero the balance the pressing the tare button
What is a random error?
An error which may be too big or too small by different amounts each time a reading is repeated
What will reduce random errors?
By taking repeated readings and then taking a mean
What is a systematic error?
An error which is always too big or too small, often by the same amount.
Where do systematic errors usually come from?
The measuring instruments
Where can systematic errors come from?
A fault with the instrument (zero error) ; there is an additional factor that has not been accounted for (friction)
How do you lessen 0 errors?
Check for a 0 error and then either adjust the instrument, or take account of this error in your measurements
What is the error in most measurements?
Half what you can read it to
What is the error when measuring length?
What you can read it to
What are some other reasons errors can occur when measuring lengths, other than the error in the ruler
it is hard to make the thing being measured straight against the ruler
How do you calculate percentage uncertainty?
uncertainty / value (or mean value) x100
Why should you try to make measurements as large as possible?
To reduce the percentage error
How do you combine uncertainties when two values are being added or subtracted?
Add the uncertainties
How do you find the most accurate measurement when asked to compare?
Calculate the percentage uncertainty and find the measurement with the lowest
How do you find the error in a reading when two separate readings, each with their own errors are multiplied or divided?
Add the percentage uncertainty then convert to absolute uncertainty
How do you find the error in a measurement when it is squared?
Multiply the percentage uncertainty by the power then convert to absolute uncertainty
How do you reduce uncertainties?
(1) Using higher resolution apparatus
(2) Calibrate the equipment
(3) Increase the size of the measurement
How do you calculate percentage difference between the measured value and the theoretical value?
(theoretical - measured) / theoretical x100
How do you find the percentage uncertainty in a gradient?
|(best gradient - worst gradient)| / best gradient x100
How do you find the percentage uncertainty in the y-intercept?
|(Best y-intercept - worst y intercept)| / best y intercept x100
What could the ‘worst gradient’ be?
The steepest gradient possible or the shallowest gradient possible
How do you check direct proportion using a table of results
Divide the corresponding sets of values and if the value is constant then the variables are directly proportional
How do you check if a values is approximately constant?
Divide max by min and is the value is < 1.10 then the values are approximately constant
How do you check inverse proportion from a table of results?
Multiply the sets of values of the two variables. If they are constant they are inversely proportional
How do you check if decay is exponential?
Look at the variables at regular time intervals t0, t2, t3, t4. if t1/t0 = t2/t1 = t3/t2 then the decay is exponential
What should log or ln values be given to?
3 d.p.
What are the advantages of data logging over traditional methods?
(1) can take two measurements simultaneously
(2) eliminates reaction time error that comes from stop clocks
(3) can measure changes in a very short time period
(4) can measure changes in a long space of time which is good because you don’t have to be there
(5) Can be automatically put on a spreadsheet
(6) Sample rate can be much greater
Define accuracy
A measurement result is considered accurate if it is judged to be close to the true value that would be obtained by an ideal measurement
Define calibration
Markig a scale on a measuring instrument. this involves establishing the relationship between indications of a measuring instrument and a standard or reference quantity values, which must be applied.
Define measurement error
The difference between a measured value and the true value
Define anomalies
Values in a set of results which are judged not to be part of the variation caused by random error
Define a fair test
Only the independent variable has been allowed to affect the dependent variable
Define hypothesis
A proposal intended to explain certain facts or observations
Define interval
The quantity between readings eg a set of 11 readings over a distance of 1 m would give an interval of 10 cm
Define precision
Precise measurements are ones in which there is very little spread about the mean value. Precision depends on the extent of random errors - it gives no indication of how close the results are to the true value.
define prediction
A statement suggesting that will happen in the future, based on observation experience or hypothesis
Define range
The minimum and maximum values or the independent variables
Define repeatable
A measurement is repeatable if the original experimenter repeats the experiment using the same method and equipment and obtains the same results
Define reproducible
A measurement is reproducible if the investigation is repeated by another person, or by using different equipment or techniques, and the same results are obtained
Define resolution
The smallest change in the quantity being measured of a measuring instrument that gives the perceptible change in the reading
Define uncertainty
The interval within which the true value can be expected to lie, with a given level of confidence or probability
Define validity
Suitability of an experiment procedure to answer the question being asked
Define continuous variable
Have values that can be given magnitude either by counting or by measurement
Define control variables
One which may, in addition to the independent variable, affect the outcome of the investigation and therefore has to be kept constant or monitored
Define dependent variable
The variable which the value is measured for each and every change in the independent variable
Define independent variable
The variable for which values are changed or selected by the investigator.
