Working Scientifically Flashcards
What is the equation from %uncertainty?
absolute uncertainty/measurement x100
What are two different ways to decrease absolute uncertainty in order to decrease % uncertainty?
- Decrease absolute uncertainty
For a single measurement increase resolution
For multiple measurements
Change equipment to reduce random error
How can you decrease %uncertainty in a result without changing absolute uncertainty
Increase the magnitude of the measurement taken - as % uncertainty is a fraction with measurement magnitude on the denominator
What does %uncertainty measure?
Precision - not accuracy
What is the measure of accuracy ? Give the equation
%difference
100 x(Measured value - true value )/true value
Whether a result has a High/low precision cannot be defined in a specific scenario - what is this scenario?
When there is only one measurement
Why do we repeat experiments?
To reduce the effect of random error
Define accuracy
how close a measured value is to the true value
Define precision
how close repeated measurements are to each other/the mean
Give 2 reasons an experimental value might not be the true value and why
- Random error - lowers precision
- Systematic error - lowers accuracy
Define random error
caused by factors that vary from one measurement to another. The measurements will be randomly spread around the true value. The results will have low precision if random error is present
Define systematic error
errors that causes all measurements to be different from the true value by the same value
Give 2 reasons we do repeats
- Reduces the effect of random error
- So the mean is more likely to be closer to the true value
What is uncertainty
the interval within which the true value is considered to lie with a given level of confidence or probability
What is the equation or absolute uncertainty for a single reading
Resolution/2
How do you determine uncertainty for repeated readings
Range of results (discounting anomalies) /2
True or false: taking repeat readings reduces absolute uncertainty
False - it increases it by increasing range
Why do we take repeat readings even though it increases absolute uncertainty
Increases the likelihood the result is accurate/confidence in the result
What does an oscilloscope graph show
Potential difference versus time
Define timebase
Scale of the x axis (time per division)
Define time period as an equation using an oscilloscope
number of divisions for one wave cycle x timebase
What is resolution of a protractor, metre ruler, stopwatch?
1 degree
1mm
10 miliseconds
If you get repeat readings of the same measurement value - what is the absolute uncertainty
Plus or minus half the resolution
Which wire terminal should you use when connecting a circuit with a current of 5A to a multimeter
Bottom = as the top one is for amperage above 10A
When you have a choice between 200V max voltage setting and 2000V max voltage setting for a 15V component which should you use?
200V as the r resolution is higher (the display would read 015.0 rather than 0015)
how many decimal places should data in a column recording measurements be to?
the same number as the resolution of the instrument that measured that data
If data has been calculated and it has the same number of sf as the raw data it was calculated from, should you round it to a lower number of significant figures than the original raw data?
No, the data that is calculated should have the same (or minimum) number of significant figures as the raw data it was calculated from
Repeat measurements in a table should always/not always be completed and be evident in the table of results
Always - unless you have an electric circuit experiment where repetition is useless as it is just looking away from the circuit and looking back again
Should the range of an independent variable in an experiment be high or low?
High
How many sets of results (at least) should be taken in an experiment
At least 6
When should the results table be created?
Before the practical begins
Which side of the results table does the independent variable go?
Left hand side
Where does the dependent variable go on a results table?
The next column after the independent variable column (second to left)
Where do quantities calculated from the dependent variable go on the results table?
The next columns after the dependent variable column
Where should you write the unit for a result when filling out a table of results?
In the table heading only
How do you separate a quantity and its unit in a table heading
Using a /
If you record data in mA, should you write the recorded data in the table in A or mA?
Always in mA, and use another column to convert to Amps (if you choose to convert to Amps, which is actually unnecessary)
When should data be recorded in tables - as you do the experiment and read the measurement, or a little while later?
As you read the measurement - it should be done contemporaneously
Is it ok to copy up results in your own table from one person in a group after the experiment?
No - every individual needs to write their own data in their own results table
- All values of current should be given to the same number of decimal places
- All values of potential difference should be given to the same number of decimal places
- All values of resistance should be given to the same number of significant figures as the raw data it was calculated from: ie. As the pd and current so all resistance values should have 2sf
- There are only 4 sets of results – there should be at least 6
- There is no evidence of repeats
- The range of values of the current is too small
When drawing a graph, data points must take up more than ___of the y and x directions
Half
What are the 4 possible acceptable scales for drawing a graph
1, 2, 5, 10
When labelling scales, do you have to label every box with a value?
Yes - scales must have frequent and regular labels
Are you allowed to plot a data point beyond the graph grid?
No
How accurate do you have to be when plotting points on a graph - what does this mean for the thickness of your plot?
Accurate to half a small square - you therefore have to plot a cross that is not too thick
Can you plot points on a. Graph using blobs?
No, only crosses
There must be a ___balance of points about the line of best fit
Reasonable
Do you need to draw a line of best fit through the origin?
No - so dont force it through the origin
The line of best fit must be ___and ___(describing its appearance)
Thin and clear
When finding the gradient of the line of best fit what piece of working must you show as present on the graph?
A LARGE triangle
What is defined as a sufficiently large triangle to be used to calculate the gradient of a line of best fit?
The length of the hypotenuse is more than half of the length of the line of best fit
Should you use data points or the line of best fit when finding the gradient of the line of best fit?
Use the line of best fit values (you can use data points if they are on the line of best fit to within half a small square)
When writing the gradient of your line of best fit what 2 things must you remember to do
State the units
Give the answer to a number of significant figures that is at least as many as the raw data the gradient was calculated from
The y intercept of a graph is read from when x = ? And what is a common mistake with this question?
X = 0,
Starting the scale at a non-zero number and then recording the y intercept as the y value at that non-zero number where the graph starts
When you have used a scale that doesn’t start at 0 for a graph of plotted points how can you find the y intercept?
Use the equation of a line y = mix + c and find the equation of the line to calculate the y intercept
Explain why dropping a ball from an upstairs window rather than one closer to the ground (when finding acceleration of free fall) improves the accuracy of the experiment
- Increasing measurement of time decreases percentage uncertainty in time because %uncertainty = absolute uncertainty/measurement value x 100
○ Given absolute uncertainty stays constant as it is equal to half the resolution of the stopwatch (as there is only one measurement taken)
Comment on this comment:
‘My result has a high level of accuracy because it has a low % uncertainty’
- If your experiment has a low %uncertainty, it may also be likely to be accurate
- When talking about accuracy you should be commenting on %difference
To what level of accuracy should raw data be recorded?
To the resolution of that instrument
What should 50cm of raw data be recorded as in metres
0.500m (as the resolution of the instrument is 1mm) and the raw data should be recorded to teh SAME RESOLUTION as it was recorded
What is the relationship between the number of significant figures in raw data versus calculated data
The number of significant figures in teh calculated data should be the minimum number of significant figures in the raw data
Which variable goes on x axis? Which on y?
Independent
Dependent
When you are asked to plot velocity against time, whcih goes on the y axis and why ?
The velocity, as you always plot y against x (the dependent against independent)
How do you describe this graph
Linear relationship with negative gradient (DO NOT say negative correlation!)
Describe this graph
Linear relationship with a positive gradient (don’t say positive correlation!!)
What form should you rearrange any equation for an experiment that you use into?
Y = Mx + c
How do you create an equation in the form y = mx + c from an equation in order to draw a graph
You can do this by identifying which variables are dependent and which are independent
- Dependent ones are (roughly) y, and independent ones are (roughly) x
- Then convert to y = mx + c form (noting that there may be some floating constants and the y variable may be squared or similar)
Is there a special rule about plotting distance vs time graphs which means you shouldn’t plot dependent on y axis and independent on x?
no there isn’t!! Normal rules apply!
What is the minimum number of measurements you need to take?
6
Draw a diagram for how a rheostat works when one wire is connected to one of the end terminals and another one is connected to the top terminal, and explain where current is going
The current flows from A (assuming you connect the positive terminal to A), through the coiled wires, up through the slider and then along the bar to the top terminal
What is the resistance of the rheostat if two wires are connected to the bottom two terminals
The total resistance the rheostat provides - as the current can only flow through the coiled wire
If the bottom two terminals of a rheostat are used, and the slider is placed 1/2 way along the rheostat, what is the ratio of potential difference from the first terminal to the slider, against the second terminal to the slider
1:1, as the resistance splits equally, and pd and resistance are directly proportion
What are the 4 marking points for graphs?
Axes
Scale
Plots
Line of best fit
Axes must have __and __
Labels and units
When answering questions on criticising a results table how can you ensure top marks?
give specific information about how many dp or significant figures the student should actually be using! (For example state 3 sf or 0.0001 dp)
How should you draw a graph: portrait or landscape
Landscape is often better
Check portrait if it is easy then do it
Where should the axes be on a graph
On the edge
What is the dependent variable
Variable you measure to see how it has been affected by the independent variable
Control variable = ?
Variable you keep the same in order to ensure that changes to the dependent variable are only caused by the independent variable
Independent variable = ?
Variable you change in order to see the effect of this change on the dependent variable
Explain how using a bridge reduces uncertainty in the investigation into speed of standing wave equation
The reading of the length of the string is more likely to be accurate (has a lower absolute uncertainty) than reading the length from the pulley
What is the dependent variable in the investigation into speed of standing wave - frequency or
Fundamental frequency!!
As the values for the fundamental frequency are subject to error - in the investigation into speed of standing wave - how should we minimise the effect of this?
Do repeats - specifically go up to the fundamental frequency and then down to the fundamental frequency - and take an average
How can a graph show the relationship between (these variables which form the equation y = mx) show the relationship is valid? (This is a very common exam question)
Example:
Why is changing mu in the investigation into speed for a standing wave a bad idea?
It requires different strings
Why is having tension as a dependent variable in an investigation into speed of a standing wave a bad idea?
Tension can only really be changed in discrete steps, and it needs to be continuous in order to work
Why is having mu as a dependent variable a bad idea in an investigation into speed of standing wave practical?
It would require a million strings, each with a different value of mu, as the value of mu will be on a continuous scale
