A2 edexcel physics unit 6 revision

A2 Edexcel Physics Unit 6
Revision

Objectives
able to:
• choose measuring instruments according to their sensitivity and precision
• identify the dependent and independent variables in an investigation and the
control variables
• use appropriate apparatus and methods to make accurate and reliable
measurements
• tabulate and process measurement data
• use equations and carry out appropriate calculations
• plot and use appropriate graphs to establish or verify relationships between
variables
• relate the gradient and the intercepts of straight line graphs to appropriate
linear equations.
• distinguish between systematic and random errors
• make reasonable estimates of the errors in all measurements
• use data, graphs and other evidence from experiments to draw conclusions
• use the most significant error estimates to assess the reliability of conclusions
drawn

Significant figures
1. All non-zero digits are significant.
2. Zeros are only significant if they have a
non-zero digit to their left.

In the examples below significant zeros are in red.
203 = 3sf 023 = 2sf 230 = 3sf
0.034 = 2sf 0.0340 = 3sf 0.0304 = 3sf
5.45 = 3sf 5.405 = 4sf 5.450 = 4sf
0.037 = 2sf 1.037 = 4sf; 1.0370 = 5sf

Example
Consider the number 3250.040
It is quoted to SEVEN significant figures
SIX s.f. = 3250.04
FIVE s.f. = 3250.0
FOUR s.f. = 3250 (This is NOT 3 s.f.)
THREE s.f. = 325 x 101 (as also is 3.25 x 103)
TWO s.f. = 33 x 102 (as also is 3.3 x 103)
ONE s.f. = 3 x 103 (3000 is FOUR s.f.)
103 is ZERO s.f. (Only the order of magnitude)

Complete:
Answers:
number s.f. number s.f.
3.24 3 2.0 x 105 2
0.0560 3 9 x 1023 1
780 3 0.073 x 103 2
400 3 10-3 0
7.83 x 105 3 030 x 106 2

Significant figures in calculations
Example: Calculate the volume of a metal of mass 3.52g if a volume of
12.3cm3 of the metal has a mass of 55.1g.

density of metal = mass / volume
= 55.1 / 12.3 (original information given to 3sf)
= 4.4797
(Intermediate calculations should be performed to at least 2sf more
than the original information – calculator had ‘4.4796747’)

volume = mass / density
= 3.52 / 4.4797
= 0.78576
volume = 0.786 cm3
(The final answer should be given to the same sf as the original
information.)

Results tables

3.05 0.15 0.13 0.14 0.14

Headings should be clear
Physical quantities should have units
All measurements should be recorded (not just the ‘average’)
Correct s.f. should be used.
The average should have the same number of s.f. as the original
measurements.

Sensitivity
The sensitivity of a measuring instrument
is equal to the output reading per unit input
quantity.

For example an multimeter set to measure
currents up to 20mA will be ten times more
sensitive than one set to read up to 200mA
when both are trying to measure the same
‘unit’ current of 1mA.

Precision
A precise measurement is one that has the maximum
possible significant figures. It is as exact as possible.
Precise measurements are obtained from sensitive
measuring instruments.

The precision of a measuring instrument is equal to the
smallest non-zero reading that can be obtained.

Examples:
A metre ruler with a millimetre scale has a precision of ± 1mm.
A multimeter set on its 20mA scale has a precision of ± 0.01mA.
A less sensitive setting (200mA) only has a precision of ± 0.1mA.

Accuracy
An accurate measurement will be close to the
correct value of the quantity being measured.

Accurate measurements are obtained by a good
technique with correctly calibrated instruments.

Example: If the temperature is known to be 20ºC a
measurement of 19ºC is more accurate than one of
23ºC.

An object is known to have a mass of exactly 1kg. It has its
mass measured on four different scales. Complete the table
below by stating whether or not the reading indicated is
accurate or precise.

scale reading / kg accurate ? precise ?
A 2.564 NO YES
B 1 YES NO
C 0.9987 YES YES
D 3 NO NO

Reliability
Measurements are reliable if consistent
values are obtained each time the same
measurement is repeated.

Reliable: 45g; 44g; 44g; 47g; 46g
Unreliable: 45g; 44g; 67g; 47g; 12g; 45g

Validity
Measurements are valid if they are of the
required data or can be used to give the
required data.

Example:
In an experiment to measure the density of a solid:
Valid: mass = 45g; volume = 10cm3
Invalid: mass = 60g (when the scales read 15g
with no mass!);
resistance of metal = 16Ω (irrelevant)

Dependent and independent variables
Independent variables CHANGE the value of
dependent variables.

Examples:
Increasing the mass (INDEPENDENT) of a material causes
its volume (DEPENDENT) to increase.

Increasing the loading force (INDEPENDENT) increases
the length (DEPENDENT) of a spring

Increasing time (INDEPENDENT) results in the radioactivity
(DEPENDENT) of a substance decreasing

Control variables.
Control variables are quantities that must be kept
constant while some independent variable is being
changed to see its affect on a dependent variable.

Example:
In an investigation to see how the length of a wire
(INDEPENDENT) affects the wire’s resistance
(DEPENDENT). Control variables would be wire:
- thickness
- composition
- temperature

Plotting graphs
Graphs are drawn to help establish the
relationship between two quantities.

Normally the dependent variable is shown
on the y-axis.

If you are asked to plot bananas against
apples then bananas would be plotted on
the y-axis.

Each axis should be
labelled with a quantity length of
name (or symbol) and its spring cm
unit.

Scales should be
sensible.
e.g. 1:1, 1:2, 1:5
avoid 1:3, 1:4, 1:6 etc…
F
The origin does not have N
to be shown.

Both vertically and horizontally your
points should occupy at least half of the
available graph paper

GOOD POOR AWFUL

Best fit lines
Best fit lines can be curves!

The line should be drawn so too steep
that there are roughly the too high
same number of points above too low
correct
too shallow
and below.

Anomalous points should be
rechecked. If this is not
possible they should be
ignored when drawing the
best-fit line

Measuring gradients
gradient = y-step (Δy)
x-step (Δx)

The triangle used to find the
gradient should be shown on the
graph. Δy
Each side of the triangle should
be at least 8cm long.

Gradients usually have a unit.
Δx

The equation of a straight line
For any straight line:
y y = mx + c
where:
m = gradient
and
gradient, m
c = y-intercept
y-intercept, c
Note:
x x-intercept = - c/m
0-0 origin

x-intercept, - c/m

Calculating the y-intercept
P Graphs do not always show the y-intercept.

To calculate this intercept:
1. Measure the gradient, m
16 In this case, m = 1.5

2. Choose an x-y co-ordinate from any point
on the straight line. e.g. (12, 16)

10 3. Substitute these into: y = mx +c,
with (P = y and Q = x)

6 In this case 16 = (1.5 x 12) + c
16 = 18 + c
12 Q
8 c = 16 - 18

c = y-intercept = - 2

Linear relationships
P W
m c
c m

Q Z

Quantity P increases linearly Quantity W decreases linearly
with quantity Q. with quantity Z.

This can be expressed by the This can be expressed by the
equation: P = mQ + c equation: W = mZ + c
In this case, the gradient m is In this case, the gradient m is
POSITIVE. NEGATIVE.

Note: In neither case should the word ‘proportional’ be used as
neither line passes through the origin.

Questions
1. Quantity P is related to quantity Q by the equation:
P = 5Q + 7. If a graph of P against Q was plotted what
would be the gradient and y-intercept?
m = + 5; c = + 7

2. Quantity J is related to quantity K by the equation:
J - 6 = K / 3. If a graph of J against K was plotted what
would be the gradient and y-intercept?
m = + 0.33; c = + 6

3. Quantity W is related to quantity V by the equation:
V + 4W = 3. If a graph of W against V was plotted what
would be the gradient and x-intercept?
m = - 0.25; x-intercept = + 3; (c = + 0.75)

Direct proportion
Physical quantities are directly
proportional to each other if when one y
of them is doubled the other will also
double. m

A graph of two quantities that are directly
proportional to each other will be: x
– a straight line
– AND pass through the origin

The general equation of the straight line in
this case is: y = mx, in this case, c = 0

Note: The word ‘direct’ is sometimes not written.

Inverse proportion
Physical quantities are inversely
proportional to each other if when one of y
them is doubled the other will halve.

A graph of two quantities that are inversely
proportional to each other will be:
– a rectangular hyperbola
– has no y- or x-intercept x

Inverse proportion can be verified by y
drawing a graph of y against 1/x.
This should be: m
– a straight line
– AND pass through the origin

1/x
The general equation of the straight line in
this case is: y = m / x

Systematic error
Systematic error is error of measurement due to readings that
systematically differ from the true reading and follow a pattern or
trend or bias.

Example: Suppose a measurement should be 567cm
Readings showing systematic error: 585cm; 584cm; 583cm; 584cm

Systematic error is often caused by poor measurement technique
or by using incorrectly calibrated instruments.

Calculating a mean value (584cm) does not eliminate systematic error.

Zero error is a common cause of systematic error. This occurs when
an instrument does not read zero when it should do so. The
measurement examples above may have been caused by a zero error
of about + 17 cm.

Random error
Random error is error of measurement due to readings
that vary randomly with no recognisable pattern or
trend or bias.

Example: Suppose a measurement should be 567cm
Readings showing random error only: 569cm; 568cm; 564cm; 566cm

Random error is unavoidable but can be minimalised by using a
consistent measurement technique and the best possible
measuring instruments.

Calculating a mean value (567cm) will reduce the effect of random
error.

An object is known to have a mass of exactly 1kg. It has its
mass measured on four different occasions. Complete the
table below by stating whether or not the readings indicated
show small or large systematic or random error.

readings / kg systematic random
1.05; 0.95; 1.02 small small
1.29; 1.30; 1.28 large small
1.20; 0.85; 1.05 small large
1.05; 1.35; 1.16 large large

Range of measurements
Range is equal to the difference between the
highest and lowest reading

Readings: 45g; 44g; 44g; 47g; 46g; 45g

Range: = 47g – 44g
= 3g

Mean value <x>
Mean value calculated by adding the readings
together and dividing by the number of
readings.

Readings: 45g; 44g; 44g; 47g; 46g; 45g

Mean value of mass <m>:
= (45+44+44+47+46+45) / 6
<m> = 45.2 g

Uncertainty or probable error
The uncertainty (or probable error) in
the mean value of a measurement is
half the range expressed as a ± value

Example: If mean mass is 45.2g and the
range is 3g then:
The probable error (uncertainty) is ±1.5g

Uncertainty in a single reading
OR when measurements do not vary

• The probable error is
equal to the precision in
reading the instrument
• For the scale opposite
this would be:
± 0.1 without the magnifying
glass
± 0.02 perhaps with the
magnifying glass

Percentage uncertainty
percentage uncertainty = probable error x 100%

measurement

Example: Calculate the % uncertainty the mass
measurement 45 ± 2g
percentage uncertainty = 2g x 100%

45g
= 4.44 %

Combining percentage uncertainties
1. Products (multiplication)
Add the percentage uncertainties together.

Example:
Calculate the percentage uncertainty in force causing a
mass of 50kg ± 10% to accelerate by 20 ms -2 ± 5%.
F = ma
Hence force = 1000N ± 15% (10% plus 5%)

2. Quotients (division)
Add the percentage uncertainties together.
Example:
Calculate the percentage uncertainty in the density of a
material of mass 300g ± 5% and volume 60cm3 ± 2%.
D=M/V
Hence density = 5.0 gcm-3 ± 7% (5% plus 2%)

3. Powers
Multiply the percentage uncertainty by the
number of the power.

Example:
Calculate the percentage uncertainty in the volume of a
cube of side, L = 4.0cm ± 2%.
Volume = L3
Volume = 64cm3 ± 6% (2% x 3)

Significant figures and uncertainty
The percentage uncertainty in a measurement or calculation
determines the number of significant figures to be used.

Example:
mass = 4.52g ± 10%
±10% of 4.52g is ± 0.452g
The uncertainty should be quoted to 1sf only. i.e. ± 0.5g
The quantity value (4.52) should be quoted to the same
decimal places as the 1sf uncertainty value. i.e. ‘4.5’
The mass value will now be quoted to only 2sf.
mass = 4.5 ± 0.5g

Conclusion reliability and uncertainty
The smaller the percentage uncertainty the
more reliable is a conclusion.

Example: The average speed of a car is measured
using two different methods:
(a) manually with a stop-watch
– distance 100 ± 0.5m; time 12.2 ± 0.5s
(b) automatically using a set of light gates
– distance 10 ± 0.5cm; time 1.31 ± 0.01s
Which method gives the more reliable answer?

Percentage uncertainties:
(a) stop-watch – distance ± 0.5%; time ± 4%
(b) light gates – distance ± 5%; time ± 0.8%

Total percentage uncertainties:
(a) stop-watch: ± 4.5%
(b) light gates: ± 5.8%

Evaluation:
The stop-watch method has the lower overall percentage
uncertainty and so is the more reliable method.
The light gate method would be much better if a larger
distance was used.

Planning procedures
Usually the final part of a written ISA paper is a question
involving the planning of a procedure, usually related to an
ISA experiment, to test a hypothesis.

Example:
In an ISA experiment a marble was rolled down a slope.
With the slope angle kept constant the time taken by the
marble was measured for different distances down the
slope. The average speed of the marble was then measured
using the equation, speed = distance ÷ time.

Question:
Describe a procedure for measuring how the average speed
varies with slope angle. [5 marks]

Answer:
Any five of:
• measure the angle of a slope using a protractor
• release the marble from the same distance up the slope
• start the stop-watch on marble release stop the stop-
watch once the marble reaches the end of the slope
• repeat timing
• calculate the average time
• measure the distance the marble rolls using a metre
ruler
• calculate average speed using: speed = distance ÷ time
• repeat the above for different slope angles

Internet Links
• Equation Grapher - PhET - Learn about graphing polynomials. The
shape of the curve changes as the constants are adjusted. View the
curves for the individual terms (e.g. y=bx ) to see how they add to
generate the polynomial curve.

Notes from Breithaupt pages 219 to 220, 223 to 225 & 233
1. Define in the context of recording measurements, and give
examples of, what is meant by: (a) reliable; (b) valid;
(c) range; (d) mean value; (e) systematic error; (f) random
error; (g) zero error; (h) uncertainty; (i) accuracy;
(j) precision and (k) linearity
2. What determines the precision in (a) a single reading and
(b) multiple readings?
3. Define percentage uncertainty.
4. Two measurements P = 2.0 ± 0.1 and Q = 4.0 ± 0.4 are
obtained. Determine the uncertainty (probable error) in:
(a) P x Q; (b) Q / P; (c) P3; (d) √Q.
5. Measure the area of a piece of A4 paper and state the
probable error (or uncertainty) in your answer.
6. State the number 1230.0456 to (a) 6 sf, (b) 3 sf and (c) 0 sf.

Notes from Breithaupt pages 238 & 239
1. Copy figure 2 on page 238 and define the
terms of the equation of a straight line graph.
2. Copy figure 1 on page 238 and explain how it
shows the direct proportionality relationship
between the two quantities.
3. Draw figures 3, 4 & 5 and explain how these
graphs relate to the equation y = mx + c.
4. How can straight line graphs be used to solve
simultaneous equations?
5. Try the summary questions on page 239

A2 edexcel physics unit 6 revision

Recommended

More Related Content

What's hot (20)

Similar to A2 edexcel physics unit 6 revision (20)

More from Samith Senadeera (8)

Recently uploaded (20)

A2 edexcel physics unit 6 revision