minimise parallax error when timing the ball bearing
as it falls through a fixed distance in the oil?
Ensure that the observer is at eye level with the ball bearing.
NOT
- Use a metre rule rather than the scale on the measuring cylinder.
- Use two parallel rubber bands around the measuring cylinder to indicate the
fixed distance. - Start and stop the clock as the middle of the ball bearing passes through the
start and finish points.
e.m.f. of the battery
V vs A graph
intercept with the potential difference axis
determine the percentage loss of kinetic energy after a ball dropped
from a fixed height has bounced once on a hard surface.
assumption: that the initial gpe of the ball will all be converted to ke just before the ball hits the hard surface.
1. Describe an experiment which could be carried out to achieve this.
2. State one precaution to improve the accuracy of the results.
rough structure
Means of measuring height (1)
• Means of determining speed (1)
• Equates (final) GPE with (final) KE (1)
• Method to determine KE after bounce (1)
• Description of appropriate experimental method (1)
• States that mass cancels OR mass stays the same OR mass
measured OR mass known (1)
•
• Indication of how (percentage) energy loss will be determined
determine the percentage loss of kinetic energy after a ball dropped
from a fixed height has bounced once on a hard surface.
assumption: that the initial gpe of the ball will all be converted to ke just before the ball hits the hard surface.
1. Describe an experiment which could be carried out to achieve this.
2. State one precaution to improve the accuracy of the results.
ANS
Drops ball of known mass vertically from known height
• Determines height of bounce using a metre rule
• States mg∆h = 1/2mv^2
- % loss in KE = loss in PE/ initial PE x 100%
- Precaution: make measurement of height of bounce at eye level
Draw a circuit diagram to show the circuit he should use to find the resistance of a fixed length of constantan wire.
apparatus
includes a length of constantan wire, an ammeter, a voltmeter, a variable resistor and a
micrometer.
Ammeter in series and voltmeter in parallel
use a range of current values to plot a graph of p.d. against current.
how to use the graph to determine a value for the resistance R of the length of wire.
Find gradient which is R (1)
Explain how plotting a graph should improve his result
- Graph enables an average result to be taken (1)
- Graph will show anomalies(1)
- Graph will show if resistance changes (1)
State and explain what additional instrument he should use to measure the length l of
the wire
Metre rule/tape(1)
• Longer length so measurement to nearest mm appropriate (1)
• [allow to nearest ½ mm]
State what further measurement needed to take to determine the
resistivity of the wire
and
Show how to use these measurements to calculate a value for the
resistivity of constantan
Diameter (1)
- Find area using A = πd^2/4 or A = πr^2
- Calculate ρ using R =ρl/A
experiment to compare the behaviour of two
wires when forces are applied to them.
first is to find the constant k in the equation
F = k∆x for each wire.
plan
Vertical length of wire with masses on end OR horizontal wire
with masses over pulley (1)
• Measure extension (1)
• Further detail (1)
• e.g. use of mark on wire OR use of F = k∆x
• OR extension = final – initial length OR comparison of
wires
experiment to compare the behaviour of two
wires when forces are applied to them.
State one variable which would have to be kept constant to make this a fair test.
State and explain one safety precaution they would need to take.
(Initial) length of wires (1)
- Use of goggles OR means of catching/avoiding falling masses
- Protection (of eyes) from snapping wire OR protection of feet/floor from falling masses (1)
F = k∆x for each wire.
The wires are going to be used to hang pictures on a wall in an art exhibition.
Explain why knowing a value for k may be useful.
k chosen so that wire wont stretch or break for a given
picture
Criticise this set of measurements.
Inconsistent precision of timing(1) • Lengths only recorded to nearest 10 cm (1) • Only 10 oscillations timed (1) • Too few values(1) • No repeats(1) • Small range(1)
The multimeter is used to measure the potential difference across the 100 : resistor in
the circuit.
Which is be the best setting to use?
20 V
investigating the variation in the emf generated by a solar cell with light intensity.
varies the distance between the solar cell and a lamp.
Which is the independent variable?
Which is the measured dependent variable?
independent variable
- distance
dependent variable
-emf
4.21, 4.20, 4.21
state the average result?
4.21
An experiment involves measuring the time taken for a ball to fall through different
distances.
The maximum distance is 1.5 m.
which is better
- light gates or sensors and a datalogger
- a stopwatch
the advantages and disadvantages of each method
- Comparison of errors
DL - Advantage fewer errors
SW - Disadvantage more errors e.g. random,systematic or parallax (not just human error) - Reaction time
DL - No need to account for reaction time
SW - Disadvantage – effect of reaction time - Precision
DL - e.g. readings to nearest millisecond
SW - e.g. insufficient precision for shortest times - Power supply
DL - Disadvantage power supply (electricity) needed.
SW - Advantage no power supply required 1 - Complexity
DL - Needs ‘training’ setting up, alignment issues, time needed to set up
SW - Simple to operate or to set up - Graph
DL - Advantage – any graph could be drawn automatically
SW - Any graph would have to be drawn manually - Availability, cost or transport
DL - Not easily available, Expensive, Not easily transportable
SW - Readily available, Cheaper ,Easily transportable
You are to plan an experiment to determine the Young modulus of a material in the form
of a long wire. You are to use a graphical method.
- a labelled diagram of the apparatus to be used,
- a list of any additional apparatus required that is not shown in the diagram,
- the quantities to be measured,
- an explanation of your choice of measuring instrument for two of these quantities,
- which is the independent and which is the dependent variable,
- how the data collected will be used to determine the Young modulus,
- the main source of uncertainty and/or systematic error,
- a comment on safety
(a) wire, support, weights, method of measuring length
(b) micrometer screw gauge/vernier callipers/digital vernier
(c) length, extension /extended length, diameter,
mass/weight/force
(d) Credit two instruments
• original length - metre rule, long length, 1 mm appropriate
• extension –metre rule/travelling microscope/vernier
scale, small length, appropriate precision
• diameter – micrometer, small length, measures to 1/100 mm
• mass – use of appropriate balance for size of mass
(e) independent - weight/force,
dependent – extension/extended length
(f) Answers should give all details including definitions of strain and stress if relevant.
• use of πd^2/4 [allow πr^2]
• use of YM equation and graph
(g) extension/diameter
(h) identifies hazard/risk and gives appropriate precaution
Examples: falling weights and toe protection , snapping wires and safety glasses
experiment to find the focal length f of a lens. He measures the distance u from an object to the lens and the corresponding distance v from the lens to
the focused image.
The equation relating f, u and v is
1/f = 1/u + 1/v
Rearrange this equation to show that:
x the gradient of the graph should be –1
x the intercept with the y axis is 1/f.
Rearranged into form y = mx + c or y = c + mx
1/v = - 1/u + 1/f or 1/v = 1/f - 1/u
comparison to show that gradient = -1
comparison to show intercept on y-axis is 1/f
Example
1/v = - 1/u + 1/f
This is comparable to y = mx + c, where y = 1/v and x = 1/u, so m = -1 and when x = 0, c = 1/f
experiment to determine the viscosity of an oil. She drops ball bearings of different diameters into a tube of the oil. She timed the ball bearings
between two fixed marks.
The student expected to obtain a straight line graph.
Suggest one possible reason for the apparent error in her measurements.
Examples
Didn’t reach terminal velocity/is not falling at constant velocity (1)
Distance between markers is too small (1)
Tube too narrow (1)
Misreading of stop-watch or micrometer (1)
Allow temperature changes in context (1)
(Ignore parallax and reaction time alone)
