Transcript Measurements and Errors

Measurements and Errors
Task: Find a textbook
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And measure it
It doesn’t matter which textbook.
It should have a length, a width and a height.
Use the meter rules in the corner.
How well did you measure your
object?
Errors in measurement
• The zero error!
• Did you use a new ruler or was the ruler old?
• Could the end of the ruler be worn down?
• The calibration error
• Lets examine what happens when we put 2
rulers that should be exactly the same
together.
• Parallax error
Types of error
• Systematic error
Calibration error and zero error are both
types of systematic error. The value that is
measured will consistently be either larger than,
or smaller than the actual value. If the
experiment is repeated with the same
equipment the error will also be repeated.
• Random error
This is an error that is not dependent on
the equipment itself but may be related to the
way that the equipment is being used (viewing
angle) or if the reading varies with time. If the
experiment is repeated these readings may well
be different.
Better than a ruler...?
• The Vernier Callipers
The micrometer screw gauge –
Better than some Vernier Callipers?
Using the micrometer
Errors on the micrometer screw
gauge
The accuracy of the equipment
• In each case with the equipment being
measured the accuracy is +/- the smallest
division on the scale.
• The ruler is accurate to +/- 1 mm
• The Vernier Callipers are accurate to +/- 0.1
mm
• The micrometer screw gauge is accurate to +/0.01 mm
AS physics practical workbook
• Pages 5, 6 and 7.
• Ignore question 9.
• Using and reading the micrometer screw
gauge and the Vernier Callipers.
Different Measuring Equipment
• The balance (for finding mass)
The current balance (does not find mass)
Beware the meniscus
• This is important for measuring manometers,
thermometers, some barometers as well as
measuring or graduated cylinders (any liquid
level).
Measuring Temperature
• Always allow time for the liquid to change temperature to the
temperature of its surroundings. Liquid thermometers are good
at measuring temperatures between about -40 and 350 degrees.
They are easily damaged and mercury is toxic!
• Thermocouple : Metals with different temperatures across them will
have a voltage across them. Different metals will have different
voltages. A combination of metals will produce a small voltage
difference that can be converted to a temperature reading. The ice is
used here as it is a known temperature. If it is not calibrated the
voltage should be measured at different temperatures to produce a
response curve. This can then be used to directly read the
temperatures from the voltage.
• The professional ones will look more like this
• Thermocouple thermometers can have a range from -200
degrees Celsius to over 1000 degrees Celsius. A normal liquid
thermometer would not survive these temperatures and give
an accurate reading.
The Kevin scale
• The Kelvin scale differs from the Celsius scale
by 273.15 degrees. Their graduations are
equal : a change of 1 degree Celsius is the
same as a change of 1 degree Kelvin. The
Kelvin temperature scale comes from the ideal
gas scale of temperature and is very important
in physics.
Measuring Current and Potential
Difference
• The galvanometer measures very small
electric currents. It is an analogue device (not
digital). Like all
Analogue meters
it is subject to
zero error.
• A centre zero scale galvanometer
Ammeter
• A galvanometer
with a resistor in parallel
is used to detect current.
The resistor in this case is known as a shunt.
The value of the resistor controls the sensitivity
of the ammeter.
Voltmeter
• A galvanometer
with a resistor in series
is used to detect voltage.
The resistor in this case is known as a multiplier.
The value of the resistor controls the sensitivity
of the Voltmeter.
• Analogue meters are subject to parallax error
as well as zero error. Be careful in your
measurement and always check for the zero
error where applicable.
• Multimeters have the advantage of being able
to measure different quantities at different
scales. Be careful with the uncertainty values
as the scale changes depending on the reading
and sensitivity. (+/- smallest unit on the scale).
Measuring magnetic flux density
• This is measured using a hall probe that is held
in the region of interest.
• A current carrying
wire in a magnetic
field will feel a
force. This is because the individual electrons
are experiencing a force.
• As the force that the wire experiences is acting
on the charge carriers there is a very small
voltage difference created between the top
and bottom parts of the wire (A and B). This
can be measured and converted to find the
magnetic flux density.
The Hall probe
• Consists of a small thin slice of semiconductor
that is placed in a magnetic field. The hall
probe will provide the voltage difference
across the semiconductor.
Oscilloscopes
Measuring frequency
Measuring voltage