Measurements and Errors

Task: Find a textbook• 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 metersit is subject to zero error.

• A centre zero scale galvanometer

Ammeter

• A galvanometerwith a resistor in parallelis 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 galvanometerwith a resistor in seriesis 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 carryingwire in a magneticfield 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