Physics – Measurements in Electricity
1
Use of Ammeters and Voltmeters
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2
How to read the ammeter and voltmeter
2.1
Using the mirror
2.2
Checking for zero error
2.3
Accounting for the zero error
2.4
Range and precision of instrument
2.5
Taking a reading
•
3
4
Quiz 1
Quiz 2
Connecting an ammeter or voltmeter to a circuit
3.1
Using crocodile clips
3.2
Connecting an ammeter to a circuit
3.3
Connecting a voltmeter to a circuit
Setting up an electrical circuit
4.1
Procedure
4.2
Trouble shooting
1
Use of Ammeters and Voltmeters
Ammeters and voltmeters are measuring instruments used to measure properties of electrical circuits.
An ammeter
•
•
An ammeter is used to measure the current flowing through
a particular point in the circuit.
The SI unit for electrical current is ampere (A) (also a base
unit).
An ideal ammeter has no resistance or R = 0 Ω (ohm).
•
The circuit symbol of an ammeter:
•
A voltmeter
•
•
•
•
•
A voltmeter is used to measure the potential difference
(p.d.) between two points in a circuit.
The SI unit for p.d. is volt (V).
When connected directly across a battery alone, the voltmeter
measures the battery’s electromotive force (e.m.f.).
An ideal voltmeter has an infinite resistance, so no current
flows through it.
The circuit symbol of an ammeter:
Quiz 1
You have a dry cell labelled 1.5 V. Which instrument would you use to confirm this label?
A
An ammeter
B
A voltmeter
C
A micrometer
2
How to read the ammeter and voltmeter
2.1
Using the mirror
•
•
A strip of mirror is placed just above the scale to help the observer avoid parallax error.
When taking a reading, look directly over the scale and the pointer. Adjust the position of the eye till the pointer coincides with (or
just covers) its image in the strip of mirror.
Incorrect: pointer does not coincide with its image.
Correct: pointer coincides with its image.
8
9
2.2
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Checking for zero error
You are familiar with the concept of zero error from using the
vernier calipers and micrometer screw gauge.
This voltmeter has no zero error.
•
Ammeters and voltmeters can sometimes also have zero
error.
This voltmeter has a zero error.
2.3
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Accounting for the zero error
You can correct for zero error using the method you have
already learnt:
actual reading – zero error = corrected reading
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Alternately, it is recommended that you manually adjust the
ammeter or voltmeter to remove (or eliminate) the zero error.
•
This is done by using a suitable screwdriver to turn the screw
located on the centre of the face of the ammeter / voltmeter.
Turn the screw until the reading reaches zero. Use the mirror
to ensure there is no parallax error.
Removing the zero error with a screw driver.
2.4
Range and precision of instrument
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There are ammeters and voltmeters with different ranges (or full scale deflections) and smallest divisions.
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The precision of each instrument is taken to be half the smallest division.
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The common types of ammeters and voltmeters available in our physics laboratory are shown in the table.
Instrument
Ammeter
Range
0-1 A
Smallest division
0.02 A
Precision
0.01 A
Ammeter
0-3 A
0.1 A
0.05 A
0.20 A, 0.25 A, 0.30 A
Voltmeter
0-1 V
0.02 V
0.01 V
0.46 V, 0.47 V, 0.48 V
Voltmeter
0-3 V
0.1 V
0.05 V
0.45 V, 0.50 V, 0.55 V
Ammeter with range 0-1 A
Example of readings
0.23 A, 0.24 A, 0.25 A
Voltmeter with range 0-3 V
2.5
•
Taking a reading
As described earlier, when taking a reading, adjust the eye position till the pointer coincides with its image in the strip of mirror.
http://youtu.be/ifrM5yHBsVY
Video 1: Adjust your eye position until the pointer coincides with its mirror reflection.
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The precision of each instrument is half the smallest division. The reading should be recorded with the same number of decimal
places as its precision.
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For example, if the precision of a voltmeter is 0.01 V (to 2 d.p.), and the reading observed is 2.5 V,
the reading should be recorded as 2.50 V (also to 2 d.p.).
Quiz 2
Q2.1
A
B
C
The ammeter shown has a range of 0-3 A. What is its precision?
0.01 A
0.05 A
0.1 A
Q2.2
A
B
C
How should the reading on this ammeter be recorded as?
0.4 A
0.40 A
0.400 A
3
Connecting an ammeter or voltmeter to a circuit
3.1
Using crocodile clips
There are two ways to connect crocodile clips to the terminals of an ammeter or voltmeter.
Method 1
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Screw the terminal’s cap down as far as it will go. Attach the
crocodile clip such that one of the metal jaws goes down the
hole in the middle of the terminal.
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Make sure it goes in as deep as possible, and that the jaw is
in firm contact with the metal inside the terminal.
http://youtu.be/LzwPhOfq3CM
Inserting a crocodile clip into a terminal.
Video 2: Inserting a crocodile clip into a terminal.
Method 2
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Unscrew the terminal’s cap to expose the lower part of the
metal contact (at the base of the terminal).
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Attach the crocodile clip to the metal contact.
http://youtu.be/3jUWUpJGZIE
Attaching a crocodile clip at the base of a terminal.
Video 3: Attaching a crocodile clip at the base of a terminal.
3.2
Connecting an ammeter to a circuit
An ammeter is always connected to a circuit in series.
Warning:
• Always make sure that there is a resistor in the circuit connected in series with the ammeter.
• Connecting an ammeter to a circuit with no resistor will damage the ammeter, as a very large current will flow through it.
•
The circuit diagram shows two dry cells in series with a
resistor, a switch and an ammeter.
A circuit diagram
•
The corresponding circuit set-up is shown below.
Ammeter connected in series to a circuit.
3.3
Connecting a voltmeter to a circuit
A voltmeter is always connected to a component or section of a circuit in parallel.
•
The circuit diagram shows a voltmeter connected across two
dry cells to measure their e.m.f.
A circuit diagram
•
The corresponding circuit set-up is shown below.
Voltmeter connected across two dry cells.
•
The circuit diagram shows two dry cells in series with a
resistor and a switch. The voltmeter is connected across the
resistor (or in parallel with the resistor).
A circuit diagram
•
The corresponding circuit set-up is shown below.
Voltmeter with two crocodile clips.
Two dry cells connected in series with a
resistor and a switch.
Voltmeter connected in parallel with the
resistor in the circuit.
4
Setting up an electrical circuit
4.1
Procedure
1. Study and understand the circuit diagram given.
2. Identify and arrange all circuit components in a manner similar to the circuit diagram.
3. Keep the switch open.
4. Connect the main components of the series section of the circuit first (including battery, switch, ammeter, resistor etc.)
ensuring all electrical connections are tightened.
5. Connect any voltmeter in parallel to the required component or section.
6. Check the final connections against the circuit diagram.
7. Close the switch when you are ready to take readings.
8. Open the switch immediately after a set of readings is taking.
4.2
Trouble shooting
Observation
No reading
Possible problems
Incomplete circuit.
2
Unsteady or varying
readings
Loose connections
3
Pointer deflects the
wrong way from zero
(anticlockwise from
zero)
Pointer deflects
beyond the range
The two wire connections (or crocodile clips) to
the ammeter or voltmeter are reversed.
Possible solutions
Check if
• Switch is closed
• If all electrical connections are complete
and tightened
• Check if all electrical connections are
correct and complete against the circuit
diagram.
• Ensure all connections are tightened.
Inter-change the two wire connections
(crocodile clips) to the ammeter or voltmeter
1. Ammeter: the current being measured is
beyond the range of the ammeter
1. Change the ammeter to one with a
greater range
2. Voltmeter: the p.d. or e.m.f. being measured
is beyond the range of the voltmeter
2. Change the voltmeter to one with a
greater range
1
4