Class:
Name:
(
Experiment 2f
) Date:
2f Using an ammeter to measure current
Objective
To study the effect of internal resistance of an ammeter in
measuring current.
Background information
1
The equivalent resistance of resistors connected in series is equal to
the sum of the individual resistances of them. It is always higher than
each individual resistance.
2
Ammeters are not ideal in reality. They have an internal resistance
which affects the current they measure.
Apparatus
❏ 1 resistor (1 Ω)
❏ 1 switch
❏ 1 ammeter
❏ several connecting leads
❏ 1 battery box (1.5 V)
Procedure
✐ The e.m.f. of the battery 1
and the resistance of the
resistor can be changed to
other values available.
Connect a 1-Ω resistor in series with an ammeter, a 1.5-V battery
box and a switch (Fig 2f-1).
battery box
ammeter
switch
A
1-8resistor
1-8resistor
Fig 2f-1
New Physics at Work (Second Edition)
© Oxford University Press 2007
37
Experiment 2f
Class:
✐ Batteries which have
been used for several
times may provide an
e.m.f. smaller than
1.5 V. In order to
show the conclusion,
Ts should buy a new
package of batteries
for the experiment, or
check out the voltage
across each battery with
a voltmeter before the
lesson, so that the e.m.f.
provided by each battery
is approximately equal to
1.5 V.
Name:
(
) Date:
2
Measure the current I′ flowing through the resistor using the
ammeter.
✎
0.6
Current I′ through the resistor = ___________
A
3
Calculate the theoretical value of current I flowing through the
resistor.
✎
1.5
Theoretical voltage V across the resistor = ___________
V
1
Resistance R of the resistor = ___________
Ω
Theoretical current I through the resistor =
V
R
1.5
1
___________
=
1.5
= ___________
A
Discussion
✎
What is the resistance of an ideal ammeter?
0Ω
✎
Compare the theoretical value I and the experimental value I′ of
current flowing through the resistor. Account for the difference.
The theoretical value I is much larger than the experimental value I′.
An ammeter has an internal resistance. When it is connected in series with the
resistor, the equivalent resistance in the circuit increases. Therefore, the current
flowing in the circuit, which is the measured current, decreases.
small
A practical ammeter has a ________________________ internal
resistance instead of zero. When it is connected in
series in a circuit, the equivalent resistance of the circuit
increases
________________________
and the measured current is
smaller
________________________
than the theoretical current of the circuit.
38
New Physics at Work (Second Edition)
© Oxford University Press 2007
Class:
Name:
(
) Date:
Experiment 2f
Further thinking
✎
How does the accuracy of an ammeter in measuring current change
with its internal resistance? Explain your answer.
The larger the internal resistance of an ammeter, the lower its accuracy.
When the internal resistance of an ammeter increases, the equivalent resistance in
the circuit increases, so the current flowing in the circuit decreases. In this case, the
difference between the measured value and the theoretical value of current increases.
New Physics at Work (Second Edition)
© Oxford University Press 2007
39