Thinking
about an ammeter and a voltmeter
An ammeter is an instrument that shows how much current passes through
it. It is designed to have a small resistance. The following problems
show why.
A. Suppose an
ammeter having a resistance of 0.5 Ω is connected in parallel
across a resistor of 50.0 Ω as shown in the figure at the
right.
 If the system
is connected up to a 12 V battery, how much current would pass through
the ammeter? (Assume you can ignore the internal resistance of the
battery.)
 How much current
would pass through the 50 ohm resistor? Is
this ammeter giving a good measure of the current in the 50 ohm resistor?
Explain why or why not.
 Is this ammeter giving a good measurement of what the current would
be in the resistor if the ammeter weren't there?


B. Suppose instead the
ammeter were in series with the resistor as shown in the figure at
the right.
 If the system is connected up to a 12 V battery, how much current
would pass through the meter?
 How much current
would pass through the 50 ohm resistor? Is
this ammeter giving a good measure of the current in the 50 ohm resistor?
Explain why or why not.
 Is this ammeter giving a good measurement of what the current would
be in the resistor if the ammeter weren't there?


A voltmeter
is also a device that shows the voltage difference across its terminals,
but it has a very large resistance. The next two problems show why.
C. Suppose a
voltmeter having a resistance
of 1500 Ω were in series with a resistor of 50.0 Ω as shown
in the figure at the right.
 If the system
is connected up to a 12 V battery, what would the voltage drop be across
the voltmeter?
 What would the
voltage drop be across the
50 ohm resistor? Is
this voltmeter giving a good measure of the voltage drop across
the 50 ohm resistor? Explain why or why not.
 Is this voltmeter
giving a good measurement of what the voltage drop across the resistor
would be if the voltmeter weren't there?


D. Suppose instead,
the voltmeter were in parallel with the resistor as shown in the figure
at the right.
 If the system
is connected up to a 12 V battery, what would the voltage drop be
across the meter? (Assume you can ignore the internal resistance
of the battery.)
 What is the voltage
drop across the
50 ohm resistor? Is
this voltmeter giving a good measure of the voltage drop across
the 50 ohm resistor? Explain why or why not.
 Is this voltmeter
giving a good measurement of what the voltage would be across the
resistor if the voltmeter weren't there?


E. So if you were to hook up an ammeter or a voltmeter to measure
the current through a resistance or the voltage drop across a resistance
respectively, how would you connect it?
Page last modified
May 1, 2009: E36