We have an e.m.f of supply
fixed at 12V. The supply has an internal resistance of 5 Ω that limits the maximum
current.
The problem is to design a laboratory experiment to investigate how
electrical power delivered to a lamp varies with the number of lamps that are parallel with the
supply.
The following apparatus is available.
12V power supply unit
Selection of
low voltage lamps Crocodile clips Joulemeter
Ammeter Connecting wires
Selection of resistors
Stopwatch Voltmeter
The maximum current from the power supply is limited to 12/5 =
2.4A.
We will assume that the power supply is fixed at 12 V, has no internal
resistance and that the lamps all operate "normally" at 12V. Since they are in parallel they all
receive the full 12V across them and the current though each will remain constant as the
number of lamps are increased. However this means that the current from the power supply
will keep on increasing and this will eventually lead to damage to the power supply. The ones
I have used are limited to 8A and if you used 24W,12V lamps after putting four in parallel the
power supply safety cut out will switch off the circuit. If the power supply is the one
suggested above I must assume that it is variable and that your lamps are low voltage (say
2.5V) and that they draw only a small current each (<0.5A).
To measure the power
delivered to any individual lamp you need to put a voltmeter across the power supply output
and put an ammeter in series with the lamp in the branch of the circuit where this lamp is
connected.
The power is then calculated from Power = voltage x current.
The
joule meter could be used to give the energy delivered directly in a time measured by your
stopwatch.
I am not too sure where you could use the micrometer, oscilloscope or
the bucket of water.
If the power supply has a significant internal resistance then the
output voltage will fall as more and more current is drawn from it. More energy will be "lost"
within the supply.
Factors to watch for during the experiment:
(a) maximum
current being drawn from the supply
(b) internal resistance of the supply
(c) contact
resistance at each junction as you insert more lamps
(d) are all the lamps
identical
(e) accuracy of your meters (ammeter, voltmeter and joule meter) (A longer time
interval will improve the accuracy of the experiment if the joule meter is being used).
(f)
tabulate readings of V,I and W as each extra lamp is inserted.