Download current has a choice

Physics
Electricity and Heat
Sensors:
Loggers:
Current
Any EASYSENSE with 4 inputs
Logging time: Meters
Teacher’s notes
32 Electric current in circuits: Parallel circuits –
current has a choice
This is one of a series of investigations into current in a circuit.
31 Series circuits – current has no choice of route.
Current is measured before and after a resistive load, the effect of a break in the circuit on current is
demonstrated.
32 Parallel circuits – current has a choice.
Current is measured as it flows out of the battery, along each possible route back to the battery and at the
common return to the battery. Current is shown to not be used up.
33 Current in a simple series circuit.
A simple series circuit has lamps and cells added to it show how the “push” of the electricity is altered.
Read
Current is the flow of electricity in a circuit. What happens when the electricity has more than one route back
the cell? Or meets more than one obstruction on its journey?
This simple investigation allows the students to measure the current in a circuit at multiple points and identify
the patterns in the current values recorded.
Potentially the investigation could use 6 Current sensors, and may be best as a demonstration experiment.
The investigation will be described using 3 or 4 Current sensors. If you have access to a 6 input logger then
the number of sensors can be increased.
In a parallel circuit, the electrical flow will be divided amongst all possible routes back to the battery. We
would predict that the current in all routes will add up to be equal to the current coming from, and returning to
the battery.
You could extend the theory by using bulbs with different resistances or use different resistances, this should
reinforce the idea that the sum of the currents in all potential paths back to the battery will always add up to
the total current available from the battery.
Apparatus
The apparatus list is based around 3 Current sensors and 2 bulbs in parallel.
1.
2.
3.
4.
5.
6.
An EASYSENSE logger with 3 inputs.
3 x Smart Q Current sensors ±1 A.
At least 2 low voltage lamp holders and bulbs.
Cell holder and 2 x 1.5 volt cells or a d.c. power supply set to 3 V.
Connecting leads or a circuit board (e.g. Locktronics, Worcester or Snap Circuits).
2 x Simple on/off switch (optional)
Electricity and Heat
T32 - 1 (V2)
Set up of the software
Using up to 4 sensors
Meters is used to display the data. This allows you to show the information from 4 sensors any one time in a
large numeric format, which is ideal for presentation on screen.
Using more than 4 sensors
When using more than 4 sensors Meters cannot show the data all in one go.
It may be preferable to open an EasyLog or Graph session and select Test Mode from Tools. The data
boxes can be made larger by clicking on the horizontal bar that separates the data boxes from the graphing
area. You will be able to drag the separation line down, which will cause the data boxes to resize. How well
this works will depend upon the screen size and resolution of the computer and / or projector.
Notes
Use the Options, Sensor Settings to change the sensor names to A1, A2, Lamp1, Lamp 2, etc. to match
the apparatus diagrams.
If possible, use lamps that have the same nominal voltage and wattage. Most low voltage lamps have similar
characteristics if they have come from the same source.
The cells used should be in good condition, weak cells can give a falling current value which will create
confusion.
You will need to use ±1A sensors, typical values collected for the current are in the region of 500 – 600 mA
when using a 6.5 V bulbs and a 3.5 V power supply.
A circuit board such as the Locktronics, Worcester or Snap Circuits type does allow the students to match
the circuit diagram to the experiment more easily. It is easy to get lost in maze of wires if loose connecters
and 4 mm plugs are used.
Using switches in the bulb path will allow you to alter the number of available paths for the current. You could
use a Light sensor to measure the bulbs output in case there is doubt that the bulbs have the same intensity.
Results and analysis
Typical results using 2 bulbs and measuring current as it flows from the battery, as it flows back to the
battery and as it flows through each bulb. If the third digit of the display is rounded, you can easily see that
the sum of the 2 bulb currents is equal to the current leaving and returning to the battery.
Electricity and Heat
T32 - 2 (V2)