Download 51 The force acting on a current carrying conductor in

Physics
Electricity and Heat
Sensors:
Loggers:
Current
Any EASYSENSE
Logging time: SnapShot with Asks for
Value function
51 The force acting on a current carrying conductor
in a magnetic field
Power
supply
Read
Current
sensor
Resistive
load lamp
Magnets
Fig 51.1
Non-Magnetic
connection
56392-654
Non-Magnetic
connection
Brass
Conductor
Balance
Wire close to one set
of poles
N poles
H-shaped
iron former
Fig 51.2
S poles
Opposite poles facing one another
What you need
1. An EASYSENSE logger.
2. A Smart Q Current sensor ±10 A
3. Low voltage power supply capable of providing a current in the range of 0 - 5 amps, and voltage 0 –
12+ volts.
4. Lamp to act as a current limiter. A 36 W 12 V lamp was used. It needs to be capable of passing a
current up to about 3 A.
5. Accurate digital top pan balance (to 0.1 g), with a tare facility. Needs to weigh up to about 500 g.
6. Non ferrous conductor – brass or aluminium rod approx 20 - 30 cm long, or a thick copper wire
mounted along the edge of a wood rule.
7. 2 retort stands with bosses and clamps.
8. 6 magnadur magnets, with pole faces measuring 50 x 19 mm or the equivalent.
9. A rectangular U section mild steel former to hold the 6 magnets, or three such formers to hold each
pair of magnets.
10. Crocodile clips and connecting wires.
Electricity and Heat
51 - 1 (V2)
What you need to do
1. Assemble the apparatus as shown in the diagram; make sure the magnets are all the same pole on
each side of the former. You need a N and S side to the former.
2. Mount the non-magnetic conducting wire close to the pole faces of one of the sets of magnets.
3. Connect the circuit to a low voltage power supply, but do not turn on.
4. Connect the Current sensor to input 1 of the logger.
5. From the EasySense software’s Home screen select Open Setup (or File, Open Setup). Open the
file Data Harvest Investigations (Edition 2) \ Setup files \ Physics_Electricity and Heat_L3 V2 \ 51
Conductor force.
6. Use the Tare function on the balance to set the mass scale reading to Zero.
7. With the power supply adjusted to zero volts turn on the low voltage unit.
8. Click on the Start icon.
9. Click on the graph area to log the first reading. This is the zero reading. A dialogue box will pop up,
type in the mass reading from the balance.
10. Adjust the voltage to give a current of 0.5 amps, and click on the graph area. Enter the mass reading
into the dialogue box.
11. Repeat increasing the voltage to give current increases of approximately 0.5 amp intervals until the
current reaches 3 amps. This will give you 7 sets of readings.
12. Use Save As to name and save the file.
13. Turn off the power.
Results and analysis
You will have obtained a bar chart showing Current and Mass data. The analysis needs to proceed as
follows:•
•
•
Generate a Force data set from the Mass data set, using a Post-log Function.
Draw the Force vs. Current graph as a x, y plot.
Copy data to Excel to draw the trendline for the Force vs. Current graph.
1. To generate a Force data set from the Mass data
•
•
•
•
•
•
From the Tools menu select the Post-log Function.
Select Preset function: General, Multiply by a constant.
Select the channel that you wish to multiply with the constant - the Mass channel.
Make the name Force, and the units as N.
Enter the number to multiply by as 0.00981
Finish.
2. Draw the Force vs. Current graph as a x, y plot
• Select Options then the X-Axis tab.
• Select the button next to Channel, OK
• When the graph has redrawn, click in the white space at the bottom or left of the graph area to
change the channel and axis name. Make the X-axis (horizontal) Current and the Y–axis
(vertical) Force.
• Make the bar chart into a line graph (Options, Graph type. Select the button at the side of Line
Graph, OK).
3. Draw a best fit line
Fleming’s left hand rule will allow us to predict the direction of the force on a wire in a magnetic field. It
is this force which drives electric motors.
The size of the Force F depends upon the strength of the magnetic field B, the current in the wire I,
and the length of the wire L, in the field.
Electricity and Heat
51 - 2 (V2)
•
•
•
•
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Select Best Fit from the Tools menu.
Select Automatic.
Select the Y–Axis as the Force channel.
Select the X-Axis as the Current
Click OK
4. Zoom in on the graph area to resize the graph.
From your experiment you will have concluded that, for a current carrying conductor in a magnetic field:
FαI
If you had completed the extension experiments you would also have shown that:
FαB
Collected together these can be expressed as:
F ∝ BIL
This equation is used to define the tesla. Using SI units for Force (newtons, N), length (metres, m) and
current (amps, A). Then:
B=
F
(teslas, T)
IL
Questions
1. From your graph of Force vs. Current, what can you conclude about the relationship between Force
and Current in a conductor in a magnetic field?
2. What happens to the force when the current flow is reversed? Use Fleming’s left hand rule to predict
your answer before investigating.
3. What is the force when the former is turned 90 degrees to have one set of magnets vertically above
the other?
Extension
1. Investigate the relationship between the Force on the conductor and the length of the conductor in
the field.
2. Investigate the relationship between the magnetic field strength B and the Force F on the current in
the conductor.
Question
1. Calculate the force acting on 100 m power-cable carrying a current of 10 A in magnetic field of 50 μT.
Electricity and Heat
51 - 3 (V2)