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Physics
Electricity and Heat
Sensors:
Loggers:
Magnetic field, Current
Any EASYSENSE
Logging time: SnapShot with Asks for
Value function
Teacher’s notes
48 How do the numbers of turns in the coil affect
the strength of an electromagnet?
Read
A wire with a current passing through it has a magnetic field around it. Unless the current is very big, the
magnetic field will be very weak.
If you take a long wire and coil it up you concentrate the magnetic field and it starts to become noticeable.
When a current is passed through the coil iron and other magnetic materials will be attracted towards it,
magnetic compasses nearby will change direction.
If the coil is wrapped around a ‘soft iron’ rod, the effect of the magnetism becomes concentrated at the ends
of the iron rod. The iron rod then behaves as if it was a bar magnet. When the current flow is stopped the
magnetism is reduced, and may disappear completely. How long the magnetism remains in the iron depends
upon the purity of the iron.
In this investigation students will make a device that becomes magnetic when a current is passed through it.
The strength of the magnetic field generated can then be measured
The strength of the electromagnet will depend upon the following factors:•
The current flowing in the coil
•
The number of turns of the coil
•
With and without the soft iron/ferrite core
In this experiment the student will investigate the effect of changing the number of turns in the coil.
Measuring the field strength will enable the students to make predictions about performance of the device
they made, and if time permits make improvements. It extends the study of electromagnets from simple
observation to prediction and refinement based on data.
Apparatus
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11.
An EASYSENSE logger
A Smart Q Magnetic field sensor ±10 mT set to the Axial range
A Smart Q Current sensor ±1 A
A length of insulated copper wire, approx. 2.0 m length. 0.6 mm ( or 24swg) insulated copper wire
works well.
Soft iron, or ferrite, core about 10 mm diameter, 100 -150 mm long
Crocodile clips
A resistor 10 R 5W / rheostat
d.c. power supply 0 to 10 V.
Connecting cables
On/off switch
Retort stand and clamps.
Set up of the software
Use the setup file 48 Electromagnet strength & turns, it uses a pre-log function to prompt the user for a
value, in this case the Number of turns being used.
Electricity and Heat
T48 - 1 (V2)
If you wish to set up the logger and software manually:Recording method
Pre log function
Name
Unit
SnapShot
General, Ask for
Value
Turns
Number
Notes
Much of the fun in this activity is making the electromagnet, but time considerations may mean that the
electromagnets need to be preformed. It would be possible to have coils wound on cores with 10, 20, 30, 40
and 50 turns. Two sets of these coils would be enough for a class, working in groups of 2 to 3, to share.
Using the Magnetic field sensor allows smaller electromagnets to be made. With 2 m of wire the magnetism
created may not be powerful enough for a magnetic attraction to be felt. The Magnetic field sensor will show
a change in the magnetic field strength as power is supplied to and removed to the coil.
For many of the students a task such as this will be novel, they may need a short “show and tell” session to
give them some idea of how to cope with the simple mechanics of the process.
Using 2 m of wire will give about 55 turns on a 9.6 mm soft iron core.
To make the electromagnet for ‘Strength vs. Turns’, you need to wind the wire around an iron core. Take
care to;
1. Bare about 3 cm at each end of the wire, for connecting into the circuit.
2. Have a length of wire about 10 cm long before you start winding onto the core, this will be the
wire to connect to the circuit once the electromagnet has been made.
3. Make sure each turn of the wire is as close to the next as is possible.
4. Keep the winding / turns neat, you don’t really want wires overlapping randomly.
5. Keep the wire tight.
6. Wind 10 turns onto the core, starting at one end of the core, and secure with tape.
7. Adjust the supply to give a current of about 0.7 A.
8. Measure the strength of field at the end of the electromagnet.
9. Wind on another 10 turns and repeat the above procedures.
10. Continue until you have wound on 50 turns.
The magnetic field sensor should be set to Axial range before being given to the students.
The axial Hall effect sensor is on the tip of the Magnetic field sensor. The sensor should be used, orientated
as shown below.
It is most important that the sensor is positioned in exactly the same position at the end of the soft iron core.
The sensor should be in contact with the core. A small move away from the end of the core will have a large
effect on the strength of the field being measured.
The power supply may create problems. The coil will have a very low resistance. 2 m of 0.6 mm copper wire
has a resistance of about 0.12 ohms. Even a 1.5V ‘D’ cell will give a large current through this resistance. It
is necessary to use a series resistor to cut down the current and use a power supply that is easily
controllable. If a 10 Ω 5 W resistor is used, then currents up to 0.7 A can run without heating up the resistor,
or coil, unduly.
The power supply used was seven 1.5 V D cells in series. This gave a current of about 0.7 A. Any variable
d.c. low voltage unit will do giving voltages up to about 10 V, and currents up to around 0.7 A.
Electricity and Heat
T48 - 2 (V2)
The resistive load does need to be a high wattage fixed value resistor, you can use a low voltage lamp but
the resistance changes with the filament temperature (brightness). A 10 R 5 W resistor proved to be ideal
when testing.
The winding wire needs to be insulated copper wire, thin and single core. The single cored wire holds its
shape when being wound. It is possible to straighten it sufficiently for reuse by pulling it backwards and
forwards round a metal rod or table leg.
Don’t be tempted to use nichrome or resistance wires, these have very high resistances and you will be
making small electric bar heaters not electromagnets! They are also un-insulated and will short out.
The Magnetic field Sensor measures the strength of the magnet in mT (milli tesla). The bigger the number,
the stronger the magnet. Any + and – signs you see, or have been recorded, indicate the polarity of the field.
A -2 mT magnet is as strong as +2 mT magnet.
Results and analysis
The results collected will be a bar chart showing the Turns and Magnetic field strength against the Reading
number. To find the relationship between turns and magnetic field we need to make a Magnetic field strength
vs. Turns graph.
Data as collected, and Auto scaled 0 to Max.
1. Make the bar chart into a line graph
• Select Options, Graph type. Select the button at the side of Line Graph, OK.
2. Produce a Magnetic field strength vs. Turns graph
• Select Options, then the X-Axis tab.
• Select the button next to Channel, OK.
Electricity and Heat
T48 - 3 (V2)
•
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) Turns and the Y–axis (vertical)
Axial Field.
3. Right click on the graph area and select Auto Scale graph 0 to Max
The relationship between turns and magnetic field strength is clearly a linear one. We can now draw a line of
best fit, using Best Fit (Tools menu).
The straight line almost passes through the origin. This is a clear indication that the Field strength is directly
proportional to the number of turns in the coil.
Extension
1. Does the coil work as an electromagnet?
2. Does the electromagnet work as well if a non magnetic metal is used for the core?
3. Does a ferrite core work as well as a soft iron core?
4. What happens if you have the same number of turns on a smaller length of the iron core?
Electricity and Heat
T48 - 4 (V2)