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Physics
Electricity and Heat
Sensors:
Loggers:
Magnetic field, Current
Any EASYSENSE
Logging time: SnapShot
Teacher’s notes
47 How does the current 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.
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For different currents
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For different numbers of turns of the coil
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With and without the soft iron core
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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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 wire, approx. 2.0 m length. Thin wire is better (0.6 mm / 24 swg)
Crocodile clips.
Soft iron core (6 inch nails work well.).
A resistor 10 R 4 W
d.c. power supply 0 to 10 V.
Connecting cables
Retort stand and clamps.
Set up of the software
Use the setup file 47 Electromagnet strength & current
Recording method
SnapShot
Electricity and Heat
T47 - 1 (V2)
Notes
Much of the fun in this activity is making the electromagnet, but time considerations may mean that the
electromagnets need to be preformed.
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 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.
Students may well cope better if they wind the complete coil, and do ‘Strength vs. Current’ first, and then
unwind the coil and do ‘Strength vs. Turns’
Using 2 m of wire will give about 55 turns on a 9.6 mm soft iron core.
To make the electromagnet you need to wind the wire around an iron core. Take care to;
1. Have a length of wire about 20 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.
2. Make sure each turn of the wire is as close to the next as is possible.
3. Keep the winding / turns neat, you don’t really want wires overlapping randomly.
4. Keep the wire tight.
5. Use tape to hold the turns in place.
6. If you reach the end of the iron core and you have wire left, secure the turns you have with sticky
tape and start to wind the extra wire over the top of the turns you have made.
7. Leave about 20 cm of wire at the end to make the other connection to the circuit.
8. Secure it all with tape to stop it unwinding and becoming loose.
To make the coil for ‘Strength vs. Turns’, you need to:1. Wind 10 turns onto the core, starting at one end of the core, and secure with tape.
2. Adjust the supply to give a current of about 0.7A.
3. Measure the strength of field at the end of the electromagnet.
4. Wind on another 10 turns and repeat the above procedures.
5. 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. The sensor needs to be ion contact with the end of the core, any movement of the sensor
will give large changes in the field strength being measured. Making the sensor touch the core gives a
reference point for the sensor positioning.
Note: There should be resistor placed in the circuit, this is not shown in the diagram
The power supply may create problems. Electromagnets draw very large currents. When testing this activity
the ‘homemade’ electromagnet used drew 7 A from 4 D cells (6 V). It would have flattened them very quickly.
Most mains power supplies will have a cut out that will trip with this level of current flow. A resistor in the
circuit to limit the current flow is essential. 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 1.5 V ‘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 resistive load does need to be a fixed value resistor, you can use a low voltage lamp but the resistance
changes with the filament temperature (brightness), this gives a non linear relationship between the voltage
and field strength. A 10 R resistor proved to be ideal when testing; the value used may be different with the
length and type of wire used to make the windings.
Electricity and Heat
T47 - 2 (V2)
The winding wire needs to be thin. When buying wire most component suppliers quote a resistance value
(usually ohms per kilometre) try to purchase a wire with a high resistance. The wire needs to be insulated,
enamelled or lacquered wire will allow very tight windings, but it does kink if you are not used to using it.
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.
Results and analysis
Magnetic field strength vs. Current
The results collected will be a bar chart showing the current and magnetic field strength against the reading
number. To find the relationship between current and magnetic field we need to make a Magnetic field
strength vs. Current graph.
Data as collected.
To make the bar chart into a line graph
Select Options, Graph type. Select the button at the side of Line Graph, OK
Data converted to line graph and Auto scaled.
To produce the Magnetic field strength vs. Current graph
• 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)
Axial Field.
• Right click on the graph area and select Auto Scale graph 0 to Max.
Electricity and Heat
T47 - 3 (V2)
Data plotted as Magnetic field strength vs. Current. A line of Best Fit has been drawn. It quite clearly shows
that the magnetic field strength is directly proportional to the current in the coil.
Extension
1. Does the coil work as an electromagnet if there is no core in it?
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
T47 - 4 (V2)