Download Teacher`s notes 19 How does the strength of an

Physics
Sensors:
Loggers:
Magnetic field, Current
Any EASYSENSE
Logging time: SnapShot
Teacher’s notes
19 How does the strength of an electromagnet
vary with an increasing current?
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 add together the fields of each coil, and the strength of the
magnetic field 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 increased,
because the iron rod becomes magnetised. 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 rod. Pure iron, which is soft, loses
most of its magnetism when the current is switched off.
In this investigation students will make a coil that becomes magnetic when a current is passed through
it. The strength of the magnetic field generated can then be measured for different currents.
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
1.
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11.
An EASYSENSE logger.
A Smart Q Magnetic field sensor, ±10 mT, set to the Radial range.
A Smart Q Current sensor, ± 10A.
A length of insulated wire, approx. 2.0 m in length. Single core 0.6 mm diameter copper wire
works well.
Soft iron or ferrite core e.g. 155 mm x 9.6 mm.
Crocodile clips.
A 36 W 12 V lamp in holder, or resistor 5 -10 R 10 – 25 W,
DC power supply 0 to 7.5 V, or up to 5 x 1.5 V cells connected in series.
Insulating tape
Connecting cables
Retort stand and clamps.
Set up of the software
Use the setup file 19 Electromagnet strength vs. current.
Physics L2
T19 - 1 (V2)
To set the logger and software up manually use,
Recording type
X-Axis
SnapShot
Channel
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
magnetic field 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 the current is increased and
decreased in 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.
The investigation can be successfully completed with 4 or 5 1.5 V cells in series, or a low voltage power
supply that can supply up to about 3 A DC. One with a continuously variable voltage control will work
best.
2.0 m of the wire will give about 55 turns on the iron core using the specifications for both given in
the ‘Apparatus’.
To make the electromagnet you need to wind the wire around a soft iron (or ferrite) core. Take care to;
1. 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.
2. Make sure each turn of the wire is as close to the next as is possible.
3. Keep the winding / turns neat, next to one another, you don’t really want wires overlapping
randomly.
4. If the core is long enough use a single layer coil. It will not get so hot.
5. Keep the wire tight.
6. After each 10 turns, use tape to hold the turns in place. Plastic insulating tape is ideal.
7. After winding on 50 turns, secure it all with tape to stop it unwinding and becoming loose.
The Magnetic field sensor should be set to Radial range before being given to the students.
Correct alignment of the
Magnetic field sensor to the
radial sensing element. Look for
the raised “spot” on the end of
the sensor. You may wish to use a
permanent marker to make it
more visible
The wire used in the tests was single core 0.6 mm diameter copper wire. It is easier to wind coils with
single core wire because it almost keeps its shape after winding. We used a 2 m length, which gives
about 55 turns on a 9.6 mm diameter soft iron core.
Such an electromagnet would pick up 2 to 4 paper clips with a current of 1.5 to 2 amps. The coil was
tested at 2.5 A for 3 minutes without undue heating.
Physics L2
T19 - 2 (V2)
The coil has a low resistance. The wire used here has a resistance of 0.07 Ω/m. A large current will
flow with only a few volts across such a coil. The coil will also heat up dangerously at such currents and
the supply fuse will blow, or the insulation will start to burn. A resistor in the circuit to limit the
current flow is essential.
The limiting resistor does not need to be a fixed value resistor, because we are measuring the current
actually flowing. You can therefore use a low voltage lamp of suitable power rating. A fixed resistor
will also work well, but its wattage rating will need to be large, .e.g. 24 or 36 W 12V lamps. A 10R
resistor will need to have a 10 W rating if currents up to 1 Amp are used and 62.5 W if 2.5A is used.
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!
Results and analysis
The results will be a line graph of Magnetic field strength in milliTeslas (mT) against the Current in
Amps (A).
Data as collected using 5 x 1.5 V cells, and a 50 turn coil on a soft iron core.
Data after selecting Auto scale graph 0 to Max. It clearly shows a linear relationship between the
Magnetic field strength and the Current.
The value for 0 A shows the effect of the residual magnetism in the soft iron core, and is probably
induced in the core by the Earth’s magnetic field.
Extension
1.
How does the electromagnet work if the iron core is missing?
2. Does the electromagnet work as well if a non magnetic metal is used for the core?
3. What happens if you have the same number of turns on a shorter length of the iron core?
Physics L2
T19 - 3 (V2)