Physics
Electricity and Heat
Sensors:
Loggers:
Magnetic field, Current
Any EASYSENSE
Logging time: SnapShot
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, you will make a device that becomes magnetic when a current is passed through it.
Such devices are called electromagnets and are very important in electronics and industrial processes.
The following factors affect the strength of the electromagnet:1. The current flowing
2. The number of turns in the coil
3. The material of the core.
In this experiment you are going to investigate the effect of changing the number of turns in the coil.
Current limiting
resistor
Electricity and Heat
47 - 1 (V2)
What you need
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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.
What you need to do
Making the electromagnet
To make the electromagnet you need to wind the wire around an iron core. Take care to;
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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 one the electromagnet has been made.
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Make sure each turn of the wire is as close to the next as is possible.
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Keep the winding / turns neat, you don’t really want wires overlapping randomly.
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Keep the wire tight.
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Use tape to hold the turns in place.
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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.
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Leave about 20 cm of wire at the end to make the other connection to the circuit.
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Secure it all with tape to stop it unwinding and becoming loose.
Setting up the magnet
1. Set up the apparatus as shown in the diagram. Connect the Magnetic field sensor to input 1 and
Current sensor to input 2 of the logger.
2. Connect the resistor between the power supply and one of the leads coming off the electromagnet.
3. Do not connect the power until you are ready to test the strength of the magnet, electromagnets are
very power hungry and they will flatten batteries very quickly.
4. Secure the electromagnet to the bench top with piece of tape to stop it moving around.
5. Place the Magnetic field sensor as shown in the diagram below. The sensor must touch the end of
the core. A slight movement of the sensor relative to the core will give changes to the readings that
are not to do with the changing variable of the experiment.
6. 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 \ 47
Electromagnet strength & current.
7. Click on Start.
8. Set the power supply to provide a current of about 0.1 A or the use the lowest setting your teacher
tells you to use.
9. Turn the power on, click in the graph area to record the magnetic field strength. Turn off the power
immediately.
10. Set the power supply to the next current, i.e. increase by 0.1 or 0.2 A and repeat.
11. Repeat until you have results for currents up to 0.8 A or what you have been told to use.
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47 - 2 (V2)
Results
The results you have collected will be a bar chart showing the Current and Magnetic field strength against
Reading number. To find the relationship between current and magnetic field we need to make a Magnetic
field strength vs. Current graph.
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. 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.
3.
Right click on the graph area and select Auto Scale graph 0 to Max.
Questions
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.
1. What happens to the strength of the magnet as the current increases?
2. What is the relationship between the current and the strength of the electromagnet?
3. Did you notice if the magnetic field disappeared totally when the power was disconnected?
4. The magnet you made is an electromagnet, what do you call the bar type magnet that does not need
electricity to make it magnetic?
5. Where do you find electromagnets? Make a list of 10 items found in the house that contain
electromagnets.
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 increase the number of turns? Or have the same number of turns on a smaller
length of the iron core?
4. Does a ferrite core work as well as an iron core?
Electricity and Heat
47 - 3 (V2)