Electromagnets
Brook Edgar & Hannah Shuter
Teachers
Explainer Video
Electromagnets
An Electromagnet is a magnet that is produced by passing an electric current through a wire. It is temporary and stops working when the current is switched off.
When current flows through a wire, a magnetic field is created. This does not mean you can start sticking magnets to your phone charger when charging your phone, as although there is a magnetic field around this current-carrying wire, it is very weak.
The magnetic field lines around a wire are circular, and, as we can see in the image below, they are closer together closer to the wire, showing that the magnetic field is strongest closer to the wire and gets weaker farther away.
The direction of the magnetic field lines around the current-carrying wire is found using your right hand, where your:
thumb = direction of current
fingers = curl in the direction of the magnetic field
To make the electromagnet stronger, the wire can be wrapped into a coil, known as a solenoid. This increases the amount of current-carrying wire in a given space and thus increases the magnetic field strength.
The electromagnet can all be strengthened by :
increasing the number of coils on the solenoid
adding more cells to increase the current
adding an iron bar/core through the solenoid as iron is an induced magnet so will become magnetic itself, increasing the magnetic field strength
The magnetic field around a solenoid is like that of a bar magnet, as shown below. The magnetic field lines come out of the right-hand side, so this is the North pole of the solenoid, and the other end is the South pole.
Remember: The cell provides energy to the circuit, and the direction of current, , is the direction of conventional current. From positive (+) to negative (-). A switch can be added to the circuit to turn the electromagnet on and off by cutting the flow of current.
Worked Example:
What do the blue arrows on the diagram below represent?
How does the diagram show the variation in field strength from the wire?
Answer:
The arrows show the direction of the magnetic field lines around the wire.
The field lines get further apart away from the wire so the field decreases.
Worked Example
Explain how the following changes would change the number of paper clips picked up:
Increasing the number of coils of wire around the iron bar.
Changing the iron bar with a wooden rod of the same thickness.
Changing the iron rod with an aluminium rod of the same length.
Increasing the resistance of the circuit.
Answer:
Increasing the number of coils on the solenoid increases the magnetic field strength as there is more wire in a given space and therefore more magnetic field lines. Therefore, there will be a stronger attraction, and more clips will be picked up.
The magnetic field would be weaker because wood is not a magnetic material, so fewer clips would be picked up.
The magnetic field would be weaker, as aluminium is not a magnetic magnet, so fewer paper clips would be picked up.
Increased resistance results in a lower current. This would decrease the strength of the magnetic field, so fewer paper clips would be picked up.
Common Errors & Exam Tips
Students often think a magnetic field around a wire has poles – it does not. It forms circles around the wire. Only when coiled into a solenoid does it have a magnetic field pattern like that of a bar magnet.
Many students confuse steel vs iron:
Steel = permanent magnet → not suitable for electromagnets as it will never lose its magnetism.
Iron = temporary magnet → ideal as the magnetic field can be turned on and off.
Students mix up the direction of current vs electron flow. AQA uses conventional current, from the positive terminal of the cell to the negative (+ → –).
Practice Questions
A student investigates the magnetic field produced by a current in a straight wire.
Describe how the magnetic effect of a current can be demonstrated.
The student increases the current in the wire. State the effect on the magnetic field strength.
-> Check out Hannah's video explanation for more help.
Answer:
Place a plotting compass near the wire and switch on the current; the needle deflects, showing a magnetic field. Reversing the current reverses the direction of compass deflection.
Field strength increases.
A solenoid is used as part of an electromagnet in a doorbell system.
Explain two ways to increase the strength of the magnetic field inside the solenoid.
Explain why adding an iron core increases the magnetic field strength of the solenoid.
-> Check out Hannah's video explanation for more help.
Increase the current through the wire by adding more cells or increasing the number of turns per unit length, increasing the number of coils and thus increasing the length of wire in a given area.
Iron is a magnetic material, so it becomes magnetised when placed inside the solenoid. This amplifies the magnetic field around the solenoid