Have you ever wondered how a massive crane in a junkyard can effortlessly lift a heavy scrap car, then drop it exactly where it needs to go with the flick of a switch? This relies on a simple coil of wire known as a solenoid. When an electric current passes through this long coil, it behaves like a magnet. However, its true power comes from the way the magnetic fields of its individual loops interact with one another.
The EDEXCEL specification requires you to explain exactly why the field inside a solenoid is so strong while the field outside is so weak. The answer lies in how the magnetic fields from each individual turn (loop of wire) combine through a process called superposition.
Because of the reinforcement effect, the magnetic field inside the solenoid is a uniform magnetic field. This means it has the exact same strength and direction at every single point. If you were to draw this, the field lines inside the core must be perfectly straight, parallel, and equally spaced.
Outside the solenoid, the overall shape of the magnetic field is identical to that of a standard bar magnet. The field lines curve out from the North pole and form large loops that re-enter at the South pole.
You can determine which end of a solenoid acts as the North pole using the Right-Hand Grip Rule:
Alternatively, you can look directly down the end of the solenoid. If the current appears to travel clockwise, that end is a South pole. If it travels anti-clockwise, it is a North pole.
While the baseline field of a solenoid is strong, you can increase its power even further to create an industrial electromagnet. The magnetic field strength () can be amplified by:
Students often write 'increase the number of coils' when asked how to make a solenoid stronger, but examiners will mark this wrong because the whole object is the coil. You must specifically say 'increase the number of turns'.
For multi-mark 'explain' questions about a solenoid's field, always provide a two-part answer: fields point in the same direction and reinforce inside, but point in opposite directions and cancel outside.
When drawing the magnetic field inside a solenoid in an exam, ensure your lines are drawn with a ruler so they are perfectly straight, horizontal, parallel, and equally spaced to secure the marks for a uniform field.
Solenoid
A long coil of insulated wire that generates a magnetic field when an electric current passes through it.
Turn
A single individual loop of wire that makes up a larger solenoid.
Superposition
The process where individual magnetic fields from different sources add together (reinforce) or cancel each other out.
Uniform magnetic field
A magnetic field that has the exact same strength and direction at every point, represented by straight, parallel, equally spaced lines.
Electromagnet
A solenoid containing a soft iron core that becomes an induced magnet when an electric current flows, allowing it to be switched on and off.
Put your knowledge into practice — try past paper questions for Physics
Solenoid
A long coil of insulated wire that generates a magnetic field when an electric current passes through it.
Turn
A single individual loop of wire that makes up a larger solenoid.
Superposition
The process where individual magnetic fields from different sources add together (reinforce) or cancel each other out.
Uniform magnetic field
A magnetic field that has the exact same strength and direction at every point, represented by straight, parallel, equally spaced lines.
Electromagnet
A solenoid containing a soft iron core that becomes an induced magnet when an electric current flows, allowing it to be switched on and off.