Wind a single loop into a long coil of many turns and something powerful happens: each turn's field adds to the next, producing a strong, almost perfectly uniform field down the middle, just like a bar magnet, but one you can switch on and off. The solenoid is the heart of every electromagnet, relay and electric motor.
You take a long wire and wind it into a tight coil of many identical loops, then pass a direct current through it.
Before reading: how do you expect the field inside the coil to compare with the field inside just one loop? Write your prediction and a one-sentence reason.
The structure of a simple coil
A solenoid is simply a long wire wound into many tight, identical loops, the same loop you met last lesson, repeated over and over.
A simple solenoid is a length of insulated wire wound into a series of closely spaced circular turns, all of the same diameter, forming a long cylindrical coil. The insulation stops adjacent turns short-circuiting, so the same current flows the whole way along the coil, looping around each turn in the same sense. The key structural features are: many turns, all wound the same way, packed closely along a length much greater than the coil's diameter. This is the arrangement that makes the internal field strong and even.
A simple solenoid is a long cylindrical coil of many closely spaced, equally sized turns of insulated wire, all wound in the same direction, carrying the same current. Its length is much greater than its diameter.
Pause, copy the highlighted description into your book before moving on.
Did you get this? True or false: a solenoid is a single circular loop of wire.
Adding many loop fields into one straight field
We just saw the structure of a solenoid. That raises a question: why does the field inside become so strong and even? This card answers it → each turn contributes a field through the centre, and side by side these merge into one uniform field along the axis.
Because every turn carries current the same way, each one pushes its field through the centre in the same direction, and overlapping these gives a single straight, uniform field.
Each turn of the solenoid produces a loop field threading through its centre, all pointing the same way along the axis. Where neighbouring turns sit side by side, their fields between the turns nearly cancel but reinforce strongly down the middle. Adding up all the turns gives field lines inside the coil that are straight, parallel, evenly spaced and densely packed, a strong, nearly uniform field along the axis. Outside the coil the lines spread out and loop around to the other end, giving a weak external field shaped exactly like a bar magnet's. That contrast, uniform inside, bar-magnet-like outside, is the answer to the hook.
Inside a solenoid the loop fields add to give a strong, uniform field, straight, parallel, evenly spaced lines along the axis. Outside, the field is weak and shaped like a bar magnet, with a north pole at one end and a south pole at the other.
Add the highlighted field description to your notes before the check below.
Quick check: The magnetic field inside a long solenoid carrying a steady current is best described as:
Pattern B, Construct then justify
In your book, draw a solenoid with a labelled current direction. Then: (1) sketch the internal field lines (straight, parallel, evenly spaced) and the external lines (looping like a bar magnet); (2) use the right-hand rule to mark which end is the north pole; (3) write one sentence explaining why the internal field is uniform and one sentence stating what happens to the poles if the current is reversed.
Match the pairs: Match each part of a solenoid's STRUCTURE to its description.
Dot point: Describe the structure of a simple solenoid
Direction of the field and how to reverse it
We just learned the shape of the field. That raises a question: which end is north, and can we change it? This card answers it → the right-hand rule reads the field direction from the current, and reversing the current swaps the poles.
Because the field comes from the current, the direction of the current decides which end is north, and flipping the current flips the poles.
To find the direction of the field (and so which end is the north pole), use the right-hand rule for a solenoid: curl the fingers of your right hand in the direction the conventional current flows around the turns, and your extended thumb points to the north pole. Inside the coil the field runs from the south end to the north end; outside it loops back from north to south. Because this is an electromagnet, reversing the current reverses the field and swaps the poles, a controllable magnet, unlike a permanent bar magnet. This switchability is exactly what makes solenoids useful in relays, doorbells and motors.
A solenoid is wound so that, viewed from the left-hand end, the current runs anticlockwise around the turns. Identify which end is the north pole, describe the field inside, and state what changes if the current is reversed.
Use the right-hand rule: curl the fingers with the current around the turns and the thumb points to the north pole. Inside, the field runs south-to-north; reversing the current reverses the field and swaps the poles.
Pause, copy the highlighted rule into your book before the check below.
Last check: Decide if the statement is true or false.
Reversing the direction of the current in a solenoid swaps its north and south poles.
Odd one out: three of these describe the FIELD produced inside a long solenoid carrying a direct current. Which one does not?
Dot point: account for the shape and direction of the magnetic field produced by a direct current passing through a solenoid
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 5(4 marks) 1. Describe the structure of a simple solenoid and account for the shape and direction of the magnetic field produced when a direct current passes through it.
1 mark: a long coil of many closely spaced, equally sized turns of insulated wire · 1 mark: the loop fields add to give a strong, uniform field inside (straight, parallel lines along the axis) · 1 mark: outside the field is bar-magnet-like, with a north and south pole at the ends · 1 mark: the right-hand rule (fingers with the current, thumb to north) gives the direction
DescribeBand 4(3 marks) 2. Describe the physical structure of a simple solenoid. Explain why the wire must be insulated and why the turns are wound close together.
1 mark: a long coil of wire wound as a helix, many turns sharing a common axis · 1 mark: insulation prevents current short-circuiting between adjacent turns, so it must follow the full helical path · 1 mark: close winding increases the turns per unit length, which strengthens the field
Dot point: Describe the structure of a simple solenoid
ApplyBand 5(3 marks) 3. Explain how you would use the right-hand rule to determine which end of a current-carrying solenoid behaves as a north pole, and describe how the external field compares with that of a bar magnet.
1 mark: curl the right-hand fingers in the direction of the conventional current around the turns · 1 mark: the extended thumb points towards the north pole end · 1 mark: outside, the field emerges from the north end and loops round to the south end, just like a bar magnet
Dot point: account for the shape and direction of the magnetic field produced by a direct current passing through a solenoid
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): A simple solenoid is a long cylindrical coil made of many closely spaced, equally sized turns of insulated wire, all wound in the same direction and carrying the same current, with a length much greater than its diameter. When a direct current flows, each turn produces a loop field threading through its centre, all pointing the same way along the axis. These individual fields add together, so inside the coil the field lines are straight, parallel, evenly spaced and densely packed, a strong, nearly uniform field along the axis. Outside the coil the field spreads out and loops from one end to the other, giving a pattern just like a bar magnet, with a north pole at one end and a south pole at the other. The direction of the field is found using the right-hand rule: curling the right hand's fingers in the direction of the conventional current around the turns makes the thumb point to the north pole. Reversing the current reverses the field and swaps the poles.
Q2 (3 marks): A simple solenoid is a length of wire wound into a long helix, so that many turns lie one after another along a single common axis, usually with the coil much longer than it is wide. The wire must be insulated because adjacent turns physically touch. If bare wire were used the current would take the short path directly across from one turn to the next rather than following the helix all the way round, so most of the turns would carry no current and the field would collapse. The insulation forces the current to follow the full helical path, so every turn contributes. The turns are wound close together to make the winding as dense as possible. The field depends on the number of turns per unit length, so packing more turns into the same length increases the field without needing any more current.
Q3 (3 marks): To find the poles, wrap the fingers of the right hand around the solenoid so that they curl in the same direction as the conventional current flows around the turns. The thumb, extended along the axis of the coil, then points towards the end that behaves as the north pole. Reversing the current direction reverses which end is north. Outside the solenoid the field pattern closely resembles that of a bar magnet of the same length: the field lines emerge from the north end, spread out through the surrounding space, and curve round to re-enter at the south end, and the lines are continuous because they carry on through the interior of the coil from south back to north. The main difference is that the solenoid's field can be switched off, reversed or strengthened simply by changing the current, whereas a permanent bar magnet cannot.
You can now describe a solenoid's structure and account for its uniform internal field and bar-magnet external field. Next lesson puts a number on the internal field with $B = \mu_0 n I$.
↩ Back to focus-area hubAt the start you predicted how the coil's field compares with one loop. The answer: it is much stronger and far more uniform, because the fields of all the turns add together along the axis. If you expected a single loop's messy field, revisit Card 2, stacking loops is what produces the even, intense internal field.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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