Solenoids
Definition
Solenoid
A long coil made of many turns of insulated wire that produces a magnetic field when a current flows through it.
Definition
Uniform magnetic field
A magnetic field that has the same strength and direction at every point, shown by straight, parallel, equally spaced field lines.
- A single straight wire produces only a weak magnetic field, and its circular field lines spread out in all directions.
- Winding the wire into a long coil called a solenoid makes the magnetic effect much stronger.
- Each turn of the coil produces its own small magnetic field, and inside the coil the fields from all the turns point in the same direction.
- These fields add together inside the solenoid, giving a strong field there.
- In the gaps between neighbouring turns, the fields of the two wires point in opposite directions and largely cancel out.
- The field inside a long solenoid is also uniform, so it has the same strength and direction almost everywhere inside and is drawn as straight, parallel, evenly spaced lines.
- Outside the solenoid, the field has the same shape as the field of a bar magnet, with field lines leaving one end and curving round to enter the other.
- One end of the solenoid therefore acts as a north pole and the other end as a south pole.
- The poles are found by curling the fingers of your right hand around the coil in the direction of the conventional current, so that your thumb points to the north pole.
- Reversing the current swaps the two poles over.
Strengthening the magnetic effect
Definition
Iron core
A bar of iron placed inside a solenoid, which becomes an induced magnet and so makes the magnetic field much stronger.
- The magnetic effect of a solenoid can be made stronger in three ways.
- Increasing the current
- A larger current makes the field around every turn stronger, so the combined field inside the coil is stronger too.
- Adding more turns
- More turns packed into the same length of coil means more individual fields adding together in the same direction.
- Adding an iron core
- The solenoid's field magnetises the iron, turning it into an induced magnet whose own field points the same way as the solenoid's field.
- The two fields add together, so a coil with an iron core is many times stronger than the same coil with only air inside.
- The current cannot be raised without limit, because a large current heats the wire and can melt its insulation.
Electromagnets
Definition
Electromagnet
A solenoid wrapped around an iron core that behaves as a magnet only while a current flows through the coil.
- A solenoid wound around an iron core is called an electromagnet.
- Iron is chosen for the core because it is easily magnetised and loses its magnetism almost as soon as the field is removed.
- Switching the current on makes the electromagnet a strong magnet, and switching it off makes it lose nearly all of its magnetism.
- The magnetism of an electromagnet can therefore be switched on and off, and its strength can be adjusted by changing the current, which a permanent magnet cannot do.
- A steel core would not be suitable, because steel keeps much of its magnetism after the current is switched off.
- For example, a crane in a scrapyard uses a large electromagnet to lift old cars and then drops them by switching off the current.
Practical
Investigating the strength of an electromagnet
- Aim: to find how the number of turns and the current affect the strength of an electromagnet, judged by how many paper clips it can lift.
- Apparatus: a large iron nail, about 2 m2\ \text{m}2 m of thin insulated copper wire, a low-voltage d.c. power supply, a variable resistor, an ammeter, a switch, connecting leads, a clamp stand and a dish of identical small steel paper clips.
- Method, number of turns:
- Wind 101010 tight turns of the insulated wire around the nail, all in the same direction, leaving long free ends.
- Connect the coil in series with the switch, variable resistor, ammeter and power supply, and clamp the nail so that its point hangs downwards.
- Close the switch, set the current to 1.0 A1.0\ \text{A}1.0 A, lower the point of the nail into the paper clips and lift it out slowly.
- Count the clips held by the nail, then open the switch and watch them fall.
- Rewind the coil with 202020, 303030, 404040 and 505050 turns, keeping the current at 1.0 A1.0\ \text{A}1.0 A, and repeat the count each time.
- Method, current:
- Keep the coil at 303030 turns and repeat the test at currents of 0.5 A0.5\ \text{A}0.5 A, 1.0 A1.0\ \text{A}1.0 A, 1.5 A1.5\ \text{A}1.5 A and 2.0 A2.0\ \text{A}2.0 A.
- Repeat every reading three times, returning all the clips to the dish between tests, and calculate the mean number lifted.
- Variables: the number of turns or the current is the independent variable, the number of paper clips lifted is the dependent variable, and the nail, the wire and the size of the clips are kept the same.
- Results: the nail lifts more clips as the number of turns increases and as the current increases, and almost all the clips drop when the switch is opened.
- Conclusion: more turns and a larger current both make a stronger electromagnet, and the iron core loses its magnetism when the current stops.
- Watch out: turns wound in opposite directions cancel each other's fields, and paper clips only give a rough measure of strength because each clip counts as a whole number.
- Safety: the thin wire becomes hot at higher currents, so switch off between readings and do not hold the coil while the current flows.
Example
Ranking three arrangements
- Three arrangements carry the same current: a straight wire, a 505050-turn solenoid with air inside, and the same solenoid with an iron core.
- The straight wire has only one weak field around it, so it produces the weakest magnetic effect.
- Each of the 505050 turns of the solenoid produces a field, and inside the coil these add together, so its effect is much stronger than the single wire's.
- The iron core becomes an induced magnet and adds its own field to the solenoid's field, so the solenoid with the core is the strongest.
- The order from strongest to weakest is therefore solenoid with iron core, solenoid with air, straight wire.
Example
Finding the poles of a solenoid
- A solenoid is viewed from its left-hand end, and the conventional current flows anticlockwise around the turns as seen from that end.
- Curling the fingers of the right hand anticlockwise as seen from the left end makes the thumb point out of the coil towards the viewer, which is to the left.
- The thumb points to the north pole, so the left-hand end is the north pole and the right-hand end is the south pole.
- The field lines outside the coil therefore leave the left-hand end and curve round to enter the right-hand end.
Exam technique
Explaining how a solenoid strengthens the field
- Build the answer as a chain, saying that each turn produces a magnetic field, that inside the coil these fields point the same way, and that they add together to make a stronger field.
- If the question asks how to make the effect stronger still, give the three ways by name, which are more current, more turns and an iron core.
- When you mention the iron core, explain that it is magnetised by the solenoid's field and adds its own field, rather than only writing that iron makes it stronger.
- Describe the field outside a solenoid by comparing it with the field of a bar magnet, and the field inside as strong and uniform.
Common Mistake
- Do not say that a solenoid is magnetic when no current flows, because its field exists only while there is a current in the coil.
- Do not say that the iron core produces the current or creates the field on its own, because it only strengthens the field after the coil has magnetised it.
- Do not suggest a steel core for an electromagnet that has to be switched off, because steel stays magnetised.
Self review
- What is a solenoid?
- Explain why the field inside a solenoid is stronger than the field around a single straight wire.
- Describe the shape of the magnetic field outside a solenoid.
- State three ways to make the magnetic field of a solenoid stronger.
- Why is iron, rather than steel, used for the core of an electromagnet?
