Magnetic Fields from Currents
An electric current flowing through a conductor generates a magnetic field in the surrounding space. Understanding the shape, direction, and strength of these fields for different conductor arrangements, like straight wires and coils (solenoids), is fundamental to how electromagnets and motors work.
Part of the ESAT Physics syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Any moving electric charge, including the current in a wire, produces a magnetic field.
- For a long, straight wire, the magnetic field lines are concentric circles centred on the wire. The field gets weaker as you move further away.
- The 'Right-Hand Grip Rule' determines the field direction: point your thumb in the direction of the conventional current (+ to -), and your fingers will curl in the direction of the magnetic field.
- A solenoid (a long coil of wire) creates a strong, uniform magnetic field inside it, similar in pattern to the field of a bar magnet, with a distinct north and south pole.
- The strength of a solenoid's magnetic field is increased by: increasing the current, increasing the number of turns per unit length, or inserting a soft iron core.
- Electromagnets use soft magnetic materials (like iron) so they can be easily magnetised and demagnetised. Permanent magnets are made from hard magnetic materials (like steel) which retain their magnetism.
Diagram
› Why does this happen?
Why does a coil (solenoid) create a strong, uniform field?
A solenoid is like many loops of wire joined together. Each loop produces its own magnetic field. Inside the solenoid, these individual fields all point in the same direction and add together, creating a strong, uniform magnetic field down the middle. Outside the coil, the fields spread out and largely cancel each other, which is why the external field is much weaker.
Why does an iron core make an electromagnet much stronger?
Iron is a magnetic material made up of tiny regions called 'magnetic domains', which act like mini-magnets. Normally, these domains point in random directions, so their magnetic effects cancel out. When the iron core is placed in the solenoid's magnetic field, the domains are forced to line up. Their individual magnetic fields now add together, making the iron core a strong magnet itself. This field adds to the solenoid's field, creating a much stronger electromagnet overall.
Definitions
- Solenoid
- A long coil of wire which, when carrying an electric current, generates a strong and nearly uniform magnetic field within its core.
- Electromagnet
- A solenoid with a soft magnetic material (typically soft iron) placed inside as a core. The core becomes strongly magnetised by the solenoid's field, creating a much more powerful, controllable magnet.
- Right-Hand Grip Rule
- A mnemonic used to determine the direction of the magnetic field produced by a current. For a straight wire, the thumb points along the current and the fingers curl in the field's direction. For a solenoid, the fingers curl with the current around the coil and the thumb points to the North pole.
Worked example
An electromagnet is made by wrapping insulated wire around a soft iron core and passing a direct current (DC) through it. What single change would reverse the electromagnet's polarity (swap its North and South poles) while keeping its magnetic field strength the same?
- 1
First decide what sets each property.
The polarity (which end is North) depends on the direction of the current, by the right-hand grip rule.
The strength depends on the size of the current, the number of turns, and the core material.
- 2
Reverse the current direction - for example, by swapping the connections to the DC supply.
This reverses the magnetic field, so the North and South poles swap over.
- 3
The size of the current, the number of turns and the iron core are all unchanged, so the field strength stays the same.
Reversing the current is therefore the change that does both.
Answer: Reverse the direction of the current in the coil.
Common mistakes
- ×Forgetting that the Right-Hand Grip Rule applies to conventional current (from positive to negative), not the flow of electrons (negative to positive).
- ×Confusing the application of the Right-Hand Grip Rule for a straight wire (thumb = current) versus a solenoid (fingers = current).
- ×Mixing up materials: soft iron is used for temporary electromagnets because it magnetises and demagnetises easily. Hard steel is used for permanent magnets because it retains magnetism.
No-calculator tips
- ✓Always physically use your right hand to determine field direction in exam questions. It is a reliable method that avoids confusion.
- ✓To quickly find a solenoid's poles, imagine looking at one end. If the current appears to be flowing anti-clockwise, that end is a North pole. If it's clockwise, it's a South pole.
- ✓Visualise the magnetic field of a solenoid as being identical to a bar magnet's field. Field lines emerge from North and enter South.