Magnetic fields around wires & solenoids
- Magnetic fields are formed wherever a current flows, such as in:
- straight wires
- solenoids
- circular coils
Magnetic field due to a straight wire
- The magnetic field lines around a straight wire are
- made up of concentric circles
- centred on the wire
- A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
- The right-hand grip rule can be used to work out the direction of the magnetic field
The direction of the field around a current-carrying wire can be determined using the right-hand grip rule
- The field lines are clockwise or anticlockwise around the wire, depending on the direction of the current
- Reversing the current reverses the direction of the field
- The direction of the magnetic field can be determined using the right-hand grip rule
- This is determined by pointing the right-hand thumb in the direction of the current in the wire and curling the fingers onto the palm
- The direction of the curled fingers represents the direction of the magnetic field lines around the wire
- For example, if the current is travelling vertically upwards, the magnetic field lines will be directed anticlockwise, as seen from directly above the wire
- Note: the direction of the current is taken to be the conventional current i.e. from positive to negative, not the direction of electron flow
Magnetic field due to a solenoid
- As seen from a current-carrying wire, an electric current produces a magnetic field
- An electromagnet utilises this by using a coil of wire called a solenoid
- This increases the strength of the magnetic field by adding more turns of wire into a smaller region of space
- One end of the solenoid becomes a north pole and the other becomes the south pole
The magnetic field lines around a solenoid are similar to a bar magnet
- As a result, the field lines around a solenoid are similar to a bar magnet
- The field lines emerge from the north pole
- The field lines return to the south pole
- The poles of the solenoid can be determined using the right-hand grip rule
- The curled fingers represent the direction of the current flow around the coil
- The thumb points in the direction of the field inside the coil, towards the north pole
In a solenoid, the north pole forms at the end where the current flows anti-clockwise, and the south pole at the end where the current flows clockwise
Magnetic field due to a circular coil
- A circular coil is equivalent to one of the coils of a solenoid
- The field lines emerge through one side of the circle (north pole) and enter through the other (south pole)
- As with a solenoid, the direction of the magnetic field lines depends on the direction of the current
- This can also be determined using the right-hand grip rule
Magnetic field lines of many individual circular coils can be combined to make a solenoid
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Worked example
The current in a long, straight vertical wire is in the direction XY, as shown in the diagram.
Sketch the magnetic field lines in the horizontal plane ABCD due to the current-carrying wire. Draw at least four field lines.
Answer:
- Concentric circles
- Increasing separation between each circle
- Arrows drawn in an anticlockwise direction