CIE A Level Physics (9702) 2019-2021

Revision Notes

25.4.1 Magnetic Fields in Wires, Coils & Solenoids

Magnetic Fields in Wires, Coils & Solenoids

  • Magnetic field patterns are not only observed around bar magnets, magnetic fields are formed wherever current is flowing, such as in:
    • Long straight wires
    • Long solenoids
    • Flat circular coils

Field Lines in a Current-Carrying Wire

  • Magnetic field lines in a current carrying wire are circular rings, centered on the wire
  • The field lines are strongest near the wire and become further part away from the wire
  • Reversing the current reverses the direction of the field
  • The field lines are clockwise or anticlockwise around the wire, depending on the direction of the current
  • The direction of the magnetic field is determined by Maxwell’s right hand screw 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 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 ie. from positive to negative, not the direction of electron flow

Field Lines in a Solenoid

  • As seen from a current carrying wire, an electric current produces a magnetic field
  • An electromagnetic makes use of this by using a coil of wire called a solenoid which concentrates the magnetic field
  • One ends becomes a north pole and the other the south pole
  • Therefore, the magnetic field lines around a solenoid are very similar to a bar magnet
    • The field lines emerge from the north pole
    • The field lines return to the south pole
  • Which is the north or south pole depends on the direction of the current
    • This is found by the right hand grip rule
  • This involves gripping the electromagnet so the fingers represent the direction of the current flow of the wire
  • The thumb points in the direction of the field lines inside the coil, or in other words, point towards the electromagnet’s north pole

Field Lines in a Flat Circular Coil

  • A flat circular coil is equal to one of the coils of a solenoid
  • The field lines will emerge through one side of the circle (north pole) and leave the other (south pole)
  • As before, the direction of the north and south pole depend on the direction of the current
    • This can be determined by using the right hand thumb rule
    • It easier to find the direction of the magnetic field on the straight part of the circular coil to determine which direction the field lines are passing through

Worked example

Magnetic_Fields_in_Wires,_Coils___Solenoids_Worked_example_-_Drawing_Flux_Lines_Question, downloadable AS & A Level Physics revision notes

Magnetic_Fields_in_Wires,_Coils___Solenoids_Worked_example_-_Drawing_Flux_Lines_Answer, downloadable AS & A Level Physics revision notes

✓ Concentric circles

✓ Increasing separation between each circle

✓ Arrows drawn in anticlockwise direction

Exam Tip

Remember to draw the arrows showing the direction of the field lines on every single field line you draw. Also, ensure that in a uniform magnetic field, the field lines are equally spaced.

Factors Affecting Magnetic Field Strength

  • The strength of the magnetic field of a solenoid can be increased by:
    • Adding a core made from a ferrous (iron-rich) material eg. an iron rod
    • Adding more turns in the coil
  • When current flows through the solenoid with an iron core, it becomes magnetised, creating an even stronger field
    • The addition of an iron core can strengthen the magnetic field up to a several hundred times more
  • When more turns are added in the coil, this concentrates the magnetic field lines, causing the magnetic field strength to increase

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