Magnetic Effect of a Current (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Revision Note

Katie M

Author

Katie M

Last updated

Did this video help you?

Magnetic fields around wires & solenoids

Extended tier only

  • 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

Field lines on wire, downloadable IGCSE & GCSE Level Physics revision notes

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

Field lines on a solenoid, , downloadable IGCSE & GCSE Level Physics revision notes

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

Direction of field solenoid, downloadable IGCSE & GCSE Level Physics revision notes

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

Field lines on circular coil, downloadable IGCSE & GCSE Level Physics revision notes

Magnetic field lines of many individual circular coils can be combined to make a solenoid

Did this video help you?

Did this video help you?

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:

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 an anticlockwise direction

Magnetic effects of changing current

Extended tier only

Magnetic field strength around a straight wire

  • The strength of the magnetic field produced around a wire can be increased by:
    • increasing the amount of current flowing through the wire
  • The direction of the magnetic field produced around a wire can be changed by:
    • changing the direction of the current
  • The strength of a magnetic field decreases with distance from the wire
    • The magnetic field is strongest near the wire and becomes weaker further away from the wire
    • This is shown by the magnetic field lines becoming further apart

Field Lines Wire, downloadable IGCSE & GCSE Physics revision notes

The greater the current, the stronger the magnetic field. This is shown by more concentrated field lines

Magnetic field strength around a solenoid

  • The strength of the magnetic field produced around a solenoid can be increased by:
    • increasing the amount of current flowing through the coil
    • increasing the number of turns on the coil
    • inserting an iron core into the coil
  • The direction of the magnetic field produced around a solenoid can be changed by:
    • changing the direction of the current
  • When a soft iron core is inserted into a solenoid, it can be used as an electromagnet
    • The iron core becomes an induced magnet when a current flows through the coils 
    • The magnetic field produced by the solenoid and the iron core will create a much stronger magnet overall

Structure of an electromagnet

Electromagnet

An electromagnet consists of a solenoid wrapped around a soft iron core

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.