Magnetic Fields Lines (OCR A Level Physics)

• 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 direction of magnetic field lines on a current carrying wire can be determined by the right hand thumb rule

• 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 electromagnet makes use of this by using a coil of wire called a solenoid which concentrates the magnetic field
• One end becomes a north pole and the other the south pole

Magnetic field lines around a solenoid are similar to a bar magnet

• 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 can be found by using the right hand grip rule

Using the right-hand grip rule to determine the direction of the magnetic field around a solenoid

• To determine the direction of the magnetic field around a solenoid:
  • Grip the coil so the fingers represent the direction of the flow of conventional current (i.e. from the positive to the negative terminal)
  • The thumb points in the direction of the magnetic field lines through the coil from north to south

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 poles depends on the direction of the current
  • This can be determined by using the right-hand thumb rule
  • It is 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

Magnetic field lines of a single circular coil are added up together to make to make the field lines of a solenoid

Factors Affecting the 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 to the coil, this concentrates the magnetic field lines, causing the magnetic field strength to increase

✓ Concentric circles

✓ Increasing separation between each circle

✓ Arrows drawn in anticlockwise direction