Magnetic Fields (OCR Gateway GCSE Physics)

• All magnets are surrounded by a magnetic field
• A magnetic field is defined as:

The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel)

Magnetic Field Lines

• Magnetic field lines are used to represent the strength and direction of a magnetic field
• The direction of the magnetic field is shown using arrows
• The strength of the magnetic field is shown by the spacing of the magnetic field lines
  • If the magnetic field lines are close together then the magnetic field will be strong
  • If the magnetic field lines are far apart then the magnetic field will be weak

• There are some rules which must be followed when drawing magnetic field lines.
• The direction of the field lines should always be from:
  • Always go from north to south (indicated by an arrow midway along the line)
  • They must never touch or cross other field lines

Magnetic Field around a Bar Magnet

• The magnetic field is strongest at the poles
  • This is where the magnetic field lines are closest together

• The magnetic field becomes weaker as the distance from the magnet increases
  • This is because the magnetic field lines are getting further apart

The magnetic field around a bar magnet

Uniform Magnetic Field

• A uniform magnetic field will be produced in the gaps between opposite poles
  • Note: Outside that gap the field will not be uniform

A uniform field is created when two opposite poles are held close together. A magnetic field is always directed from North to South

• A uniform magnetic field is one that has the same strength and direction at all points
  • To show that the magnetic field has the same strength at all points there must be equal spacing between all magnetic field lines
  • To show that the magnetic field is acting in the same direction at all points there must be an arrow on each magnetic field line going from the north pole to the south pole

• The magnetic field lines are the same distance apart between the gaps of the poles to indicate that the field strength is the same at every point between the poles

Investigating Field Lines

• The shape and direction of a magnetic field may be investigated using plotting compasses
• A plotting compass is like a small bar magnet, with a north and south pole
• The arrow of the plotting compass represents the north pole

Step 1:

• • Place the magnet on top of a piece of paper
  • Draw a dot at one end of the magnet (near its corner)

Step 2:

• • Place a plotting compass next to the dot, so that one end of the needle of the compass points away from the dot
  • Use a pencil to draw a new dot at the other side of the compass needle

Step 3:

• • Move the compass so that it points away from the new dot, and repeat the process above

Step 4:

• • Keep repeating the previous process until there is a chain of dots going from one end of the magnet to the other
  • Then remove the compass, and link the dots using a smooth curve – this will be the magnetic field line

Step 5:

• • Repeat the whole process several times to create several other magnetic field lines

Compasses around a bar magnet show the direction of the magnetic field from north to south