A MAGNETIC FIELD is a region around a magnet (or current-carrying conductor) where a magnetic material experiences a force.
MAGNETIC FIELD LINES:
Shown as lines with arrows pointing from NORTH to SOUTH outside the magnet.
Inside the magnet, arrows point from south to north (completing the loop).
Closer field lines → STRONGER field.
Field lines NEVER CROSS each other.
The STRENGTH of a magnetic field is shown by the SPACING of field lines:
Close together → strong field (near the poles).
Far apart → weak field (far from the magnet).
A COMPASS placed in a magnetic field aligns with the field — the north pole of the compass points in the direction of the field line at that point.
Field Patterns for Different Magnets
BAR MAGNET:
Field lines emerge from the NORTH pole, curve around, and enter the SOUTH pole.
Strongest field at the POLES (field lines closest together).
Field extends in 3D around the magnet.
ATTRACTING MAGNETS (opposite poles facing):
Field lines go from the N pole of one magnet to the S pole of the other.
In the space between — field lines go directly across → strong, attractive force.
REPELLING MAGNETS (like poles facing):
Field lines curve AWAY from each other.
A NEUTRAL POINT exists between them — where fields cancel (no net field).
HORSESHOE MAGNET:
Field lines go directly between the two poles (N and S facing each other).
Strong, concentrated uniform field in the gap.
Useful in motors and generators.
Plotting Magnetic Fields
PLOTTING WITH A COMPASS:
1. Place bar magnet on paper.
2. Place a compass at a point near the magnet.
3. Mark the position of the NORTH END of the compass needle with a dot.
4. Move the compass so its SOUTH end is where the previous dot was.
5. Mark the new position of the north end.
6. Repeat to trace the field line.
7. Draw a smooth curve through the dots with an arrow pointing from the original position towards the current one (north to south direction outside the magnet).
PLOTTING WITH IRON FILINGS:
Scatter iron filings around the magnet on a sheet of paper.
Filing align with field lines → reveals field pattern.
Faster than compass method but doesn't show direction.
Earth's magnetic field:
Earth has a magnetic field that acts like a giant bar magnet.
Geographic north pole corresponds to a magnetic SOUTH pole (compass north points there).
Compass needles align with Earth's field — used for navigation.
⚠️ Common Mistake
Field lines go from NORTH to SOUTH outside the magnet — this shows the direction a north pole would move. Field lines NEVER cross. Closer field lines = STRONGER field — not weaker. The Earth's geographic north pole is actually a magnetic SOUTH pole (that's why compass north is attracted to it).
📌 Key Note
Magnetic field: region where magnetic material experiences force. Field lines: N to S outside magnet, never cross, closer = stronger. Plot with compass (shows direction) or iron filings (shows pattern, no direction). Strongest field at poles. Earth's field: like a giant bar magnet.
🎯 Matching Activity — Magnetic Field Lines
Match each field line property to its meaning. — drag the symbols on the right to match the component names on the left.
Arrow direction
Drop here
Closely spaced lines
Drop here
Widely spaced lines
Drop here
Lines that never cross
Drop here
Weak magnetic field — far from the magnet
Strong magnetic field — near the poles of a bar magnet
Field lines represent unique directions — two directions at one point is impossible
Points from north pole to south pole outside the magnet
🧪 Required Practical
🔬 RP21 (Physics) — Plot the magnetic field pattern around a bar magnet using a plotting compass. Mark direction of field lines from N to S.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. A student plots a magnetic field using iron filings around a bar magnet. The filings cluster densely near the poles. What does this show?
2. Which direction do magnetic field lines point outside a bar magnet?
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