Magnetic Effects of Electric CurrentMagnetic Field and Field Lines
Why does a magnet affect a compass at all? Because a compass needle is itself a small bar magnet, balanced on a pivot so that it can turn freely. Left alone, one end points roughly north and the other roughly south. The end that points north is called the north-seeking end, or simply the north pole; the other end is the south-seeking end, or south pole. In earlier classes you also found out how two magnets treat each other: like poles repel and unlike poles attract.
Sort each pair: do the two poles attract or repel each other?
So when a compass comes near a bar magnet, the magnet's poles pull on one end of the needle and push on the other, and the needle turns. That is the answer to the first question about this section: a compass needle is deflected near a bar magnet because the needle is a small magnet itself.
Activity 12.2: Iron Filings Draw a Picture
A magnet pushes and pulls without touching anything, so something must fill the space around it. Iron filings let you see that something. In the real activity you fix a sheet of white paper on a drawing board, place a bar magnet in the middle, sprinkle iron filings evenly all round it (a salt-sprinkler is handy), and tap the board gently.
Sprinkle the filings, then tap the board.
The filings arrange themselves along curves that run from one end of the magnet to the other. Each filing is a tiny piece of iron, and the magnet exerts a force on it even from a distance; that force turns the filings until they line up. The region around a magnet in which its force can be detected is called its magnetic field. The lines along which the filings line up represent the magnetic field lines. Notice also that far from the magnet the filings hardly line up at all: the field gets
Activity 12.3: Draw a Field Line with a Compass
Iron filings show the shape of the field, but not its direction. A compass shows both. Here is the real activity. Place a bar magnet on a sheet of paper and mark its outline. Put a small compass near its north pole: the south end of the needle points towards the north pole of the magnet, and the north end points away from it. Mark the position of the needle's north end with a dot. Now move the compass so that its south end sits exactly on that dot, and mark the new position of the north end. Keep going, step by step, until you reach the south pole of the magnet. Join the dots with a smooth curve: that curve is a field line.
Your turn. Drag the compass to a starting point near the magnet, then tap the red button to mark the north end and step the compass on. Complete all three missions.
Look at the lines you have drawn. Each one leaves the
The Rules of Field Lines
A magnetic field has both a direction and a magnitude. The direction of the field at any point is taken to be the direction in which the north pole of a compass needle points when it is placed there. That is why, by convention, field lines are drawn with arrows that come out of the north pole and go into the south pole.
Inside the magnet the field lines carry on, from its
The relative strength of the field is shown by how close together the field lines are. Where the lines are crowded, the field is stronger: the force on the pole of another magnet placed there is
Finally, no two field lines ever cross. Suppose two lines did cross at a point. A compass needle placed exactly there would have to point along both lines, in
Tip
To draw the field of a bar magnet in the exam: draw the magnet with N and S marked, then several smooth lines from N to S on both sides, closer together near the poles and never crossing, with an arrow on every line pointing from N to S outside the magnet. Show a line or two through the magnet as well, running from S to N.
Speed Round: True or False?
Read each statement about magnets and field lines and call it. After each answer you will see why.
Get five right to move on.