Magnetism
If a substance is attracted to a magnet it is said to be magnetic.
A magnetic substance must contain iron, cobalt or nickel.

Magnets have two poles.
The north seeking pole (N) and the south seeking pole (S).

Iron filings sticking to the poles of a bar magnet

The pole of the magnet is where the magnetism is strongest.

  • Rules about the behavior of poles
  • Like poles repel
  • unlike poles attract

(ie a North pole will be attracted to a South Pole or to another piece of iron, cobalt or nickel)
(ie a north seeking pole is repelled by another north seeking pole)



Investigating the strength of a magnet

Charlotte’s younger brother, George, was playing with magnets.
He thought that a larger magnet would be stronger than a small one

Charlotte set up an experiment to find out if this was, in fact, the case.

Aim: To investigate the strength of a bar magnet

Prediction: Larger magnets are stronger than small magnets

Method:
Place a magnet 10cm from a steel ball bearing.
Slide the magnet towards the ball and record the distance the magnet is from the ball when the ball starts to move.
This measurement is made 3 times for each magnet and the average distance calculated.

The magnet is replaced for a another and the experiment repeated.

The results are recorded in a table and a bar graph drawn.

  • Independent variable (The variable we are changing): the size of the magnet
  • Dependent variable (the variable we are recording): The distance of the magnet from the ball when the ball is first attracted to the magnet
  • Other variables (which are to be kept the same): The size, mass and material  of the steel ball. The shape of the magnet.

 

Results:

Size of magnet Distance of magnet from steel ball
reading 1
cm
reading 2
cm
reading 3
cm
Average
cm
Magnet A  3cm 4.2 4.1 4.3 4.2
Magnet B  5cm 5.8 5.6 5.9 5.77
Magnet C  6cm 3.4 3.4 3.5 3.43
Magnet D  2cm 7.3 7.8 7.6 7.57

 

Conclusion:
Magnet D is the strongest (because it attracts the ball over the greatest distance) however it is not the largest (it is in fact the smallest).

There seems to be no correlation between the size of a magnet and its strength so we conclude that the larger magnets are not necessarily the strongest.


Lines of magnetic force

Lines of invisible force called magnetic field lines run from the N pole to S pole.
Notice that the arrows point away from the North seeking pole, towards the South seeking pole

bar-magnet
For another image click here

 

We cannot actually see the magnetic field lines but we can find out where they are by using a plotting compass.

  1. Place the magnet on a piece of white paper
  2. Place a plotting compass anywhere near the magnet
    (a plotting compass is a tiny compass, often with a transparent base)
  3. Using a pencil mark two dots on the paper either end of the needle.
  4. Remove the compass and, using a ruler, draw a short line to join the two dots.
  5. Repeat instructions 2, 3 and 4 a lot of times, every so often placing an arrow on the line to mark the ‘North’ end of the compass (coloured red).
  6. All the short lines can be joined together to give us a complete picture.


The magnetic effect of an electric current

A  length of wire formed into a coil and carrying an electric current will produce a similar magnetic field to a bar magnet.

 

Making an electromagnet

We can make an electromagnet by wrapping a coil of wire around an iron nail (or soft iron bar) and connecting the ends of the wire to a battery.
coilWhen an electric current flows through the wire there will be a magnetic field around the iron bar.

The strength of the electromagnet can be increased by

  • Increasing the number of turns in the coil of wire
  • increasing the current flowing through the coil

 

  • Uses for an electromagnet:
  • For lifting iron scrap (attached to a crane)
  • Used in the construction of small electric motors
  • Used to make a relay (see below)

The Relay is a switch that makes use of an electromagnet

A relay consists of two parts:

  • i.    A coil of wire that acts as an electromagnet
  • ii.   Contacts that are pulled together by the electromagnet
switches4

For more information see CIRCUITS