A region of space around an object that is a magnet in which another magnet will experience a repulsive/attractive force that extends to infinity, and decreases over distance, as per the inverse square law
If an object is truly neutral, it will not be affected
If it is neutral because the positive/negative values are balanced, the charges will move, and it will be affected
At a fundamental level, atoms produce their own magnetic field by spinning
Because they are made of charged particles, as atoms move, charged particles in motion will produce their own magnetic field
Domain Theory: If you have a ferromagnetic (can be turned into a magnet) material (e.g. iron, cobalt, nickel, neodymium), the it was conceived that they consisted of magnet domains which, initially randomised in terms of their direction
All magnets have a north pole and a south
The earth behaves like a giant magnet
Charged particles (many of which are high energy particles) are attracted to the earth
When they hit the Earth’s magnetic field, they undergo circular motion and go towards the poles
This high density charge causes the Auroras
MAGNETIC NORTH/SOUTH IS NOT TRUE NORTH/SOUTH
This is because the ferromagnetic material in the centre of the Earth is undergoing motion
Thus a magnetic field is produced
Thus the Earth behaves as a magnet, with poles 15˚ from the normal
Technically the Earth’s geographic north pole is its [magnetic] south pole, in terms of its polarity, and vice versa
MAGNETIC FIELD LINES GO OUT OF THE MAGNET’S NORTH POLE AND INTO ITS SOUTH POLE
Minimum number of field lines is 3
One to show direction
2 to show that distance is increased between multiple field lines
Handout Notes
The Magnetic Field
Oersted discovered the connection between electricity and magnetism, when he found out that switching an electric current on or off caused deflection of a nearby compass needle
A magnet has 2 poles (north and south), where the magnetic field is strongest. If a magnet is broken, each piece still has a north and a south pole. Like poles repel, unlike poles attract
All magnets are dipolar (monopoles are theorised but are yet to be demonstrated)
Earth’s Magnetic Field
The Earth has its own magnetic field produced by its north and south poles. It is thought that this magnetic field is produced as a result of its core being made of iron, floating in a molten iron outer core. The rotation of the core acts like a giant dynamo and the resultant electric currents produces a magnetic effect
The Earth acts as a huge magnet, with its poles near (but not at) the geographic poles. The north magnetic pole is situated near the south geographic pole
A magnet suspended in the Earth’s magnetic field experiences a force that aligns it with the Earth’s magnetic field
A compass needle arrow (its north pole) will point towards the north of the Earth, showing that the magnetic polarity of the pole located int he geographic north is such that it is a south pole. The compass needle arrow end should be called the “North-seeking pole” because it points towards the north - a bit confusing, but remember that magnetic lines of force go from the geographic south into the north of the Earth
The Earth’s magnetic dip at a particular point is the angle that the field line makes with the Earth’s surface. Magnetic dip is measured with a dip needle. At Perth, the magnetic field ‘points’ upwards at about 66˚
Every magnet is surrounded by a magnetic field, where objects made of ‘ferromagnetic’ metals (iron, cobalt, nickel) will experience a magnetic force which may be attractive (unlike poles). Non-magnetised ferromagnetic metals will experience an attractive force regardless of polarity
The pattern of a magnetic field can be visualised using iron filings (powder). It can also be plotted using plotting compasses (a compass is just a small light magnetised arrow-shaped needle that can swivel around, and it lines up with the direction of a magnetic field)
Magnetic fields are represented by lines that ‘leave’ from a north pole and ‘enter’ a south pole. These poles may be on the same magnet, or on separate magnets
Magnetic field, or flux density, (B) is a vector quantity - it has magnitude and direction. The direction of a field line at a particular point is defined as the direction of the force exerted on a ‘single north pole’ at that point. B is expressed in tesla, T, or webers per square meter, Wb m−2
Magnetic field lines never cross over (otherwise a compass would try to line up in two or more directions at once!!!)
Relative magnetic field strengths are shown by the closeness of the field lines. Lines close together represent a stronger field
Interaction of magnetic fields
Magnetic field lines can interact just like forces do. To find the resultant flux density and direction we must construct a vector diagram. Applying this principle to two magnets placed together, we can map the resultant field pattern which is distorted due to interaction between the fields
Domain theory of magnetism (not explicitly in syllabus)
1. Ferromagnetic materials have their atoms grouped in isolated fragments called domains. In each domain the atoms have their magnetic fields (due to spinning unpaired electrons) lined up so that each domain is a microscopic magnet. These domains normally have their N-S axes pointing in random directions, so they cancel out. However, during magnetisation, their directions line up so they reinforce
Magnetic substances (especially iron, cobalt and nickel) are strongly affected by magnetic fields. A magnet near an unmagnetised iron bar induces magnetism in the iron. The end of the bar nearest to the magnet has an opposite pole induced in it, and so is attracted to the bar