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Atoms and static electricity | Magnets
Atoms and static electricity
All substances are made of atoms.
Atoms are neutral because they have the same number of protons and electrons.
A proton has an electric charge of +1; an electron’s charge is –1.
Neutral objects have an equal number of protons (+) and electrons (-).
The electron (-) is the only particle in an atom that moves.
Neutral objects become charged by gaining or losing electrons.
Gain electrons: become negatively charged i.e. electrons (-) > protons (+)
Lose electrons: become positively charged i.e. electrons (-) < protons (+)
Law of attraction and repulsion of electric charge.
Like charges repel, unlike charges attract. |
Protons repel one another because they are positively charged.
Electrons repel one another because they are negatively charged.
Protons and electrons attract each other because they have unlike charges.
Electrification by friction
Some materials become electrically charged when rubbed together. This charging involves the transfer of electrons.
The material losing electrons becomes positively charged and the material gaining the electrons becomes negatively charged.
The rubbed materials are considered to be charged with static electricity because their charge remains in position, the charge does not move.
A perspex rod becomes positively charged when rubbed with fur. The fur becomes negatively charged.
Electrons (-) transferred from the perspex to the fur.
A polythene rod becomes negatively charged when rubbed with cotton. The cotton becomes positively charged.
Electrons (-) transferred from the cotton to the polythene.
The perspex, fur, polythene and cotton remain charged because they are insulators and do not allow electrons (-) to flow through them.
Electrical conductors allow electrons (-) to flow through them e.g. metals and graphite are good conductors.
A charged body attracts neutral materials e.g. a plastic pen rubbed in your school jumper will attract small pieces of paper.
Static electricity is used in fingerprinting, spray painting, photocopying and in the removal of dust from chimneys.
Magnets
A magnet is an object that has the ability to attract iron.
One end of a freely hanging magnet always points north.
If a material is easily magnetised it is described as magnetic. Iron, nickel and cobalt are good magnetic elements. Steel is a magnetic alloy.
A permanent magnet retains its magnetism when the magnetising agent has been removed. Steel and alnico are alloys used to make permanent magnets.
A temporary magnet quickly loses its magnetism when the magnetising agent has been removed. Soft iron and mumetal are used to make temporary magnets.
Every magnet has two centres of attraction close to the opposite ends of the magnet. The two centres of attraction are called the north magnetic pole and the south magnetic pole.
Law of attraction and repulsion of magnetic poles.
Similar poles repel one another. Like charges repel, unlike charges attract. |
A magnet's area of influence is its 'magnetic field'.
Magnetic fields are mapped using magnetic field lines. The closer the field lines, the stronger the magnetic force.
The magnetic field lines can be mapped with iron filings or plotting compasses.
A magnetic compass is a suspended magnet which lines up along a field line. The compass indicates the direction of the magnetic field line.
The earth is a magnet and has a magnetic field pattern similar to a bar magnet. A magnetic compass can be used as a navigation aid.
Earth's magnetic poles don't coincide with geographic poles. The north magnetic pole of the earth is about 8° west of north in Ireland.
Magnets are used in scrap yards for moving iron and steel, electric bells, electric motors, compasses, fridge doors, cash and credit cards, audio and videotapes.
Magnetism and electric current
When an electric current is flowing in a wire, it produces a magnetic field.
The magnetic field pattern around a straight piece of wire looks like a set of rings with the wire as the centre. The right hand thumb rule gives the direction of field lines – thumb pointing in the current direction, close the fingers to show the field line direction.
The magnetic field of a solenoid is similar to that of a bar magnet.
An electromagnet is a solenoid with a soft iron core at its centre. It becomes a much stronger magnet when current passes through it. When the current is switched off, it loses its magnetism.
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