Electric charge
Charging an insulator
Moving charges.
When you rub two different insulating materials against each other they become electrically charged. This only works for insulated objects- conductors direct the charge flow to earth
When the materials are rubbed against each other:
- Negatively charged particles called electrons move from one material to the other.
- The material that loses electrons becomes positively charged.
- The material that gains electrons becomes negatively charged.
- Both materials gain an equal amount of charge, but the charges are opposite.
Attraction and Repulsion
Electrically charged object may attract or repel each other, and they can attract small uncharged object placed close by.
Detecting charge
If two charged objects are brought close together, they repel each other if they have the same type of charge- positive or negative. They will attract each other if they have the positive charge.
How does a Van de Graff generator work?
- Produces a charge
- The charge travels onto the person touching the generator
- The charges are all the same, so they repel
- The strands of hair repel each other, so they stand on end.
Photocopiers
Electrostatic charges (Electric charges that accumulate on an insulator) are used in a photocopier or a laser printer.
How it works:
- Original document
- The image of the document is projected onto a positively charged copying plate
- Where lights falls onto the plate, the electrical charge leaks away
- Negatively charged black toner particles are attracted to the remaining positive areas
- Paper is placed over the coping plate: toner is transferred to the paper and heated to make it stick.
- The photocopy is taken off the copy plate and turner over the reveal the text.
Electrostatic precipitators
Many power stations burn fossil fuels (Fuels made from the remains of ancient plants and animals, such as coal, oil and natural gas.) such as coal and oil. Smoke is produced when these fuels burn. Smoke comprises tiny solid particles such as unreacted carbon, which damage buildings and cause breathing difficulties. To avoid this happening, the smoke is removed from the waste gases before they pass out chimneys. The electrostatic precipitator is the device used for this job.
- Smoke particles pick up a negative charge at a metal grid
- Smock particles are attracted to the positively charged collecting plates.
- Collecting plates are knocked to remove the smock particles.
Electrolysis
Why chemical compounds are able to conduct electricity when they are melted or dissolved in water.
Moving ions.
Certain compounds are made up of electrically charged particles called ions. Ions can be positive or negative. These particles are unable to move around in solids, but can move around in liquid or when a solid is dissolved in water.
When a potential difference is applied, the positively charged ions move to the negative terminal (cathode). Meanwhile, the negatively charged ions move the positive terminal (anode) the movements of these ions allow an electric current to flow.
Metals
Metals atoms (the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons.) release some of their electrons, and these electrons are free to move through the structure of the metal. This is why metals are good conductors of electricity.
Factors affecting electrolysis
During the electrolysis of a compound, simpler substances are deposited at the electrodes. These substances can be solids, liquids or gases. The amount of substance (their mass or volume) deposited is directly proportional to:
- the current flowing
- How long the current flows.
If the current is doubled, the amount of substance deposited doubles.
Electromagnetic forces
Magnets and electromagnets (magnets made by wrapping a coil of wire around an iron bar and passing an electric current through the coil) have a north-seeking pole and a south-seeking pole. Their magnetic fields can be shown using diagrams with field lines. The arrow heads on the field lines go from north to south. Opposite poles of magnet attract each other and like poles repel.
A wire that carries an electric current in a magnetic field experiences a force and may move. The size of the force can be increased by increasing the size of the current flowing or the strength of the magnetic field. The dc electric motor uses this electromagnetic effect.
Field lines around a magnet
North and south poles
- Two like poles repel each other
- Two unlike poles attract each other
- Magnets will attract magnetic materials such as iron or steel
- We can only be sure that a material is magnetized if it repels a know magnet.
Magnetic fields.
A magnetic field is the space around a magnetized object, such as a bar magnet, where the effects of magnetism can be detected.
The magnetic field is shown as a series of lines, with arrowheads pointing from N to S.
Coils of wire.
When electricity is passed through a coil of wire, the coil acts like a bar magnet.
Electromagnets
An electromagnet consists of coil of wire with an electric current passing though it. The poles of the electromagnets are reversed if the current flowing through it is reversed. If the coil is wrapped around a soft iron core, the magnetic field is concentrated and becomes stronger.
Stronger still
The strength of an electromagnet can be increased by:
- using a soft iron core
- increasing the number of turns on the coil
- Increasing the current in the coil.
Electric wires in a magnetic field.
It experiences a force and may move. If the current or the magnetic fields is reversed, the direction of the force is reversed and the wire moves in the opposite direction.
Increasing the size of the force
- increasing the size of the current
- increasing the strength of the magnetic field.
The dc electric motor
A simple dc electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.
The directions of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or commutator. A coil of wire is used with lots of turns to increase the effect of the magnetic field.