Download Electricity and Magnetism Summary Notes

Static Electricity
Electricity and Magnetism
Summary Notes
Conductors, such as metals, allow an electric charge to pass through them. Insulators, such as plastic, do not. A
substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes
positively charged.
Objects can be positively charged, negatively charged or neutral.
A substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes
positively charged.
When a charged object comes near to another object they will either attract or repel each other.
• If the charges are the same - they repel
• If the charges are opposite - they attract
• If one is charged and the other is not - they attract
•
Like charges repel. Unlike charges attract.
Substances can become charged by rubbing or by induction. For example, when an acetate rod is rubbed with a
cloth, electrons are transferred from the rod onto the cloth, making the rod positively charged.
Current voltage and resistance.
Current:
The flow of charge around the circuit. Measured in amps (A) by an
ammeter in series.
Voltage
The difference in energy between two parts of a circuit. Can also be defined as the
strength or ‘push’ that a power supply, such as a battery, gives a circuit. Measured in
volts (V) by a voltmeter in parallel .
Resistance
The resistance to flow and is measured in ohms (Ω).
Circuits.
For a circuit to work there must be:
•
A power supply
•
A device
•
Wires to make a complete circuit, with no gaps, so current can flow all the way around.
Note:
Conventionally current flows from positive (+ve) to negative (–ve), but the charges flow from negative to
positive.
 CRD 2016
Circuit symbols:
Cell
Battery
Bulb
Buzzer
OR
Motor
OR
Open Switch
Closed Switch
Resistor
OR
Variable Resistor
Wires
Fuse
Ammeter
Voltmeter
Ammeter and voltmeter in circuit
 CRD 2016
Ohm’s law.
V = IR
Where V= voltage in volts
I = current in amps
R = resistance in ohms
Series and parallel circuits
Series -
only one continuous loop.
Parallel -
branched with more than one route for the current to flow.
Series
Parallel
Current
Same
Different
Bulb added
Dimmer
Same brightness
Bulb blows
Circuit broken
Other bulbs still light.
Diagram
Electricity and safety.
When electricity flows through a wire the wire can get hot. This can be dangerous as it can create an electrical fire. It
is also used in electric fires, irons, stoves and light bulbs.
As electricity can be dangerous we need a method to ensure that we stay safe even if something goes wrong. Also
some electrical appliances are expensive and we don’t want them damaged when there is a power surge. We protect
equipment by using a fuse. A fuse is a thin piece of wire which melts hen too much electricity flows through. When it
melts it breaks the circuit so no more electricity can flow.
Electricity and the body
Electricity flows through the body in nerves. Electrical impulses are also involved in the control of the heart.
It is current not voltage that damages the body. Electrical shocks can be fatal.
Electricity can also be used to monitor the heart or restart it.
 CRD 2016
Magnetism
Magnetism is a non-contact force.
There are three magnetic elements. They are:
• Iron
• Nickel
• Cobalt.
Mixtures that contain any of these three elements are magnetic. Steel is an example of a mixture that contains iron.
Some compounds are also magnetic. Magnetic materials are attracted to magnets and can block magnetism.
Magnetic materials contain small groups of atoms called domains, which have direction. When those domains all line
up, the magnetic material becomes a magnet.
Non-magnetic materials are not attracted to a magnet, but will let magnetic forces pass through.
The law of magnetism
Magnets have two poles: North and South. The north pole of a magnet is the pole, which when the magnet is allowed
to move freely points towards the Earth’s North Pole.
The law of magnetisms states that:
•
Like poles repel
•
Unlike poles attract.
Making and destroying magnets
Iron can be magnetised by stroking a piece of iron with a permanent magnet. The iron must be stroked in the same
direction with the same end of the magnet, many times.
The magnetism of a magnet can be destroyed by:
•
Dropping or hitting – this knocks the domains out of position.
•
Heat – this causes the domains to vibrate faster and lose their orientation.
 CRD 2016
Magnetic fields
The space around a magnet where a magnet will have an effect is called
the magnetic field.
•
Magnetic fields are three-dimensional
•
A magnetic field is strongest around the magnet’s poles.
•
The stronger the magnetic field the further it extends.
•
Magnetic fields can be drawn with magnetic field lines.
•
Field lines have arrows which point north to south.
•
The stronger the magnetic field the more field lines that are drawn.
•
Magnetic fields can be shown with plotting compasses or iron filings.
The shape of magnetic field lines between two magnets depend on the closest two poles of the magnet.
The Earth produces a magnetic field as if it had a giant bar magnet going through its centre. The Earth’s magnetic
field extends into space. A compass, it a weak magnet, which can move freely and so lines up with the Earth’s
magnetic field. It does not point to true North but to magnetic North.
Electromagnets
When electricity flows through a wire, it has a small magnetic field around it. Electromagnets are made by coiling a
wire around an iron core. When electricity flows through the wire, magnetism is produced. When the electricity is
turned off the magnetism disappears.
The strength of an electromagnet can be increased by:
•
Increasing the number of coils of wire
•
Increasing the size of the current (by increasing the voltage, as V=IR)
•
Using an iron core
 CRD 2016
Electromagnets are used in electric bells, telephones, relays, videos and lifting things in scrap yards.
Relay Switches
A relay is a switch operated by electricity. Relays allow one circuit to switch a second circuit which can be
completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC
mains circuit. Relay switches normally use electromagnetism. Relays are used in the ignition of cars so that the
driver does not need to touch any part of the circuit which has a high current.
 CRD 2016