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Transcript
SUMMARY
Module 12: Electricity and Magnetism
•
Ferrous materials are attracted by magnets and are made from iron, steel, cobalt or
nickel.
•
Like poles of magnets repel and unlike poles attract.
•
You can destroy a magnet by heating it or by continually dropping it.
•
Iron is easily magnetized but looses it magnetism quickly.
•
Steel is permanently magnetized.
•
Induced magnetism is the effect when a ferrous material can become a magnet when it
is in the magnetic field of a magnet.
•
The magnetic field direction is from north to south.
•
An electromagnet can be made by putting an iron nail in a coil of wire which is connected
to d.c. or a.c. supply.
•
Electromagnets can be used in scrap yards, electric bells and relay switches.
•
Permanent magnets can be used for loudspeakers, refrigerator doors and the removal of
magnetic objects from people’s eyes.
•
Static electricity is produced by friction e.g. when rubbing a rod with a cloth.
•
Like charges will repel each other and unlike charges will attract each other.
•
Electric charge is measured in Coulomb (C).
•
The real charge on an electron is 1.6 x 10-19 C.
•
An electroscope is an instrument which can be used to detect small amounts of electric
charge.
•
The direction of an electric field line shows the path that would be taken by a positive
charge which was free to move in the field.
•
Electrostatics can be useful in photocopiers and electrostatic precipitators in chimneys.
•
A Van der Graaf generator is a device that generates static electricity continuously to the
inside of a hollow metal dome by means of a rotating rubber belt.
•
Lightning is likely to hit the highest point of the ground, or it may strike a tall building or
tree.
•
A building can be protected against lightning by a lightning conductor.
•
Electric current is the rate of flow of charges and can be given by the formula
•
The unit used for current is Ampere (A) and is measured with an ammeter.
•
Electromotive force refers to the energy supplied by a source in driving charge round a
complete circuit. Emf is measured in volts.
•
Potential difference is the difference in electrical potential between two points through
which a current flows and is also measured in volts with a voltmeter.
•
Resistance is the resistance to the flow of electric current and is measured in ohms (Ω).
•
Ohm’s Law states that the current flowing through a metallic conductor is directly
proportional to the potential difference across its ends provided that the temperature
remains constant.
•
The resistance of a wire depends on:
1. Length.
2. Cross section area.
3. Temperature.
The Institute of Information Technology, 2008
SUMMARY
Module 12: Electricity and Magnetism
4. Type of material.
•
In a parallel circuit the voltages across all the branches are the same.
•
The sum of the voltages in the external part of the circuit is equal to the voltage across
the cell.
•
The current in a series circuit is the same everywhere.
•
The total resistance for the resistors in series is the sum of the resistance of each.
•
The current from the source in a series circuit is split up in the branches.
1
1
1
=
+
R
R1 R2
•
The total resistance for the resistors in parallel is calculated by
•
A diode can be used to change alternating current to direct current.
•
A capacitor is a device which is used to store electric charge.
•
The measure of the extent to which a capacitor can store charge is called capacitance.
•
Capacitance =
•
Capacitance is measured in farads (F).
•
Power = Current x Potential Difference
•
The unit of power is Watt (W).
Ch arg e
Potential.difference
•
P = I2R
•
Electrical energy
•
E = I2Rt
P=
C=
Q
V
P = IxV
V2
R
E = IVt
E=
V 2t
R
•
Electromagnetic induction is the process of inducing a voltage when a wire is moved
through a magnetic field cutting the magnetic field at a right angle.
•
The size of the induced e.m.f. depends upon:
a. The speed at which the wire cuts the magnetic field
b. The number of wires or coils cutting the magnetic field
c. The strength of the magnetic field
•
Lenz’s Law states that the direction of the induced e.m.f. or current is such as to oppose
the change that is causing it.
•
An a.c. generator is a generator which produces electrical current in the form of
alternating current.
•
Root mean square value (r.m.s) is the value of an alternating current which would give
the same electrical power as a similar d.c. value.
•
A d.c. generator produces electrical current in the form of direct current.
•
A step-up transformer has more turns on the secondary coil and is used to increase the
voltage.
•
A step-down transformer has less turns on the secondary coil and is used to lower the
voltage.
The Institute of Information Technology, 2008
SUMMARY
Module 12: Electricity and Magnetism
VS N s
=
Vp N p
I pV p = I sVs
•
•
The advantages of high voltage transmission are:
a. A small current at high voltage delivers the same power as a large current at low
voltage.
b. At a smaller current, the heating effect is less and therefore less power is lost. For
calculating power loss use P = I 2 R
The d.c. motor changes electrical energy to kinetic energy.
•
The size of the force can be increased by:
a. Increasing the strength of the magnetic field
b. Increasing the size of the current
c. Increase the number of turns on the coil
•
Thermionic emission happens when a metal is heated and it gives off electrons.
•
A beam of electrons coming from the cathode is called a cathode ray.
•
The X-plates are used to deflect the beam from side to side and the Y-plates deflect the
beam up and down.
•
The beam is deflected whenever there is a potential difference across a set of plates.
The Institute of Information Technology, 2008