Magnetic field can be produced by Permanent magnet, current carrying wire, current
carrying coil or electromagnet.
Advantages of electromagnet :
The magnetic field can be turned on or off.
Strength of magnetic field can be controlled by current.
Can be formed in different shapes.
Magnetic field always starts at N-pole and ends at S-pole. (There must be two poles
existed).
Strength of magnetic field can be expressed by the number of field lines per unit area.
Determine the pole of magnetic field : right hand grip rule. The thumb represent the
North pole.
Magnetic field strength can be combined together. A uniform magnetic field and a
circular magnetic field forms a catapult magnetic field and a force is produce.
A current carrying wire produces a circular magnetic field. If this wire is placed inside a
uniform magnetic field, a force is then produced. The direction of this force is determined
by the Fleming left hand rule (F-B-I).
Motor is using this property. (A coil with two straight edges produce circular magnetic
fields and placed within two slab (or half circular) magnet (producing uniform magnetic
field).
D.C. Motor needs a commutator to keep the coil rotating continuously by changing the
direction of current in the coil every half revolution. The split ring touches different
carbon brushes every half revolution.
A.C. motor needs a commutator and a electromagnet. It behaves the same as a d.c. motor
except that when the current change its direction, the magnetic field also changes its
direction.
There is no free lunch, energy cannot be created by a power plant. Energy can only be
converted from other forms to electrical form. Normally K.E. is being converted to
electrical energy in the turbine of a power station. K.E. is converted from downpouring
water (hydroelectric plant), huge amount of high-pressure steam (nuclear power plant or
coal-fire, petrol power plant).
In the conversion process, energy can be lost to friction, internal energy …..etc.
K.E. is visible in rotating coils or magnetic fields.
Electromagnetic induction : When the strength of the magnetic field inside a coil is set
changing or a wire cut passed a magnetic field at right angle, a voltage is induced.
If the coil or the wire is part of a complete circuit, a current is also induced.
The magnitude of the induced current or voltage directly related to the rate of
change of magnetic field (stronger field line, changing faster), the rate of number of
field line cut and the number of wires or number of turn of the coil.
The strength of magnetic field change inside a coil can be due also to
A coil rotating inside a magnetic field (K.E. rotating coil)
Or the magnetic field rotating around the coil. (K.E. rotating magnet)
(In both cases, the two sides of the coil are cutting (K.E. moving wire) the magnetic
field lines like a wire (K.E. moving wire) cut passed a magnetic field at right angle
but in opposite directions.)
Power Generator is a device either rotating coil inside magentic field or rotating magnetic
field around a coil. Rotating magnetic field is easier as the coil (the connecting wire) will
not move.
If the coil is set moved, avoiding the twisting of the connecting wire becomes very
important. There are two ways to do this. One is using commutator for D.C. and the other
is using slip rings for a.c. To ensure that the output voltage remains almost the same and
the wire cut most magnetic field in each rotation, practical generator uses three coils
(rotor) inside the magnetic field and the magnet is a curve magnet.
When two coils are put together side by side or connected in series, one is connected
to a supply source and the other to a circuit without any source, and current is allowed to
flow in the first coil and is varied. Then this first coil becomes an electromagnet and its
field strength is varied. When this changing magnetic field goes passed the second coil.
A voltage or a current is induced in the second coil. This is called mutual induction.
Since the magnitude of the induced voltage depends on the number of turns of the
coil. The voltage can be adjusted by varying the number of turns.
Transformer is made according to this principle. The voltage of the first coil can be
changed into different voltages by simply changing the number of turns of coil in the
second one.
Vp/Vs = Np/Ns
If there is no energy lost during this process. Then
VpIp = VsIs
However, there usually is energy lost due to leakage of magnetic field line.(Not all
mangetic field lines can go passed the second coil (secondary coil).
To preserve the energy, we can use soft iron to group all field lines together.
Transmission of electrical power : power lost is in the transmission cable is P = I2 R or
P = IV (V is potential difference across the cable, NOT potential)
I in the cable is limited by two factors.
1. The potential difference across the cable and its resistance. (I = p.d. /R)
However, the p.d. of the cable is not certain. It depends on the current in it.
2. How large the current can be due to the power of the power plant.
( I=P/V)
Consider the western cross-harbor tunnel. The number of car expected to go
passed this tunnel can be far more than the existing number.
Is it because the tunnel is built unsatisfactory so that the actual number of car is
allowed?
What are the other reasons?
A power plant can generate power of 6MW. If, before the power is transmitted,
the potential is raised to 600kV (each C of charge must carry 600 kJ of energy),
then there can only be at most 10 C of charge allowed in one second (or 10A)
and this current will go to the transmission cable.
If this 10A current travels through the cable of 20 ohm, then at the two ends of
the cable, the potential difference is 10A x 20ohm = 200V or 0.2kV.
The potentials at the beginning and the end of the cable will be 600 kV and
599.8 kV. The difference is 0.2kV.
The power loss is then P = I2R = 10A x 10A x 20 ohm = 2000 W. The loss is
about 2000W/6MW = ~ 1/30 or 0.03 %.
20 ohm resistance is very small if the length of the cable is over 1 km. If the
potential difference is 1 kV, it allows 50 A current through. But the amount of
charge is limited by the power plant.
(The situation is like the western cross-harbor tunnel, although the tunnel is
wide and can allow many more cars to go passed. There is simply not enough
car can pay the fee. [carry enough energy])