EMI

1. Electromagnetism
Topic 11.2 Power generation and
transmission

2. Rotating Coils
Most of our electricity comes from huge
generators in power stations.
There are smaller generators in cars and on
some bicycles.
These generators, or dynamos, all use
electromagnetic induction.
When turned, they induce an EMF (voltage)
which can make a current flow.

3. The next diagram shows a simple AC generator.
It is providing the current for a small bulb.
The coil is made of insulated copper wire and is
rotated by turning the shaft.
The slip rings are fixed to the coil and rotate with
it.
The brushes are two contacts which rub against
the slip rings and keep the coil connected to the
outside part of the circuit.
They are usually made of carbon.

4. AC Generator

5. When the coil is rotated, it cuts magnetic field
lines, so an EMF is generated.
This makes a current flow.
As the coil rotates, each side travels upwards,
downwards, upwards, downwards... and so on,
through the magnetic field.
So the current flows backwards, forwards... and so
on.
In other words, it is AC.

6. The graph shows how the current varies
through one cycle (rotation).
It is a maximum when the coil is horizontal
and cutting field lines at the fastest rate.
It is zero when the coil is vertical and cutting
no field lines.

7. AC Generator Output

8. The Sinusoidal Shape
As the emf can be calculated from
ε = - N Δ (Φ/ Δt)
and Φ = AB cos θ
It can be clearly seen that the shape of the curve
must be sinusoidal.

9. The following all increase the maximum
EMF (and the current):
increasing the number of turns on the coil
increasing the area of the coil
using a stronger magnet
rotating the coil faster.
(rotating the coil faster increases the
frequency too!)

11. Alternating Current
The graph shows the values of V and I
plotted against time
Can you see that the graphs for both V and I
are sine curves?
They both vary sinusoidally with time.
Can you see that the p.d. and the current
rise and fall together?
We say that V and I are in phase.

12. The time period T of an alternating p.d. or current
is the time for one complete cycle. This is shown
on the graph
The frequency f of an alternating pd or current is
the number of cycles in one second.
The peak values V0 and I0 of the alternating p.d.
and current are also shown on the graph

13. Root Mean Square Values
How do we measure the size of an
alternating p.d. (or current) when its value
changes from one instant to the next?
We could use the peak value, but this occurs
only for a moment.
What about the average value?
This is zero over a complete cycle and so is
not very helpful!

14. In fact, we use the root-mean-square
(r.m.s.) value.
This is also called the effective value.
The r.m.s. value is chosen, because it is the
value which is equivalent to a steady
direct current.

15. You can investigate this using the apparatus
in the diagram
Place two identical lamps side by side.
Connect one lamp to a battery; the other to
an a.c. supply.
The p.d. across each lamp must be
displayed on the screen of a double-beam
oscilloscope.

17. Adjust the a.c. supply, so that both lamps
are equally bright
The graph shows a typical trace from the
oscilloscope We can use it to compare the
voltage across each lamp.

19. Since both lamps are equally bright, the d.c.
and a.c. supplies are transferring energy to
the bulbs at the same rate.
Therefore, the d.c. voltage is equivalent to
the a.c. voltage.
The d.c. voltage equals the r.m.s. value of
the a.c. voltage.
Notice that the r.m.s. value is about 70%
(1/√2) of the peak value.

20.  In fact:

21. Why √2
Why The power dissipated in a lamp varies
as the p.d. across it, and the current passing
through it, alternate.
Remember power,P = current,(I) x p.d., (V)
If we multiply the values of I and V at any
instant, we get the power at that moment in
time, as the graph shows

23. The power varies between I0V0 and zero.
Therefore average power = I0V0 / 2
Or P = (I0 / √ 2) x (V0 / √ 2)
Or P = Irms x Vrms

24. Root Mean Square Voltage

25. Root Mean Square Current

26. Calculations
Use the rms values in the normal equations
Vrms = Irms R
P = Irms Vrms
P = Irms
2 R
P = Vrms
2 / R

27. Transformers
A transformer changes the value of an
alternating voltage.
It consists of two coils, wound around a
soft-iron core, as shown

29. In this transformer, when an input p.d. of 2 V
is applied to the primary coil, the output pd.
of the secondary coil is 8V
http://www.allaboutcircuits.com/worksheets/t
rans1.html
Transformer simulation

30. How does the transformer
work?
An alternating current flows in the primary coil.
This produces an alternating magnetic field in the
soft iron core.
This means that the flux linkage of the secondary
coil is constantly changing and so an alternating
potential difference is induced across it.
A transformer cannot work on d.c.

31. An Ideal Transformer
This is 100% efficient
Therefore the power in the primary is equal
to the power in the secondary
Pp = Ps
i.e. Ip Vp = Is Vs

32. Step-up Step-down
A step-up transformer increases the a.c.
voltage, because the secondary coil has
more turns than the primary coil.
In a step-down transformer, the voltage is
reduced and the secondary coil has fewer
turns than the primary coil.

34. The Equation
Note:
• In the transformer equations, the voltages and currents that
you use must all be peak values or all r.m.s. values.
Do not mix the two.
Strictly, the equations apply only to an ideal transformer, which
is 100 % efficient.

35. Energy losses
Energy is lost when it is transmitted, at the power
transmission lines and the Transformers
Large amounts of electrical energy Are transmitted
each second, from the power stations to the
consumers, often over large distances.
Since power = current x voltage, we could use:
either a) a low voltage and a high current,
or b) a high voltage and a low current

36. Why does the National Grid always use
method (b)?
Remember that a current always produces
heat in a resistor.
If the cables have resistance R, and carry a
current I, the energy converted to heat each
second is I2 R
P = I2 R

37. This means that in method (a) the high current
produces a lot of heat in the cables and little of the
energy from the power station gets to the
consumer.
Method (b) is used because the low current
minimises the power loss.
Transformers at each end of the system step the
voltage up and then down.

38. Losses in transformers
Copper losses: the wires have some
resistance
Hystereis loss: Magnetising and
demagnetising uses power
Eddy currents: small currents form in the
core

39. Transmission of Power

40. At the power station side:
Voltage is stepped up with a transformer to
275000 V
This reduces electrical loss in the
transmission lines

41. At the end of the line
Voltage is stepped-down with a transformer
to
33000 V: heavy industry
11000 V: Light industry
230 V : Homes

42. Health risks
How many transformers are there in your
home?
How many electric fields are you exposed to
everyday?
What about wireless internet?
Can these pose a threat to our health?

43. Electric fields from power lines and mobile
phone masts are all around us
Electric fields are known to interact with
tissues by inducing electric fields and
currents in them.
Some studies have found a higher rate of
cancer in people living close to power lines

44. How can these fields do this?
Results from animal studies conducted so
far suggest that electric fields do not initiate
or promote cancer.
Electric fields and magnetic fields were
classified as possibly carcinogenic to
humans based on epidemiological studies
of childhood leukaemia

45. "Possibly carcinogenic to humans" is a
classification used to denote an agent for
which there is limited evidence of
carcinogenicity in humans and less than
sufficient evidence for carcinogenicity in
experimental animals.

46. What about high-voltage power
lines?
Do not touch them!!
Again no risk of cancer has been found

47. Rectifying AC
AC is great for transmission purposes as
we’ve seen, however many domestic
devices require DC to operate.
This can be dealt with using a power pack,
which is really a rectifier.

48. Half wave rectification
A diode is a semiconductor that allows
current to flow one-way only. Its schematic
symbol shows the direction of the
conventional (+) current flow.
This means that if an AC current is passed
through a diode only half the time.

49. The output signal is intermittent and
therefore it is common for it to be smoothed
by adding a capacitor into the circuit.

50. Full wave rectification
If we arrange 4 diodes in a particular way
then we can achieve full wave rectification.

51. With smoothing capacitor