And why do we use AC?
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| Public domain image by FelixMittermeier on Pixabay.com |
To
understand the absolute basics of electricity, you might like to read
my guide to electricity basics on the Medium.com website first which is
more detailed and has diagrams. This post on Facebook is a summary of
that article.
Electricity 101: A Complete Beginner's Guide at this link:
Volts and amps
If
we call the voltage in a circuit V (measured in volts) and the current I
(measured in amps), the power P in watts used by a connected electrical
load is simply calculated by multiplying the voltage by the current.
I.e:
P = VI
Amps are denoted by the symbol A, volts by V and power in watts by W.
Question. A toaster takes 4 amps (4 A) at 230 V. How much power does it use?
Answer. Power P = VI = 230 x 4 = 920 watts (920 W)
Electrical resistance
Electric
cables and loads also have a property called resistance, analogous to
the resistance in a water hose. When a hose is reduced in diameter or
made longer (or someone steps on it), resistance to flow increases.
Resistance to electricity flown in electrical cables, or any conductor
for that matter (something that carries electricity) results in power
dissipation as heat when electricity flows through the cables. (That's
why we use cables with greater cross sectional area or CSA to carry
heavier current, because it reduces resistance).
If we denote the resistance of a cable or electrical load by R and the current again by I, then the power dissipation in the cable is given by:
P = I²R
If we denote the resistance of a cable or electrical load by R and the current again by I, then the power dissipation in the cable is given by:
P = I²R
So
we square the current (multiply it by itself) and multiply by the
resistance to calculate the power dissipation. Resistance is measured in
ohms.
Question.
An incandescent lightbulb has a resistance of 100 ohms. A current of
200 milliamp (200 mA or 0.2 A) passes through it. How much power does it
use?
Answer. P = I²R = 0.2 x 0.2 x 100 = 4 W
So
current flowing through a resistance produces heat. This isn't of any
advantage unless it's a desired effect. Devices such as electric
heaters, toasters, elements in kettles, immersion elements and electric
blankets make use of the property of resistance to create heat which is
useful. Incandescent light bulbs also use an electric current to make a
filament white hot. However in this case it's the light produced that's
of benefit and the heat is just a wasteful by-product (95% of the energy
used is converted to heat).
Resistance of electrical cables
We
saw above that the power dissipated in a resistance is P = I²R. It
doesn't matter whether the resistance is that of an electrical device,
the resistance of a cable or an electrical component called a resistor
(You might have seen these if you've opened a device with electronics
inside. They look like cylinders with coloured stripes).
When
Eirgrid or ESB Networks transmit power, they want to reduce power
losses in cables, i.e. reduce P = I²R). This power loss is known as
copper loss (although transmission cables are primarily aluminium to
reduce weight). To reduce P, we need to make I and/or R smaller. R can
be reduced by sizing cables accordingly. Making them thicker reduces R,
but obviously there are practical limits because of cost and weight. A
more sensible option is to reduce I, the current. Because of the squared
term in the equation, the effect of reducing current is exponential. So
for instance if current is made 10 times smaller, for the same
resistance R, power loss is reduced to one hundredth of what it was
previously. So this is exactly the technique used by transmission
networks: Step up voltage by a factor of 100 or more and reduce current
proportionately.
So what are transformers for ?
Devices called transformers,
which are the electrical analog of a mechanical gearbox, convert low
voltage high current to higher voltage lower current. (They also do the
reverse). This is easy to do when alternating current
or AC is used. The lower current then reduces the power loss in cables
over distance. It also reduces voltage drop because drop is proportional
to current. the third advantage is lower current allows lighter gauge
cables to be used, reducing cost and weight on supporting pylons or
poles. Very high voltages are used for transmission countrywide and
voltage is then reduced or stepped-down to a more practical level at
substations. It's then further reduced by additional transformers (often
pole mounted) before electricity is delivered to consumers' premises.
The transmission line from Moneypoint Power Station on the Shannon
Estuary that runs to the substation at Dunnstown near Carnalway operates
at 400,000 volts or 400 kV.
See this guide for an explanation of the difference between AC and direct current DC
Also
this Wikipedia article about the late 19th century War of the Currents
which was a series of events surrounding the introduction of Edison's DC
and George Westinghouse's competing AC electric transmission systems.
Guess which won out?