And Why Do We Use AC?
To understand the absolute basics of electricity, you might like to read my electricity 101 guide first, which is more detailed and has diagrams.
How to Understand Electricity: Volts, Amps and Watts Explained on AppliancesVolts 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 × 4 = 920 watts (920 W)
Electrical resistance
Electric cables and loads 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 flow 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 thicker cables whose copper cores have greater cross-sectional area (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
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 × 0.2 × 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 for distribution. It's then further reduced by additional transformers (often pole mounted) near where it's 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.
What's the Difference Between AC and DC?
Also see 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?