CURRENT, VOLTAGE AND RESISTANCE
Electric Current (Direct and Alternating Current) Electric current is the rate of flow of electric charge. It is measured in amperes, or amps (A). Electric charge is carried by electrons – particles with a negative charge, which are normally attached to atoms. When the electric current flows through a conductor, the electrons move from one atom to another. If the number of electrons flowing through a conductor increases, then the amperage, or amprage (current) increases. When electrons flow, carrying the current, the can be called charge carriers. There are two types of electric current:: direct current (DC for short) and alternating current (AC for short). Direct current. The current from a cell (battery, fuel cell, solar cell) is direct current. Direct current (DC) is a constant flow of electricity which travels around a circuit in one direction. Photovoltaic cells (PVs), or solar cells, are an effective way of generating your own electricity from sunlight. The current they produce can be used immediately, may be stored in rechargeable batteries (like the ones in cars), or can be fed into the power grid and sold to the electric company. But PVs produce direct current. This is fine for charging batteries, but it is not suitable for powering household appliances, which require alternating current. For this, the DC supply from PVs and batteries needs to go through an inverter – a device which converts DC to AC. Alternating current. The electricity supplied to homes and other buildings – called mains electricity – is alternating current (AC). Unlike a DC supply, an AC supply flows backwards and forwards, its direction continually alternates. The rate at which the current alternates (called the frequency) is measured in hertz (Hz). For example, in the UK, AC supply is 50 Hz – it alternates 50 times per second. Mains electricity is generated at power stations, which use large generators. A voltage is produced when a magnet moves into a coil of wire. This principle is used in generators to produce electricity – either a coil of wire rotates in a magnetic field, or a magnet rotates in a coil of wire. A generator converts mechanical energy to electrical energy. A generator rotates a magnet within an iron surround. Current from the generators leaves the power station and enters the power grid (a network of power lines (cables) which transmit it around the country). At the point where it enters the grid, the electricity flows through transformers – especially step-up transformers, which increase voltage and decrease amperage. This reduces the energy lost from the power lines over long distances. Before the supply is used by homes and other buildings, it passes through several step-down transformers, which reduce its voltage and increase its amperage. The supply may be stepped up to over 400,000 volts at the point where it enters the large transmission lines leaving the power station. It is normally then stepped down in stages, first passing through a wider network of lower-voltage transmission lines, and finally through the small distribution lines which supply streets and houses.
The current supplied to most homes is single-phase. In factories and large buildings, which have powerful electrical equipment, the supply is often three-phase – effectively three currents, each with a different phase. This provides a smoother supply as it reduces the gaps between the voltage peaks.
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