Page 199 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 3 Scientific principles

Direct current

In a direct current (DC) circuit the electrons always flow from the negative (−) p Figure 3.37 Direct current symbol
pole towards the positive (+) pole. The polarity, or direction, of the electrons
never reverses. DC can be produced from a number of sources, including
electrochemical, photovoltaic cells and batteries. DC can be stored in batteries
and cells.

Alternating current

Alternating current (AC), unlike DC, does not travel in a constant direction.
It alternates. In other words, it reverses its direction of travel constantly and
uniformly throughout the circuit at 50 times a second. This rapid movement _ +
(50 times/second) is called the frequency and is measured in Hertz (Hz). In the Battery
UK, the frequency of AC power is 50 Hz.
p Figure 3.38 Simple direct
1 cycle = 1/50 of a second current circuit
50 cycle/second = 50Hz
KEY TERM
+ Hertz (Hz): the SI unit
of frequency, measuring
the number of cycles
per second in alternating
current.

−

p Figure 3.39 The AC sine wave p Figure 3.40 Alternating current symbol

The advantage that AC has over DC is that AC can easily be transformed to
higher or lower voltages. DC voltages are difficult to transform. Changing AC
voltages is done by the use of a transformer, which uses the properties of AC
electromagnets to change the voltages. AC generator
Another advantage is that AC can easily be transported over long distances
without excessive voltage loss and is, therefore, much more efficient than DC. p Figure 3.41 Simple alternating
current circuit showing the
AC is generated at power stations and portable electricity generators. It cannot alternating direction of
be stored. electron flow

Material conductivity and resistance

As we have already seen, the atom is orbited by electrons. Electrons carry a
negative charge and can move from atom to atom. The direction of movement
between atoms is random unless a force causes the electrons to move in one

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