Engineering Science for Technical Secondary Schools – Full Notes
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FORM THREE
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Electric current can flow in two ways in a circuit. DC is when the current (or voltage) is a constant level in one direction, as seen with batteries. AC has flow in one then the opposite direction in a sinusoidal wave. This is the seen in mains electricity, which switches from positive to negative pattern 50 times per second (50 Hz frequency). The AC wave is quantified using root mean square (RMS) of the waveform. RMS is calculated by squaring the waveform which converts any negative values to positive values. The mean is calculated and the square root taken, giving mains UK current as 230 RMS. Higher frequencies do not transmit economically over long distances. Unfortunately lower frequencies such as 230 RMS carry a higher risk of excitation of muscles and nerves, increasing danger from electrical shocks.
Like electricity, magnetism produces attraction and repulsion between objects. While electricity is based on positive and negative charges, no known magnetic monopoles exist. Any magnetic particle or object has a “north” and “south” pole, with the directions based on the orientation of the Earth’s magnetic field. Like magnet poles repel each other (e.g., north repels north), while opposite poles attract one another (north and south attract).
Familiar examples of magnetism include a compass needle’s reaction to Earth’s magnetic field, the attraction and repulsion of bar magnets, and the field surrounding electromagnets. Yet, every moving electric charge has a magnetic field, so the orbiting electrons of atoms produce a magnetic field, there is a magnetic field associated with power lines, and hard discs and speakers rely on magnetic fields to function. Key SI units of magnetism include the tesla (T) for magnetic flux density, weber (Wb) for magnetic flux, ampere per meter (A/m) for magnetic field strength, and henry (H) for inductance.
