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Electromagnetic fields

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# Direct and alternating voltage - direct and alternating current

• Wires and appliances to which a voltage is applied are surrounded by electric fields. If a current flows, magnetic fields are additionally produced.
• A basic distinction is made between direct and alternating voltage or, as the case may be, between direct and alternating current.
• Many appliances and machines are operated from the public electricity network providing an alternating voltage at a frequency of 50 hertz (Hz).

### Polarity

Polarity describes the mutual orientation of two opposite poles, e.g. of the negative and positive electric pole of battery, or the north and south pole of a magnet.

### Frequency

Frequency specifies the time rate of changes of a periodic quantity, e.g. the instantaneous field strength of a low-frequency electric or magnetic field. It is a measure of the number of cycles per unit time; the unit of frequency is hertz (Hz): 1 Hz = 1 cycle per second = 1/s.

Ever since the discovery of the dynamo principle by Werner von Siemens in the second half of the 19th century, electricity has extended to all areas of human life. Whether in industry, traffic, research, medicine or obviously in the household – these days you can find electrically operated machines and appliances everywhere. All these machines and appliances with their electrical wiring are surrounded by electric and magnetic fields.

## Electric and magnetic fields

Wires and appliances to which a voltage is applied are surrounded by electric fields. If a current flows, magnetic fields are additionally produced.

A basic distinction is made between direct and alternating voltage or, as the case may be, between direct and alternating current. Alternating voltage changes the polarity of the connections at regular intervals, in other words, the negative pole turns into the positive pole and vice versa. When a current flows, the electrons forming the current change their direction of flow accordingly. With direct voltage and current, the polarity and the direction of flow of the electrons are not reversed.

## Electricity networks

Many appliances and machines are operated from the public electricity network providing an alternating voltage at a frequency of 50 hertz (Hz). Railway operators (for example the German company Deutsche Bahn) use a separate power network operated at a frequency of 16.7 Hz.

The fields generated have the same frequency as the voltages and currents in the relevant networks. As the two aforementioned frequencies are at the low-frequency end of the electromagnetic spectrum, these fields are referred to as "low-frequency alternating fields" or simply as "low-frequency" fields. The fields produced by direct voltages and currents are called static fields. Their frequency is 0 Hz.

State of 2018.01.10

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