Question

1.A current flowing through you of more than 5 ma is considered dangerous.

Why do we see warnings about high voltage, rather than high current?

What voltages are dangerous even if the skin is dry? (Dry skin has a resistance of 100 kohms while wet skin can be much less.)

An electroencephalogram (EEG) is a recording of differences in voltage observed on the scalp by attaching electrical contacts. If voltage differences that are observed are about 0.5 mv, and the resistance of the skin is about 10 kohms, what size currents are involved?

Answer 1

Warnings about high voltages is provided because the resistance of different bodies may vary and hence the current and since the voltages provided remains constant, hence warnings for voltages are given.

Since the current of more than \(5 \mathrm{~mA}\) can be dangerous and the dry skin has a resistance of \(100 \mathrm{kohms}\), the voltages that can be dangerous is, or the maximum safe voltage,

$$ \begin{aligned} V &=I R \\ V_{\max } &=(5 \mathrm{~mA})\left(100 \times 10^{3} \Omega\right) \\ &=500 \mathrm{~V} \end{aligned} $$

The current size in EEG is,

$$ \begin{aligned} I &=\frac{V}{R} \\ &=\frac{0.5 \times 10^{-3} \mathrm{~V}}{10 \times 10^{3} \Omega} \\ &=50 \times 10^{-9} \mathrm{~A} \\ &=50 \mathrm{nA} \end{aligned} $$