Explain the mechanism of electric shock when touching a car battery or electric wire.
■Mechanism of current flow
First, let us consider how the current flows. For example, in the following case, will the current flow through the light bulb?
Since the positive and negative of the battery terminals are not connected above, current does not flow.
Then, what about when the terminal on the plus side is grounded to the ground as shown in the left figure below?
No current flows in this case either. The figure on the right below is a circuit diagram that means that current flows, but in reality the plus and minus are connected and the state is different from the figure on the left.
If the negative terminal is also placed on the ground as shown below, the ground becomes a conductor and the positive and negative sides are connected, allowing current to flow.
In other words, the circuit must be a closed circuit for current to flow.
In this case, the current does not flow with the voltage of a dry battery, and a voltage of several hundred volts is required.
■Electric shock case with electric wire
A bird perched on a wire will not be electrocuted, but that is because the wire has a lower resistance than the bird and the current flows in the wire as shown below.
However, if you touch the negative wire at the same time or your fishing rod gets caught in the wire, you will get an electric shock.
Even in the case of humans, the resistance is several thousand ohms, so you may think that the current will not flow like the above, but since it becomes a parallel circuit as shown below, electricity will flow through the ground and you will be electrocuted.
＜Whereabouts of the electric current flowing through the ground＞
I explained that the current flowing to the ground returns to the wire again, but how does the current know the ground position of the wire?
Since current is an exchange of electrons, it is sufficient if the total amount of electrons given to the ground and received by the wire is equal, and the wire does not receive the actual electrons themselves (I think).
＜No electric shock if the wire is not grounded＞
If there is no ground wire on the wire side as shown below, the circuit cannot be closed, so there is no risk of electric shock.
However, the ground wire is always installed.
■Electric shock case with car 12V lead battery
The 12V lead-acid battery in the car is grounded to the body (which reduces the cost of the harness).
If you touch the positive terminal as shown below, no current will flow through the human body because the circuit is not closed.
However, if you touch the positive terminal with one hand and the body with the other, a closed circuit will be created and current will flow.
Calculate how much current will flow. The resistance of the human body is calculated as approximately 4000 Ω when dry and 400 Ω when wet (depending on the conditions).
The relationship between the current flowing through the human body and the effect on the human body is as follows.
1mA：Feel tangy
5mA：Feel pain
10mA：Unbearable pain
20mA：Muscle spasms, stiffness and inability to move.
50mA：A very dangerous condition, even if it continues for a short time, it will lead to death.
100mA：Instant death
Based on the above, the flowing current is as follows from Ohm's law.
When it's dry, it feels painful, but when it's wet, it's in a very dangerous state, so don't touch it with wet hands.
＜When removing the battery, remove the negative terminal first＞
When removing the battery, remove it from the negative terminal side so as not to create a closed circuit, and you will not get an electric shock.
If the positive side is removed and the tool accidentally touches the positive terminal and the body side, a closed circuit will be created and a large current will flow.
＜When doing a battery booster, install it from the positive terminal＞
If the battery goes up, how to connect the battery booster is as follows.
For the same reason as above, installing from the positive terminal is the same as removing from the negative terminal.
(1) connect red cable
The positive terminal of the gone up battery → the positive terminal of the normal battery
(2) connect black cable
Negative terminal of normal battery → Negative terminal of gone up battery
■Electric shock from car lithium-ion battery
Lithium-ion batteries used in electric vehicles have a higher voltage (e.g. 200V) than lead-acid batteries, and when supplying current to a motor, the voltage is further boosted, resulting in a high voltage (e.g. 500V). Therefore, the structure of the car is handled more strictly than the lead battery, and unlike the lead battery, the car body is not grounded. Therefore, even if you touch only one terminal, you will not get an electric shock.
(Electric vehicles also have a lead battery, so if you touch that path, you will get an electric shock.)
If you get an electric shock, either you touch the positive and negative terminals at the same time, or there is a leak for some reason.
In addition, since the conductor is tightly covered, it is usually not possible to touch it.
If you touch the positive terminal and negative terminal, the current flowing will be as follows, and even with dry hands there is a danger of life, so you should never touch it.
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