Physics 7 Lesson 29: Safety when using electricity
1. Summary of theory Tóm
1.1. Electric current through the human body can be dangerous
– Human (or animal) bodies are conductors of electricity. So when touching an electrical circuit, in the human (or animal) body, an electric current will flow through. It can then cause muscle convulsions, cardiac arrest, suffocation, and nerve paralysis that can even be fatal to humans (or animals).
– Depending on the strength of the current strong or weak, the effect of the current on the human body (or animals) has different levels. The electric current passing through the human (or animal) body has the magnitude:
- Above 10 mA causes convulsions of the muscles.
- Above 25 mA (through the chest) causes heart damage.
- 70 mA or more and with a voltage of 40 V or more will cause the heart to stop beating.
Note: An electric current of reasonable intensity can be used through the human body to treat some diseases such as electric acupuncture, electric massage, …
1.2. Short circuit phenomenon and fuse effect
a) Short circuit phenomenon (short circuit)
– Definition of short circuit: A short circuit (short circuit) is a short circuit or short circuit.
– Comment: When there is a short circuit, the current in the circuit has a maximum value. ( \(I_2\) >> \(I_1\)).
Excessive amperage can melt or burn the insulating sheath and other parts in contact with or near it. That could cause a fire.
The filament of the light bulb is broken, the wire in the electric fan is melted and broken, the electrical circuits in the machine are damaged, etc.
b) Effect of fuse
– The concept of fuse: A fuse is a device that automatically disconnects an electrical circuit when the current has an excessive increase, especially when a short circuit occurs.
- When a short circuit occurs with the circuit shown in Figure 29.3, the fuse heats up, melts and blows, and the circuit cuts itself off.
C4: Meaning of the Ampere number written on each fuse:
If the current exceeds that limit, the fuse will blow.
⇒ Fuse current \(\leq\) number on each fuse.
1.3. Safety rules when using electricity:
Experiment with a power source with a voltage of less than 40V.
Use insulated conductors.
Do not touch the phase wire of the civil circuit.
When someone is electrocuted, do not touch that person, but need to find a way to immediately turn off the power switch and call an ambulance.
2. Illustrated exercise
Question 1: Describe the effects of short circuits.
When there is a short circuit, the current in the circuit increases in magnitude.
Effects of a short circuit:
Excessive amperage can burn or burn the insulating sheath and other parts in contact with it or near it. That could cause a fire.
If part of the circuit is short-circuited, power tools in the rest of the circuit may be damaged.
Verse 2: Please indicate the meaning of the number of amps written on each fuse.
The meaning of the ammeter number written on each fuse: If the current exceeds that value, the fuse will blow.
For example: The number on the fuse is 1A, which means that the fuse will blow when the current through it is more than 1 A.
3.1. Essay exercises
Question 1: Why can electric current pass through the human body?
Verse 2: Why when using electricity, even with a plastic cover, we should not hold hands directly on the power cord?
Question 3: What is a short circuit?
Verse 4: Fill in the blanks with suitable words: Electric current……flows through the human body when it touches an electrical circuit at a ….. position of the body.
3.2. Multiple choice exercises Bài
Question 1: What happens when there is a short circuit?
A. The voltage is constant.
B. The voltage spiked.
C. Amperage spikes.
D. Amperage remains constant.
Verse 2: How much voltage can the power network have to cause death?
A. Under 220 volts
B. Over 40 V
C. Over 100 V
D. Over 220 volts
Question 3: When passing through the human body, electric current can
A. Causes burns
B. Causing the heart to stop beating
C. Nerves are paralyzed
D. Both A, B and C
Verse 4: Which statement below is incorrect?
A. Human and animal bodies are electrical conductors.
B. Human and animal bodies do not allow electric current to flow.
C. There will be no current flowing through the body if we accidentally touch the wire if our feet wear plastic sandals, stand on the table (insulated from the ground).
D. Do not approach high voltage power lines.
Through this lesson on Safety when using electricity, students need to complete some of the objectives given by the lesson, such as:
State the dangerous limits of voltages and currents to the human body.
List and implement some rules to ensure safety when using electricity.