Physics 7 Lesson 22: Heat and luminous effects of electric current của
1. Summary of theory Tóm
1.1. Heat effect
– Current passing through all normal conductors causes the conductor to heat up.
Application of the heating effect of electric current:
- Manufacturing irons, electric stoves, incandescent lamps, rice cookers,…
- Making fuses used when using electricity, it automatically disconnects the circuit when there is a problem to ensure electrical safety.
1.2. Luminous effect
a) Electric tester bulb
An electric current flows through the gas in the bulb, causing the gas to glow
b) Light Emitting Diodes (LEDs)
For an incandescent light bulb to glow, it takes some electrical energy to heat the filament to a high temperature.
So to save electricity, people used tube lights. Thanks to a special mechanism, the powder coating inside the lamp tube glows. This lamp heats up less and consumes less electrical energy than incandescent bulbs.
Today, people are still constantly researching and producing lamps that consume less electrical energy such as compact lamps, LED lights…
These are reasonable solutions to save energy. Nowadays, light emitting diodes (LEDs) are being produced more and more cheaply.
LED has many advantages, so in the near future, it will replace other lamps
2. Illustrated exercise
2.1. Form 1: Determination of electrical phenomena
If in a circuit with copper conductors a piece of lead wire (called a fuse) is interlaced, in some cases due to the thermal effect of the current, the conductor can heat up above 327°C. What happens then to the lead wire and to the circuit?
Copper and lead both conduct electricity, so current flows through the piece of lead connected to the copper wire.
Due to the heat effect of electric current, both copper wire and lead wire are hot. When the piece of wire heated to 327°C (equivalent to the melting point of lead) the lead wire broke, breaking the circuit.
2.2. Form 2: Show how to use light-emitting diodes
For a circuit with a diagram as shown in Figure 22.5, the power source is a battery with unknown (+) and (-) terminals. Please show how when using light emitting diodes to determine if A or B is the (+) terminal of this battery and the direction of current flows in the circuit.
Connect the small metal plate of the LED to the A terminal of the power supply and close the switch K. If the LED is on, the A terminal is the positive terminal of the power supply and the current flows as shown in the figure:
If A is the positive terminal, the direction of current flows through the circuit as shown in the figure:
If the light is not on, terminal A is the negative terminal of the power supply and terminal B is the positive terminal of the power supply.
3.1. Essay exercises
Question 1: Fuses work on what effect of current?
Verse 2: Does the light come on because the two ends of the string are hot or because the gas in between the two ends of the wire glows?
Question 3: Name some commonly used tools and equipment that are heated when an electric current flows through.
Verse 4: Why does electric current have a thermal effect?
3.2. Multiple choice exercises Bài
Question 1: The thermal effect of electric current in which of the following devices is beneficial?
A. Rice cooker B. Electric fan
C. Television (television) D. Water pump
Verse 2: Which of the following bulbs glows when an electric current flows through a gas?
A. Pronged bulb
B. Light Emitting Diodes
C. Motorcycle bulbs
D. Flashlight bulb
Question 3: Electric current has a luminous effect when flowing through which of the following devices, when they are in normal operation?
A. Electric water pump
C. Household electrical wiring
D. TV indicator light
Verse 4: When current flows through a filament light bulb, which of the following statements is true?
A. The bulb just heats up.
B. The light bulb just glows.
D. The light bulb both glows and heats up.
C. The bulb glows but does not heat up.
Through the lecture The heat effect and the luminous effect of this current, students need to complete some of the objectives given by the lesson such as:
Name the heating and luminous effects of electric current
Give examples of heat and light effects
Name and describe the luminous effect of electric current for 3 types of light bulbs: flashlight (filament lamp), electric tester bulb, light emitting diode (LED) bulb.