Physics 7 Lesson 26: The potential difference between the two ends of the power tool
1. Summary of theory Tóm
1.1. The potential difference between the two ends of a power tool
In a closed circuit, the potential difference between the two ends of a conductor causes an electric current to flow through that conductor.
– For a certain conductor (light bulb, rice cooker, electric iron…) if the potential difference between the two ends of the conductor is larger, the magnitude of the current flowing through that conductor is greater.
– The number of volts on each power tool indicates the rated voltage of that tool. The rated voltage is the maximum voltage that the tool can withstand.
- When the potential difference between the two ends of the bulb is zero, no current flows through it.
- Between the two terminals of the power supply, whether open circuit or closed circuit, there is a potential difference.
- Each power tool will work properly when the applied voltage is exactly equal to its rated voltage.
- If the applied voltage is greater than the rated voltage, the power tool will fail.
- If the voltage used is smaller than the rated voltage, appliances that operate on the heating effect of electric current such as filament light bulbs, irons, electric stoves, etc. can still work but are weaker normal; and for electrical tools such as: electric fans, water pumps, refrigerators, televisions, computers… may be damaged. Therefore, people often use a device called a voltage stabilizer with an adjustment effect to always have the base voltage equal to the rated voltage.
1.2. Analogy between voltage and water level difference
When there is a difference in water level between two points A and B, there is a flow of water from A to B.
When there is a potential difference between the two ends of the bulb, current flows through the bulb.
A water pump produces a water level difference similar to that of an electric power source
2. Illustrated exercise
Question 1: A light bulb is marked with 2.5 V. What voltage can this light be hooked up to so that it won’t fail?
In order for the 2.5V bulb to light normally and not be damaged, we connect the bulb to the voltage of 2.5V.
Verse 2: Observe the voltage test when the lamp is not connected to the circuit and when the lamp is connected to the circuit.
Observe the readings of the voltmeter. Comment on the potential difference between two lamp oils when not connected to the circuit.
The voltmeter always reads 0 when the bulb is not connected to the circuit.
3.1. Essay exercises
Question 1: Does the voltmeter in the diagram in figure 26.5 have a different reading?
Verse 2: For a given light bulb, if the voltage across the two ends of the bulb increases, how does the current through the bulb change?
Question 3: Given the circuit as shown in the diagram below. Know that when the switch is closed, the light is on. When the switch is off, between which two points is there a voltage difference (non-zero)?
3.2. Multiple choice exercises Bài
Question 1: In order to form a concept that starts with the same method, in the lesson, students have seen the author compare the voltage difference with the difference in water level. Based on that, tell me that the negative (-) pole of the power supply can be compared with which of the following?
A. High water level
B. Water pump
C. Water flow
D. Low water level
Verse 2: Know that the rated amperage of an electric stove is 4.5A. Given the currents of the following magnitudes flowing through the stove, in which case the kitchen’s solenoid will break?
A. 4.5A B. 4.3A C. 3.8A D. 5.5A
Question 3: Which of the following statements is not correct?
A. The greater the current, the brighter the lamp.
B. The brightness of the lamp depends on the amperage.
C. Amperage is too small, the lamp will not light.
D. Light not on means zero amperage.
Verse 4: Which of the following means 220V on a light bulb?
A. Between the two ends of the bulb there is always a voltage of 220V.
B. The lamp only lights up when the voltage between the two bulbs is 220V.
C. That bulb can generate a voltage of 220V.
D. For the lamp to light normally, the voltage at both ends of the bulb must be 220V.
Through the lesson The potential difference between the two ends of this power tool, students need to complete some of the objectives given by the lesson, such as:
State that when there is a potential difference between the two ends of a light bulb, current flows through the bulb.
State that a power tool will operate normally when used according to the rated voltage indicated on the tool.
Use ammeter to measure amperage, voltmeter to measure the potential difference between the two ends of a light bulb in a closed circuit.