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How many ohm resistor should be connected to 120 white LED lamps connected to 12 volt dry battery? The voltage will drop if there are too many
According to the typical specification of white LED, the typical value of forward voltage is 3.5V when the current is 20mA. An LED should be connected with 420-430 ohm resistor, and 120 white LED lamps should be connected with a resistor, that is 3.542 ohm, and it should be more than 25W. In fact, no one should connect like this. One by one, considering the voltage drop of dry battery
Lili's father uses a kind of razor (resistance is 4 Ω) which needs two dry batteries for power supply. My father thinks that using dry batteries is too wasteful and uses a 4.5V regulated power supply instead. I would like to ask: (1) what method can make the razor work normally; (2) calculate the physical value of your method
A: (1) a constant resistance R can be connected in series on the circuit outside the regulated power supply to achieve the function of voltage division, so that the voltage applied to the shaver is just 3 V. (2) the normal working voltage of the shaver is provided by two dry batteries. The voltage of one dry battery is 1.5V, and the voltage of two dry batteries is 1.5V × 2 = 3V
There are three score problems
（）×3/5=40 （）×2/9 =45 140÷（）=4/5
（200/3）×3/5=40 （405/2）×2/9 =45 140÷（175）=4/5
Simple fraction application questions
Xiao Ming takes 5 hours to walk from school to supermarket, and Xiao Zhang takes 7 hours to walk from school to supermarket. Now Xiao Zhang starts from school to walk to supermarket, and Xiao Ming goes from supermarket to school. How long do they meet?
Answer: 1 / (1 / 5 + 1 / 7) = 35 / 12 hours
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