As shown in the figure, after s is closed, R1 = 50 Ω, and the current indication is required to vary from 60mA to 600mA, what is the power supply voltage lower than? What is the maximum structure of the rheostat? The figure is a series circuit with a constant resistor R1, a sliding rheostat and an ammeter Make it clear!

As shown in the figure, after s is closed, R1 = 50 Ω, and the current indication is required to vary from 60mA to 600mA, what is the power supply voltage lower than? What is the maximum structure of the rheostat? The figure is a series circuit with a constant resistor R1, a sliding rheostat and an ammeter Make it clear!

1, u = IR is obtained from I = u / R, and the allowable power supply voltage is obtained
U = I large R1 = 0.6A × 50 Ω = 30V
2. Get r = u / I from I = u / R, and calculate the maximum total resistance of the circuit
R large = u / I small = 30V / 0.06a = 500 Ω
3. R2 = r-r1 is obtained from R = R1 + R2, and the maximum resistance of sliding rheostat is calculated
R2 = RDA - R1 = 500 Ω - 50 Ω = 450 Ω

1. During the 100m race in the sports meeting, if the timekeeper hears the sound of the gun, is it earlier or later than when he sees the smoke? How long is it earlier or later? Which is more accurate?
2. When a student was traveling by boat, when the boat was between the two mountains, he yelled. After one second, he heard an echo. After three seconds, he heard another echo. He asked: how far is the distance between the two mountains? (the temperature was 15 ℃)
3. On a thunderstorm night, Yuhong saw the lightning for 5 seconds and heard the thunder. How far is the thunder from her?
4. After the "5.7" air crash, the salvage of the "black box" has always been the focus of people's attention. In order to salvage it out of the water as soon as possible, experts carried out exploration in the wrecked sea area. The measuring ship launched an ultrasonic wave with a wave speed of 1450 M / s from the sea surface to the seabed, and measured that the time from the launch to the reflection back to the sea surface was 0.2 S. what was the depth of the sea area where the "black box" was located?
You good brothers and sisters, who can teach me next, please
.......................

1. About two seconds later, it's more accurate to see the smoke
2. T1 * V + T2 * V = 1 * 340 + 3 * 340 = 1360m
3. T * V = 5 * 340 = 1700m
4. T * V / 2 = 1450 * 0.2 = 290m

① Can an object with a weight of 2.94n be weighed by Cheng Liang's 200g balance? (calculation instructions) page 4
② How much pressure does the cube copper block with the volume of 1dm & # 179; put on the horizontal desktop with the area of 1m & # 178;? Page 16
(g = 10 / kg, P copper = 8.9x10 & # 179; kg / M & # 179;)
③

1、2.94N=0.294kg=294g,
200g is beyond the range of the balance. Of course, it can't be weighed
2. G copper block = 1 * 0.001M & # 179; * 8.9x10 & # 179; kg / M & # 179; * 10 / kg = 89n,
So the pressure is 89n (independent of the contact area, the pressure is related to the contact area)

On some problems of physics in grade two of junior high school,
series connection:
What is the proportion between resistance and current in series circuit? What is the proportion between electric power and current? What is the proportion between electric power and voltage? What is the proportion between electric power and resistance?
parallel connection:
What's the proportion between resistance and current in parallel circuit? What's the proportion between electric power and current? What's the proportion between electric power and voltage? What's the proportion between electric power and resistance?

Because junior high school does not involve the problem of power internal resistance, so it is relatively simple to solve
But as the second floor guys said, your question is not comprehensive. In the series and parallel circuits, the relationship between voltage, resistance and power supply can not be expressed by one-sided ratio. The teacher may have taught you that, but if you learn the electrical part of high school physics, you will understand the knowledge of junior high school. Some are wrong, some are one-sided
Here is my summary of series and parallel circuits.)
Series: the sum of the resistors in a series circuit, r = R1 + R2
The current in series circuit is equal everywhere I = u / R
The calculation of electric power in series circuit is: P = w / T = I * IR (input problem is the square of I multiplied by R) P = IU = u square / R
The above three formulas of electric power with respect to current resistance voltage are derived from I = u / R
Parallel: the sum of resistance in parallel circuit: R1 = R2 / R1 * R2, that is, the reciprocal of the total resistance of parallel circuit is equal to the sum of the reciprocal of the resistance of each branch
The current in parallel circuit is equal to the sum of all branch currents, i.e. I = I1 + I2 In
The calculation method of electric power in parallel circuit is basically the same as that in series circuit, but the calculation should pay attention to the relationship between the electric power and consumers

1. Why does any object always have a certain temperature in general? 2. What is diffusion phenomenon? What factors do you think the diffusion speed may have? Why? 3. What are the similarities and differences between the internal energy and the mechanical energy of an object?

1. Because objects always have internal energy
2. Only when the molecules are moving irregularly (note that the diffusion phenomenon is invisible) factor: it is related to temperature
3. The same is the energy of molecules, the different is realized by doing work and heat transfer, and the other is realized by machinery

1. Connect the bulb marked "2.5V 0.3A" to the 6V power supply. How much resistance should be connected to the bulb to make it shine normally?
2. Xiaoming's family has an electric furnace marked with "220 V 880 W". What is its electric power in normal operation? What is the current through the electric furnace wire at this time? When it is used alone again, the electric energy meter marked with "1200 revs / kW * H" in Xiaoming's family turns 30 turns in 2 minutes, and the electric power of the electric furnace is?

Analysis: 1, from the topic can know, the requirement is normal lighting, so, what we have to do is to control the voltage at both ends of the bulb to the rated voltage of the bulb, that is, 2.5V, series voltage, so, the voltage of the resistor in series is 6v-2.5v = 3.5V, the current in the circuit is the rated current of the bulb, 0.3A, so, series voltage is 0.5V

The world of movement··········

3. Known: the propagation speed of laser v = 3 * 10 octave M / S = 300000000 M / s time t = 2.56s ask: the distance from the earth to the moon s from v = s / T, s = VT = 300000000 M / s * 2.56s = 384000000 m answer: the distance from the earth to the moon is 384000000 M. 4. Known: the average velocity of bullet through persimmon v = 900

Section 3

I only remember the second semester of the second grade of junior high school is mechanics from Chapter 8. Ha ha

Review outline of the second chapter in Physics

Didn't your teacher send you a review paper? Although I have this paper, I don't want to type it because there are too many words
1： Straight line propagation of light
2： Reflection of light
3： Planar mirror: the characteristics of imaging
4： Refraction of light
5： Dispersion of light
6： Invisible light
It should be to copy these contents. There are too many specific ones. Let's have a look later. Anyway, the teacher will give them to you, so you don't need to search online!
You go to this net first!

A plastic suction cup has an area of 10 square centimeters. Press it vertically and tightly in the center of the plate glass with a mass of 2 kg and lift it up slowly. Can the plastic suction cup lift the glass? (at this time, the air pressure in the suction cup is ignored)

It is known that: S = 10cm2 (square centimeter) = 0.001m2 (square meter), P = 101300pa, M = 2kg
Find: F
Solution: F = PS = 101300pa times 0.001m2 = 101.3n
G = mg = 2kg times 10N / kg = 20n
101.3N>20N
So, yes
A: plastic sucker can lift the glass