When a bicycle tire is inflated with an air pump, after a while, the cylinder wall will heat up. When explaining this phenomenon, classmate a said, "this is caused by the piston moving up and down and the cylinder wall friction, and the temperature of the cylinder wall rises due to the heat generated by friction." according to the same theory, "this is because the piston compresses the air in the cylinder, which increases the internal energy and temperature of the air in the cylinder, and also due to heat transfer Make the wall of the cylinder warm up too. "Who's right? Explain your reasons

When a bicycle tire is inflated with an air pump, after a while, the cylinder wall will heat up. When explaining this phenomenon, classmate a said, "this is caused by the piston moving up and down and the cylinder wall friction, and the temperature of the cylinder wall rises due to the heat generated by friction." according to the same theory, "this is because the piston compresses the air in the cylinder, which increases the internal energy and temperature of the air in the cylinder, and also due to heat transfer Make the wall of the cylinder warm up too. "Who's right? Explain your reasons

Student B is right. Because the piston presses down, the more work is done on the air in the cylinder, the more mechanical energy is consumed, and the more internal energy is converted, the higher the air temperature in the cylinder. Due to heat transfer, the temperature at the bottom of the cylinder is also higher, so student B is right

As shown in the figure, there are two identical beakers, a cup containing 800 g of water, B cup containing 400 g of water and 400 g of broken copper block (C copper < C water), under the condition of the same initial temperature and heating method, they are heated at the same time () A. A the water in the cup boils first B. B the water in the cup boils first C. The water in two cups boils at the same time D. It's impossible to tell who is boiling first

According to the heat calculation formula q = cm △ T, △ t = q absorption
The heat absorbed by the substance in the two beakers is the same, and the mass is the same; however, there are copper blocks in cup B whose specific heat capacity is less than water, which is absorbed according to △ t = Q
The results show that the smaller the specific heat capacity is, the higher the temperature is; therefore, the water in cup B boils first;
So B is correct; ACD is wrong;
Therefore, B is selected

A hollow copper ball with a mass of 89g is suspended when immersed in water_______ (ρ copper = 8.9 × 103kg / m3)

V copper = m / ρ copper = 89g / 8.9g/cm3 = 10cm3,
If the ball is suspended in water, then G = f, that is, Mg = ρ water, GV ball
VSphere = m / ρ water = 89g / 1.0g/cm3 = 89cm3,
So V empty = 89cm3-10cm3 = 79cm3

The heat released from the complete combustion of 0.42 cubic meters of natural gas is absorbed by 200 kg of water with an initial temperature of 20 ℃ (1) What is the heat released from the complete combustion of natural gas? (2) How many ℃ can these water temperatures rise?

The calorific value of natural gas is not given in the question?
1. Q discharge = VQ = 0.42 × q = J
2. Q absorption = q discharge
Because Q absorption = cm (T1-T2)
So T1 = q suction / cm + T2 algebra is OK

Junior high school physics problem, seek solution step! Urgent wait! The tractor with a power of 24kw, ploughing at a constant speed for 20min, has advanced 1800m (1) How much work does the tractor do? (2) How much resistance does the tractor suffer when moving forward? (3) What measures should be taken to speed up the speed in 3600 m and keep the power constant in 3600 m? Electric bicycles are light, safe and pollution-free, and they are very popular among people. On weekends, a student rode an electric bicycle to the seashore and islands for sightseeing, passing through the 7000m long Lianyungang seawall, which is known as "the first dyke of Shenzhou". If the electric bicycle runs at a constant speed of 18km / h, the traction power of the electric bicycle is 150W (1) The time required for the student to pass through the dike; 2) the traction force and the work done by the traction force of the electric bicycle through the dike How much power does a pump pump pump when it puts the water pressure of 7.5m3 to a 10m high water tower in 15s? (g = 10N / kg)

According to the equilibrium condition f = w / S = 8000 * 3600j / 1800m = 16000n3. P, if t remains unchanged, w remains unchanged, while s increases twice as much. It can be seen that if F should be 1 / 2 of the original, the resistance should be reduced to the original 1 / 22.1. T = s / v = 7 / 18h2. F = w / S = Pt / VT = P / v = 150W / 5m / S = 30nw = FS =

In an experiment, a student found that the nameplates of two small light bulbs on the table were fuzzy. After careful observation, they found that "3V? 3V" was marked on L1? W ", L2 is marked with"? In order to get the missing data on the name plate, he connected the two lamps to the power supply with voltage UA in some way, at this time, both lamps were normally on; then, when they were connected to the power supply with voltage UB in another way, such as L2, the actual power of L1 was 4 / 9 of the rated power. Assuming that the resistance value of L1 was greater than that of L2 and remained unchanged, the rated power and l of L1 were calculated What are the rated voltages of 2?

Suppose that L1 and L2 are connected in parallel and then in series. When L1 and L2 are connected in parallel, U1 = U2 and R1 > R2, u12r1 < u22r2, i.e. P1 < P2. ① when L1 and L2 are in series, I1 = I2 and R1 > R2, i12r1 > i22r2, i.e., P1 ′ > P2 ′. ② since L2 is normal before and after L2, namely P2 = P2 ', P1

It's resistance and power

It takes patience to do this
You read more physics books. Junior high school physics is very simple
It's basically about Ampere
When doing experiments in experimental class, we must listen carefully. We must do it. There is no problem

In the circuit as shown in the figure, the supply voltage remains unchanged. When the key s is closed, the circuit works normally. After a period of time, the ammeter suddenly has no indication. If there is only one fault in the circuit, and it only occurs on the resistance R1 or R2 (1) The fault in this circuit may be______ ; (2) To further determine the fault, Xiao Qiang replaced the resistance R1 in the circuit with a good lamp L, closed the key s, and observed the light on and off. If the light l was on, it indicated that the fault was______ If the light L is not on, the fault is______ .

(1) The analysis shows that one of the resistance is open, i.e. R1 or R2
(2) When a good lamp L is used to replace the resistance R1 in the circuit, close the key s, and the lamp L and the resistance R2 are connected in series. Due to the mutual influence of the series circuits, if the lamp L is on, it means that R2 is working normally, and R1 is open circuit. If the lamp L is not on, it means that R2 is open circuit
So the answer: (1) R1 open circuit or R2 open circuit, (2) R1 open circuit, R2 open circuit

When the resistance R1 R2 is connected in parallel, the current representation number is 1.5A. When only R1 is connected, the current representation number changes by 0.5A. When R1 R2 is connected in series, the power of R2 is 2W (1) Resistance of R2 (2) Supply voltage

Since I is always 1.5A in parallel, and the voltage at both ends of r1r2 is equal to the power supply voltage, and only R1 is also the power supply voltage, I1 remains unchanged, while the changing current is I2, so I1 = itotal-i2 = 1.5a-0.5a = 1a. Since I1: I2 = 2:1, u is constant, R1: R2 = 1:2, then r total = 3R1, then I = 1 / 3i1

1. The lamp L is marked with "6V 4W". If the resistance and supply voltage of L remain unchanged, when the slide P of sliding rheostat moves from the middle point of rheostat to the maximum resistance, the ratio of the two readings of the voltmeter is 3 to 4 The ratio of the power of the lamp before and after the lamp when p is at the midpoint of the rheostat and at the end of the maximum resistance What is the maximum resistance of sliding rheostat I can't figure it out L and sliding rheostat in series. Voltmeter measures voltage at both ends of sliding rheostat The picture shows It's very simple. It can be done in the brain In the direction of the resistance, of course

Suppose that the maximum resistance of the sliding rheostat is RX and the power supply voltage is u. ① set the voltage at both ends of the sliding rheostat at the front and the back as 3uo and 4uo respectively, because P = I ^ 2R, because r does not change, so pfront / pback = I ^ 2 front / I ^ 2 back = [(3uo) / (1 / 2RX)] ^ 2: [(4uo) / RX] ^ 2 = 9:4. ② first, calculate the resistance of lamp L, r = u ^ 2 / P