High School Physics, some questions about voltmeters

Answer: The voltmeter is essentially a large resistance that can show the voltage at both ends of itself, but it is connected in parallel with a certain resistance, and the parallel voltage is also displayed, because the resistance is very large, so the general resistance is relatively small compared to it, so the current flowing through the voltmeter is very small when it is connected in parallel, and the current is ideally considered to be 0, that is, this part of the circuit of the voltmeter is regarded as an open circuit.

Back to the question you asked, closed circuit, the voltmeter is connected to both sides of the power supply to measure the voltage of the external circuit (also known as the road-end voltage - the voltage between the two poles of the power supply); When the circuit is disconnected, the road-end voltage is also measured, but at this time, the voltage is closer to the power supply electromotive force (called the power supply voltage in junior high school), if it is an ideal voltmeter (infinite internal resistance), then the power supply electromotive force is measured.

It can be understood that the external resistance of the circuit is the internal resistance of the voltmeter, and the voltmeter shows the voltage at both ends of itself, because the internal resistance of the voltmeter RV>>R (internal resistance of the power supply), so the voltage division of the voltmeter RVE (RV+R)) is close to the voltage electromotive force.

Non-ideal power supply (power supply with internal resistance): It can be regarded as an ideal power supply and internal resistance (R) in series, so that the circuit knowledge of junior high school and high school can be unified.

Directly on both sides of the power supply, of course, is the road-end voltage, which is the same as the effect when disconnected, but it should be noted that the road-end voltage is not equal to the power supply voltage (considering the internal resistance), the mobile phone plays, I hope it can help, thank you!

1. Turn the gear switch to the appropriate range of the AC voltage gear.

2 .The measuring pen is searched in parallel into the circuit under test.

Missing calendar 3 digital watch chain forest, direct reading voltage value. The pointer table, find the corresponding tick mark, and read the voltage value.

Range selection: 1. When you know the voltage range to be measured, you can directly select the appropriate range.

2. When you don't know the range of the voltage to be measured, select the maximum range file first and make a preliminary measurement; Select the appropriate range based on the results.

3. When switching the range, you need to turn off the power or remove the measuring pen, and then switch the range.

Only the middle tick mark of the multimeter is DC and AC common, and the zero tick mark is on the left, and the reading increases evenly from left to right. It is used to measure the value of DC current, DC voltage, and high-voltage alternating current.

The measured value at the time of reading (dial hand indication dial full bias indication number) magnification.

The second row of scales has a total of 50 divisions, and the range is artificially selected according to the actual situation. And only one person is allowed to be an estimator when reading. For example, the range is 250V, then each small cell is 5V, the first line is 0, the second is 5, there is no line between 0 and 5, then it is estimated.

The principle of measuring electrical resistance.

The ohmmeter is made according to Ohm's law of closed circuits, and its principle is shown in the figure below, G is the ammeter (meter head), the internal resistance is RG, the full bias current is IG, the electromotive force of the battery is E, and the internal resistance is R. The resistor r is a variable resistor, also called a zeroing resistor.

1. When the red and black watch pens are connected, it is equivalent to the resistance rx=0 to be measured, adjust the resistance value of r, so that the pointer of the meter head points to the full scale, so the pointer points to the full deviation of the current, which is set as the zero point of the resistance scale. RG+R+R is the internal resistance of the ohmmeter.

2. When the red and black watch pens are not in contact, it is equivalent to the measured resistance rx=, there is no current in the ammeter, and the meter pointer is not deflected, and the position pointed by the pointer is set as the point of the resistance scale.

3. When the red and black pen are connected to the measured resistance Rx, through the current of the meter head, change the Rx, the current I changes with the change, each Rx value corresponds to a current value, and the Rx value corresponding to the I value is directly marked on the dial, and the resistance value of the measured resistance can be directly read out from the dial.

Illustrate.

Looking at this figure, the lower left corner ma looks up at the DC voltage range of 1mA, 10mA, and 100mA respectively, and the bottom layer is the DC voltage range with the range respectively.

There are three pointers for ABC. A refers to the DC voltage range of the range, in order to facilitate reading we choose the bottom scale on the dial, the one from 0-250, the reading is the indication divided by 10, it should be.

b refers to the DC current in the range of 100mA, the scale is 0-50, and the reading is multiplied by 2, which should be 48mA

C is the ohmic file. Got it.

There is a knob to turn to the DC voltage level to measure the voltage, and turn to the current level to measure the current. The wiring is connected at both ends of the area to be measured.

First of all, it's important to be clear: at all times, a voltmeter measures its own voltage.

What is Voltmeter?

The voltmeter is actually a meter head (sensitive ammeter G), which is modified with a large resistance in string. Therefore, the essence of a voltmeter is a large resistor that can display its own voltage.

Why can voltmeters measure the voltage of other electrical appliances when they are connected in parallel?

Again, the voltmeter measures its own voltage at any given time. When it is connected in parallel to other resistors, the voltage of the resistor is equal to the voltage of the voltmeter according to the characteristics of parallel connection. Strictly speaking, it is not the voltmeter that measures the voltage of the resistor, but the voltage of the resistor is equal to the voltage of the voltmeter.

What does it mean when a voltmeter is connected in series?

As mentioned above, a voltmeter is actually a large resistor that displays the voltage. The voltmeter is connected in series, which is actually equivalent to a large resistance in the circuit in series, the current is very, very small, and the voltage obtained by other electrical appliances on the branch is basically zero. In this case, the voltmeter voltage is the total branch voltage.

Summary: A voltmeter can be understood as a large resistor that can display the voltage, and it will display its own voltage at any time.

Note: In the same way, an ammeter can be understood as a small resistor that can display the current, and it will display its own current at any time. Therefore, in order to obtain the voltage and current of other electrical appliances, according to the characteristics of the series-parallel circuit, the voltmeter is connected in parallel, and the ammeter is connected in series.

1. For instruments with a minimum index of "1", the measurement error appears in the next digit, and the next digit is estimated by one-tenth.

For example, voltmeters and ammeters with ranges of 3 V and 3 A have a minimum division of V and A, and the reading should be estimated to one-tenth of the minimum division, and the estimated reading is possible).

2. For instruments with a minimum index of "2" or "5", the measurement error appears in the same digit, and the half-grid estimation method is used (i.e., the same digit is estimated by one-half or one-fifth respectively).

For example, the range of a galvanometer, its minimum division is a, if the estimate is read to one-tenth of the minimum division, the estimated reading is that half of the error is in the same position as the minimum division, so the reading should be estimated to the minimum division (less than the minimum division half a grid is omitted, equal to or more than the minimum division half a grid to be read by half a grid), so if the final reading is in amperes, there are two decimal places after the decimal point, and the mantissa can only be .

3. The ohmmeter in the multi-purpose meter, the data it reads out by itself is inaccurate, so the ohmmeter can only be used to estimate the approximate range of the resistance of an electrical appliance, and cannot be used to accurately measure the instrument. Therefore, when the ohmmeter is read, even if it is read accurately, it is not accurate.

If it is an exam, then he will definitely let the pointer be on a scale value, so that the number can be read directly, and there is no need to estimate.

If it is or 1 or the like, it needs to be estimated, if it is 2 or or the like, it does not need to be estimated (generally?). 1 or? 2)

D sliding rheostat slides from left to right, the resistance becomes larger, the voltage division becomes larger, the current indication becomes smaller, and the voltage at both ends of R1 on the voltmeter side becomes smaller.

Select B, when the sliding blade of the sliding rheostat slides from left to right, the total resistance increases, so the trunk current decreases, the current representation number decreases, the voltage at both ends of R1 decreases, and the voltage representation number = power supply electromotive force - R1 voltage at both ends, so the voltage representation number becomes larger.

Select D, the resistance changes from small to large, the total resistance of the circuit increases, the current indicator must decrease, the voltage is measured in the parallel section, which can be approximately regarded as, when the dicing is on the left, the voltage is measured by the power supply voltage, which must be the maximum, when the dicing is on the right, the resistance dividers, and the voltage indicator decreases, so choose D, all decrease.

Think of the voltmeter as an open circuit first.

Therefore, when P moves to the right, the total resistance becomes larger, and the current representation number becomes smaller, however, the voltmeter measures the voltage at both ends of R1, which is also the case for many students (including some who are not students!). Got it wrong!

As the current increases, the voltage at both ends of R1 increases.

Therefore, choosing B is absolutely correct!

I choose DThe slider moves from left to right, the resistance becomes larger, the current indicator becomes smaller, and the ammeter is used as a wire (this, the physics teacher should have talked about) after the switch is closed, the voltmeter measures the voltage of R1, because the resistance of R1 remains unchanged, the current becomes smaller, so the voltage representation number becomes smaller.

Definitely choose D's.

Select D, first of all, to determine which voltage is measured by the voltmeter, and the voltage is measured R1.

Because this is a series circuit, the current is equal everywhere, the ammeter measures the total current, and the series resistance is equal to R1+R2.

When the sliding rheostat slides to the right, the R2 resistance becomes larger, and the resistance of the whole system also increases, so the current becomes smaller.

In a series circuit, the supply voltage does not change (known), so the voltage is equal to the sum of the resistor voltages at both ends (series divider), the greater the resistance, the greater the divided voltage, and R2 becomes larger, so the R2 voltage becomes larger.

When the power supply voltage remains the same, the R2 voltage becomes larger, so the R1 voltage becomes smaller, and the voltmeter becomes smaller.

So, the ammeter, the voltmeter are all smaller, the answer d

The sliding rheostat slides from left to right, the total resistance becomes larger, the total current becomes smaller, and the ammeter measures the total current, so the current representation number becomes smaller.

The voltmeter measures the partial voltage of R1, so U1=I*R1 becomes smaller.

The correct answer is d

This question examines the law of the closed Electrician Neulum and can be solved by the basic formula: E r2 (r+r2+r1)=4

e•r2/(r+r2)=9

r1:r2=3:2

Considering r+r2 as a whole, we can find e=

You can also use the power electromotive force template e=u1+u1+u2e=u2 to bury 2+u2 in the internal annihilation code'

u2 u1=

u2'U within 2

Find e=

You are asking which voltage is measured by the voltmeter in the first question?

Analysis: In the first question, when CD indirectly a voltmeter, in general, the internal resistance of the voltmeter is very large, and it can be regarded as an open circuit in this circuit, then there is no current passing through the resistor R3 (the potential at both ends of the resistor is equal), so the voltmeter is to measure the voltage at both ends of the resistor R2.

That is, the resistor r3 can be replaced by a wire.

Since it is an ideal voltmeter, the internal resistance is infinite, and the branch where R3 is located is open, and R3 can be regarded as the resistance of the ideal voltmeter, so the voltmeter measures the voltage at both ends of R2.

Remember: the voltmeter is connected to the circuit in parallel, so R2 is measured

Choose C. If the bulb does not light up, it means that there is either a short circuit or a short circuit in the circuit. If there is a short circuit, then the ammeter will definitely deflect very badly, because a large current is generated.

The voltmeter is definitely not deflection, because it measures the voltage of L1, and if it is short-circuited, then L1 has no voltage, so it is completely contradictory to the title. So for the open circuit. The open circuit may exist in any or more places of the ammeter, L1, L2, because the voltmeter is obviously deflected, indicating that there is a large voltage at L1, then only the open circuit is possible at L1.

Therefore, there is a broken circuit at L1.

Choose a place where there is a break between the cabs, such as the lights are burning.