Why is the electromotive force 3V in this question, and what does this question mean?

Because the voltmeter is connected in parallel at both ends of the battery, it examines the concept that when the circuit is open, the reading of the voltmeter is approximately equal to the electromotive force of the battery (ignoring the current passing through the voltmeter at this time), so the electromotive force of the battery is 3V.

Then the subsequent solution should not be a problem, that is, the property of the series circuit and Ohm's law are used to solve the problem.

Why is the electromotive force 3V in this question, and what does this question mean?

In this problem, when s is disconnected, the circuit is open, and the voltmeter reads the electromotive force e, e = 3 v.

When S is closed, the circuit is turned on, and the voltmeter reads the voltage v at both ends of the resistor r, where v = 2 V.

The voltage drop on the internal resistor of the power supply is u' ，u' = e - u = 1 v。

Internal resistance r = u'/ i = 1 ω。

That's what this question means.

After the switch is disconnected, the current in the circuit is 0, so the voltage on the internal resistance and resistance r of the power supply is 0, so the reading of the voltmeter at this time is the power supply voltage, that is, the power supply electromotive force.

Because after disconnecting the S switch, the resistance below will have no effect, and the meter is the original voltage of the power supply, so the measured volts of electromotive force are how many volts.

a. When the power supply is not connected to the circuit, between the two poles.

The voltage is equal to the electromotive force of the power supply, and the voltage between the two inner poles of the power supply in the closed circuit is the end-of-road capacitance voltage, which is less than the electromotive force of the power supply, so a is wrong;

b. The greater the electromotive force, the greater the ability to convert other forms of energy into electrical energy, so b is correct;

c. When a battery with an electromotive force is connected to the circuit, if the charge of 1c passes through the circuit, the chemical energy is converted into electrical energy, so c is wrong;

d. The unit of electromotive force and the electric potential difference are the same, but they are two different physical quantities, and the electromotive force is not essentially the electric potential difference, so d is wrong;

Therefore, choose B

When the BC external circuit is disconnected, the electromotive force of the power supply is equal to the output voltage of the power supply, and A is wrong. The electromotive force of the power supply is numerically equal to the work done by the electric field force to move the unit positive charge along the closed circuit for one revolution, b pair. Electromotive force is a physical quantity that describes the ability of a power source to convert other forms of energy into electrical energy, c pairs.

The size of the battery is different, the resistance is different, the current is different, the work is different, and D is wrong.

a, c according to Ohm's law of closed circuits: the value of electromotive force is equal to the sum of the internal and external voltages, when the power supply is not connected to the circuit, there is no current in the circuit, the internal voltage of the power supply is zero, and the external voltage is equal to the electromotive force between the two poles of the power supply; When the power supply is connected to the circuit, the circuit has current, the power supply has an internal voltage, and the voltage between the two poles is less than the electromotive force, so A is wrong, C is correct

B. The power supply is a device that converts other forms of energy into electrical energy, and the electromotive force characterizes the size of this conversion ability, so the greater the power supply with greater electromotive force, the greater the ability to convert other forms of energy into electrical energy, so B is correct

d. The electromotive force is determined by the characteristics of the power supply itself, and has nothing to do with the composition of the external circuit

A. According to Ohm's law of closed circuit e=u+ir, it is known that when i=0, u=e, that is, when the power supply is not connected to the external circuit, the voltage between the two poles is equal to the electromotive force of the power supply, so a is wrong

b. The electromotive force characterizes the power supply's ability to convert other forms of energy into electrical energy, and the greater the electromotive force, the greater the power supply's ability to convert other forms of energy into electrical energy, so b is correct

c. When measuring the electromotive force of the power supply with a voltmeter, the voltmeter becomes an external circuit, there is a certain current in the circuit, and there is a voltage in the inner circuit, so the voltage measured by the voltmeter between the two poles of the power supply is less than the electromotive force of the power supply, so c is wrong

d. The electromotive force characterizes the characteristics of the power supply, which is determined by the power supply itself, and has nothing to do with the external circuit

bc is only equal to the electromotive force of the power supply only when the power supply does not constitute a closed loop, and a is wrong; The power supply electromotive force is the property of the power supply itself, which has nothing to do with the external circuit structure, d wrong;

a. The electromotive force is the ability of the reaction power to convert other forms of energy into electrical energy, which is different from the intrinsic meaning of the electric potential difference; Therefore, a error is accommodated;

b. Do more work when moving with the same amount of electromotive force with large electromotive force; It's not that the power supply does more work with a large electromotive force; Therefore berror;

c. The power supply with large electromotive force does more work when moving the same amount of power, but the work is not necessarily fast; Therefore c is wrong;

d. The magnitude of the electromotive force is equal to the work done by the non-electrostatic force to move the positive charge of 1c from the negative electrode to the positive electrode in the power supply; Therefore d is correct;

Therefore, d