What is the electromotive force of the power supply equal to the voltage between the two poles of the power supply when the power supply is open Such as the title Why is it not the voltage between the two poles of the power supply when the circuit is connected?

An open circuit is a circuit state indicating an open circuit
The electromotive force of the power supply is equal to the voltage between the two poles of the power supply when the power supply is open
It means the voltage at both ends of the power supply when the circuit is disconnected
Because the usual method is to directly use a voltmeter to connect the two sections of the power supply
This is because we regard the internal resistance of the voltmeter as infinite, but in fact, the internal resistance of the voltmeter is not infinite
Therefore, there will be very weak current in the circuit, so the voltmeter will also divide the voltage, resulting in the measured value is too small
If an ideal voltmeter is used to measure the voltage, it is equal to the open circuit. It is the voltage at the two poles of the power supply. If the channel voltmeter is short circuited, its reading is zero. Is that clear

In a closed circuit, is the electromotive force always equal to the sum of internal and external voltages?
Proper explanation

I don't know the details
In high school, it can be safely said that electromotive force is equal to the sum of internal and external circuit voltage

Is the short circuit voltage zero?
ha-ha

There are two kinds of short circuit, local short circuit and power short circuit
If there is a local short circuit, the local voltage is 0, because the current is 0. According to Ohm's law, the voltage is 0
The power supply is short circuited. In general, it seems that the voltage is equal to the power supply voltage, and it will not be very large. Because the resistance is very small, the current will be very large. The voltage is certain, but it will not be very large, and the power supply is easy to burn out
To sum up: may be zero, may be equal to other, but must be less than the power supply voltage, mainly in that case of short circuit

What is the electromotive force of the power supply equal to the voltage between the two poles of the power supply when the power supply is open Such as the title Why is it not the voltage between the two poles of the power supply when the circuit is connected?