What is internal resistance and ideal voltmeter?

Hint: Recall the resistance which is present within the battery that resists the current flow when connected to a circuit. Ideal voltmeter does not influence the circuit while measuring voltage in a circuit.

Complete step by step answer:
Internal Resistance is the resistance which is present within the battery that resists the current flow when connected to a circuit. Thus, it causes a voltage drop when current flows through it. It is the resistance provided by the electrolyte and electrodes which is present in a cell.

So Internal resistance is offered by the electrodes and electrolyte which oppose the current flow inside the cell.

The ideal voltmeter is a theoretical concept of a voltmeter that does not influence the circuit, because the current to the ideal voltmeter is zero. According to Ohm's law the internal impedance of the ideal voltmeter needs to be infinite.

In the Physics laboratory, there is no such thing as the ideal voltmeter, but students should know the concept and can make calculations how a given voltmeter will influence their circuit. So that students can know that about the major error, by not measuring with an ideal voltmeter.

Additional Information:
The voltage dropped due to internal resistance in a circuit is given by,

V = (V+) +( V-) – Ir

We know that,

ε = (V+) + (V-). = I (R + r)
ε = IR + Ir
ε = V + Ir
So, V = ε – Ir

Where V is the potential difference across the circuit, ε is the emf, I is the current flowing through the circuit, r is internal resistance.
Usually internal resistance of a cell is not considered because ε >> Ir. The value of internal resistance changes from cell to cell.

Note: Students should understand these both terms i.e., internal resistance and ideal voltmeter. Students should keep in mind that an ideal voltmeter is a completely theoretical concept and an ideal voltmeter has infinite impedance and no capacitance that makes perfect readings of voltage. The internal resistance of an ideal voltmeter is infinity which is practically not possible.