Chose the correct statement:

Options

A. Number of collisions per second between free electrons in a wire is a measure of electric resistance of the wire
B. When a finite current passes through a conductor, no heat can be dissipated in it
C. e.m.f can never be electrostatic in nature
D. An Ohmic resistor must obey Ohm.s law

Answer

To solve the question of choosing the correct statement from the given options, we will analyze each statement one by one.

### Step 1: Analyze the First Statement
**Statement 1:** "The number of collisions per second between free electrons in a wire is a measure of electric resistance of the wire."

- **Analysis:** This statement is incorrect. The number of collisions per second between free electrons relates to conductivity and mobility of electrons, but electric resistance is defined by the material's resistivity and geometry, not merely the number of collisions.

**Hint:** Consider the definitions of resistance and conductivity in relation to electron movement.

### Step 2: Analyze the Second Statement
**Statement 2:** "When a finite current passes through a conductor, no heat can be dissipated in it."

- **Analysis:** This statement is also incorrect. According to Joule's law, when a current flows through a conductor, heat is produced due to the resistance of the conductor. The heat produced is given by \( H = I^2 R t \), where \( H \) is the heat, \( I \) is the current, \( R \) is the resistance, and \( t \) is the time. Therefore, if a finite current passes, heat will definitely be dissipated.

**Hint:** Recall Joule's law and how it relates current to heat generation in a conductor.

### Step 3: Analyze the Third Statement
**Statement 3:** "EMF can never be electrostatic in nature."

- **Analysis:** This statement is misleading. Electromotive force (EMF) can arise from electrostatic sources, such as in the case of a battery, which generates EMF due to electrostatic potential differences. Therefore, this statement is incorrect.

**Hint:** Think about the sources of EMF and how they can include electrostatic potentials.

### Step 4: Analyze the Fourth Statement
**Statement 4:** "An ohmic resistor must obey Ohm's law."

- **Analysis:** This statement is correct. An ohmic resistor is defined as a resistor that follows Ohm's law, which states that the current through it is directly proportional to the voltage across it, provided the temperature remains constant.

**Hint:** Review the definition of ohmic and non-ohmic conductors and their relationship with Ohm's law.

### Conclusion
After analyzing all four statements, we find that the fourth statement is the correct one: "An ohmic resistor must obey Ohm's law."

### Final Answer
The correct statement is: **"An ohmic resistor must obey Ohm's law."**