Problem 51 - Master Resource Book in JEE Main Physics

Question

Electric conduction takes place in a discharge tube due to movement of (a) positive ions (b) negative ions (c) electrons (d) photons

Step-by-Step Solution

Answer

Electric conduction in a discharge tube is due to both positive ions and electrons.

Step 1: Understanding the Discharge Tube

In a discharge tube, a gas is ionized by a high voltage. This ionization results in free electrons and positive ions inside the tube due to the gas atoms losing electrons.

Step 2: Role of Electrons

The free electrons move towards the anode (positive electrode) because they have a negative charge. This movement constitutes one part of the electric conduction in the tube.

Step 3: Role of Positive Ions

Positive ions, formed due to the ionization, move towards the cathode (negative electrode). Their movement also contributes to the electric conduction.

Step 4: Understanding Electric Conduction

Electric conduction in the discharge tube is primarily due to the movement of charged particles, which are electrons and positive ions. Photons do not carry charge and negative ions are not the main carriers in this context.

Step 5: Identifying the Answer

Considering the contributors to electric conduction in the discharge tube, the correct answers are electrons and positive ions.

Key Concepts

Discharge Tube

A discharge tube is a fascinating device used to study electric conduction in gases. It is a sealed glass tube filled with gas at low pressure. When a high voltage is applied across electrodes at each end, the gas inside becomes ionized. This ionization process occurs because the high voltage provides enough energy to free electrons from their atomic bonds. As electrons escape, they create charged particles known as ions within the gas. By studying a discharge tube, we can observe and learn about the behavior of these ions and electrons, as well as how electric conduction happens in gases.

Ionization

Ionization is a key process that occurs inside a discharge tube when a gas becomes electrically conductive. This happens when a high voltage is applied to the gas, providing enough energy to knock electrons out of their orbits around atoms. This process transforms neutral atoms into ions, which are atoms with an electric charge.

• Neutral atoms lose electrons and become positively charged ions.
• The freed electrons gain enough energy to move freely in the gas.

Ionization is fundamental to the operation of a discharge tube and initiates the electric conduction by creating a mixture of charged particles within the gas.

Charged Particles

In the context of a discharge tube, the term 'charged particles' refers to both electrons and ions generated through ionization. These particles are crucial for electric conduction in the tube. Their movement under the influence of an electric field is what allows current to pass through the gas.

• Electrons possess a negative charge and move toward the anode.
• Positive ions move in the opposite direction toward the cathode.

The simultaneous movement of these charged particles forms the basis of electric conduction in a discharge tube, making it possible for the current to flow.

Electrons

Electrons play an essential role in the process of electric conduction within a discharge tube. They are negatively charged subatomic particles that, upon being freed from atoms, can move throughout the gas. Because electrons are negatively charged, they are attracted to the anode, or positive electrode, in the tube. This attraction causes electrons to move toward the anode, creating a substantial part of the electric current. Their journey across the tube is not just a random process; it's driven by the electric field generated by the applied high voltage. Electrons contribute significantly to electric conduction due to their small mass and ability to move quickly when energized.

Positive Ions

Positive ions are the other half of the equation in the electric conduction of a discharge tube. When gas atoms lose electrons during ionization, they become positively charged ions. These ions, having a positive charge, are naturally attracted to the cathode, or negative electrode. As they move towards the cathode, they help carry current through the gas, complementing the movement of electrons. It's important to note that while electrons and positive ions both contribute to electric conduction, their movement is in opposite directions due to their opposite charges. This synchronized movement of electrons to the anode and positive ions to the cathode is what sustains the electric conduction in a discharge tube.