Topic 1 of 13 • Chapter: Electron, Photon, Photoelectric Effect and X-rays • Physics

Electric Discharge Through Gases – Complete Notes, Revision, Important Questions & Downloads

Electric Discharge Through Gases is the entry point to the chapter because it tracks how a discharge tube changes as pressure falls from normal pressure to 10 mm Hg, 4 mm Hg, 1.65 mm Hg, 0.8 mm Hg, 0.05 mm Hg, and finally about 10^-2 mm Hg. The exact textbook subtopic here is Discharge Phenomena at Different Pressures, so NEET uses this page to test whether you can connect each named region such as dark discharge, negative glow, Faraday's dark space, Crooke's dark space, cathode glow, and striations to the correct pressure window. The topic matters because cathode rays appear only after the pressure becomes low enough for the mean free path to grow, which is the physical bridge to the later electron-discovery topics in the same chapter. A fast exam check is to remember that dark discharge appears near 4 mm Hg, negative glow is named at 1.65 mm Hg, and cathode rays become evident near 10^-2 mm Hg.

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1 SubtopicTheoryMedium Difficulty

Expected QuestionsQ

0-1

Usually tested as a conceptual setup question or as the background condition needed before a cathode-ray or positive-ray question is solved correctly.

Time Required⏱

35-45 min

Enough for one pass through the pressure sequence and one recall drill in which you reproduce the named glow or dark-space changes in order.

Difficulty⚡

Medium

The facts are short, but students lose marks by swapping the pressure values or by treating all glow regions as one undifferentiated discharge stage.

NRI USA Curriculum GapUS

Medium

US high-school physics usually treats gas discharge historically, but NEET expects exact tube-region labels at fixed low pressures, which is a more memory-linked demand.

1Subtopics

5Practice Questions

4Free Downloads

35-45 minPrep Time

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NEET Weightage - Electric Discharge Through Gases

Electron, Photon, Photoelectric Effect and X-rays (Chapter 25)

NEET Year	Questions from this Topic	Bar	Marks
2024	0	0 Q	0
2023	0	0 Q	0
2022	0	0 Q	0
2021	0	0 Q	0
2020	1	1 Q	4
2019	0	0 Q	0
Recent Exam Trend	0-1		0-4

Electric Discharge Through Gases is mostly a recognition topic: NEET asks which visible region belongs to which pressure, or why cathode rays do not appear at ordinary pressure.
The topic is usually indirect because it supports Cathode Rays, Positive Rays or Canal Rays, and J.J. Thomson's Experiment more often than it appears as a standalone long question.
The safest scoring habit is to learn the discharge sequence as a pressure ladder rather than as isolated facts, because the tube regions evolve in order as pressure is reduced.

📊

0-1

Avg Questions / Year

🎯

0-4

Total Marks (6 yrs)

📈

Indirect

Pattern

⚠️

Medium

Difficulty

Exam Strategy for Electric Discharge Through Gases

Memorise the pressure ladder before the named regions Write the sequence normal pressure -> 10 mm Hg -> 4 mm Hg -> 1.65 mm Hg -> 0.8 mm Hg -> 0.05 mm Hg -> 10^-2 mm Hg. Then attach dark discharge, negative glow, Faraday's dark space, Crooke's dark space, cathode glow, striations, and cathode rays to the correct step.

Check whether the question is asking for appearance, not formula Most stems in Electric Discharge Through Gases are identification questions. Before solving anything, decide whether the asked quantity is a named glow region, the onset of cathode rays, or the reason mean free path has increased.

Do one trap check on pressure direction Students often reverse the process and assume that more glow always means higher pressure. In this topic, the important changes happen because pressure is reduced and electrons gain longer mean free paths between collisions.

Link this topic to the next electron topics After revising the discharge sequence once, immediately connect the 10^-2 mm Hg stage to Cathode Rays. That makes the chapter flow logical instead of leaving this topic as a list of disconnected tube colors.

Download Study Notes - Electric Discharge Through Gases

PDF · Cheat Sheet · MCQ Set · PYQ

📘

Electric Discharge Through Gases - Full Notes

Compact notes on the low-pressure discharge sequence, with each named glow or dark space placed against the correct pressure and one recall check at the end.

1 subtopicPressure sequenceNEET focus

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📗

Electric Discharge Through Gases - Formula Sheet

Quick sheet for the pressure markers 10 mm Hg, 4 mm Hg, 1.65 mm Hg, 0.8 mm Hg, 0.05 mm Hg, and 10^-2 mm Hg, with the matching discharge features.

1 pagePressure map

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📙

Electric Discharge Through Gases - MCQ Practice

Five topic-specific MCQs that force you to match pressure reduction with the correct visual region or the appearance of cathode rays.

5 MCQsDetailed solutions

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📕

Electric Discharge Through Gases - Previous Year Questions

Previous-year-style practice on discharge tubes, pressure effects, and the conditions under which cathode rays become visible in the tube.

PYQ-styleAnswer key included

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Subtopics in Electric Discharge Through Gases

2-Column Table

Column A	Column B
Discharge Phenomena at Different Pressures↗

Rapid Revision - Electric Discharge Through Gases

Concept → Trap → Example

1) Discharge Phenomena at Different Pressures

Pressure ladder

As the pressure in a discharge tube is reduced, the visible pattern changes in order: dark discharge near 4 mm Hg, negative glow and Faraday's dark space near 1.65 mm Hg, Crooke's dark space and cathode glow near 0.8 mm Hg, striations near 0.05 mm Hg, and cathode rays near 10^-2 mm Hg.

Use this subtopic when the stem asks which glow or dark region appears after the pressure is lowered to a named value in the discharge tube.
The physical condition to check is low pressure, because longer mean free path lets electrons gain enough kinetic energy between collisions to change the tube appearance.
Trap: many students remember the names but swap Faraday's dark space and Crooke's dark space, or they place cathode rays too early before the pressure has reached about 10^-2 mm Hg.

Example (NEET-style)Example: if the pressure is reduced to 1.65 mm Hg, the tube shows a sky-coloured negative glow at the cathode, the positive column shrinks toward the anode, and the dark region between them is Faraday's dark space.

US Curriculum Gaps - Electric Discharge Through Gases

NRI students often know the historical idea of discharge tubes, but NEET asks for exact low-pressure staging and named tube regions.

AP Physics courses usually do not require exact glow-region recall

A US classroom may describe gas discharge qualitatively, but NEET expects you to map 4 mm Hg, 1.65 mm Hg, 0.8 mm Hg, 0.05 mm Hg, and 10^-2 mm Hg to the correct visible feature without hesitation.

Build a one-line pressure ladder and revise it aloud until the sequence becomes automatic.
Keep Faraday's dark space and Crooke's dark space on separate flashcards so their positions do not merge.
Revise cathode-ray appearance as the final low-pressure stage, not as a generic property of all gas discharge.

NEET treats this topic as the foundation for later atomic discoveries

The exam expects you to see why reduced pressure enables cathode rays and then connect that condition to electron discovery, canal rays, and mass-spectrograph experiments in the same chapter.

After this page, immediately revise Cathode Rays so the chapter sequence feels causal rather than historical only.
When a question mentions low-pressure discharge tube, ask first whether the answer belongs to glow regions or to the charged particles that appear later.
Use NCERT-style wording rather than broader popular-science descriptions when memorising this topic.

NEET-style Practice Questions - Electric Discharge Through Gases

2 NEET-style application questions

1In a discharge tube, the pressure has been lowered to 1.65 mm of Hg. Which observation matches the textbook sequence most closely?Pressure identification

Only zig-zag red spark is seen from one electrode to the other

Sky-colour negative glow appears near the cathode and the dark region between it and the positive column is Faraday's dark space

Crooke's dark space appears and cathode rays already strike the opposite wall directly

No discharge at all is observed in the tube

Option 2 is correct because the local OCR page states that at 1.65 mm of Hg, sky colour light is produced at the cathode, called negative glow, the positive column shrinks toward the anode, and the dark region between them is Faraday's dark space. Option 1 belongs to a much higher pressure near 10 mm of Hg, where a zig-zag spark and cracking sound are observed. Option 3 mixes the 0.8 mm of Hg stage, where Crooke's dark space and cathode glow are named, with the later 10^-2 mm of Hg stage, where cathode rays become evident. Option 4 belongs to normal pressure or the extremely low pressure limit near 10^-4 mm of Hg, not to 1.65 mm of Hg.

2Why do cathode rays become visible only when the pressure inside the discharge tube is reduced to nearly 10^-2 mm of Hg?Physical reason

Because gas molecules become heavier at low pressure

Because the mean free path increases, so electrons gain larger kinetic energy between collisions and can emerge effectively from the cathode side

Because electric field disappears and leaves only magnetic field

Because the anode becomes positively charged only at that exact pressure

Option 2 is correct. The discharge sequence is controlled by low pressure because fewer gas particles are present, so the mean free path of electrons increases. Those electrons can then gain enough kinetic energy between collisions to produce the later-stage effects culminating in cathode rays near 10^-2 mm of Hg. Option 1 is false because molecular mass does not change with pressure. Option 3 invents a field change that the textbook never states. Option 4 mistakes a fixed feature of the electrode setup for the actual variable, which is pressure and therefore collision frequency inside the tube.

Practice Problems - Electric Discharge Through Gases

Click "Reveal Answer" after attempting

1A discharge tube at 4 mm of Hg shows illumination at the electrodes while the rest of the tube appears dark. What is this stage called?

Negative glow

Dark discharge

Striations

Cathode glow

👁 Reveal Answer

Option 2 is correct. The OCR page states that at 4 mm of Hg, illumination is observed at the electrodes and the rest of the tube appears dark, and this is called dark discharge. Negative glow belongs to 1.65 mm of Hg, striations belong to 0.05 mm of Hg, and cathode glow belongs to 0.8 mm of Hg.

2At which pressure does the positive column split into dark and bright discs of light called striations?

10 mm of Hg

1.65 mm of Hg

0.8 mm of Hg

0.05 mm of Hg

👁 Reveal Answer

Option 4 is correct. The local text says that at a pressure of 0.05 mm of Hg, the positive column splits into dark and bright discs of light called striations. The other options correspond to red spark, negative glow with Faraday's dark space, and Crooke's dark space with cathode glow, respectively.

3If a question states that the dark space is between the cathode and the negative glow, which named region is being referred to?

Faraday's dark space

Crooke's dark space

Positive column

Dark discharge

👁 Reveal Answer

Option 2 is correct. Crooke's dark space is the dark space created between the cathode and the negative glow at 0.8 mm of Hg. Faraday's dark space lies between the positive column and the negative glow, so confusing those two names is the most common trap in this topic.

4Which observation is associated with the pressure level near 10^-2 mm of Hg?

The whole tube is filled with bright positive column only

A zig-zag thin red spark runs from one electrode to the other

Invisible particles from the cathode strike the opposite glass wall and make it glow

No discharge is possible at all

👁 Reveal Answer

Option 3 is correct. At about 10^-2 mm of Hg, invisible particles move from the cathode and on striking the opposite wall of the glass tube make it glow; these are called cathode rays. Option 2 belongs to around 10 mm of Hg, option 1 is an earlier lower-than-4 mm stage, and option 4 belongs to the still lower limit near 10^-4 mm of Hg.

5Why is this topic usually revised immediately before Cathode Rays?

Because the formulas are identical

Because the discharge sequence explains the low-pressure condition under which cathode rays appear

Because both topics are asked only numerically

Because both topics belong to thermodynamics

👁 Reveal Answer

Option 2 is correct. Electric Discharge Through Gases supplies the physical tube conditions, especially the role of reducing pressure and increasing mean free path, that make the later Cathode Rays topic intelligible. The connection is causal, not formula-based, and that is why NEET often treats them as consecutive conceptual steps in the same chapter.

Physics - Electric Discharge Through Gases Revision Checklist

Check off chapters as you revise

Use this section for quick chapter tracking before mocks, part tests, and final NEET revision.

Discharge Phenomena at Different Pressures: write the full pressure ladderReproduce from memory the sequence normal pressure, 10 mm Hg, 4 mm Hg, 1.65 mm Hg, 0.8 mm Hg, 0.05 mm Hg, and 10^-2 mm Hg without looking at the book.

Discharge Phenomena at Different Pressures: separate Faraday's dark space from Crooke's dark spaceWrite one line for each dark region and state which one lies between positive column and negative glow, and which one lies between cathode and negative glow.

Discharge Phenomena at Different Pressures: mark the first appearance of negative glowState the pressure 1.65 mm of Hg and attach the phrase sky-colour light at cathode so that the label is not memorised in isolation.

Discharge Phenomena at Different Pressures: mark the first appearance of striationsRevise that striations appear at 0.05 mm of Hg and describe them as dark and bright discs of light in the positive column.

Discharge Phenomena at Different Pressures: link the final stage to cathode raysState why the tube wall glows at about 10^-2 mm of Hg and connect that condition directly to the next topic on cathode rays.

FAQs - Electric Discharge Through Gases

Notes · Downloads · Revision · Important Questions

Why are gases poor conductors at normal atmospheric pressure in this topic?

At ordinary pressure the gas molecules are close enough to produce frequent collisions, so free charge carriers do not gain enough energy between collisions to sustain the discharge pattern described in the tube. This is why the chapter begins by lowering pressure before any structured glow sequence is observed.

What is the easiest way to remember dark discharge?

Attach it to the sentence '4 mm of Hg gives illumination at electrodes but the rest of the tube is dark.' That one sentence captures both the pressure and the appearance, which is better than memorising the label alone.

How do I distinguish Faraday's dark space from Crooke's dark space in NEET questions?

Use position, not only name. Faraday's dark space lies between positive column and negative glow at 1.65 mm of Hg, while Crooke's dark space lies between cathode and negative glow at 0.8 mm of Hg. The pressure and the location must be recalled together.

Why does the colour of the positive column depend on the gas in the tube?

The OCR page explicitly notes that the whole tube is filled with bright light called positive column and that the colour depends on the nature of the gas. Different gases emit different characteristic glow colours when excited under low-pressure discharge conditions, so the observed colour is not universal.

At what stage do cathode rays actually become evident?

The local page places their appearance near 0.01 or 10^-2 mm of Hg, when invisible particles move from the cathode and make the opposite glass wall glow. If you place cathode rays earlier, you are skipping the ordered pressure stages that the textbook builds first.

Why is this topic usually called indirect in NEET preparation?

Because the facts here often act as the background for later chapter questions on cathode rays, positive rays, and electron experiments. NEET may ask a direct identification question, but more often it expects you to know these discharge conditions while solving the subsequent particle-based topics.

Does the tube keep glowing more and more as pressure keeps decreasing forever?

No. The sequence changes with decreasing pressure, but the OCR page finally states that when pressure drops to nearly 10^-4 mm of Hg, there is no discharge in the tube. So the topic is not monotonic glow increase; it is a staged change followed by disappearance of discharge at extremely low pressure.

What is the fastest last-minute revision method for this topic?

Revise it as a pressure table. Write the pressure in one column and the visible effect in the next column: red spark, dark discharge, negative glow and Faraday's dark space, Crooke's dark space and cathode glow, striations, and cathode rays. That table form matches the way NEET tests the topic.

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Electric Discharge Through Gases

Electric Discharge Through Gases – Complete Notes, Revision, Important Questions & Downloads

1) Discharge Phenomena at Different Pressures

AP Physics courses usually do not require exact glow-region recall

NEET treats this topic as the foundation for later atomic discoveries

Physics - Electric Discharge Through Gases Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

Discharge Phenomena at Different Pressures

Subtopics

Discharge Phenomena at Different Pressures

NEET > Physics > Dual Nature of Matter and Radiation Chapters

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Electric Discharge Through Gases

Overview content

Electric Discharge Through Gases – Complete Notes, Revision, Important Questions & Downloads

1) Discharge Phenomena at Different Pressures

AP Physics courses usually do not require exact glow-region recall

NEET treats this topic as the foundation for later atomic discoveries

Physics - Electric Discharge Through Gases Revision Checklist

NEET NRI Counseling & Admission eBook Download

Schedule Trial Session For NEET Prep

Discharge Phenomena at Different Pressures

Subtopics

Discharge Phenomena at Different Pressures

NEET > Physics > Dual Nature of Matter and Radiation Chapters

Comments

Leave a comment

Comments (0)