Key relationship
Induced sphere F=−2αk²q²/r⁵; contact guide F=+kq(0.1q)/r²; tanθ=F/(mg).
Physics · Practical apparatus
Pith-ball polarization and contact
A neutral ball is attracted by the nonuniform field through polarization. In the imposed contact state it receives the rod’s sign and repels. Increasing separation rapidly weakens the induced-force guide.
Open the interactive experiment →Pith-ball polarization and contact
நடுநிலைப் பந்து மின்தூண்டலால் ஈர்க்கப்படும். தொடர்புக்குப் பிறகு அதே குறி மின்னூட்டம் பெற்றால் விலகும்.
Key notes, relationships and model limits
Practical tip
A neutral ball is attracted by the nonuniform field through polarization. In the imposed contact state it receives the rod’s sign and repels. Increasing separation rapidly weakens the induced-force guide.
Common trap
An attracted neutral object need not have acquired a net charge.
Model limits
Point-charge field and2 mm dielectric sphere with relative permittivity3. Attraction occurs without net charge transfer; contact state imposes a10% same-sign charge share. Rod is left, right-positive force.
What you will learn
Distinguish induced polarization from charge transfer.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
Before contact net ball charge stays0; force changes sign after the imposed same-sign transfer.
Why this matters
The model connects pith-ball polarization and contact with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
Attraction of a neutral ball before contact proves?
- It already has the same-sign net charge
- Polarization can cause force
- Its mass is zero
Read the explanation
Opposite induced charge nearer the rod feels the stronger field.
Course connection: Virtual apparatus preparation. Check exact board/exam membership in the syllabus sources. Physical practical work remains necessary.