PHY058 · Electrostatics · Class 12 + College
Electric field
Objective: To understand the electric field of a point charge as the force per unit positive test charge.
🧰 Apparatus
- Charged sphere
- Small test charge (on a light pith ball)
- Field-line demonstration with semolina in oil
- Scale
📐 Formulas and SI units
E = F / q (N/C)
E = kQ / r² for a point charge
💡 Theory
The electric field at a point is the force on a small positive test charge divided by its charge: E = F/q. Around a point charge E = kQ/r², pointing away from positive and towards negative charges. Field lines show the direction; their spacing shows the strength.
🪜 Procedure
- Place the source charge and a small test charge at a distance r.
- Measure (or read) the force on the test charge.
- Calculate E = F/q.
- Repeat at other distances.
- Observe the radial line pattern with semolina in oil.
📋 Observations Simulated values
✅ Result and conclusion
20 nC gives E ≈ 2000 N/C at 0.3 m, so a 1 µC test charge feels about 0.002 N.
🔬 In a real lab
The test charge must be small so it does not disturb the source; real pith balls also feel gravity and air currents.
⚠️ Safety
High-voltage sources used to show field lines must be operated by the teacher.
❓ Quick check
1. Field lines around a positive charge point…
2. E at 2r compared with r?
3. Unit of electric field?
🏋️ Practice
EasyF = 0.01 N on 2 µC. E?
Answer: 5000 N/C
MediumE = 1000 N/C on 3 µC. F?
Answer: 0.003 N
ChallengeDo field lines cross?
Answer: No