Demonstrating and Visualising Electric Fields

Electric fields are often described in theory, but they can also be made visible in the lab. With a simple setup using castor oil, semolina, and electrodes, students can see how invisible forces act between charges — a striking and memorable demonstration of field lines in physics.

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The Experiment

Equipment:

• 90 mm petri dish

• Castor oil (enough to cover the base)

• A pinch of semolina grains

• Two metal electrodes (pins, rods, or plates)

• High-voltage DC power supply (around 1–5 kV, current-limited for safety). We find that a Wimshurst machine works best as we can turn the handle a few times to see the effect

Method:

1. Pour a thin layer of castor oil into the petri dish — this acts as an insulating medium that allows particles to move freely without conducting electricity directly.

2. Sprinkle a light dusting of semolina grains evenly over the surface.

3. Insert the two electrodes in the oil — start with parallel plates for a uniform field.

4. Apply voltage gently and observe the movement of semolina grains.

   • The grains align themselves along the electric field lines, forming a visual map of the field.

Repeat the demonstration with different configurations:

• Parallel plates: uniform straight lines.

• Point and plate: radial pattern showing divergence from a point charge.

• Two points: curved lines showing attraction or repulsion.

• Parallel wires: more complex patterns with symmetrical curves.

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The Science

The semolina grains become polarised in the electric field. One side becomes slightly positively charged, the other negatively charged, so each grain aligns with the direction of the field. The castor oil slows the motion, allowing the pattern to form clearly and remain stable.

This visualisation shows how electric field lines represent direction and strength:

• Lines are closer together where the field is stronger.

• Lines curve smoothly, never crossing.

• The pattern changes shape with each electrode configuration, matching textbook diagrams almost perfectly.

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Skills Highlight

• Constructing experimental setups safely for electrostatics

• Observing and recording field patterns qualitatively

• Linking visual evidence to theoretical field diagrams

• Understanding polarity, potential difference, and charge interaction

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Why It Works in Teaching

Seeing the invisible is always powerful in physics. The semolina-in-oil demonstration makes electric fields concrete, a vivid link between diagrams on the board and the real behaviour of charges in space. Students grasp the geometry and symmetry of electric fields far more effectively when they can see them form before their eyes.