Practica de fisica

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ELECTRIC FIELDS

Demonstration
This shows the shape of electric fields, in much the same way that magnetic fields are demonstrated with iron filings.
Apparatus and materials

• Power supply, EHT, 0-5 kV
• Electric fields apparatus
• Semolina
• Castor oil

Technical notes
A Van de Graaff generator can be used in place of the EHT power supply. If both these supplies areused in turn, students will see that electrostatic charges make the same field patterns as the charges provided by a power supply which can drive currents.

The ‘electric fields apparatus’ consists of two electrodes mounted in a glass dish – see the illustration. The electrodes can be made from aluminium sheet or can be purchased complete with dish. Apart from the care which needs to be takenwith the insulation, this unit is readily improvised.
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Safety
Do not be tempted to try an HT (0 – 350 V) power supply. The current which can be delivered by such a unit is too high to be used with bare electrodes.
Read our standard health & safety guidance
Procedure
a Fill the electrode unit with a layer of castor oil to a depth of about 0.5 cm. Sprinkle a thin layer of semolina overthe surface. (A thin piece of glass tubing drawn out to give a fine pointed stirrer is helpful so that the semolina is evenly distributed.) It is better to start with too little semolina than to start with too much. You can always increase the quantity later.

b Place the electrodes in the castor oil. Connect the positive and negative terminals of the EHT power supply to the electrodes. Adjustthe supply to give 3,000 to 4,000 volts. When the voltage is switched on, the field lines will be clearly visible.

c Try electrodes of different shapes. For example, one can be a 'point' electrode whilst the other is a plate, or two point electrodes can be used. A wire circular electrode with a point electrode at the centre will show a radial field. The field with two plates quite closetogether should also be shown.
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Teaching notes
1 Just as scientists talk of a magnetic field in the space around a magnet, they talk of electric fields in the space around an electric charge. The grains of semolina behave like electric compass needles (electric dipoles), and line up to show the direction of the electric field.

2 There is an electric field spreading out from any electriccharge, ready to grip on any other charge and exert a force on it. This is similar to the Earth’s readiness to grip another mass such as the Moon, or a student, or a mug on the edge of a table, with a gravitational force. However, the force that an electric field exerts is not there until there is a charge for the field to push or pull on. You could say ‘charged’ just means ‘ready to make forces’ inthe same way as soldiers say that a ‘gun is charged with explosive ready to make forces on a bullet’.

3 The illustrations show some electric field patterns which can be modelled in this demonstration.

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CAMPO ELECTRICO

Demostración

Esto muestra la forma de campos eléctricos, en gran parte de la misma manera que los campos magnéticos se demuestran con limaduras de hierro.Aparatos y materiales

* Fuente de alimentación, EHT, 0-5 kV
* Campo eléctrico aparatos
* Sémola
* Aceite de ricino

Notas técnicas

Un generador Van de Graaff se puede utilizar en lugar de la fuente de alimentación EHT. Si estas dos fuentes se utilizan a su vez, los estudiantes verán que las cargas electrostáticas que los patrones mismo campo que los gravámenes contempladospor una fuente de alimentación que pueden conducir corrientes.

«Aparatos eléctricos campos 'El consta de dos electrodos montados en una placa de vidrio - véase la ilustración. Los electrodos pueden ser en chapa de aluminio o se puede comprar completo con plato. Aparte de los cuidados que hay que tener con el aislamiento, esta unidad es fácil de improvisar.
Campo eléctrico

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