Ideas for using your EHT power supply

Some pieces of equipment get dusted off once a year, used for one practical (the same one each year) and then packed away again. The EHT power supply is often one such item. Here we have a couple of ideas for getting extra use from yours.

EHT – Extra High Tension (power supply)

• Capable of producing 5000 V DC.

• School EHT supplies contain the option of an internal resistor limiting the number of amps to around 0.1 mA (depending on size of resistor). This will increase to 5 mA without the resistor.

• Shrouded leads are used which provides added protection from shock (no metal exposed).

• EHT supplies tend to be safer than HT supplies as the current output is generally lower.

• Make all connections/disconnections with the unit switched OFF.

• Depending on the age and make of the EHT the input and output terminals may look different. Make sure you are clear which terminals you are using and that the unit is safe before using. Check manufacturer’s instructions for your particular unit.

Remember when doing any experiments using the EHT: safety first, always risk assess the procedure, and trial before demonstrating!

GIANT (BIN BAG) CAPACITOR

This is a good way to introduce capacitors to the class, using foil and plastic sheets.

EQUIPMENT:

• 2 x large pieces of bin bag plastic (insulator)

• 2 x large pieces of tin foil (conductor): these must not be larger than the plastic sheets

• EHT power supply

• Microammeter with centre zero, connecting wires, crocodile clips

METHOD:

1. Lay one sheet of plastic on the table.

2. Lay one sheet of foil on top of this, followed by a second sheet of plastic and then a second sheet of foil.

3. Connect one lead to the bottom sheet of foil using a crocodile clip, and the other end to the microammeter and then to the negative terminal of the EHT supply.

4. Connect a second lead to the top sheet of foil using a crocodile clip, and the other end to the positive terminal of the EHT supply.

5. Before switching on make sure you can reach the EHT without touching the foil sheet.

6. Turn on the EHT and increase the power until you start to hear/see sparks on the foil. Turn the power back down until just before this point and switch off the power supply.

7. Switch on the power supply and watch the microammeter.

8. Switch off the supply, remove the leads from the power supply and connect the leads together to complete the circuit (excluding the EHT). Watch the microammeter again.

9. Once you have shown what happens to the microammeter you can re-connect the EHT and increase the power, showing the sparking effect of the capacitor breaking down, electrons are getting across the insulating layer to the second piece of foil so the capacitor cannot store charge.

10. Once you have finished the demonstration, discharge the capacitor as before, remove the EHT from the circuit and connect the leads together. You can then dismantle the circuit completely.

Ref: Institute of physics: creating a bin bag capacitor

EHT and capacitance:

Capacitors are circuit components capable of storing and then discharging charge, like a backup battery - except they won’t generate charge.

Each wire sticking out of the capacitor attaches to a thin metal sheet, with an insulating, dielectric, material between them. The metal sheets charge up but charge cannot pass through the dielectric and is held (stored) until allowed to discharge. You can do this by disconnecting from the power source and re-joining the circuit wires. The charge will be released back into the circuit, until the capacitor is ‘empty’. It can be recharged by replacing the power source.

ADAPTATION OF MILLIKAN’S OIL DROP EXPERIMENT

This experiment, a version of the (Millikan) oil drop experiment, demonstrates how charged particles will move when they are between 2 charged plates, and that they can be held there, in the generated electric field.

EQUIPMENT:

• EHT power supply (Max 5 kV), appropriate EHT safe (shrouded) wires, and crocodile clips

• Metal plates x 2, Insulating pillars x 4

• Thin aluminium foil, cut into small pieces

• Ensure all health and safety guidelines on using the EHT are followed (see page 30). All equipment should be turned off whilst setting up, take care not to touch the plates once the demonstration has started, and switch off before disconnecting the apparatus at the end.

METHOD:

1. Set up the apparatus as shown in the diagram. The plates should be parallel.

2. Attach the bottom plate to the earth connection and the negative EHT port, and the top plate to the positive EHT connection.

3. Place some foil pieces on the bottom plate.

4. Switch on the EHT supply, turn it up until the foil is held (‘dancing’) between the plates.

5. Switch off and allow the charge time to dissipate (voltage output from the EHT to fall to zero) before disconnecting.

SCIENCE:

This is an example of a capacitor with air as the dielectric. When current flows, it charges up the metal plates (top becoming positive, and the bottom negative). As in a capacitor, the electric field lines between the parallel plates will be straight and uniform.

As the foil sitting on the bottom plate becomes charged, electrons will be repelled and the foil will be attracted upwards to the positive plate, where it will be repelled again (like charges) and pushed back towards the negative plate. This will continue until the circuit is switched off and the electric field has gone.

In the original Millikan experiment, an oil drop was held stationary between the 2 plates, and the force acting on the charge on the oil drop was calculated and, by extension, a calculation of size of the electron could be made, making the Millikan experiment a fundamental experiment in the history of the atom.

Ref: Institute of physics