Van de Graaff GeneratorVan de Graaff Generator

GoalsGoals

Learn how a Van de Graaff generator Learn how a Van de Graaff generator works and what it is used forworks and what it is used for

Explain demonstration/experiments Explain demonstration/experiments that involve the Van de Graaff that involve the Van de Graaff generatorgenerator

What is it?What is it?

The Van de Graaff generator is a The Van de Graaff generator is a device that separates electric device that separates electric charges mechanically.charges mechanically.

This produces a high voltage with a This produces a high voltage with a low current.low current.

Low current means its safe to learn Low current means its safe to learn with!with!

How much voltage?How much voltage?

VoltageVoltage is… is…is commonly used as a short is commonly used as a short name for name for electrical potential differenceelectrical potential difference

Which means Which means is a measurement of the is a measurement of the energy contained within an electric field, energy contained within an electric field, or an electric circuit, at a given point or an electric circuit, at a given point

This VdG generator can produce anywhere This VdG generator can produce anywhere from 10-200 thousand volts (V).from 10-200 thousand volts (V).

It is safe because the current is quite low.It is safe because the current is quite low.

What does it look like What does it look like inside?inside?

Base (Electric motor)

Insulating tube

Metal Sphere

Conveyor belt (rubber)

Metal Combs

Electrons jump onto the belt at the bottom

Electrons jump off the belt at the top

How does it work?How does it work?

The VdG generator is a charge separator.The VdG generator is a charge separator. When turned on, the belt begins to move, When turned on, the belt begins to move,

picking up negative charges deposited by picking up negative charges deposited by the metal comb at the base.the metal comb at the base.

The belt then transfers the charges to the The belt then transfers the charges to the metal comb at the top of the generator.metal comb at the top of the generator.

The metal comb is attached to the metal The metal comb is attached to the metal sphere and thus transfers the negative sphere and thus transfers the negative charges to the sphere.charges to the sphere.

How is the VdG like the How is the VdG like the cashier’s check out at the cashier’s check out at the

grocery store?grocery store?

Think of the groceries as charges.

Base (Electric motor)

Insulating tube

Metal Sphere

Discharging The VDG Discharging The VDG

By completing the circuit, we By completing the circuit, we neutralize the static charges!neutralize the static charges!

Touch the wand to the VDGTouch the wand to the VDG

SafetySafety

1.1. Do not allow students to use the Van de Graaff generator Do not allow students to use the Van de Graaff generator unsupervised. unsupervised.

2.2. People with cardiac pacemakers should never operate the People with cardiac pacemakers should never operate the generator or come in contact with it. generator or come in contact with it.

3.3. Still, we are dealing with high voltage here. Stay about Still, we are dealing with high voltage here. Stay about three feet away from the collector while it is charged, or three feet away from the collector while it is charged, or unless instructed otherwise.unless instructed otherwise.

4.4. Always discharge the collector dome between Always discharge the collector dome between experiments and when you are finished. experiments and when you are finished.

5.5. The motor produces a lot of heat that could damage the The motor produces a lot of heat that could damage the belt or the motor itself. Do not run the generator belt or the motor itself. Do not run the generator continuously for long periods of time. continuously for long periods of time.

6.6. Leave the upper and lower combs alone. Leave the upper and lower combs alone. 7.7. Keep the entire device clean and dry. Keep the entire device clean and dry. 8.8. Handle the aluminum parts with care. Handle the aluminum parts with care.

What is it used for?What is it used for?

Used during early research into Used during early research into radioactivity and atomic structure, used as radioactivity and atomic structure, used as power source for particle accelerators.power source for particle accelerators.

Still used to power high energy x-ray Still used to power high energy x-ray machinesmachines

Useful in laboratory study of Useful in laboratory study of electrostaticselectrostatics

VdG DemosVdG Demos

1.1. Mop HeadMop Head

2.2. Sparks!Sparks!

3.3. Flying PieFlying Pie

4.4. Crazy HairCrazy Hair

5.5. Shock LineShock Line

Mop HeadMop Head

The charge The charge staystayss there when the there when the power is turned off – that’s why it’s power is turned off – that’s why it’s called called staticstatic electricity!electricity!

When there is no net charge present, When there is no net charge present, the mop strands fall down - the the mop strands fall down - the charges has been neutralized, or charges has been neutralized, or rather, rather, dischargeddischarged

VdG DemosVdG Demos

1.1. Mop HeadMop Head

2.2. Sparks!Sparks!

3.3. Flying PieFlying Pie

4.4. Crazy HairCrazy Hair

5.5. Shock LineShock Line

Sparks!Sparks!

You can see a brief glimpse of static You can see a brief glimpse of static discharge!discharge!

The light bulb lights up because on The light bulb lights up because on electrons are need to carry electricity electrons are need to carry electricity – – No protons requiredNo protons required

VdG DemosVdG Demos

1.1. Mop HeadMop Head

2.2. Sparks!Sparks!

3.3. Crazy HairCrazy Hair

4.4. Flying SaucersFlying Saucers

5.5. Shock LineShock Line

Electrons Flow out!Electrons Flow out!

Hair stands up because the electrons Hair stands up because the electrons trapped in your hair are repelling each trapped in your hair are repelling each

other!other!

VdG DemosVdG Demos

1.1. Mop HeadMop Head

2.2. Sparks!Sparks!

3.3. Crazy HairCrazy Hair

4.4. Flying SaucersFlying Saucers

5.5. Shock LineShock Line

Flyin’ Pie!Flyin’ Pie!

The pie tins fly away because as they The pie tins fly away because as they gain a negative charge due to gain a negative charge due to transferring electrons – the repel transferring electrons – the repel each other!each other!

Back to law of charges!Back to law of charges!

VdG DemosVdG Demos

1.1. Mop HeadMop Head

2.2. Sparks!Sparks!

3.3. Crazy HairCrazy Hair

4.4. Flying SaucersFlying Saucers

5.5. Tinsel TimeTinsel Time

6.6. Shock LineShock Line

Push/Pull?Push/Pull? When I hold them, the tinsel has a When I hold them, the tinsel has a

positive charge, so it pulls towards positive charge, so it pulls towards the VdG generator.the VdG generator.

When it touches and gains a When it touches and gains a negative charge – it immediately is negative charge – it immediately is repelled by the negative charges!repelled by the negative charges!

VdG DemosVdG Demos

1.1. Mop HeadMop Head

2.2. Sparks!Sparks!

3.3. Crazy HairCrazy Hair

4.4. Flying SaucersFlying Saucers

5.5. Tinsel TimeTinsel Time

6.6. Shock LineShock Line

Shocking!Shocking!

They human body makes an They human body makes an excellent conductor – not because of excellent conductor – not because of water, but because of the water, but because of the electrolytes electrolytes stored in our bodies’ stored in our bodies’ water!(We are about 70 percent water!(We are about 70 percent electrolitic water!)electrolitic water!)

When the discharge occurs, When the discharge occurs, everyone gets discharged!everyone gets discharged!

Questions for Test on Questions for Test on MondayMonday

1.1. Explain in your own words how the VdG Explain in your own words how the VdG generator works.generator works.

2. 2. Be sure you can explain in words and using a Be sure you can explain in words and using a diagram how:diagram how:

a.a. Someone’s hair can stand upSomeone’s hair can stand up

b.b. How a shock can occurHow a shock can occur

c.c. How something can be repelled by the VdG How something can be repelled by the VdG