MILLIKAN’S OIL DROP

EXPERIMENT

Historical Setting

In 19th century starting Rutherford had done his alpha scattering experiments. Structure of atom known.

Nobody know the charge on an electron.

Millikan modified experiments tried by others and did very careful work.

Used nonvolatile oil rather than volatile water.

AIM

To determine the charge of oil drop is an integral multiple of elemental electron charge.

APPARATUS

Design of the Experiment

• Plates can be charged. Charge can be varied. Oil drops fall through hole in top plate.

• Electrons are present in these oil drops

• Microscope used for observing drops.

Procedure

Spray the oil drops into the oil drop box from the spraying hole by means of sprayer.

Ensure the change-over switch in the middle gear, measure the time tg required for falling motion (it looks as if rising motion) of the same oil drop as above mentioned in a certain distance.

Apply about 200V on the parallel polar plates to drive unnecessary oil drops until only several slow moving oil drops remain.

Measure the time t E required for rising motion of the oil drop (it looks as if falling motion) in the same distance.

PRINCIPLE

Free fall of the oil drop with mass m and charge Q

Oil drop falls down under the force of gravity. Then force of gravity will be balanced against air resistance.

mg=f1, f1 is the air resistance that is acting upward

According to Stokes law f1=6∏ηrVg η= Coefficient of viscosity for air r= Radius of oil drop

So, mg= ∏r3ρairg(Volume*Density*g) 6∏ηrVg= ∏r3ρairg

From that we will get radius of oil drop as, r= 3

Motion of the oil drop in Electric field E.

Force due to E= QE

When the oil drop elevates at a uniform speed, the force of the electric field is balanced against gravityand resistance of air . The following relationship can be obtained:f2+mg=QE,f2 = Resistance of the air when oil drop is elevated

So we got,

Since air is not a continuous medium,Applying a correction factor,We get the Total charge of Oil drop

• Coefficient of viscosity for air: 5kg/ms

• Density of oil drop =981kg/m3

• Distance between two parallel polar plates d = 5 X 10-3m.

In this experiment the information that should be provided is,

OBSERVATIONTg(s)

Vg (*10-5) (m/s)

Radius(*10-5) (m)

Tp(s)

Ve(m/s)

ChargeOf drop(*10-17)

(C)

Charge/Charge of e-

22.41 4.46 3.48 7.15 1.39*10-4 4.01 250.97

5.27 37.9 .17 13.36 1.48*10-4 0.80 50.09

26.18 3.81 3.31 2.62 3.81*10-4 8.54 534.02

4.08 40.91 0.20 15.09 1.32*10-4 1.07 67.01

RESULT

Charge carried by an oil drop is an integral multiple of elementary electron charge ie 1.6*10-19 C

Found out the quantised nature of charges

Discussion

Obtained Q/e ratios that are not always integer in this case . This might be due to manually aligning the scales, noting down rising and falling time that may caused the error.

Bigger and smaller oil drop give less accuracy.

Reference

Millikan oil drop experiment Mittal.

http://en.wikipedia.org/wiki/Oil_drop_experiment

http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/millikan.html

Presented by,Suhas K RameshRollno-10094IISER BHOPAL