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School Class (No.) ( )

Name Subject AL Practical Physics

Date Mark

D5 Measurement of e/m using deflection tube

Procedure

1. Measure the separation (d) of the two metal plates inside a deflection tube.

Separation (d) = ________0.05_________ m

2. Record the radius (r) and the number of turns (N) of Helmholtz coils.

Radius (r) = ___________0.07_________ m

Number of turns (N) = ______320 turns_______

8. Adjust the voltage of the EHT supply and the current through the Helmholtz coils until the

cathode ray is horizontal. It means the null deflection has been obtained. Record the

voltage (V) across the metal plates and the current (IB) through the Helmholtz coils.

Voltage (V) = _________3.5 ___________ kV

Current (IB) = __________0.5__________ A

Results and Discussion

1. From the results in step 2 and IB in step 8, calculate the magnetic flux density of the

magnetic field produced by the Helmholtz coils by using the equation B = ,

where N is the number of turns of each coil, r is the radius of the coil and o is the

permeability of free space (4 107 T m A1).

Magnetic flux density B =

= = 20.68 x 104 T

New Way Physics for Advanced Level © Manhattan Press 2005

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2. From the results in steps 1 and 8, calculate the specific charge (e/m) of an electron by

using Equation (3). Compare the experimental result of e/m with the standard e/m value

(1.76 1011 C kg1).

Specific charge = = 1.64 x 1011 C kg1

This experimental value is about 7 % smaller than standard value, thus the result is

acceptable.

3. In step 7, what should you do if the cathode ray is curved upwards further?

If the cathode ray is curved upwards further, reverse the connections to the coils and/or

the

low voltage power supply.

4. State the sources of error and suggest improvements for this experiment.

The major source of error is due to the non-uniform magnetic and electric fields making

the electron bean diverged when approaching the end region.

5. Give a conclusion to this experiment.

By employing the null deflection method, the specific charge (e/m) of an electron can be

measured.

New Way Physics for Advanced Level © Manhattan Press 2005