Q: If the velocity of an electron in a cathode ray tube is $$2.0 \times 10^7$ m/s, the electric field is 1.0$$ \times $$10^4 V/m$$, and the magnetic field is $$0.5 T$$, does the electron travel undeflected? Use $$v = \frac{E}{B}.$$

No, because $$v = \frac{1.0 \times 10^4$}{0.5} = 2.0 \times 10^4$ m/s, which does not match the electron's velocity

Question 1

Which experiment provided the first direct evidence that atoms contain smaller, negatively charged particles?

Accepted Answer

Thomson's cathode ray tube experiment

Question 2

In Thomson’s cathode ray tube experiment, what observation led to the conclusion that cathode rays are negatively charged?

Accepted Answer

The rays were attracted to the positive plate

Question 3

Which of the following best describes the 'plum pudding' model of the atom proposed by J.J. Thomson?

Accepted Answer

Electrons embedded in a sphere of positive charge

Question 4

In the cathode ray tube, what is the effect of applying a magnetic field perpendicular to the path of the cathode rays?

Accepted Answer

The rays are deflected in a direction depending on their charge

Question 5

Thomson determined the charge-to-mass ratio of the electron using the following relationship. Which equation did he use to relate the velocity of the electron to the electric and magnetic fields?

Accepted Answer

$$v = \frac{E}{B}$$

Question 6

If the charge of an electron is $$-1.60 	imes 10$$^{-19}$$ C and the charge-to-mass ratio is -1.76$$ 	imes $$10^{11} C/kg$$, what is the mass of an electron?

Accepted Answer

$$9.11 	imes 10$$^{-31}$$ kg

Question 7

Millikan’s oil drop experiment determined the charge of a single electron by:

Accepted Answer

Balancing the gravitational and electric forces on oil droplets

Question 8

An oil drop in Millikan’s experiment is suspended when the electric force equals the gravitational force. Which equation represents this balance?

Accepted Answer

$$qE = mg$$

Question 9

Suppose an oil drop has a charge of $$-4.8 	imes 10$$^{-19}$$ C. How many electrons does it contain?

Accepted Answer

3

Question 10

Which of the following statements is NOT a conclusion from Thomson’s cathode ray experiments?

Accepted Answer

Atoms are indivisible solid spheres

Question 11

A student claims that increasing the voltage in Millikan’s oil drop experiment will always increase the charge measured on the oil drop. Is this correct?

Accepted Answer

No, because the charge on the drop is quantized and only changes in multiples of the electron charge

Question 12

Which real-world application relies on the principle of electron deflection in electric and magnetic fields, as demonstrated in Thomson’s experiments?

Accepted Answer

Television and computer CRT screens

Question 13

If the velocity of an electron in a cathode ray tube is $$2.0 	imes 10^7$ m/s, the electric field is 1.0$$ 	imes $$10^4 V/m$$, and the magnetic field is $$0.5 T$$, does the electron travel undeflected? Use $$v = \frac{E}{B}.$$

Accepted Answer

No, because $$v = \frac{1.0 	imes 10^4$}{0.5} = 2.0 	imes 10^4$ m/s, which does not match the electron's velocity

Question 14

Which of the following best explains why all metals emit cathode rays in Thomson’s experiments?

Accepted Answer

All metals contain electrons that can be emitted as cathode rays

Question 15

A cathode ray tube is set up with an electric field of $$2.0 	imes 10^4$ V/m and a magnetic field of 0.8$ T. What velocity must the electrons have to travel undeflected?

Accepted Answer

$$2.5 	imes 10^4$ m/s

Question 16

Which of the following is a direct result of Millikan’s oil drop experiment?

Accepted Answer

The determination of the fundamental charge of the electron

Question 17

If an oil drop has a mass of $$6.4 	imes 10$$^{-15}$$ kg and is suspended in an electric field of 1.0$$ 	imes $$10^3 V/m$$, what is the charge on the drop? (Use $$g = 9.8 m/s$^2$$ and $$qE = mg)$$

Accepted Answer

$$6.27 	imes 10$$^{-17}$$ C

Question 18

Which statement best summarizes the significance of the charge-to-mass ratio ($$\frac{e}{m}) $$determined by Thomson?

Accepted Answer

It showed that electrons are much lighter than atoms

Question 19

A scientist observes that the charge on oil drops in Millikan’s experiment always occurs in multiples of $$1.6 	imes 10$$^{-19}$$ C. What does this observation support?

Accepted Answer

Charge is quantized

Question 20

Which of the following is NOT a key contribution of J.J. Thomson?

Accepted Answer

Discovering the neutron

The Discovery of the Electron: Key Experiments and Models

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