When cathode rays are deflected onto the electrode of an electrometer, the instrument becomes?

Step 1: Analyze the question The question asks what charge an electrometer gains when cathode rays strike its electrode.

Step 2: Explain the concept Cathode rays are streams of electrons, which carry a negative charge. J.J. Thomson showed this in his experiments. He deflected the rays with electric and magnetic fields. The rays bent toward the positive plate, which shows they are negatively charged. He also directed the rays into an electrometer, and it registered a negative charge.

Step 3: Solution

• Cathode rays are made of electrons, so they carry negative charge.
• When they fall on the electrode of an electrometer, the negative charge is transferred to it.
• The electrometer therefore becomes negatively charged.

Check the other options:

• A. Bipolar: The rays carry only one type of charge, so this is wrong.
• C. Positively charged: This would need positive particles, such as the canal (anode) rays.
• D. Neutral: This is wrong because charge is being delivered to the electrode.

Step 4: Answer The correct option is B. Negatively charged.

Exam tip: Remember "Cathode rays = Charge is negative (electrons)". Anode (canal) rays are positive. Any question on Thomson's experiment points to electrons, which are negative.