Fig. 1 shows a physics club demonstration using a thin aluminium bead suspended by an insulating thread. A negatively charged acrylic sheet is moved slowly ...
Question 1 Report
Fig. 1 shows a physics club demonstration using a thin aluminium bead suspended by an insulating thread. A negatively charged acrylic sheet is moved slowly towards the bead. At first the bead swings towards the sheet. It is then allowed to touch the sheet and swings away. The bead has a very small mass, so the electrostatic force causes a clear movement.
Fig. 1
(a) State the overall charge on the aluminium bead before the sheet is brought close to it. [1]
(b) Describe the movement of electrons in the bead while the negative sheet is close but has not touched it. [2]
(c) Explain why the initially neutral bead moves towards the sheet. [2]
(d) Explain why the bead moves away after touching the sheet. [2]
Answer Details
(a) Before the sheet approaches, the aluminium bead is neutral, with no overall charge. [1]
(b) Electrons in the bead are repelled by the negatively charged sheet. They move to the side of the bead furthest from the sheet. [2]
(c) The near side of the bead is left with a shortage of electrons, so it becomes positive. Opposite charges attract, producing a force that moves the bead towards the negative sheet. [2]
(d) When the bead touches the sheet, electrons transfer from the sheet onto the bead. The bead and sheet are then both negatively charged, so they repel and the bead moves away. [2]
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