Forces between charged and uncharged objects
3 marks · NormalA pupil rubs a balloon with a woolen cloth and then holds it next to a wall. The balloon sticks to the wall.
Explain, in terms of charges, why the balloon is able to stick to the wall.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
The balloon becomes charged (likely negative) by gaining electrons from the cloth. When the charged balloon is brought near the wall, it repels electrons in the surface of the wall, leaving that surface slightly positive. The attraction between the opposite charges holds the balloon to the wall.
Common mistakes: Saying both are charged the same way so attract
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Balloon becomes charged by rubbing with the cloth
B2 · 1 mark · Explains that charges in the wall are rearranged (induced charge separation)
B3 · 1 mark · Describes that opposite charges attract, causing the balloon to stick
Worked solution
Rubbing the balloon with the woolen cloth makes it charged (usually negative, because it gains electrons). When the charged balloon is placed next to the wall, electrons in the wall are repelled away from the surface, leaving the surface of the wall slightly positive. The attraction between the positively charged area on the wall and the negatively charged balloon makes the balloon stick.