A student rubbed a plastic rod with a cloth.
The rod became negatively charged and the cloth became positively charged.
i- Explain why the cloth became positively charged.
[ 3 marks]
SOLUTION
When the student rub a plastic rod with a cloth, electrons are removed from the rod and transferred to the cloth due to friction. The cloth now has fewer electrons than protons so it has a positive charge. The plastic rod has more electrons than protons, so the rod becomes negatively charged.
Figure 6 shows the negatively charged rod on a balance.
ii- Explain why the reading on the balance increases.
[3 marks]
SOLUTION
Both rods are like charged (negatively). As a result, when the the upper rod is brought close to the rod placed on balance, they both experiences repulsive force. This repulsive force behaves as an additional downward force on balance.
iii- The balance had a zero error.
The zero error is not important in this experiment.
Give the reason why.
[3 marks]
SOLUTION
In this experiment we are concerned about the change in the reading of mass, not about the exact mass of the rod placed on the balance.
iv- A negatively charged rod is held near an earthed conductor.
Explain why a spark jumps between the negatively charged rod and the earthed conductor.
[3 marks]
SOLUTION
There is a large potential difference between charged rod and earthed conductor due to which the negative charges flows through the air to the earthed conductor.
