Edexcel IGCSE Physics · Spec 2.22P-2.23P
Conductors, Insulators and Charging by Friction (Paper 2)
Covers classify common materials, Investigate charging by friction and Control and interpret the test.
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Conductors, Insulators and Charging by Friction (Paper 2)
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Explained
Conductors, insulators, and testing charging properly
A conductor lets charge move through it. An insulator does not. That single difference decides which materials can be charged by rubbing and which cannot.
Why metals conduct
In a metal, some electrons are delocalised and free to move through the whole structure. Apply a potential difference and they drift, which is a current.
Copper, aluminium, steel and graphite conduct. Graphite is the odd one out, being a non metal, and it conducts because of the spare delocalised electron on each carbon atom.
Why insulators do not
In plastics, rubber, glass and dry air, all the electrons are held in bonds and none are free to move. Charge placed on an insulator therefore stays where it is put, which is exactly why static builds up on them.
The word dry matters for air. Damp air conducts a little, which is why static effects are much weaker on a humid day, and that is a legitimate answer when a test gives a poor result.
Charging by friction
Rub two insulating materials together and electrons are transferred from one to the other. The material that gains electrons becomes negatively charged; the one that loses them is left positive.
Only electrons move. Protons are held in nuclei and do not transfer between materials, so an object never becomes positive by gaining positive charge.
The two charges are equal in size and opposite in sign, because every electron that arrived on one object left the other.
Investigating it fairly
A typical test rubs different rods with the same cloth and measures how strongly each is then charged, by seeing how many small pieces of paper it picks up or by using a leaf electroscope.
To make it fair, keep the number of rubs the same, the pressure roughly the same, and the same cloth throughout. Use rods of the same size, and hold them by an insulating handle so charge does not flow away through your hand.
Then control the room. Dry conditions give a much larger effect, so the humidity should be the same for every material tested, and results taken on different days are not comparable.
Repeat each material and take a mean, and discharge each rod between trials by earthing it, otherwise charge left over from the previous test adds to the next.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme is explicit that only electrons are transferred in electrostatics, and it records a response that referred to the transfer of positive charge as scoring zero.
The second mark in that question is separate. The report notes that fewer candidates gave a suitable reason for the transfer by referencing the force of friction or the rubbing, and that a response mentioning electrons with no reason scored one mark rather than two.
On the conductor and insulator side, a mark scheme asks why a plastic covering is used and credits that plastic is an electrical insulator, allowing the idea that it does not conduct electricity. Its second mark is for saying it prevents an electric shock, and the note instructs the examiner to ignore references to current transferring to the surroundings.
The examiner report on that paper adds that some candidates discussed thermal rather than electrical insulation. Plastic happens to be both, so the word electrical is worth writing.
Interpreting the results
A rod that picks up more paper carries more charge. A rod that picks up none was either not charged or has lost its charge, and if it is a metal held in your hand the second is the reason: the charge flowed straight through you to earth.
That result is not a failed experiment. It is the clearest demonstration in the whole topic of what a conductor does.
Spec 2.22P-2.23P
What you need to know
- Classify common materials
- Investigate charging by friction
- Control and interpret the test
Active recall
Quick check
Answer each question before opening the answer.
Name two common insulators
Any two of plastic, rubber, glass or dry air
Why is repulsion useful evidence?
Only like charges repel; attraction can also involve a neutral object
Why use dry conditions?
Moisture allows charge to leak away
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