Edexcel IGCSE Physics 2(d) Electric Charge — Notes
Written by an examiner for Pearson Edexcel International GCSE Physics (4PH1), linear and modular. All of 2(d) is examined only on Paper 2P (and in 4XPH1 Unit 1).
Which parts do I need?
Everything on this page is Physics only. Science Single Award (4SS0) and Double Award (4SD0) candidates can skip the whole page. Physics (4PH1) candidates need all of it for Paper 2P.
What this page covers
| Spec | You need to be able to… | Route |
|---|---|---|
| 2.22P | name everyday materials that conduct electricity and materials that insulate, such as metals and plastics | Physics only · 2P |
| 2.23P | carry out an investigation into charging insulators by rubbing them (practical) | Physics only · 2P |
| 2.24P | explain how a material becomes positively or negatively charged by losing or gaining electrons | Physics only · 2P |
| 2.25P | know that unlike charges attract and like charges repel | Physics only · 2P |
| 2.26P | explain static electricity effects by describing how electrons move | Physics only · 2P |
| 2.27P | explain why static charge can be dangerous, for example when refuelling aircraft and tankers | Physics only · 2P |
| 2.28P | explain how static charge is put to use, for example in photocopiers and inkjet printers | Physics only · 2P |
Modular candidates: these are statements 2.22P–2.28P of 4XPH1 Unit 1 (4WPH1). They are not in 4XSD1.
In one minute
- Conductors (metals, graphite) let electrons move through them. Insulators (plastics, rubber, glass, dry wood) do not.
- Rubbing two insulators together moves electrons from one to the other. The one that gains electrons becomes negative; the one that loses electrons becomes positive. Positive charges do not move.
- Like charges repel; unlike charges attract. A charged object also attracts small uncharged objects.
- Static charge is dangerous when a spark could ignite fuel vapour — so aircraft and tankers are earthed while refuelling.
- Static charge is useful in inkjet printers (charged ink drops are steered by charged plates) and photocopiers (charged toner sticks to a charged image on a drum).
Which materials are conductors and which are insulators? Physics only · 2P
| Conductors | Insulators |
|---|---|
| metals — copper, aluminium, iron, steel, gold, silver | plastics — polythene, PVC, acetate, Perspex, nylon |
| graphite (carbon), for example in a pencil | rubber |
| the human body and the ground (Earth) conduct well enough to carry charge away | glass, dry wood, dry paper, wool, dry air |
- Metals conduct because they contain free electrons that can move through the material. In an insulator the electrons are held in place.
- Only insulators keep a static charge where it has been put. If you rub a metal rod held in your hand, any charge flows through the metal and your body to the ground, so the rod does not stay charged.
- This is why wires are copper (conductor) coated in PVC (insulator).
How does rubbing make a material charged? Physics only · 2P
All materials contain atoms with positive charge in the nucleus and negative electrons around it. An uncharged (neutral) object has equal amounts of positive and negative charge.
- When two different insulators are rubbed together, friction transfers electrons from the surface of one to the surface of the other.
- The material that gains electrons has more negative charge than positive: it becomes negatively charged.
- The material that loses electrons has more positive charge than negative: it becomes positively charged.
- The two charges are equal and opposite, because the electrons lost by one are gained by the other — charge is not created.
- For example, polythene rubbed with wool becomes negative (it gains electrons); acetate rubbed with a cloth becomes positive (it loses electrons).
A plastic comb is pulled through dry hair. Afterwards the comb is negatively charged. Explain how the comb became negative, and state the charge on the hair.
Friction between the comb and the hair transferred electrons from the hair to the comb. The comb now has more electrons than protons, so it is negative.
The hair has lost electrons, so it is positively charged, with a charge equal in size to the comb's.
An answer that says “positive charges moved from the comb to the hair” scores nothing — only electrons move.
What forces act between charges? Physics only · 2P
- Two negatively charged polythene rods repel; a negative polythene rod and a positive acetate rod attract.
- The forces act without the objects touching, and they are stronger when the charged objects are closer together.
- Repulsion is the only sure test that an object is charged: attraction could also happen between a charged object and an uncharged one (see “Explaining static effects” below).
Practical: how do you investigate charging insulators by friction? Physics only · 2P
This is a specification practical. Questions ask you to describe how to charge the rods, how to show the forces between them and how to tell the sign of a charge.
Investigating how insulating materials can be charged by friction
- Aim: show that insulators become charged when rubbed, and find whether like or unlike charges are produced on different materials.
- Apparatus: two polythene rods, two acetate rods, a dry woollen or cotton cloth, a paper stirrup, nylon thread, a clamp stand, small pieces of paper.
- Independent variable: the materials used (which rod and which cloth).
- Dependent variable: whether the suspended rod is attracted or repelled (the direction it turns).
- Control variables: the same cloth, the same number of rubs, the same distance the second rod is brought to; keep everything dry.
Method
- Rub a polythene rod firmly with the cloth about 20 times along its length. Hold it near small pieces of paper to check it is charged — they jump up and stick to it.
- Place the charged polythene rod in the paper stirrup hanging from the nylon thread, and let it come to rest.
- Charge the second polythene rod in the same way and bring it close to one end of the hanging rod, without touching. Record what happens. (It turns away: like charges repel.)
- Charge an acetate rod and bring it near the hanging polythene rod. Record what happens. (It turns towards it: unlike charges attract.)
- Repeat with two acetate rods, and with different cloths, to build up a table of results.
Results, errors and improvements
- Polythene rubbed with wool is negative; acetate rubbed with a cloth is positive. Two rods that repel must have the same type of charge.
- Moisture in the air or on your hands lets charge leak away, so the effect weakens with time. Use dry rods and cloths and test straight after rubbing.
- Do not touch the charged part of a rod — your hand conducts the charge away.
- The thread must be an insulator (nylon), otherwise the hanging rod discharges through it.
- Safety: low risk. Handle the rods carefully to avoid breaking them, and keep the clamp stand stable.
How do moving electrons explain everyday static effects? Physics only · 2P
Every static effect can be explained by electrons moving — either being transferred by friction, moving within an object, or flowing to or from the ground.
Why does a charged balloon stick to a wall?
- Rubbing the balloon on a jumper transfers electrons to the balloon, so the balloon becomes negative.
- When the balloon is held near the wall, its negative charge repels electrons in the surface of the wall slightly away from the balloon.
- This leaves the surface of the wall nearest the balloon slightly positive.
- Unlike charges attract, and the positive surface is closer than the displaced electrons, so the balloon is attracted to the wall.
The same idea explains why a charged comb picks up small pieces of paper, and why a stream of water bends towards a charged rod.
Why do you sometimes get a shock from a car door?
- As you slide across the seat, friction transfers electrons between your clothes and the seat, and your body becomes charged.
- When you touch the metal door, electrons flow quickly between you and the car body (and to earth). This sudden flow of charge is a small current that you feel as a shock, sometimes with a spark.
What does “earthing” do?
Connecting a charged conductor to the ground with a conducting wire lets electrons flow until it is uncharged. A negative object loses its extra electrons to earth; a positive object gains electrons from earth. Never say “positive charge flows to earth”.
An acetate rod is rubbed with a cloth and becomes positively charged. It is held near small pieces of uncharged paper, which jump up to the rod. Explain why the uncharged paper is attracted to the rod.
- The positive rod attracts electrons in the paper towards the side nearest the rod.
- That side of the paper becomes negative, and the far side is left positive.
- The negative side is closer to the rod, so the attraction is stronger than the repulsion of the far side.
- So there is an overall force of attraction, and the paper moves to the rod.
Why can static charge be dangerous? Physics only · 2P
Static charge is dangerous when it builds up and then discharges suddenly as a spark near something that can burn or explode.
- Refuelling aircraft and tankers: fuel flowing quickly through a hose rubs against the pipe, so electrons are transferred and charge builds up on the fuel, the hose and the vehicle.
- If the charge builds up, the voltage between the vehicle and its surroundings can become large enough for a spark to jump.
- A spark in the air full of fuel vapour can ignite it, causing a fire or explosion.
- Prevention: the aircraft (or tanker) and the fuel supply are connected by a bonding cable and connected to earth before fuelling starts. Charge then flows away continuously through the cable instead of building up, so no spark can form. Hoses are made of material that conducts slightly for the same reason.
- Other dangers: sparks near flammable gases or dust (for example in grain silos and flour mills), and electric shocks from objects that have built up a large charge.
Before a road tanker unloads petrol at a filling station, the driver connects a cable from the tanker to an earthing point. Explain why.
- Petrol flowing through the pipe rubs against it, so friction transfers electrons and the tanker can become charged.
- A large charge could discharge as a spark.
- A spark could ignite the petrol vapour and cause an explosion.
- The earthing cable lets electrons flow between the tanker and the ground, so charge cannot build up and no spark occurs.
A charged metal object is connected to earth. A charge of 12 μC (12 × 10⁻⁶ C) flows to earth in 4.0 ms. Calculate the average current during the discharge.
Use charge = current × time, \(Q=I\times t\), rearranged to \(I=\dfrac{Q}{t}\).
Convert: \(Q=12\times10^{-6}\ \text{C}\), \(t=4.0\ \text{ms}=4.0\times10^{-3}\ \text{s}\).
\(I=\dfrac{12\times10^{-6}}{4.0\times10^{-3}}=\) 0.0030 A (3.0 mA)
The flow of charge during a discharge is a current, and it stops as soon as the object is uncharged. Remember that ms means milliseconds, not minutes.
How is static charge used in printers and photocopiers? Physics only · 2P
Inkjet printer
- The print head forces out a stream of tiny ink droplets through a nozzle.
- Each droplet passes a charging electrode, which gives it a charge (in the diagram, a negative charge — it has gained electrons).
- The droplets pass between two deflecting plates. A negative droplet is repelled by the negative plate and attracted to the positive plate, so it is deflected.
- The size of the deflection depends on the charge on the droplet and on the voltage between the plates. A computer changes these for each droplet, so the droplets land in exactly the right places to form letters and images.
Photocopier
- A drum coated with a special material is given a positive charge all over its surface.
- An image of the page being copied is shone onto the drum. Where light falls (the white parts of the page), the coating conducts and the charge leaks away. Where the page is dark (the writing), no light reaches the drum and the charge stays.
- Fine black powder called toner is given a negative charge. It is attracted to the positively charged parts of the drum only, forming a copy of the image.
- A sheet of paper is given a positive charge, so it attracts the toner off the drum.
- The paper is heated, which melts the toner and fixes the image permanently.
In an inkjet printer, the droplets have a negative charge. (a) The top deflecting plate is made positive and the bottom plate negative. State and explain which way a droplet moves. (b) Suggest how the printer makes a droplet land further from the centre of the page.
(a) It moves upwards: the negative droplet is attracted by the positive top plate and repelled by the negative bottom plate (unlike charges attract, like charges repel).
(b) Increase the voltage between the plates, or give the droplet a larger charge, so the force on it is larger and it is deflected further.
Examiner tips for 2(d)
In every static question, say that electrons are transferred. Answers that move “positive charges” or “protons” lose the mark, even if the final charge is right.
When explaining charging, say where the electrons go from and to: “electrons move from the cloth to the rod, so the rod becomes negative and the cloth positive”.
For refuelling questions, link four ideas: friction → charge builds up → spark → ignites fuel vapour. Then give the precaution and how it works (earthing lets charge flow away).
If a question shows a current that rises and then falls to zero during a discharge, the current is the flow of charge; it stops when the object is no longer charged. \(Q=I\times t\) still applies — watch for μC and ms.
If asked how to show that an object is charged, use repulsion by an object with a known charge. Attraction does not prove it, because charged objects also attract uncharged ones.
Common misconceptions
| Misconception | Why it is wrong | Correct idea |
|---|---|---|
| A positive object has gained positive charges. | Protons are fixed in the nuclei and do not move. | A positive object has lost electrons. |
| Rubbing creates charge. | Charge is only moved from one object to another. | The two objects gain equal and opposite charges. |
| Metals cannot be charged. | A metal can hold charge if it is insulated from the ground. | A metal held in the hand discharges through the body, so it does not stay charged. |
| A charged object only attracts objects with the opposite charge. | It can also attract uncharged objects by moving electrons inside them. | Only repulsion proves that an object is charged. |
| Earthing makes positive charge flow into the ground. | Only electrons move. | Electrons flow to or from earth until the object is neutral. |
Key facts
| Idea | Key fact |
|---|---|
| conductors / insulators | metals and graphite conduct (free electrons); plastics, rubber, glass and dry wood insulate |
| charging by friction | electrons transferred; gains electrons → negative; loses electrons → positive |
| forces between charges | like charges repel; unlike charges attract |
| attracting uncharged objects | a charged object moves electrons within the uncharged object, so the near side has the opposite charge |
| danger | build-up of charge → spark → ignites fuel vapour; prevent by bonding and earthing |
| uses | inkjet printers (charged droplets deflected by charged plates); photocopiers (charged toner attracted to charged drum) |
| discharge as a current | \(Q=I\times t\) (on the Equation List, all papers; from 2(c)) |
Check your understanding
1. Name two electrical conductors and two electrical insulators. Physics only · 2P
Conductors: e.g. copper, aluminium (any metal), graphite. Insulators: e.g. polythene, rubber, glass, dry wood.
2. A polythene rod rubbed with wool becomes negative. Explain why, in terms of electrons. Physics only · 2P
Friction transfers electrons from the wool to the rod (1). The rod has more electrons than protons, so it is negative (1).
3. Two charged balloons hang next to each other and move apart. What can you say about their charges? Physics only · 2P
They have the same type of charge (both positive or both negative) — like charges repel.
4. Explain why a metal rod held in your hand does not become charged when you rub it. Physics only · 2P
Metal is a conductor and so is your body (1); any charge flows through the rod and your body to earth, so none stays on the rod (1).
5. Explain why aircraft are connected to earth before they are refuelled. Physics only · 2P
Fuel rubbing in the pipe causes charge to build up (1); this could cause a spark (1) that ignites the fuel vapour (1); earthing lets the charge flow away so it cannot build up (1).
6. In a photocopier, why does toner stick only to some parts of the drum? Physics only · 2P
The toner is charged negatively (1) and is attracted only to the parts of the drum that are still positively charged — the dark parts of the image, where light did not reach (1).
7. A charge of 6.0 μC flows to earth in 0.50 ms. Calculate the average current. Physics only · 2P
\(I=\dfrac{Q}{t}=\dfrac{6.0\times10^{-6}}{0.50\times10^{-3}}=\) 0.012 A (12 mA)
← Previous: 2(c) Energy and voltage in circuits · Next: 3(b) Properties of waves →