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Physics · 4 Electricity and magnetism

4.2.1 Electric charge

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Before you start

  • An atom has a nucleus of protons and neutrons, with electrons outside it.
  • Protons are positive, electrons negative, and a neutral atom has equal numbers of each.
  • The idea of a field, from magnetism in 4.1.
Syllabus objectives covered (10)
  • State that there are positive and negative charges
  • State that positive charges repel other positive charges, negative charges repel other negative charges, but positive charges attract negative charges
  • Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges
  • Explain that charging of solids by friction involves only a transfer of negative charge (electrons)
  • Describe an experiment to distinguish between electrical conductors and insulators
  • Recall and use a simple electron model to explain the difference between electrical conductors and insulators and give typical examples
  • State that charge is measured in coulombs
  • Describe an electric field as a region in which an electric charge experiences a force
  • State that the direction of an electric field at a point is the direction of the force on a positive charge at that point
  • Describe simple electric field patterns, including the direction of the field: (a) around a point charge (b) around a charged conducting sphere (c) between two oppositely charged parallel conducting plates (end effects will not be examined)

Part 1 of 6

Two kinds of charge

  1. There are two kinds of electric charge, positive and negative. Everything electrical in this unit follows from that and from one rule.
  2. Like charges repel, unlike charges attract. Positive repels positive, negative repels negative, positive attracts negative.
  3. That is the same shape of rule as magnetic poles in 4.1, and it is worth noticing the parallel. But charges and poles are different things — do not mix the vocabulary.
  4. An object with equal amounts of positive and negative charge is uncharged or neutral. It is not empty of charge; the charges simply cancel.
  5. Charge is measured in coulombs, symbol C.

Part 2 of 6

Charging by friction — and what actually moves

  1. Rub a polythene rod with a dry cloth and it becomes charged. Rub a perspex rod and it charges the other way. This is charging by friction.
  2. Here is the point the syllabus is really testing: only electrons move. The positive charges are protons, locked in the nuclei of a solid, and they cannot go anywhere.
  3. So a solid becomes negatively charged by gaining electrons, and positively charged by losing them. Nothing positive ever travels.
  4. Polythene rubbed with a cloth gains electrons from the cloth and becomes negative; the cloth, having lost them, becomes equally positive.
  5. Perspex rubbed with a cloth loses electrons to the cloth and becomes positive; the cloth becomes negative.
  6. Charge is always conserved. Whatever one object gains, the other loses — an equal and opposite amount, every time.
  7. This is why an answer such as 'the rod gained positive charge' is marked wrong even when the final sign is right. Say which way the electrons went.

Part 3 of 6

Detecting charge

  1. The simplest detection experiment: bring a charged rod near small pieces of paper, or near a thin stream of water from a tap. Both are attracted.
  2. Suspend a charged rod on a thread and bring a second charged rod near it. Attraction or repulsion tells you whether the charges are alike.
  3. Remember the same logic as magnets: repulsion is the reliable test. A charged rod attracts uncharged objects too, so attraction alone tells you nothing definite.
  4. A gold-leaf electroscope is the standard instrument. Touching a charged rod on its metal cap charges the leaf and the stem alike, so they repel and the leaf rises.
  5. A bigger charge means a bigger repulsion and a wider divergence of the leaf, so the electroscope gives a rough measure as well as a detection.
    capinsulatorstemleaflike charges repel, so the leaf lifts
    Stem and leaf carry the same charge, so they repel and the leaf lifts.

Part 4 of 6

Conductors and insulators

  1. A conductor allows charge to flow through it. An insulator does not.
  2. The electron model explains why. Metals have free electrons — electrons not bound to any one atom, able to move through the whole material. Push them and charge flows.
  3. Insulators have no free electrons. Every electron is held in place around its own atom, so charge cannot travel through the material.
  4. Typical conductors: copper, aluminium, all metals, and graphite. Typical insulators: plastic, rubber, glass, dry wood, polythene, perspex.
  5. Graphite is worth remembering because it is the odd one out — a non-metal that conducts.
  6. The experiment to distinguish them. Build a simple circuit with a cell, a lamp and two crocodile clips, leaving a gap. Clip the material under test across the gap.
  7. If the lamp lights, charge is flowing and the material is a conductor. If it stays off, the material is an insulator. Replacing the lamp with an ammeter lets you compare how well different conductors conduct.
  8. This also explains why you can only charge an insulating rod by rubbing it while holding it in your hand. Charge on a metal rod would flow through you to the ground and be lost immediately.

Part 5 of 6

Electric fields

  1. An electric field is a region in which an electric charge experiences a force. Compare it with the definition of a magnetic field — same structure, different thing being acted on.
  2. The direction of an electric field at a point is the direction of the force on a positive charge placed at that point.
  3. That single convention determines every arrow you will ever draw in this topic, so it is worth memorising as a sentence rather than as a picture.
  4. It follows immediately that field lines point away from a positive charge, since a positive test charge would be repelled, and towards a negative charge.
  5. As with magnetic fields, closer lines mean a stronger field, and field lines never cross.

Part 6 of 6

The three field patterns

  1. Around a point charge. Straight lines radiating outwards in all directions, like the spokes of a wheel. Arrows point outwards for a positive charge and inwards for a negative one.
  2. The lines spread out with distance, so they are further apart further away — correctly showing that the field gets weaker as you move away.
  3. Around a charged conducting sphere. Outside the sphere the pattern is identical to that of a point charge: straight radial lines, starting perpendicular to the surface.
  4. Start the lines at the surface, not at the centre. The charge on a conductor sits on its outside.
    point charge+charged sphere+parallel plates (uniform)+−
    Radial for a point charge and a sphere; uniform between parallel plates.
  5. Between two oppositely charged parallel plates. Straight, evenly spaced, parallel lines running from the positive plate to the negative plate.
  6. Evenly spaced means the field has the same strength everywhere between the plates — a uniform field. That is the distinctive feature and the thing to say if asked to describe it.
  7. The lines must meet both plates at right angles. You will not be examined on what happens at the edges of the plates.

Worked examples

Example 1. A polythene rod becomes negatively charged when it is rubbed with a dry cloth. Explain, in terms of electrons, what happens to the rod and to the cloth.

Show the working
  1. State the rule that governs it: in a solid, only electrons can move, because protons are fixed in the nuclei.
  2. The rod ends up negative, so it must have gained electrons. Electrons carry negative charge, so gaining them makes an object more negative.
  3. Those electrons came from the cloth, which has therefore lost the same number.
  4. Having lost negative charge, the cloth is left with an excess of positive charge, so it becomes positively charged.
  5. Note that the two charges are equal in size and opposite in sign — charge is conserved.

Answer: Electrons are transferred from the cloth to the rod. The rod gains electrons and becomes negatively charged; the cloth loses the same number and is left equally positively charged. Only electrons move, because protons are fixed in the nuclei.

Example 2. Describe an experiment to determine whether a material is an electrical conductor or an insulator, and explain the result using an electron model.

Show the working
  1. Set up a series circuit containing a cell and a lamp, with a gap left in the wire and a crocodile clip on each side.
  2. Clip the sample across the gap, completing the circuit through the material under test.
  3. If the lamp lights, charge is flowing through the sample, so it is a conductor. If the lamp does not light, it is an insulator.
  4. Explain using electrons: a conductor such as copper contains free electrons that can move through the material, carrying charge.
  5. In an insulator such as plastic, every electron is bound to its own atom and cannot move through the material, so no current flows.

Answer: Connect the material into a series circuit with a cell and a lamp. The lamp lights for a conductor and stays off for an insulator. Conductors contain free electrons that can move through the material; in insulators the electrons are bound to their atoms and cannot flow.

Example 3. Draw and describe the electric field between two oppositely charged parallel plates, and state what the pattern tells you about the strength of the field.

Show the working
  1. Draw the two plates parallel and horizontal, labelling the upper one positive and the lower one negative.
  2. Draw straight lines from the positive plate to the negative plate, meeting both plates at right angles.
  3. Put arrows on the lines pointing from positive to negative, because that is the direction of the force on a positive charge.
  4. Space the lines evenly. That is the key feature, not a drawing convenience.
  5. Equal spacing means equal field strength everywhere between the plates, so the field is uniform.

Answer: Straight, parallel, evenly spaced lines running from the positive plate to the negative plate and meeting both at right angles. The even spacing shows the field is uniform — the same strength everywhere between the plates.

Where people go wrong

Mistake: Saying an object becomes positively charged by gaining positive charge.

Instead: It becomes positive by losing electrons. Positive charges never move in a solid.

Protons are held in the nuclei. The syllabus objective specifically says charging by friction involves only a transfer of negative charge.

Mistake: Treating attraction as proof that an object is charged.

Instead: A charged object attracts uncharged objects too. Repulsion is the reliable test.

The same reasoning as magnets — the charged object induces opposite charge on the near side of a neutral object.

Mistake: Drawing electric field lines from negative to positive.

Instead: Lines run from positive to negative, out of a positive charge and into a negative one.

The direction is defined by the force on a positive test charge, which is repelled by positive.

Mistake: Drawing radial field lines starting from the centre of a charged sphere.

Instead: Start them at the surface, perpendicular to it.

The charge on a conductor resides on its outer surface, and there is no field inside.

Mistake: Drawing the field between parallel plates with unevenly spaced lines.

Instead: Space them evenly to show a uniform field.

The spacing carries the information about strength, and uniformity is the whole point of this pattern.

Check yourself

  1. State the rule for the forces between electric charges.
    AnswerLike charges repel; unlike charges attract.
  2. A perspex rod becomes positively charged when rubbed. What has happened to its electrons?
    AnswerIt has lost electrons to the cloth, leaving an excess of positive charge.
  3. In what unit is charge measured?
    AnswerThe coulomb, symbol C.
  4. Describe how a gold-leaf electroscope shows that an object is charged.
    AnswerThe charge spreads over the cap, stem and leaf. The leaf and stem carry the same charge, so they repel and the leaf rises; a larger charge gives a larger divergence.
  5. Explain the difference between a conductor and an insulator using the electron model, and give an example of each.
    AnswerA conductor such as copper has free electrons that can move through the material, so charge flows. An insulator such as plastic has no free electrons, so charge cannot flow.
  6. Define an electric field and state how its direction is defined.
    AnswerA region in which an electric charge experiences a force. Its direction is the direction of the force on a positive charge placed at that point.
  7. Describe the field pattern around a negative point charge.
    AnswerStraight radial lines with arrows pointing inwards, towards the charge, spreading further apart with distance.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student rubs a polythene rod with a dry woollen cloth. The rod becomes negatively charged. The rod is then held near a thin stream of water from a tap, and the stream bends towards the rod.

(a) Explain how the rod becomes negatively charged. 2 marks No calculator

Mark scheme
  • B1 electrons are transferred from the cloth to the rod
  • B1 the rod has an excess of electrons / negative charge; only electrons move, ORA

(b) State the charge on the cloth after rubbing, and give a reason. 2 marks No calculator

Mark scheme
  • B1 positive
  • B1 it has lost electrons to the rod; the charges are equal and opposite

(c) Suggest why the experiment does not work well on a humid day. 1 mark No calculator

Mark scheme
  • B1 moisture on the surfaces conducts, allowing the charge to leak away, owtte

Question 2. Two identical metal spheres on insulating stands are given opposite charges of equal size.

(a) Define an electric field. 1 mark No calculator

Mark scheme
  • B1 a region in which an electric charge experiences a force

(b) Sketch the electric field pattern around the positively charged sphere, taken on its own. 2 marks No calculator

Mark scheme
  • B1 straight radial lines starting at the surface of the sphere
  • B1 arrows pointing outwards, away from the sphere

(c) Explain why the field lines point outwards from this sphere. 2 marks No calculator

Mark scheme
  • B1 the field direction is the direction of the force on a positive charge
  • B1 a positive charge would be repelled by the positively charged sphere

Question 3. A student tests four materials — copper, polythene, graphite and glass — by connecting each in turn into a circuit containing a cell and a lamp.

(a) Identify which materials cause the lamp to light. 1 mark No calculator

Mark scheme
  • B1 copper and graphite, both required

(b) Explain the difference between the two groups of materials in terms of electrons. 2 marks No calculator

Mark scheme
  • B1 conductors contain free electrons that can move through the material
  • B1 in insulators the electrons are bound to atoms and cannot move through the material, ORA

(c) Suggest one change to the apparatus that would allow the student to compare how well the two conducting materials conduct. 1 mark No calculator

Mark scheme
  • B1 replace the lamp with an ammeter (accept: measure the current for each sample and compare)

Words to know

Coulomb (C)
The unit of electric charge.
Charging by friction
Transferring electrons between two solids by rubbing, leaving them with equal and opposite charges.
Conductor
A material containing free electrons, through which charge can flow.
Insulator
A material with no free electrons, through which charge cannot flow.
Electric field
A region in which an electric charge experiences a force.
Uniform field
A field of the same strength everywhere, shown by evenly spaced parallel field lines.
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