Full chapter
Static electricity
All 5 topics and the revision summary on one page.
01
Electric charge and interactions
Electric charge has two signs: positive and negative. Charged objects can exert forces on one another without touching.
Charge is represented by q or Q and is measured in coulombs, C. A positive sign means a net positive charge; a negative sign means a net negative charge.
Neutral does not mean no charged particles
Atoms contain positively charged protons in their nuclei and negatively charged electrons. A neutral object has equal total positive and negative charge, so its net charge is zero.
An object with an excess of negative charge has a negative net charge. An object with an excess of positive charge has a positive net charge. Positive and negative describe the charge balance, not whether an object is made only of protons or only of electrons.
Predict attraction and repulsion
Predict the electric force on each charged object
These small objects carry the shown net charges. Only their electric forces on one another are shown; orange arrows are forces, not motion.
What the observation establishes
Two charged balls repel
They have the same sign. They could both be positive or both be negative; repulsion alone does not distinguish those possibilities.
If a ball repels a known positive charge in this model, that additional comparison identifies the ball as positive.
Attraction has more than one possible explanation. A charged object can attract an oppositely charged object, but it can also attract a neutral object. Attraction alone does not prove that both objects carry opposite net charges.
Optional check Two small charged balls, separately suspended and isolated from other charged objects, repel. What can be concluded about their charge signs?
Rubbing can transfer electrons between objects. Induction can redistribute charge without the charged object touching the conductor.
02
Charging by rubbing
Rubbing can transfer electrons from one material to another. It separates charge rather than creating it.
An electron carries negative charge. A neutral object has balanced total positive and negative charge. During the ordinary charging processes here, protons remain bound in their atomic nuclei; they do not move from the cloth to the rod.
Follow the electron transfer
Contact and separation as two materials are rubbed can move electrons between them. The final sign depends on which material gains electrons. Use the stated direction of transfer; do not assume every kind of plastic rod always gains or always loses electrons.
| Change to an initially neutral object | Result |
|---|---|
| Gains electrons | Excess negative charge: the object becomes negative. |
| Loses electrons | Electron deficit: the object becomes positive. |
Count the electrons before and after transfer
Each material starts with four positive charge symbols and four electron symbols in this small model. The symbols are not a measured particle count. Positive charges remain in their original material.
The cloth loses electrons and the rod gains them. With no other charge transfer, the pair's total charge remains zero.
Charge accounting
Electrons move from cloth to rod
In the drawing, each material begins with four positive-charge symbols and four electrons. Each positive symbol balances one electron. After two electrons transfer:
- Rod: four positive symbols and six electrons give a negative excess.
- Cloth: four positive symbols and two electrons give a positive excess.
- Whole pair: there are still eight positive symbols and eight electrons. Its total charge has not changed.
These are schematic charge counts, not measured particle counts in real materials. If both objects began neutral and no charge entered or left the pair, their final net charges are equal in magnitude and opposite in sign.
A positive rod would instead indicate that it had lost electrons in the stated process. Becoming positive does not require positive particles to arrive from the cloth.
Why a charged object may need insulation
In a conductor, some electrons can move through the material. In an insulator, excess charge is much less free to move through the material, so it can remain near the region that was rubbed.
Holding a conductor directly can provide a path through the hand to Earth, allowing charge to leak away. An insulating handle or support helps separate the charged conductor from that path. Insulation does not mean the material contains no charged particles.
Separate the observation from its explanation
Compare a rod's effect before and after rubbing, using similar distances and the same small test objects. A rubbed rod may attract small neutral paper pieces, or repel an appropriately charged suspended object.
The movement is the observation. Electron transfer is the explanation for the rod's changed net charge. Attraction of neutral paper alone does not identify whether the rod is positive or negative; the charge balance inside a neutral object can respond to a nearby charged object.
Damp surfaces and unintended contact can allow charge to leak away. A weak or short-lived effect does not establish that the material contains no charge. Keep the support, surroundings and comparison distance similar when comparing trials.
Check the boundary of the charge account. Equal opposite final charges follow for an initially neutral pair with no other charge transfer. Charge lost to the hand or surroundings must also be included in a wider account.
Optional check An initially neutral cloth and rod are rubbed. Electrons move from the cloth to the rod, and no charge is exchanged with their surroundings. What are their final charges?
03
Charging by induction
A nearby charged object can move electrons within a conductor without touching it. To leave a net charge afterwards, separate the charge while that influence remains.
Electrons are negative and can move through a metal. The positive ions remain in its structure. Earth can act as a large reservoir that accepts or supplies electrons through a conducting connection.
Redistribution is not yet a change of total charge
Start with a neutral metal sphere on an insulating stand. Hold a negative rod near its left side without contact. The rod repels mobile electrons towards the far side. The near side has an electron deficit, and the far side an excess.
The sphere is still neutral overall: electrons have moved within it, but none have entered or left. Its nearer positive region is attracted more strongly than the farther negative region is repelled in this arrangement, so the neutral sphere can be attracted towards the rod.
If the rod is simply taken away, the electrons redistribute and the sphere remains neutral. A separated charge pattern while a rod is present is not proof of a retained net charge.
Use Earth to leave the sphere positive
Disconnect Earth before removing the negative rod
Signs on the sphere show local surface charge imbalance, not individual protons moving. The unmarked neutral sphere still contains charged particles. Blue arrows identify electron movement.
1. Start with an isolated neutral sphere
Net charge: zero. The metal sphere stands on an insulating support. There is no nearby rod and no conducting connection to Earth. Neutral means balanced charge, not an absence of charged particles.
2. Bring the negative rod close
Net charge: still zero. The negative rod does not touch the sphere. Some mobile electrons shift towards the far, right-hand side. The near side has an electron deficit. No electron leaves the isolated sphere, so its total charge remains zero.
3. Keep the rod close and connect Earth
Net charge: positive. A conducting wire connects the sphere to Earth at the right. Some electrons leave through the wire while the negative rod remains in place. The sphere has a net electron deficit. The blue arrow shows electron movement, not conventional current or positive particles entering.
4. Disconnect Earth first
Net charge: positive and isolated. The Earth connection is opened while the negative rod is still close. The gap prevents further charge exchange along that path. The sphere retains its net positive charge. No electron-flow arrow crosses the open gap.
5. Remove the rod
Net charge: remains positive. The charged rod is removed after Earth has been disconnected. The isolated sphere remains positively charged. In the ideal model with no other nearby objects, its excess positive surface charge is distributed symmetrically. This is a surface electron deficit, not protons moving through the metal.
- Start neutral and isolated. Place the neutral metal sphere on an insulating stand, with no rod nearby and no Earth connection.
- Bring the negative rod close without touching. Electrons move towards the far side. The total charge of the sphere is still zero.
- Keep the rod still and connect the sphere to Earth. Some electrons leave the sphere through the conducting path. The sphere is left with an electron deficit; positive particles do not flow in from Earth.
- Disconnect Earth first, with the rod still close. The sphere is now isolated. Its net positive charge is retained because the electrons that left no longer have that return path.
- Remove the rod. Charge redistributes over the sphere, which stays positive. An ideal isolated spherical conductor with no other nearby objects has a symmetric final surface-charge distribution.
The inducing rod never touches the sphere and retains its own charge in this ideal model. The electrons lost by the sphere go to Earth, so the wider charge account includes Earth.
Explain the order
Why disconnect Earth before removing the rod?
While the negative rod is close, it repels electrons from the sphere. Disconnecting Earth at that point isolates an electron deficit.
If the rod is removed while Earth is still connected, electrons can return through the available path. The sphere becomes neutral in this model. Disconnecting Earth then simply isolates a neutral sphere.
A positive inducing rod reverses the electron movement: electrons enter the sphere from Earth. Disconnect Earth while that rod is still nearby, then remove the rod, and the sphere retains a negative charge. The order stays the same even though the signs reverse.
See the sequence with a gold-leaf electroscope
A gold-leaf electroscope has a conducting cap connected to a metal stem and a thin conducting leaf. When the leaf and the adjacent stem acquire like charge, their repulsion makes the leaf rise away from the stem. Its insulating support helps prevent unintended leakage.
Use an initially uncharged instrument as the conductor in the negative-rod experiment. Bring the rod near the cap without touching: electrons move away from the cap towards the stem and leaf, and the leaf rises. With the rod held in place, briefly connect the cap to Earth; electrons leave and the leaf falls in this standard arrangement.
Disconnect Earth while keeping the rod close. Then remove the rod: the remaining positive charge redistributes onto the stem and leaf, and the leaf rises again. This retained divergence after the rod has gone indicates that charge remains. Leaf divergence alone does not identify its sign; the electron account explains why it is positive here.
| Sequence | Final observation and explanation |
|---|---|
| Disconnect Earth, then remove the rod | The leaf remains diverged after removal. The instrument was isolated with an electron deficit, so it retains positive charge. |
| Remove the rod, then disconnect Earth | The leaf ends down in the ideal arrangement. Electrons could return while the instrument was still earthed, leaving it neutral. |
| Approach and remove the rod without earthing | The leaf rises temporarily and returns down. There was redistribution while the rod was close, but no net charge transfer. |
Discharge the instrument before each new trial and keep the rod's sign, approximate position and support arrangement comparable. Do not touch the rod to the cap when testing induction. Small residual movement, leakage or limited sensitivity can affect what is seen; leaf angle is not a direct measurement in coulombs.
Induction without an Earth connection
Two initially neutral conducting spheres A and B touch each other on insulating supports. Place the negative rod close to A. Electrons are repelled from A into B through their contact, so A has a positive charge balance and B a negative one while the pair remains neutral overall.
Induction can also separate charge without Earth
A and B start neutral. The large signs label each sphere's net charge, not individual particles or a detailed surface distribution. Assume no charge escapes.
1. Keep the spheres touching; bring the rod close
Two initially neutral conducting spheres A and B touch on insulating supports. A negative rod is brought near A without contact. Electrons move through the contact from A to B. A develops a net positive charge and B a net negative charge, while the combined charge remains zero.
2. Separate A and B with the rod still close
The spheres are moved apart while the negative rod is kept near A. There is now an insulating gap between them, so electrons cannot move back through the former contact. A remains positive and B negative. Neither sphere is earthed.
3. Remove the rod after separation
After the spheres have been separated, the negative rod is removed. A retains positive charge and B negative charge. If no charge escaped, their net charges have equal magnitudes and opposite signs, so the combined charge is still zero. Large signs label net charge, not a surface distribution.
- Bring the negative rod near A without touching either sphere. Keep A and B in contact while electrons redistribute.
- Separate the spheres using their insulating supports, with the rod still held nearby.
- Remove the rod. A retains a positive net charge and B a negative net charge.
No electrons had to travel to Earth. They moved from A to B, and separating the spheres removed the path between them before the inducing influence was removed. If the rod is removed before the spheres are separated, electrons redistribute through their contact and both are left neutral in this model.
A charge indicator can show that each isolated sphere retains charge afterwards. Their signs follow from the electron account, or can be checked with suitable known-charge comparisons. Attraction between the spheres by itself would not establish every part of that charge account.
Ask where electrons can move at each stage. A metal contact or Earth lead provides a path; an insulating gap removes it. The correct order isolates a charge imbalance before removing the nearby rod.
Optional check A negative rod is held near a metal sphere without touching it. The sphere is connected to Earth. Which next steps leave the sphere positively charged?
04
Electric fields and force direction
An electric field is a region in which an electric charge experiences a force. A field diagram describes the effect of the source charges at different positions.
Unlike charges attract and like charges repel. A field-line arrow uses one fixed convention: it points in the direction of force on a positive test charge.
Draw the required point-charge patterns
A point-charge model treats the source's size as negligible compared with the distances being considered. For the test-charge interpretation, imagine a charge small enough not to change the source arrangement significantly.
A field arrow gives the force direction on a positive test charge
These are schematic field lines, not visible threads or particle tracks. Lines that reach a frame continue beyond the drawing. Their number is not a supplied measure of field strength.
One isolated positive charge
One isolated negative charge
Equal unlike charges
Two equal positive charges
Two equal negative charges
- One isolated positive charge
- Draw radial lines pointing outwards. A positive test charge is repelled from the source.
- One isolated negative charge
- Draw radial lines pointing inwards. A positive test charge is attracted towards the source.
- Two equal unlike charges
- Lines run from the positive charge towards the negative charge, with curved lines above and below the line joining them. Arrowheads must agree with the positive-test-charge convention.
- Two equal positive charges
- Lines leave both charges and curve away from the middle region. They do not connect positive to positive.
- Two equal negative charges
- The pattern has the corresponding shape to two equal positive charges, with arrows reversed to point into the negative charges.
Field lines do not cross: crossing would give two different field directions at one position. Lines may leave the edge of a diagram to represent a field extending beyond the drawing; they should not simply end at an unlabelled point in empty space. At a position where the resultant field is zero, there is no nonzero field-direction arrow to draw.
The lines are a representation, not visible threads around the charges. Their drawn number is not a count of particles. Do not infer an exact numerical field strength by counting an illustrator's lines.
A negative charge has the opposite force direction
Keep the field direction fixed when changing the test charge
Green arrows show the supplied field; orange arrows show electric force. Blue arrows or paths show motion. A force direction need not be the current direction of motion.
Apply the same rule to a particle separator
The field changes sideways motion. It does not instantly replace the particles' upward motion. Gravity and air resistance are ignored in this comparison.
Read the convention
The field arrow points right at a marked point
- A positive test charge has electric force right.
- A negative test charge has electric force left.
Changing the test-charge sign reverses its force. It does not reverse the existing field established by the source charges.
Force changes motion; it does not specify the velocity an object already has. A charge moving left can experience force right, so it may initially slow before reversing. If electric force is the only unbalanced force, its direction gives the acceleration direction, consistent with resultant force and acceleration.
Apply a supplied field to a new arrangement
In the separator diagram, positive and negative particles enter moving upwards. The field points left. Ignore gravity and air resistance for this comparison.
The positive particles experience force left and begin to bend towards a left collector. Negative particles experience force right and begin to bend towards a right collector. Both initially continue moving upwards while their horizontal velocities change. The force does not instantly turn either velocity through a right angle.
A field line is not automatically a moving particle's path. It shows force direction for a positive test charge at each point. The charge's sign, existing velocity and other forces also matter when predicting its motion.
Optional check An existing electric field points to the right at a marked position. A small negative test charge is placed there without significantly changing the field. Which statement is correct?
05
Discharge hazards and particle collection
Accumulated charge can lead to an unwanted spark. Controlled charging can also give suspended particles an electric force that removes them from a gas stream.
Electrons carry negative charge. A conducting Earth connection can accept or supply electrons, and an electric-field arrow gives force direction for a positive test charge.
Explain how the hazard develops
Contact and separation between materials can separate charge. If the charge cannot flow away, it can accumulate and produce a strong electric field across an air gap. If the air becomes conducting, a rapid discharge through it can produce a spark.
- Flammable vapour during liquid handling
- Flow and contact between liquid and surfaces can separate charge. A discharge spark can ignite a suitable flammable vapour-air mixture. The dangerous combination is an ignition source and an ignitable mixture, not merely the existence of any charged object.
- Combustible dust in air
- Particle movement and contact can lead to charge accumulation. A spark can ignite a suspended cloud when the combustible dust and air form an ignitable mixture.
A suitable conducting path to Earth allows excess charge to flow away and can reduce accumulation. Connecting conductors together, called bonding, allows charge to redistribute between them and reduces the conditions for a spark between them. These connections must actually conduct; an insulating coating or broken connection can leave the intended path unavailable.
This reasoning is different from induction's deliberate isolation step. Induction disconnects a path at a chosen stage to retain charge; reducing a static hazard often requires a path that prevents charge from building up.
Collect particles with an electrostatic precipitator
A dry electrostatic precipitator removes suspended dust or smoke particles from a gas stream. The model here uses negative discharge wires between earthed collecting plates.
A negative dust particle is forced towards a collecting plate
This is a cross-sectional force model of a dry wire-and-plate precipitator. A representative negative discharge wire lies between two earthed collecting surfaces.
Charged gas particles transfer negative charge to suspended dust. The electric force drives it towards a collecting surface while gas continues through. A dust grain does not have to touch a wire to become charged.
Later, mechanical shaking removes the deposited dust into a hopper. This is particle removal; it does not remove every gaseous pollutant.
- Charge the suspended particles. The strong field near the discharge wires ionises gas, producing charged particles. Electrons can attach to gas molecules, forming negative ions that transfer negative charge to dust as they collide with it. Dust grains need not touch a wire to become charged.
- Exert a sideways electric force. In the space between the negative wires and earthed collectors, negatively charged dust experiences force towards the collecting plates, opposite the local field direction.
- Deposit the particles. Dust collects on the plates while gas continues through the device with fewer suspended particles.
- Remove the deposit. Periodic mechanical shaking dislodges the collected material into a hopper, allowing collection to continue.
Earthing a plate does not make it electrically inactive. The connection allows charge to redistribute between the conducting collector and Earth. Its facing surface can attract negative dust in the field produced by the negative wires. This model does not require a separate positive supply attached to the collecting plates.
The collected particles can move across the gas flow because an electric force acts on them. Removing particles does not mean that every gaseous pollutant has been removed, or that every particle is captured.
Use the same reasoning in a different separator
Application to a supplied design
Particles enter upwards through a leftward field
For the supplied field in the separator example, positive particles have force left and negative particles force right. Side collectors can therefore receive different charge signs.
The design reasoning is: identify the charge sign, read the field direction, predict the force, then consider how it changes the existing motion.
Explain charging and collection separately. The gas first transfers charge to the dust; the field then exerts a force on that charged dust. Neither step is explained by saying only that the particles are attracted to a plate.
Optional check In a separator, positive and negative particles enter moving upwards. The electric field points left. Ignore gravity and air resistance. How does the electric force begin to deflect the two kinds?
Revision summary
- Signs, units and interactions
- Charge q or Q is measured in coulombs, C. Like charges repel; unlike charges attract. Neutral means equal total positive and negative charge. Attraction alone can also involve a neutral object.
- Rubbing
- Electrons transfer between materials. Gaining electrons makes an initially neutral object negative; losing them makes it positive. Protons do not transfer in these processes. An initially neutral isolated pair gains equal opposite net charges.
- Induction with Earth
- Bring the rod near without contact, connect Earth while holding the rod in place, disconnect Earth first, then remove the rod. A negative rod drives electrons out and leaves a positive conductor; a positive rod draws electrons in and leaves a negative conductor.
- Electric field
- A region where an electric charge experiences a force. Field arrows give force direction for a positive test charge. A negative charge has force opposite the arrow; the existing field does not reverse when the test sign changes.
Distinguish three changes
| Process | What to explain |
|---|---|
| Transfer by rubbing | Electrons pass from one material to another. Include every object that gains or loses charge. |
| Redistribution in an isolated conductor | Electrons move within it while the rod is nearby. Total charge stays unchanged; removing the rod alone does not leave an initially neutral conductor charged. |
| Retained induction charge | Disconnect Earth, or separate touching conductors, while the inducing rod remains. Removing the rod afterwards redistributes the retained charge without removing its net amount. |
With two touching neutral spheres and a negative rod near A, electrons move from A to B. Separate them before removing the rod: A remains positive and B negative. Removing the rod while they still touch allows the imbalance to disappear.
A gold leaf rises when it and the stem have like charge. Retained divergence after the inducing rod has gone indicates remaining charge; it does not alone establish the sign. Keep observations separate from the electron explanation.
Draw field patterns with their arrows
- Positive isolated point charge: radial arrows outwards. Negative point charge: radial arrows inwards.
- Unlike pair: lines run from positive to negative. Like pair: lines do not join like signs; they curve away from the region between the charges.
- For a pair of negative charges, reverse the arrows of the corresponding positive-pair pattern so they enter the charges.
- Lines do not cross. A field line represents a force-direction convention, not a guaranteed particle trajectory.
Follow the application chain
Hazard: charge separation and accumulation can cause a discharge spark, which may ignite a suitable vapour-air or combustible-dust mixture. A conducting path can reduce unwanted accumulation.
Precipitator: ionised gas transfers charge to suspended particles; the electric field drives them towards collectors; particles deposit; mechanical shaking removes the deposit. Gas flow, field direction and force on a negative particle are different directions or quantities.
New separator: use the stated field and charge sign before predicting deflection. A positive particle is forced along the field arrow; a negative particle is forced opposite it. Its previous motion does not vanish instantly.
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