P9: Static Electricity
Static charge by friction, induction, electric fields, sparking, dangers and uses of static, Van de Graaff generators and lightning.
Static Charge by Friction
When two insulating materials are rubbed together, electrons can be transferred from one material to the other. This creates static charge.
How Static Charge is Created
- Only electrons can move — positive charges (protons) remain in the nucleus
- The material that gains electrons becomes negatively charged
- The material that loses electrons becomes positively charged
- The net charge on both materials is always equal and opposite
- Static charge only builds up on insulators because charge cannot flow away
Worked Example
A polythene rod is rubbed with a cloth duster. Electrons transfer from the duster to the rod. The rod becomes negatively charged and the duster becomes positively charged. The total charge is still zero — charge has been conserved.
Forces Between Charges
Charged objects exert forces on each other and on uncharged objects.
Rules of Electrostatic Forces
- Like charges repel — two positives or two negatives push each other away
- Unlike charges attract — a positive and a negative pull towards each other
- A charged object can also attract an uncharged object by induction
Never say "opposites attract" without specifying charges. Always state that like charges repel and unlike charges attract. Also remember that a charged object attracts a neutral one, which is not the same as unlike charges attracting.
Induction
A charged object can attract an uncharged object without touching it. This process is called induction.
How Induction Works
- A negatively charged rod is brought near a neutral object
- Electrons in the neutral object are repelled to the far side
- The near side becomes positively charged and the far side becomes negatively charged
- The positive side is closer to the rod, so the attractive force is stronger than the repulsive force
- The neutral object is attracted towards the charged rod
Charging by Induction (Permanent)
- Bring a negatively charged rod near a conductor
- Electrons in the conductor are repelled to the far side
- Connect the conductor to earth — the repelled electrons flow to ground
- Remove the earth connection first, then remove the charged rod
- The conductor is left with a positive charge
Electric Fields
An electric field is the region around a charged object where a force is exerted on other charged objects.
Properties of Electric Fields
- Electric field lines point away from positive charges and towards negative charges
- The field is strongest where the lines are closest together (near the charge)
- The field gets weaker with increasing distance from the charge
- A charged object in the field experiences a force in the direction of the field lines (if positive) or opposite (if negative)
- Between two parallel plates, the field is uniform — the field lines are parallel and equally spaced
When drawing electric field lines around a point charge, they should radiate outwards from a positive charge or inwards towards a negative charge, and they must never cross.
Sparking
A spark is a sudden flow of charge through the air. It occurs when the electric field becomes strong enough to ionise the air.
How Sparks Form
- When a charged object has a very high potential, it creates a strong electric field around it
- If the field is strong enough, it pulls electrons out of air molecules — this is ionisation
- The ionised air becomes conducting, allowing charge to flow rapidly through the gap
- This rapid discharge is seen as a spark
- Sparks are more likely with sharp points because the charge concentrates there, creating a very strong local field
Dangers of Static Electricity
Static electricity can be dangerous when sparks ignite flammable materials.
| Situation | Danger | Precaution |
|---|---|---|
| Fuelling aircraft or tankers | Sparks could ignite fuel vapour | The fuel tanker and aircraft are earthed with a conducting wire before fuelling begins |
| Refuelling a car | Static on the person could cause a spark near the fuel | Touch a metal part of the car before handling the nozzle to discharge any static |
| Operating theatres | Sparks could ignite oxygen-rich atmosphere | Antistatic clothing and flooring are used; equipment is earthed |
| Grain silos and flour mills | Dust particles can become charged by friction; sparks could cause explosions | Silos are earthed and the air is kept humid to reduce static build-up |
| Lightning | Charge builds up in clouds; a discharge to ground can be devastating | Lightning conductors provide a safe path for the discharge to reach the ground |
When explaining precautions against static dangers, always mention earthing (connecting to the ground) so that charge can flow safely to earth instead of building up.
Uses of Static Electricity
Static electricity has useful applications in industry and everyday life.
Electrostatic Paint Spraying
- The paint is given a positive charge as it leaves the spray gun
- The object being painted is given a negative charge (or is earthed)
- The paint is attracted evenly to all surfaces, even the back
- Less paint is wasted and the coating is more even
Electrostatic Precipitators
- Used in power stations to remove smoke particles from chimney emissions
- The smoke particles pass through a negatively charged grid and gain a negative charge
- The charged particles are attracted to positively charged metal plates on the chimney walls
- The particles stick to the plates and are collected, while clean gas exits
- This significantly reduces air pollution
Photocopiers and Laser Printers
- A negatively charged drum is exposed to light from the image — illuminated areas lose their charge
- Positively charged toner particles stick to the remaining negatively charged areas
- The toner is transferred to paper and fixed by heating
Van de Graaff Generators
A Van de Graaff generator produces very high voltages by building up static charge on a metal dome.
How a Van de Graaff Generator Works
- A motor drives a rubber belt around rollers inside the machine
- Friction between the belt and rollers transfers charge onto the belt
- The charged belt carries the charge up to the metal dome
- A comb at the top removes the charge from the belt and transfers it to the dome
- Charge builds up on the dome, creating a very high voltage
- If the voltage is high enough, a spark jumps from the dome to a nearby earthed conductor
Van de Graaff Demonstration
When a person touches a Van de Graaff generator while standing on an insulating stand, charge builds up on their body. Their hair stands on end because each strand becomes similarly charged and repels the others. When they step off the insulating stand or touch an earthed conductor, the charge flows to ground and they are discharged.
Lightning
Lightning is a dramatic natural example of static electricity discharge.
How Lightning Forms
- Strong air currents in a thundercloud cause ice particles and water droplets to collide
- Electrons are transferred between particles, creating regions of positive and negative charge
- The bottom of the cloud becomes negatively charged
- This induces a positive charge on the ground below
- When the charge is large enough, the air ionises and a massive spark (lightning) discharges the cloud to the ground
Lightning Conductors
- A thick metal strip runs from the top of a tall building to the ground
- The top has a sharp point which ionises the air, providing a low-resistance path
- If lightning strikes, it follows the path of the conductor to earth rather than through the building
- This protects the building from damage
Earthing
Earthing is the process of connecting an object to the ground using a conductor so that charge can flow safely to earth.
Purpose of Earthing
- Prevents static charge from building up on an object
- Any excess charge flows through the earth wire to the ground
- Reduces the risk of sparks that could cause fires or explosions
- The earth wire in a plug provides a path for fault currents to flow, blowing the fuse and disconnecting the supply
Practice Questions
1. A polythene rod is rubbed with a woollen cloth. The rod becomes negatively charged. Explain what has happened in terms of electrons.
Electrons have been transferred from the woollen cloth to the polythene rod. The rod gains electrons and becomes negatively charged. The cloth loses electrons and becomes positively charged with an equal magnitude of charge.
2. Two negatively charged balloons are hung from strings. Describe the force between them and explain why.
The balloons repel each other because they have like charges (both negative). Like charges always repel.
3. Explain how a charged balloon can attract small pieces of uncharged paper.
The charged balloon induces a charge in the paper. Electrons in the paper are repelled or attracted, creating a separation of charge. The side of the paper nearer the balloon has an opposite charge, so the attractive force is stronger than the repulsive force from the far side, and the paper is attracted.
4. Explain why aircraft are earthed during refuelling.
Friction between the fuel and the pipe can transfer charge, creating static. If charge builds up, a spark could ignite the fuel vapour. Earthing provides a path for the charge to flow to ground, preventing any build-up of static charge.
5. Describe how an electrostatic precipitator removes smoke particles from a chimney.
Smoke particles pass through a negatively charged grid and gain a negative charge. They are then attracted to positively charged metal plates on the chimney walls. The particles stick to the plates and are collected, while clean gas exits the chimney.
🔢 Maths Skills
Mathematical Skills for this Topic
Minimal mathematics in this topic: Static electricity is primarily a conceptual topic rather than a mathematical one. However, you may still need to apply basic mathematical skills. You should be able to describe charge quantitatively: charge is measured in coulombs (C), and the charge on a single electron is approximately 1.6 × 10−19 C. If an object gains n electrons, the total charge gained = n × 1.6 × 10−19 C.
Interpreting diagrams and qualitative data: You may need to interpret electric field line diagrams. Closer field lines indicate a stronger field. The spacing between field lines gives a visual representation of field strength. Parallel equally-spaced lines between two plates indicate a uniform field. Field lines that radiate from a point charge indicate a non-uniform field that weakens with distance.
Using proportionality: The force between two charged objects follows an inverse square relationship with distance. As distance doubles, the force decreases to one quarter. As distance triples, the force decreases to one ninth. This is qualitative at GCSE level but understanding the pattern is important for describing how force changes with distance.
⚠️ Common Misconceptions
Watch Out!
Students often think that static electricity is a completely different type of electricity from current electricity. Wrong: Static electricity and current electricity are two completely different phenomena with nothing in common. Correct: Both static and current electricity involve the movement or presence of electrons. The difference is that in static electricity, charge builds up and stays in one place (on insulators), whereas in current electricity, charge flows continuously through a conductor. They are both caused by electrons — the distinction is whether the charge is stationary or moving.
Students often think that only insulators can become charged. Wrong: Only insulating materials can hold static charge — conductors can never become charged. Correct: Conductors can also become charged if they are isolated from earth (not in contact with a conductor that leads to ground). For example, a metal sphere on an insulating stand can hold static charge. The key requirement for static charge to build up is that there is no path for the charge to flow away — this is achieved by isolation, not by the material being an insulator. In practice, insulators are easier to charge by friction because charge cannot flow through them even without isolation.
✍️ 6-Mark Question
Extended Answer Question
6 marks: Explain how static electricity is generated when two insulating materials are rubbed together and discuss the dangers and uses of static charge.
When two insulating materials are rubbed together, friction causes electrons to be transferred from one material to the other. Electrons are transferred because the two materials have different tendencies to hold onto their electrons. The material that gains electrons becomes negatively charged because it now has more negative charges than positive charges. The material that loses electrons becomes positively charged because it has fewer electrons than protons. The total charge is always conserved — the negative charge gained by one material equals the positive charge left on the other. Only electrons can move because positive charges (protons) are locked in the nuclei of atoms. The charge stays on the surface of the materials because they are insulators, meaning the charge cannot flow away.
Static charge can be dangerous when it leads to sparking. Sparks can ignite flammable materials such as fuel vapour. When refuelling aircraft, friction between the fuel and the pipe can create static charge. If a spark occurs near fuel vapour, it could cause an explosion. To prevent this, the aircraft and fuel tanker are earthed with a conducting wire before fuelling begins, allowing any charge to flow safely to ground. Dust explosions in grain silos are another danger — dust particles become charged by friction and a spark can ignite the dust-air mixture.
Static electricity also has useful applications. In electrostatic paint spraying, the paint is given a charge and the object being painted is earthed, so the paint is attracted evenly to all surfaces including edges and backs. In electrostatic precipitators, smoke particles are charged and then attracted to opposite-charged plates, removing pollution from chimney emissions. Both dangers and uses rely on the same principle of charge attraction and the ability of charge to build up on insulating materials.
Mark scheme: 2 marks for explaining how static is generated (electron transfer, conservation of charge), 2 marks for discussing dangers with specific examples (sparks, fuel, earthing), 2 marks for discussing uses with specific examples (paint spraying, precipitators)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
During refuelling of an aircraft, fuel flows through a pipe at high speed. A safety inspector observes the following conditions at a small airfield: the fuel truck is not connected to the aircraft by an earthing wire, the refuelling nozzle is made of plastic (an insulator), and the operator is wearing non-conductive shoes on a dry day.
Evaluate the risks in this situation and explain what precautions should be taken. Refer to the process of charge generation, the danger of sparking, and the role of earthing in your answer.
Answer: This situation presents a very high risk of a static discharge causing a fire or explosion. As fuel flows through the pipe, friction between the fuel and the pipe walls transfers electrons, creating static charge on both the fuel and the pipe. Without an earthing wire connecting the fuel truck to the aircraft, charge can build up on both vehicles to a high voltage. The plastic nozzle is an insulator, so any charge that builds up on it cannot flow away, increasing the risk. The operator wearing non-conductive shoes on a dry day means they could also accumulate static charge from their own movement, and discharging this when touching the nozzle could create a spark near fuel vapour.
The necessary precautions are: (1) Connect an earthing wire between the fuel truck and the aircraft before fuelling begins, so any charge that builds up can flow safely to ground instead of accumulating. (2) Use a conductive (metal) nozzle that is connected to the earthing system, allowing charge to dissipate rather than build up. (3) The operator should wear conductive (antistatic) shoes or use an earthing strap to prevent personal static build-up. (4) Keep the fuel nozzle in contact with the aircraft filler port to maintain electrical continuity throughout refuelling. These precautions ensure that at no point can a large potential difference build up between any two parts of the system, preventing the risk of a spark that could ignite fuel vapour.
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