Static Electricity, Charging by Friction & Electric Field Patterns
In Triple Science GCSE Physics, static electricity occurs when two electrical insulators are rubbed together, causing negatively charged electrons to be transferred by friction from one material's surface to the other. The material that gains electrons becomes negatively charged, and the material that loses electrons is left with an equal positive charge (protons in the nucleus never move). Every charged object creates an electric field in the space around it, where another charged object experiences a non-contact electrostatic force of attraction or repulsion, and where a high enough potential difference can ionise air molecules to cause a spark.
- Charging Insulators by Friction
- When certain insulating materials (such as a polythene rod or an acetate/perspex rod and a dry woollen cloth) are rubbed together, friction transfers negatively charged outer-shell electrons from one material to the other. The material that gains electrons becomes negatively charged; the material that loses electrons is left with an equal positive static charge.
- Law of Electrostatic Attraction & Repulsion
- Electrostatic force is a non-contact force. Two objects that carry the same type of charge (two positive charges or two negative charges) repel each other. Two objects that carry opposite types of charge (one positive and one negative) attract each other. A charged object can also attract a small uncharged object (like bits of paper or a thin stream of water) by inducing an opposite charge on the nearest surface.
- Sparking, Ionisation of Air & Earthing
- As electric charge builds up on an isolated object, the potential difference between the object and the Earth (at 0 V) increases. If the potential difference is large enough, the strong electric field causes electrons to be removed from air particles (ionisation), turning air (normally an insulator) into a conductor so charge flows across the gap as a spark. Earthing connects the object to the ground with a metal conductor so excess electrons safely flow to or from the Earth, preventing sparking.
Electric Field Simulator & Interactive NPN Transistor Builder
Adjust charges and distances to visualize Coulomb’s Law field lines, or switch to the Transistor Lab to wire and test an NPN circuit.
1. Coulomb’s Law Point-Charge Calculation
Formula: F = k · |Q₁ · Q₂| / r² where k = 1/(4πε₀) = 8.99 × 10⁹ N·m²·C⁻²
3.60 N
Attractive (towards each other)7.19e+6 N/C
At r/2 = 0.10 m2. Uniform Parallel-Plate Field Calculator
Formula: E = V / d (units: V m⁻¹ or N C⁻¹)
Key Concepts & Exam Definitions (6)
- Charging Insulators by Friction
- When certain insulating materials (such as a polythene rod or an acetate/perspex rod and a dry woollen cloth) are rubbed together, friction transfers negatively charged outer-shell electrons from one material to the other. The material that gains electrons becomes negatively charged; the material that loses electrons is left with an equal positive static charge.
- Law of Electrostatic Attraction & Repulsion
- Electrostatic force is a non-contact force. Two objects that carry the same type of charge (two positive charges or two negative charges) repel each other. Two objects that carry opposite types of charge (one positive and one negative) attract each other. A charged object can also attract a small uncharged object (like bits of paper or a thin stream of water) by inducing an opposite charge on the nearest surface.
- Sparking, Ionisation of Air & Earthing
- As electric charge builds up on an isolated object, the potential difference between the object and the Earth (at 0 V) increases. If the potential difference is large enough, the strong electric field causes electrons to be removed from air particles (ionisation), turning air (normally an insulator) into a conductor so charge flows across the gap as a spark. Earthing connects the object to the ground with a metal conductor so excess electrons safely flow to or from the Earth, preventing sparking.
- Electric Fields & Field Strength
- A charged object creates an electric field around itself. A second charged object placed in this field experiences a non-contact electrostatic force. The electric field is strongest closest to the charged object, and gets weaker as distance from the charged object increases (because the field lines spread further apart).
- Radial electric field lines (isolated spheres)
- Around an isolated spherical point charge, the electric field pattern is radial (lines look like spokes of a wheel). The lines must be drawn perpendicular (at 90°) to the surface of the sphere, evenly spaced, and never crossing. For a positive (+Q) sphere, arrows point radially outwards; for a negative (-Q) sphere, arrows point radially inwards.
- Uniform Electric Field (Parallel Plates)
- Between two oppositely charged parallel metal plates, the electric field is uniform: the field lines are straight, parallel, equally spaced, perpendicular to the plates, and have arrows pointing from the positive plate to the negative plate.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
When an acetate rod is rubbed with a cloth, the acetate rod becomes positively charged. Explain how this happens in terms of particles.
How could you experimentally prove whether a charged rod is charged rather than just magnetic or uncharged? (Repulsion vs Attraction).
Explain in 3 steps how a high potential difference causes a spark across an air gap.
Why must aircraft and fuel tankers be connected by a metal bonding/earthing cable before refuelling?
In which direction do the arrows on electric field lines always point?
Explain how electrostatic paint spraying gives an even coat of paint on a metal bicycle frame and reduces paint waste.
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
A child with dry hair slides down a plastic playground slide. Explain why the child's hair stands on end at the bottom of the slide, and why the child receives a small electric shock when they subsequently touch a metal gate post. (Triple Science) [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. As the child slides down the plastic slide, friction between the child's clothing/hair and the insulating plastic transfers negatively charged electrons from one to the other [1 mark]. 2. Every strand of the child's hair gains the same type of electrostatic charge (e.g., all strands become negatively charged) [1 mark]. 3. Because like charges repel each other, the individual strands of hair experience a non-contact electrostatic repulsive force and push apart (stand on end) [1 mark]. 4. This build-up of static charge creates a large potential difference between the child and the earthed metal gate post (which is at 0 V) [1 mark]. 5. When the child touches (or gets very close to) the metal gate, electrons flow rapidly through the child's hand to the Earth (discharging/earthing the child), felt as a small electric shock [1 mark].
- Q2
Explain how a lightning strike occurs between a storm cloud and the ground in terms of charge, potential difference, electric fields, and air molecules. (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Friction between rising water droplets and falling ice particles inside the storm cloud causes a massive build-up of static charge at the base of the cloud [1 mark]. 2. This creates a huge potential difference (millions of volts) and a very strong electric field between the charged cloud and the ground (0 V) [1 mark]. 3. The strong electric field causes electrons to be ripped away from air molecules, ionising the air [1 mark]. 4. Although air is normally an electrical insulator, ionised air is a conductor, allowing a huge current (electrons) to flow rapidly between the cloud and the ground as a lightning flash [1 mark].
- Q3
Describe the shape, spacing, and direction of the electric field lines around an isolated positively charged metal sphere, and explain what happens to the electrostatic force on a negatively charged dust particle as it is moved closer to the sphere. (Triple Science) [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The electric field lines are straight radial lines extending outwards in all directions, perpendicular (at 90°) to the surface of the sphere [1 mark]. 2. The arrows on the field lines point outwards, away from the positive sphere [1 mark]. 3. As the negatively charged dust particle is moved closer to the positive sphere, it experiences an attractive electrostatic force towards the sphere [1 mark]. 4. The magnitude of this attractive force increases as the particle gets closer because the electric field is stronger closer to the sphere (where the field lines are closer together) [1 mark].
- Q4
Explain how a negatively charged polythene rod is able to pick up small, uncharged pieces of tissue paper from a desk. (Triple Science) [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. When the negatively charged polythene rod is brought close to the uncharged tissue paper, its electric field repels the negatively charged electrons in the surface atoms of the paper slightly away from the rod [1 mark]. 2. This leaves the top surface of the tissue paper closest to the rod with an induced positive charge [1 mark]. 3. Because opposite charges attract (and the induced positive charges are closer to the rod than the repelled negative charges), the upward electrostatic attraction is greater than the weight of the tissue paper, lifting it up [1 mark].
Common Exam Questions (4)
What is the golden rule of static electricity that examiners always check?
Only negatively charged electrons are ever transferred when insulators are rubbed together. Never say that 'positive charges' or 'protons' move or are rubbed onto an object; protons are locked tightly inside the atomic nuclei. An object becomes positively charged only because it has lost negatively charged electrons.
Why can you charge a plastic polythene rod by rubbing it with a cloth while holding it in your hand, but you cannot charge a copper metal rod the same way?
Polythene plastic is an electrical insulator, so the electrons transferred onto its surface cannot flow away and remain trapped in one place ('static'). Copper is an electrical conductor containing delocalised electrons; any electrons transferred onto the copper rod immediately conduct through the metal and through your body to the Earth.
How does a build-up of static charge cause a spark (or lightning)?
As more static charge builds up on an isolated object, the potential difference (voltage) between the charged object and the Earth (at 0 V) increases enormously. If the potential difference becomes high enough, the strong electric field pulls electrons out of air molecules (ionising the air). Ionised air becomes an electrical conductor, allowing a sudden flow of electrons (current) to jump across the air gap as a spark.
Which way do the arrows point on an electric field diagram, and how do you show field strength?
Electric field lines always have arrows pointing away from a positive charge and towards a negative charge (the direction a small positive test charge would be pushed). The strength of the electric field is shown by how close together the field lines are: closer lines mean a stronger electric field.
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