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Static Electricity (AQA GCSE Physics)
Revision Note
Charging by Friction
- When certain insulating materials are rubbed against each other they become electrically charged
- This is called charging by friction
- The charges remain on the insulators and cannot immediately flow away
- One gains a net positive charge and the other gains a net negative charge
- An example of this is a plastic or polythene rod being charged by rubbing it with a cloth
- Both the rod and cloth are insulating materials
A polyethene rod may be given a charge by rubbing it with a cloth
Movement of Electrons
- Charging by friction occurs due to the movement of electrons
- When a polyethene rod is rubbed with a cloth, negatively charged electrons are transferred from the rod to the cloth
- The cloth has gained electrons, so it becomes negatively charged
- The rod has lost electrons, so it becomes positively charged
When electrons are rubbed onto a cloth, it gives it a negative charge and leaves the rod with a positive charge
Examiner Tip
At GCSE level, if you are asked to explain how charge is gained or lost, you must reference electrons. If an object gains electrons, it gains negative charge and if it loses electrons it loses negative charge (and hence, gains positive charge)
Remember when charging by friction, it is only the electrons that can move, not any 'positive' charge, therefore if an insulator gains a negative charge, the other insulator must have gained a positive charge
Static Electricity
- Electricity is caused by the movement of charge in a circuit
- This occurs in conductors
- Static electricity, on the other hand, is caused by a build up of stationary charge on a surface
- This occurs on the surfaces of insulators
- Some examples of static electricity are:
- The accumulation of dust particles on surfaces
- Hair sticking up after combing it with a plastic comb or going down a plastic slide
- Rubbing a balloon and sticking it to a wall
- Sparking
Examples of Static Electricity
Static electricity can be seen in everyday scenarios when two or more insulators interact
Demonstrating Forces between Charges
- All objects are initially electrically neutral
- This means the negative and positive charges are evenly distributed within the object
- When charged objects are brought near to each other, forces of attraction or repulsion can be observed
- When two objects with different types of charge come together, a force of attraction is observed
- When two objects of the same type of charge come together, a force of repulsion is observed
- One example is sticking a balloon to the wall:
- Rubbing a balloon on a woollen jumper transfers electrons onto the balloon by friction
- The balloon becomes negatively charged and the jumper becomes positively charged
- The wall is uncharged initially, but when the balloon is placed near the wall, the electrons on the balloon's surface repel the electrons on the wall's surface
- Furthermore, the positive charges on the wall's surface are attracted to the negatively charged balloon which allows the it to stick to the wall
Sparking
- The build-up of electrostatic charge can lead to sparking
- An example of this is the small electric shock sometimes felt when a person touches a metal door handle
Sparking can occur when a charge builds up on a person which is allowed to flow when they touch an earthed metallic object, such as a door handle
- A spark occurs between two objects when
- There is a large potential difference between the two objects
- Which causes a current to flow between them
- Another example of sparking is the small electric shock felt from touching another person after walking on a vinyl floor or nylon carpet with rubber shoes
Sparking can occur when a person with a large build up of charge comes into contact with an uncharged person as there is a large potential difference between them
- Sparking often occurs between a charged insulator and an earthed conductor
- An earthed conductor is a wire, usually made from copper, that allows a current to flow to the Earth
- A current will always take the path of lower resistance
- Since copper has a lower resistance than, for example, a person, the current will flow from the insulator to the Earth through the copper wire rather than the person
- Sometimes, when a potential difference between two objects becomes very large, the electric field may become strong enough to cause the breakdown of air
- As a result, an electrical discharge (large spark) can travel through the air to allow the current to flow
- This can be seen in a Van de Graff generator or a lightning strike
Dangers of Sparking
- Sparking can become dangerous in certain situations such as:
- Electrocution e.g. by lightning
- Ignition of a fire or explosion by a spark
Lightning
- An extreme example of sparking is lightning
- In a storm, ice crystals in clouds rub against each other causing a movement of electrons between them
- The top of the cloud becomes positively charged, and the bottom becomes negatively charged
- The electrons on the ground are strongly repelled by the negative charge on the cloud, which causes it to become positively charged
- The potential difference between the cloud and the ground becomes increasingly large (~106 V)
- Eventually, the cloud discharges a large spark as the negative charges jump to meet the positive charges on the ground
Lightning is caused by the build-up of charge in clouds
Ignition by Sparking
- A spark may ignite an explosion or fire when close to a flammable gas or liquid
- For example, when refuelling aeroplanes, a build-up of static charge can pose a significant danger
- As the fuel passes through a pipe, the friction between them causes static charge to build up
- If the potential difference becomes too large, it could cause a spark
- A spark could then ignite the fuel and cause an explosion
Sparking can be reduced when refuelling a plane by connecting a bonding line to the Earth to allow any excess build up of charge to dissipate and travel to the ground
- The risk can be reduced by connecting the fuel tank to the Earth with a wire called the bonding line
- The conductor earths the aeroplane by carrying excess charge through to the Earth and removes the risk of any sparks
Examiner Tip
When answering exam questions on static electricity, make sure to use keywords such as 'friction' and the transfer of 'electrons'. Avoid saying the transfer of 'charge' since this is too vague and will not get you full marks unless you state it is the movement of negative charge
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