Static Electricity AQA

Electric charge

​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 becomes positive and the other negative

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Friction

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• This occurs because negatively charged electrons are transferred from one material to the other

• The material, in this case, the rod, loses electrons

• Since electrons are negatively charged, the rod becomes positively charged

  • As a result, the cloth has gained electrons and therefore is left with an equal negative charge

Electric Forces Between Charges

• The charge of a particle is either:

  • Positive

  • Negative

  • Neutral (no charge)

• Attraction and repulsion between two charged objects are examples of a non-contact force

  • This is a force that acts on an object without being physically in contact with it

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​Static electricity

• Static electricity is the stationary electric charge which is produced by friction which causes sparks, or the attraction of other small objects such as dust or hair

  • This is caused by the imbalance between negative and positive charges in two objects

• This only works for insulators (conductors allow the charge to move through them)

• This applies to solids, liquids and gases.

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​One example is sticking a balloon to the wall

• Rubbing a balloon on a woolen jumper transfers electrons onto the balloon by friction

• The balloon is now negatively charged whilst the jumper is left positively charged

• The wall is still neutral, however, when the balloon is placed near the wall, the positive charges in the wall are brought to the surface because they are attracted to the negative charge of the balloon

• Since opposite charges attract, the balloon sticks to the wall from only the electrostatic attraction

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• These experiences are most common in dry air conditions

• This is because it is much harder for the charges to dissipate so they build up instead, and make static electricity more noticeable

• Some of these can cause sparking

​Sparking

• The build-up of electrostatic charge can be quite dangerous and can cause sparking (also known as an electric shock)

• A static electric spark occurs when two objects which are charged by friction and become oppositely charged and have a surplus of electrons so large that the electrons 'jump' across to an object that is neutral

• Since a current is the flow of electrons, this causes a small current to flow between the objects, called a spark

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• In a storm, clouds move over each other causing them to become charged when electrons are transferred between them

• Since the ground is neutral, the negative charge jumps to meet the positive charges on the ground creating a big spark

• This is what is known as lightning

extreme sparking

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Electric fields

• A charged object creates an electric field around itself

  • This is similar to the way in which magnets create magnetic fields

• This can be shown by electric field lines

• Fields lines always point away from positive charges and towards negative charges

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• The strength of an electric field depends on the distance from the object creating the field:

  • The field is strongest close to the charged object - this is shown by the field lines being closer together

  • The field becomes weaker further away from the charged object - this is shown by the field lines becoming further apart

• Objects in an electric field will experience an electric force

  • Since force is a vector, the direction of this force depends on whether the charges are the same or opposite

electric force

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• The size of the force depends on the strength of the field at that point

• This means that the force becomes:

  • Stronger as the distance between the two charged objects decreases

  • Weaker as the distance between the two charged objects increases

• The relationship between the strength of the force and the distance applies to both the force of attraction and force of repulsion

  • Two negative charges brought close together will have a stronger repulsive force than if they were far apart

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• The electric field lines for a charged, isolated sphere, such as a spherical conductor:

  • Point away from the centre of a positive sphere

  • Point towards the centre of a negative sphere

fields and charge

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fields and charge

• A uniform electric field, such as that between two parallel plates, are straight parallel lines from positive to negative:

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fields and charge

• The electric field pattern between two oppositely charged spheres (or point charges) is slightly different and looks like this:

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Draw

draw the electric field pattern around the Van de Graaff dome

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