topic 11

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  When insulating materials are rubbed together, they can become (statically) electrically charged
• Why insulators become electrically charged when rubbed together

  1. Electrons transfer from one material onto the other

  2. The material gaining electrons becomes negatively charged

  3. The material losing electrons becomes equally positively charged
• Two electrically charged objects are brought close together


  They exert a force on each other
• Two identically charged objects are brought close together

  They exert a repulsive force on each other and repel
• Two oppositely charged objects are brought close together


  They exert an attractive force on each other and attract
• Non-contact force


  The repulsive or attractive force acting between two electrically charged objects
• Why we feel an electric shock

  1. When an object builds up an electrostatic charge, it needs to be 'earthed' via a conductor

  2. Electrons flow to/from the earth to balance the charge

  3. This can also occur if you touch a charged object whilst you are earthed

  4. If earthing occurs through a person/animal, it causes an electric shock
• Why lightning occurs


  1. Clouds can build up an electrostatic charge

  2. When this charge becomes large enough, the clouds discharge via the air to the earth
• A charged balloon is placed next to a wall

  The positive charges near the balloon get repelled to other parts of the wall, leaving an area of the wall with a negative charge, which attracts the positively charged balloon towards the wall, making the balloon stick
• How earthing removes excess charge


  It provides a path with a low resistance for the electrons to flow either from the device or towards the device to reduce the charge build up
• How static electricity causes danger when fuelling cars


  1. Static charge can build up due to the friction between the pipes and the fuel

  2. When the charge builds up, a spark may be discharged

  3. The spark could start a fire if it reacts with the flammable fuel

  4. This is resolved by earthing the vehicle and pump
• Electric field


  A region where an electric charge will experience a non-contact force
• Direction of electric field lines

  In the direction that a positive charge would move (experience a force)
• Appearance of electric field lines in a parallel field

  • Straight lines, which are parallel to each other and point from the positive plate to the negative plate
  • At the edges, the lines are partially curved
• Comparing electric field strengths

  The field lines being closer together shows a stronger field
• How electric fields explain static electricity

  1. The object that is statically charged will generate an electric field

  2. This field attracts the electrons on other objects causing a flow of charge resulting in sparks