Electrostatics and Magnetism

• There are two kinds of charge

  Positive and negative
• All objects are made up of atoms, and all atoms contain positive particles (protons) and negative particles (electrons)
• Atoms do not have an overall charge. They are neutral. That is because atoms contain the same number of protons and electrons. So the positive and negative charges are cancelled out. Static electricity or electrostatic charge is caused by an atom having too many or too few electrons. Protons or the positive charges cannot move as they are located in the nucleus.
• Electrostatic charge means that charge (electrons) can sit on the surface of an insulator and remain stationary i.e. 'electro'-electron, 'static' not moving. Only insulators can have electrons sitting on the surface or 'static', as conductors would allow the electron to flow or conduct which will stop charge from building up on the surface of the conductor.
• Plastic is a good insulator and it is easily charged. Electrons can be removed from surfaces or added to surfaces by rubbing two insulators together. This is called charging by friction (rubbing).
• Charging a polythene rod with a cloth

  Method

  1. Hold a polythene rod
  2. Rub the polythene rod with a duster
  3. Hold the end of the rod near small pieces of paper

  Observations: The pieces of paper stick to the charged polythene rod.

  Explanation: The polythene rod is charged by friction. Electrons transfer from the cloth to the rod. The polythene rod is charged negatively as it has gained electrons. The paper is attracted to the charged rod because a charge is induced in the paper which attracts it to the rod.
• Charging an acetate rod with a cloth

  Method

  1. Hold an acetate rod
  2. Rub the acetate rod with a duster
  3. Hold the end of the rod near small pieces of paper

  Observations: The pieces of paper stick to the charged acetate rod

  Explanation: The acetate rod is charged by friction. Electrons transfer from the rod to the cloth. The acetate rod is charged positively as it has lost electrons. The paper is attracted to the charged rod because a charge is induced in the paper which attracts it to the rod.
• Charing a metal rod

  Method

  1. Hold a metal rod
  2. Rub the metal with a duster
  3. Hold the end of the rod near small pieces of paper

  Observations: The pieces of paper are not affected by the rod. They are not attracted to the rod.

  Explanation: The metal rod is a conductor. Any charge that is transferred to the rod will conduct through the metal to your hand. Your hand acts as an Earth wire and no charge builds up on the rod. The metal rod remains neutral. The paper is not attracted to the rod as it is not electrostatically charged.
• Repulsion of two polythene rods

  Method

  1. Charge a polythene rod by friction
  2. Place it carefully on the top of an upside down watch glass
  3. Charge a second polythene rod and hold it next to but not touching the balanced polythene rod.
  4. Move it closer to the rod and further from the rod.

  Observations: The polythene rod on the watch glass will twist away or is repelled by the second polythene rod.

  Explanation: Polythene rod is charged negatively by friction. Like charges repel. The closer the rods are the greater the electrostatic force of repulsion.
• Attraction of polythene and acetate rods

  Method

  1. Charge a polythene rod by friction
  2. Place it carefully on the top of an upside down watch glass
  3. Charge an acetate rod and hold it next to but not touching the balanced polythene rod.

  Observations: The polythene rod on the watch glass will twist towards or is attracted to the acetate rod.

  Explanation: Polythene rod is charged negatively by friction. Acetate is charged positively by friction. Unlike charges attract.
• Balloon sticking to wall

  Method

  1. Rub a balloon on a head or polyester sweater
  2. Hold balloon near wall or ceiling

  Observations: The balloon sticks to the wall or ceiling

  Explanation: The balloon charges negatively. Electrons transfer from the sweater to the balloon. When the balloon is held near the wall, electrons in the wall are repelled away from the surface. This induces a charge in the wall and the surface becomes positively charged. Opposite charges attract. The negative balloon is attracted to the positively charged surface of the wall.
• What is 'electro'-'static' charge?

  Electrons which are stationary (stuck) on the surface of an insulator or an isolated metal object.
• How can insulators such as plastic be charged?

  Rub them together - the friction between them will transfer electrons from one surface to the other
• Does a polythene rod become positively or negatively charged when it is rubbed with a cloth?

  Polythene becomes negatively charged. When rubbed together the electrons transfer from the cloth to the rod.
• Does an acetate rod become positively or negative charged when it is rubbed with a cloth?

  Acetate becomes positively charged. When rubbed together the electrons transfer from the rod to the cloth.
• Can positive charges in an insulator move? Explain.

  No, they are bound to the nucleus.
• Opposite charges attract. Describe one experiment to prove this fact.

  Attraction of polythene and acetate rods.

  1. Charge a polythene rod by friction
  2. Place it carefully on the top of an upside down watch glass
  3. Charge an acetate rod and hold it next to but not touching the balanced polythene rod.
• How does the force of attraction depend on the distance between the charges?

  As distance between the charged objects decreases, the force of attraction increases.
• Describe what happens as electroscope is charged by friction with a polythene rod.

  The negatively charged polythene rod is rubbed across the electroscope. The electrons transfer from the rod to the electroscope. The negatively charged electrons are trapped on the electroscope and they repel and spread out. The post is negative and the gold leaf is negative - they repel and the gold leaf moves upward.
• How can a charged object be attracted to a neutral object? Describe how a balloon can stick to a neutral wall.

  The negatively charged balloon is moved towards the wall. The electrons in the neutral wall are repelled from the surface, leaving the surface of the wall positive. This is called an induced charge. The negatively charged balloon is attracted to the positively charged wall.
• Danger of refueling

  Problem: Aircraft become electrostatically charged during flight (air rubbing up against the aircraft body). The build-up of charge on the plane can discharge to the ground or to the fuel nozzle. This discharge or spark is very dangerous near a flammable fuel. Any spark may result in an explosion.

  Solution: Earth the aircraft, the fuel tanker and the fuel nozzle so that this electrostatic charge is discharged through the wire to Earth. If there is no build-up of charge there is no risk of a spark and no risk of an explosion.
• Lightning rod

  A metal rod which is Earthed
• Solution to danger of lightning

  1. Connect a lightning rod to the highest point on a building

  2. When the lightning bolt hits the rod the charge travels down to the Earth and into the ground

  3. This greatly reduces the risk of fire
• Grain shoot

  Grain travels down a shoot when delivered into a silo
• Danger of grain shoot

  1. Grain rubs up against the shoot

  2. Causes a build-up of charge on the shoot

  3. Electrostatic charge can discharge to the silo

  4. Discharge or spark is very dangerous in a high dust area

  5. Moving grain produces a very fine dust

  6. Dust has an extremely high surface area

  7. Spark may result in an explosion
• Paint spraying

  Nozzle has positive (+) charge.

  Object has negative (-) charge.

  Positively charged paint droplets repel each other and spread out.

  Advantage to this: you get an even coat.

  Positively charged paint droplets are attracted to the negatively charged object.

  Advantage: no paint is wasted and paint droplets stick to all sides of the object (back and front).
• Photocopier

  The semi-conductive drum is positively charged.

  Light is shone on the paper. The dark image absorbs light. The white paper reflects light onto the drum. This makes the drum conductive and charge on white areas leak away, leaving only the area of the image charged. Negatively charged toner is attracted to the positively charged image. Heating the paper fuses the toner onto it.
• Soot removal

  The gas particles pass through the negatively charged grid. This makes the gas particles negatively charged.

  The negatively charged gas particles are attracted to the positively charged metal plates on the side of the chimney. Waste gases without particles continues up chimney. The metal plates are knocked to remove the build-up of soot.
• When a plastic rod is rubbed with a cloth, the plastic rod gains electrons.

  After the plastic rod has been rubbed with the cloth, the plastic rod has a negative charge.
• Electrostatic charges can be useful during paint spraying.

  1. The droplets of paint are given the same charges as they leave the sprayer. Explain why this is an advantage.

  • The droplets repel each other and they spread out as like charges repel
  • This creates a fine spray and even coat

  2. The droplets of paint are positively charged. The object being painted is given a negative charge. Explain why this is an advantage.

  • The droplets are attracted to the object as opposites attract.
  • Less paint is wasted - all the paint goes to the object - none on the floor.
• Solution for grain shoot

  Earth the grain shoot, tractor and silo so that this electrostatic charge is discharged through the wire to Earth. If there is no build-up of charge, there is no risk of a spark and no risk of an explosion.
• You cannot isolate a single magnetic pole; they always occur together. If you break a magnet in two, you get two smaller dipole magnets. If you continue to break the magnet into two over and over again then you would end up with an atom which acts like a little magnet. Adjacent atoms, and large groups of atoms line up with each other. These groups of aligned atoms are called magnetic domains.
• An ordinary piece of iron by itself is not a magnet. This is because the magnetic domains are randomly orientated, and their effects cancel. In the presence of a magnetic field, the domains are induced into alignment, and the iron becomes magnetised. The degree of magnetism depends on the degree of alignment. When all the domains are aligned in the same direction then the magnetism is at its strongest and the magnet is said to be saturated.
• Magnetic or not?

  A bar magnet easily picks up nails, paper clips and iron filings, and we say that these are magnetic materials. On the other hand, a magnet has no observable effect on nonmagnetic materials such as wood or aluminium. The magnetic properties of a material depend on the magnetic field of its electrons. Common magnetic materials, called ferromagnetic materials, are iron, nickel and cobalt. There are more nonmagnetic metals than magnetic.
• Magnetically soft: Iron, use: electromagnets.

  Magnetically hard: Steel, use: permanent magnets.
• Copper: non-magnetic

  Steel: magnetic

  Iron: magnetic

  Tin: non-magnetic

  Aluminum: non-magnetic

  Nickel: magnetic

  Brass: non-magnetic

  Magnesium: non-magnetic

  Zinc: non-magnetic
• If the magnetic field is removed, thermal motion (vibration of particles) causes the domains to go back into a random orientation, and the magnetism is lost. Iron is considered to be magnetically soft because it loses its magnetism easily when it is removed from a magnetic field. Other material such as steel (an alloy of iron) is magnetically hard since its domains are locked in their alignment. These magnets can be destroyed by heating them or striking it on the hard surface to effectively jumble the aligned domains.
• Law of magnetism: Opposite poles attract, like poles repel
• The force that is felt by a magnet or a piece of magnetic material by another magnet is called the magnetic force. Magnetic materials feel a magnetic force when they are in a magnetic field.
• A magnetic field is a region where a magnetic material experiences a force.