9. Electricity and magnetism

• Forms of energy: atoms and electromagnetic radiation
• Define: static electricity
• How do we know that it is a force?
• Positive and negative charges
• Coulomb’s law
• Compare strength with gravity
• Define electric field
• How are natural magnets, i.e. lodestone, formed?
• Magnetic field
• Define dipole

• What happens when you walk across a rug and then touch another person? You get a shock. Why? Static electricity.
• E.g. (1) Lightning, (2) When you comb your hair and it stands up. (3)Walk across a rug on a dry day and your clothes stick to you.
• These are examples of the same phenomena, static electricity.
• What is going on here?
• We recognize that we are dealing with a new force. It is not gravitational.
• Newton did not know this force, but his laws tell us how to recognize it. We see that an object is being accelerated. This means that a force is responsible.
• We know that if you rub amber with cat’s fur then touch other objects with the amber then these objects repel each other. The reason is static electricity. (The Greek word for amber is “Electro”). In these experiments the electrical charge does not move once it has been placed on an object so it is called static electricity.
• It is clearly different from gravitation. For instance, gravity is only attractive. Electricity can be both attractive and repulsive. The electrical force is much more powerful than the gravitational. For instance, a comb can lift a piece of paper against the entire gravitational force of the planet.
• We recognize two forms, negative and positive.
• Before 1750, the nature of electricity was little more than a curiosity, and it was poorly understood.
• Most people thought that electrical effects were the result of mixing of two different “electrical fluids”.
• Benjamin Franklin started studying electricity in 1742. He became convinced that there was only one single electric fluid and that objects could have excess or deficiency of this fluid. Therefore, he invented the terms positive and negative, referring to the deficiency.
• In 1752, he conducted his famous kite experiment in which he flew a kite in a thunderstorm and lightning hit it and passed along the string, producing an electric spark. From this observation, he invented the lightning rod.
• We understand now that he was wrong in thinking about one fluid. There are both positive and negative charges. But, in his defense, he was seeing the movement of the negative charge, i.e. electrons, only.
• Negatively charged electrons move around a positively charged nucleus, and the electric force holds the atom together, much like gravitation holds the planets around the sun. The result is a neutral atom.
• The outermost electrons are not held in with strong forces, so they can easily be removed from the atom, resulting in free electrons and positive ions. If the electrons end up residing in another body then that body will become negatively charged.
• When you run a comb through your hair, then your hair loses electrons to the comb. The hair is then positively charged and the comb is negatively.
• All the atoms in the sun are so excited because of the high energy that no electrons are attached to them.

• In the 1780’s Coulomb undertook measurements wherein he obtained objects with different charges and measured their forces. The result is known as Coulomb’s law

where k is a constant, and q₁ and q₂ are the charges of two bodies that are at a separation of d. k is a constant anywhere in the universe.
• The 1/d² part is similar to that in gravity. The units for q are coulombs which is equal to 6.3x10¹⁸ electrons.
• Gravity is 10³⁹ orders of magnitude weaker than electricity. So we always ignore the effects of gravity within an atom.

• Imagine a charged body sitting at some point. If you brought a second charged body near it then it would feel a force. The strength and direction of the force would vary, depending upon the position of the second object.
• A collection of all the possible strengths and directions of the force can be constructed to represent the electric field (force field). It is what a second object would feel if it were brought to a spot.
• We think of this force field as surrounding every charged object.

• We define the electric field as a representation of the way bodies, not in contact, can influence each other.
• E.g. gravitational field. Electric field.

• Another force, as demonstrated by the acceleration of nails under the influence of a magnet.
• Natural magnets have been known for 1000s of years. The most common is a form of magnetite, Fe₃O₄, called lodestone. We still are not certain as to their origin, but most scientists believe that lodestone is magnetite that has been hit by lightning.
• Chinese first put magnetism to practical use by inventing the compass. Europeans extensively used it as a guide to navigation. They took a sliver of lodestone, and balanced it so that it could freely rotate. It always points north south.
• In the 1500’s Gilbert studied magnets and found that they contained two poles, labeled N and S. The N pole points north. He learned how to make magnets by stroking them with a piece of lodestone. He found that iron could be made magnetic by hammering it. Heating destroyed the magnet. He was the first to realize that earth itself was a giant magnet. He discovered that each magnet has two poles, and that like poles repel, unlike poles attract.
• So if the earth is a giant magnet, then which pole is in Canada?

• A magnetic field exists around each magnet. Its orientation is shown by metal filings.

• We know that birds fly north for the summer and fly south for the winter without problems. How do they know which way to go?
• The first demonstration that animals had internal magnet came in 1975. A single-celled bacteria has a series of small magnetite grains built into their bodies. This allows them to navigate within the sludge of a pond bottom.

• Magnets have been found in some birds.
• The Australian silvereye has a modification in their eye that allows them to see the magnetic field.

• Any field with two poles is called a dipole. All magnets are dipoles. Cut a magnet in half and we end up with 2 magnets, each with their own N and S pole.
• Question? Is the resulting magnetic force smaller?