Electricity

Electrical Charge and Forces  Electrical charge is the property that causes protons and electrons to attract or repel one another.  There are electric charges in clothes that stick together from the dryer.  The attraction or repulsion between electrical charges is called electric force.  Like charges repel each other and opposite charges attract each other.

 An electric field is the effect an electric charge has on other charges in the space around it.  The strength of an electric field is dependant on 2 things.  1. amount of charge producing the field.  2. distance from the charge.  The further the 2 charges are away from each other, the less force they feel.  The closer the 2 charges are, the more force they feel.

Electric Field Lines  Lines pointing in the direction of the electric field.  Represent the strength and direction of the field.  Electric field is stronger where lines are closer together.  Lines point away from a positive charge and towards a negative charge.

Electric Materials  There are 2 ways to describe whether a substance is able to conduct electricity.  1. Conductors – materials in which electric charges move easily.  Copper, iron, and most other metals.  2. Insulators – materials in which electricity does not move easily.  Glass, rubber, plastic.

Charge Transfer  Static electricity is the study of the behavior of electric charges, including how it is transferred between objects.  Charge can be transferred by:  1. friction.  2. contact.  3. induction.

Friction  Rubbing a balloon on your head is an example of charging by friction.  Electrons move from your hair to the balloon because the atoms in the balloon attract electrons much more strongly than the atoms in your hair.  The balloon gets a negative charge and your hair gets a positive charge.

Contact  A Van de Graaf generator charges a metal sphere and when you touch it you acquire a charge large enough to make your hair stand on end.  The sphere is still charged, but its net charge is reduced.

Induction  Induction is a transfer of charge without contact between materials.  Ex: When you walk across the carpet your hand picks up a net negative charge.  When you reach for a door knob, the negative charge of your hand causes the electrons in the doorknob to all move to the base of the doorknob.  The doorknob still has a net charge of zero, but the charges have moved.

Homework Questions  1. What determines whether charges attract or repel?  2. Name 2 factors that affect the strength of an electric field.  3. List the 3 methods of charge transfer.  4. Explain and give an example of charging by friction.  5. When a glass rod is rubbed with silk, the glass becomes positively charged. What is the charge on the silk? Explain your answer.