Electric Charges

Intro

Have you ever walked across a carpet in stocking feet and reached for the doorknob only to be “zapped” by an unexpected shock? Why does your hair mysteriously stand on end after you pull off a wool hat on a winter day? These event are actually caused by the same thing!

Shocking, Isn’t It?

• To explain why you get a shock by touching a doorknob after walking across the carpet, we need to become familiar with static electricity

• Particularly:– Electric Charges– Law of Attraction & Repulsion– How objects become charged– Insulators & Conductors

Charges

The atom….

Eureka

What Is A “Charge”?

• All objects are made of atoms

• Atoms contain protons, neutrons and electrons

• An atom must keep their protons and neutrons “trapped” inside their nucleus.

• Electrons are “free” to move around.

What Is A “Charge”?

• When an object contains an unequal number of protons and electrons it is said to be charged– Negatively charged means there are more electrons than

protons– Positively charged means there are more protons than

electrons

• When an object contains an equal number of protons and electrons it is said to be neutral– Neutral objects do not carry a charge

• Benjamin Franklin named charges “positive” and “negative” about 100 years before the electron was discovered

Law of Attraction & Repulsion• Scientists found that a charged object

will either attract or repel another charged object

• Two objects with the same charge will repel each other

• Two objects with opposite charges will attract each other

• This is summarised in the “Law of Attraction and Repulsion”

Like charges repel, unlike charges attract

• This force of attraction or repulsion is known as electric force.

• Object that exert an electric force are said to have an electric charge.

• A neutral object has an equal number of protons and elections.

• An object that has more electrons than protons is negatively charged.

• An object that has fewer electrons that protons is positively charged.

PLAY: Electric Field Hockey

Time to Experiment with Charges