17.1 Electric charges Concept of charge What will happen when you comb your hair on a dry day? The hair is attracted to a plastic comb.

17.1 Electric charges How about bringing a rubbed plastic ruler near small pieces of aluminium foil? The ruler picks up the pieces of aluminium foil.

17.1 Electric charges in the comb when you comb your hair What causes these phenomenon? in the ruler when you rub the ruler Electric charges are produced:

17.1 Electric charges The objects carrying electric charges are said to be charged.

17.1 Electric charges The rubbed end of the ruler exerts an electric force on the small pieces of paper. The unrubbed end of the ruler cannot pick up the pieces of paper.  These phenomena are called static electricity.  Electrostatics is the study of charges at rest. F

17.1 Electric charges Forces between charges They repel each other. They have the same kind of electric charges (negative). Why? What happens when you bring two rubbed polythene strips together?

17.1 Electric charges They repel each other. They have the same kind of electric charges (positive). Why? How about bringing two rubbed acetate strips together?

17.1 Electric charges The rubbed polythene strip attracts the rubbed acetate strip. They have different kinds of electric charges. Why? How about bringing a rubbed acetate strip and a rubbed polythene strip together?

17.1 Electric charges Unlike charges attract. Like charges repel. Video: Interaction between electric charges

17.1 Electric charges What determines the magnitude of eletric force between two charges?  The distance between charges  The quantity of charges For a fixed quantity of charges: For a fixed distance: distance force quantity of chargesforce

17.1 Electric charges A microscopic view of charges  All substances are made of atoms.  Each atom consists of: EXTENSION proton neutron electron (positively charged) (negatively charged) (uncharged) nucleus

17.1 Electric charges  number of protons = number of electrons  An uncharged object gains electrons. It becomes negatively charged. atoms are neutral

17.1 Electric charges  An uncharged object loses electrons. It becomes positively charged. - –

17.1 Electric charges  The transfer of electrons can be caused by rubbing. Electrons are transferred from the acetate strip to the duster. acetate strip loses electrons duster gains electrons – – – – – – –

17.1 Electric charges Electrons are transferred from the duster to the polythene strip. polythene strip gains electrons duster loses electrons – – – – – – –

17.1 Electric charges Attraction of uncharged objects plastic ruler (negatively charged) aluminium foil (uncharged) – – – – – – – – – – – – – – – – – – – – – – – – – induced -ve charges repelled by the ruler induced +ve charges attracted by the ruler induced charges When a negatively charged ruler is brought near a piece of uncharged aluminium foil...

17.1 Electric charges plastic ruler (negatively charged) aluminium foil (uncharged) – – – – – – – – – – – – – – – – – – – – – – – – – attractive force repulsive force Top end of the foil is closer to the ruler attractive force > repulsive force The foil is attracted by the ruler.

17.1 Electric charges plastic ruler (negatively charged) – – – – – – – – – – – – – – – – – – When a negatively charged ruler is brought near a piece of paper... molecules + – + – + – + – + – + – + – + – + – The charge distribution of the molecules are distorted. a piece of paper (insulator)

17.1 Electric charges plastic ruler (negatively charged) a piece of paper (insulator) – – – – – – – – – – – – – – – – – – – – – – induced -ve charges repelled by the ruler induced +ve charges attracted by the ruler attractive force repulsive force attractive force > repulsive force The paper is attracted by the ruler.

17.1 Electric charges cotton plastics Conductors and insulators Materials through which charges cannot flow easily — electrical insulators Can you give some examples of good insulators? glass

17.1 Electric charges copper Materials allow charges to flow through easily — electrical conductors Can you give some examples of good conductors? silver Free electrons can move freely in metals. Metals such as:

17.1 Electric charges That’s the end of Section 17.1 Check Point Key Ideas Previous Page Section 17.2 Exit