Chapter 21 Physics

Electric charges

• Two types of Electric charge one called positive and the other negative.

• When charges are the same in an object it is neutral.

• An object can lose or gain an electric charge therefore it has a net charge

Charge stability

• Static electricity is a tiny imbalance of charges and can be transferred easily to help restore a zero charge.

• Structure of an atom need to know the charge in coulombs and the mass of the parts of an atom.

Parts of the atom

Mass and charges• Proton has a mass of • 1.673 x 10-27 kg

• Proton has a charge of + 1.602 X 10 -19 C

• Neutron has a mass of 1.675 X 10-27 kg

• Neutron has a charge of 0

Mass and charges• Electron has a mass of 9.109

X 10-31 kg

• Electron has a charge of -1.602 X 10-19 C

• Coulomb was named in honor of Charles Coulomb who made the first fine measurement between the forces between charges.

• Coulomb = C and has a charge of 6 X 10 18 Protons or electrons

• The unit of C can apply to both positive and negative charge

Electric forces

• Like charges repeal each other and unlike charges attract each other

• Works with electroscope

Electric current

• Formula is:• I = q/t• I = current in amps• . Q = charge that flows• . t = Time for charge to flow in seconds

Electric current

• Say that current is the movement of electric charge through a circuit.

• Eg. A current of 10 amperes means that 10 amperes of charge flow through the wire every second.

• Drift velocity is caused by an applied voltage added to random bouncing thus causing electrical current.

Conductors and insulators

electrons in a conductor with current move opposite the current flow

Electrons in an insulator do not move at all

Electrons in a semiconductor has a few free electrons to move with the current flows.

Static electricity and Polarization

• Charging by friction causes the attraction of objects

• Polarization of objects causes the insulated material to have electrons go to one side.

• Conductors the entire object becomes polarized.

Coulomb’s law

• F = K q1 q2/ r2

• F = force in Newton's • K = a constant 9 X 109 N m2 / C2

• . q = charges• . r = Distance (m)

Coulomb’s Law

• The force between charges is directly proportional to the magnitude or amount of each charge.

• The force between charges is inversely proportional to the square of the distance between them. Or the inverse square law goes into effect.

Coulomb’ s law

• Used to calculate point charges

• Electric forces always occurs in pairs according to Newton’s third law of motion

Electric fields

• Gravitational field strength = F = mg

• Electric field strength = F= qE• F = Force• . q = charge• E = electric field• Strength of the field = N/kg

Information

• One volt is one joule/coulomb• A joule is equal to a newton-meter• Newton/C = one volt per meter

Capacitors

• Is a storage device for electric charges• Can be charged by connecting it battery• Can be discharged by connecting It to any

closed circuit that allows current to flow

Capacitors

• The current flowing into or out a particular capacitor depends on four things:

• 1. The amount of charge already in the capacitor.

• 2. The voltage applied to the capacitor by the circuit.

• 3. Any circuit resistance that limits the current flowing in the circuit

Capacitors

• 4. The capacitance of the capacitor.

• The amount of charge in a capacitor depends on the following:

• 1. The voltage applied to the capacitor• 2. The insulating ability of the material

between the positive and negative plates

Capacitors

• 3. The area of the two plates ( larger > smaller)• 4. The separation distance between the plates

Capacitance

• . q = CV• . q = Charge (C) • C = Capacitance C?V• V = voltage (V)