Physics Topic 4

Electricity and Magnetism

Electrostatics

Charged Objects

• Charges come about from electrons moving

Conservation of Charge

• Total charge of a system remains constant

Coulomb’s Law

• Unit for charge = Coulomb (C) = 6.25 x 1018

• Charge of an electron = -1.6 x 10-19 C• Charge must be a whole number multiple of

the charge of an electron!!!

Electric Fields

• Electric field line – imaginary line along whicha positive test charge would move in anelectric field.

Potential Difference• If a charged particle’s motion opposes the

direction of an electric field, work must bedone to move it.

• Amount of work to move an elementarycharge against an electric field with a potentialdifference of 1V

= 1 electronvolt (eV) = 1.60 x 10-19 J

Electric Current• Current – the rate at which charges pass a

given point in a circuit• Circuit – closed path along which charges

move• Switch – device tomake, break, orchange connectionsin a circuit

Current (I)

• Measured by ammeter• Units are Amperes, A

Conductivity• Conductors – material in which electrons

move readily• Insulator – material in which electrons do not

move readily

Ohm’s Law

• Resistance (R) – opposition that a device offersto the flow of electrons.

• Measured in ohms (Ω)

Resistance• Resistivity, ρ, depends on electronic structure

and temperature.• Resistance is directly proportional to resistivity

and length of wire• Resistance is inversely proportional to the

cross-sectional area

Resistors

• Resistor – device with a definite amount ofresistance. Used to limit current.

• Variable Resistor – amount of resistance canbe changed

Electric Circuits

Series Circuits

• All parts are connected end to end to providea single path for the current

Parallel Circuits

• Elements are connected between two points,with one of the two ends of each componentconnected to each point

More Equations

Magnetism• Magnets have 2 Poles (North and South)• Magnetism – force of attraction between

magnetic poles• FUN FACT – Earth’s South Magnetic Pole is the

Geographic North Pole and vice-versa

• Strength of field = density of field lines

Electromagnetic Induction

• Induced Potential Difference – a voltagecreated in a conductor due to its motion in amagnetic field.

• An electromagnetic wave is formed when oscillating oraccelerating electric charges produce changing electricand magnetic field