Physics Term 1

Static electricity

DC Circuits

Practical Electricity

Current of electricity

Charging

Determining charge
Electroscopes
Metal knob connected to two thin lightweight pieces of metal foil, called leaves
1.charge electroscope
2.if leaves remain, object is neutral
3.if leaves open further, object is of the same kind of charge of elctroscope
4.if leaves close, its the opposite

Induction

  • two metallic objects
  • charged rod brought near
  • electrons attracted/repelled to the further/nearer object
  • separate objects
  • spheres have an equal and opposite charge

Conduction

  • Contact between objects
  • electron flows from the more negative object to the more positive object

Friction (for insulators)

  • energy from friction removes outer electrons from wool
  • extra electrons on shoes causes a net negative charge
  • charge conserved > the wool gets a net positive charge

Grounding
Removing excess charge

Coulomb
Unit for charge
charge of 6.24 x 10^18 electrons

Resistance

V/I

Measurements

P = VI = (I^2)R = V^2/R

Kilowatts hour
(3.6MJ)

Calculate resistance

Series
R(eff)=R1+R2+R3+...

Parallel
R(eff)=1/(1/R1+1/R2+1/R3+...)

Definitions

Discharge

Ground (Earth)

Charges

Neutral

No charge

Something either +or -

Reservoir of free electrons to absorb/supply electrons

Movement of charges

Something + or - moving around

Charging

Something charged becomes neutral

Charges "appear"

Potential difference

Current

Parallel

Series

Parallel

Series

v1 = v2 = v

v = v1 + v2

i = i1 + i2

i = i1 = i2

RI

V/R

Current

  • rate of flow of electrical charges (ie how many electrons flow per second)
    I = Q/t
  • Q in Coulombs
  • t in seconds

Discharging (insulators)

Heating

  • heat causes air around object to be ionised,
  • the ions in the air can neutralise the charges in the object

HUMID CONDITIONS water vapour, a polar molecule, removes excess charges on the insulator
(negative charges "leak off" as they as attracted to the positive end of water molecules)

Electrostatic Hazards

Lightning

  • friction builds up between water molecules and air in thunderclouds
  • thunderclouds become charged
  • the air becomes ionised
  • provides conducting oath down to nearest/sharpest object on ground

Electrostatic Discharge

  • friction (tires on trucks)
  • rubbing (rugs)

Electric Fields

  • lines are denser in regions of stronger electric field (near the charges)
  • NO LINE SHOULD INTERSECT
  • number of lines is proportional to magnitude of charge
  • +ve -> -ve

Conventional Current
Opposite to direction of flow of electrons (from -ve)

Ammeter

  • Connected in series
  • No/Very little resistance
  • Measures current

Electromotive Force (EMF)

  • work done by source per unit charge driven around a complete circuit (ie J/C)
    ℰ = W/Q
  • ℰ in Volts (V)
  • W is work done is Joules (J)
  • Q is Coulombs (C)

Potential Difference

  • work done to drive a unit charge through two points
    V = W/Q

Voltmeter

  • Infinite/Very high resistance
  • Connected in parallel
  • Measures potential difference

Resistance

  • ratio of potential difference across a component to the current that flows through it
    R = V/I
    VICTORY FOR RI

Resistors

Fixed Resistor

  • Fixed resistance

Variable Resistor

  • move slider
  • change length
  • change resistance

Factors affecting resistance

  • length (as length increases, resistance increases)
  • cross sectional area (as cross sectional area increases, resistance decreases)
    R = ρ(l/A)
  • ρ is resistivity of material in Ω m
  • l is length
  • A is cross sectional area

Ohm's Law

  • current passing through a metallic conductor is directly proportional to the potential difference across the ends provided physical conditions are the same

Household electricity

Parts

Earth wire(Green yellow)

  • No potential no voltage
  • Touches metal casing
  • Safety mechanism

Neutral wire(Blue)

  • No potential no voltage

Live wire(Brown)

  • high potential and voltage
  • Contains fuse(melts and opens circuit after certain Current)

When the circuit breaks, the electricity goes via the earth wire, causing the fuse to break (safety)

Alternative : double insulation

  • has insulation other than wire covering to prevent any exposed metal