Flashcards in Electricity Deck (33):
how is static electricity formed?
charging of insulators by friction
how are positive and negative charges formed?
movement of electrons from one material leaves positive charge and produces negative charge on material gaining electrons
how does static electricity occur by rubbing rod with cloth?
- with polythene rod, cloth transfers electrons onto rod
- with acetate rod, electrons are transferred onto cloth
name three uses of static electricity
- paint sprayers
- dust precipitators
how do paint sprayers work?
- paint gun is charged which charges paint
- paint drops repel each other giving fine spray of paint
- object is opposite charge so attracts fine spray of paint all over equally
how do dust precipitators work?
cleans up emissions from chimney
dust passes through charged grid
atttact to metal plates so then no pollutants released to air
what is the main danger of electrostatics?
large build up of charge produces sparks
how can sparks be prevented?
earthing so charged object is connected to ground using a conductor so easy route for charge to go straight into ground without causing a build up
what is current
flow of charge
formula for current
current = charge/time
units of current:
A - amperes
units of charge:
C - coulombs
what is voltage
amount of energy transferred per unit of charge
what is another term for voltage
formula for voltage
voltage = energy/ charge
units of voltage
formula for resistance
resistance = voltage/ current
units for resistance
how does current vary with voltage through a fixed resistor?
describe curve on V-I graph for a filament lamp
s- shaped curve
how does resistance link with temperature
resistance increases with temperature because as electrical energy is converted to heat energy the ions vibrate more so current cannot flow as easily hence resistance increases
how is voltage and current distributed in a series circuit?
- V: shared
- I: same
how is voltage and current distributed in a parallel circuit?
- V: same
- I: shared
how is resistance calculated in series and parallel circuits?
series: adds up
parallel: 1/Rt= 1/R1 + 1/R2 + 1/R3
what is power?
how much energy transferred per second
unit of power
formula for power
current x voltage
formula for energy transferred
power x time
what is electromagnetic induction?
creation of p.d. across a conductor experiencing a changing magnetic field
how do ac generators work?
rotating coil in magnetic field - every half turn current changes direction
- higher current when coil is perpendicular to field
factors affecting induced voltage
- number of turns
- speed of turns
- cross-section of coil
- strength of magnetic field
formulas for step up/down transformers
Vp Ip = Vs Is (power in = power out) as a consequence of 100% efficiency
Vp/Vs = Np/Ns