P4 - Electricity Flashcards
electric field
region in which an electric charge experiences a force
for a positive charge, what direction would the force be experienced?
outwards
for a negative charge, what direction would the force be experienced?
inwards
electric current
charge passing a point pre unit time
electrostatic charge
materials’ charge
where does electric charge come from?
batteries and generators / if rubbed
what 2 materials can used to create charge?
polythene (becomes -ve) and perspex (becomes +ve)
closer the charges, _____ the _____ between them
greater force
what does rubbing materials do?
seperates charges that are already there
conductors and examples?
materials that let electrons pass through them as they have free electrons between indivisual atoms
-ex copper, gold, metals, carbon
insulators and examples?
materials that dont conduct as electrons are tightly held by atoms and are not free. (but it can be transferred by rubbing and once transferred, it stays. and its easy to charge)
-ex: plastics, glass, dry air
semi-conductors and examples?
materials that are poor conductors when cold, good conductors when warm
-ex: silicon, germanium
direction of conventional current? flow of electrons?
positive to negative / negative to positive
earthing
connected to the ground by conducting materials so that unwanted charges flow away. if this is not done, electrons may be pulled through the air and cause dangerous sparks
voltage
cause of current in the circuit
induced charges
appear on an unchanged object because there’s a charged object nearby (opposite charges)
what can electrostatic charge be detected by?
leaf electroscope
static electricity
charges that are not free to move (build up in one place, shock when they move) (build up of charge on insulating materials)
static electrcity uses [3]
-electrostatic smoke precipitators (dust attracts to the charged plates)
-spray painting (spray drop is +ve, drops repela and spread out, earth is -ve and the car is connected to earth, drops attracted to the car)
-photocopiers (black toner attracted to paper)
emf
electricatl work done by a source in moving a unit charge around a complete circuit (V) (across cell)
pd
work done by a unit charge passing between two points in a circuit (V) (across device)
resistance
ratio of voltage to current in an appliance (measures load)
how to measure emf?
voltmeter across cell
how to measure pd?
voltmeter across appliance (device)
if resistance is greater, what happens to voltage and current?
resistance increases, voltage increases, current decreases
resistance formula
voltage / current
current formula
charge / time (I=Q/t)
what is Ohm’s law?
current is directly proportionate to Voltage (V increases, I increases)
what are ohmic conductors? examples? graph?
conductors that follow Ohm’s law
-examples: metallic conductors at room temperature
-I-V graph is a straight line through the origin
Non-Ohmic conductors? graph?
conductors that don’t follow Ohm’s law
-I-V graph is not a straight line through the origin
relation between length and resistance
resistance is directly proportionate to length (longer the wire, more resistance)
relation between area and resistance
resistance is inversely proportionate to area (thin wire, large resistance / thick wire, small resistance)
what happens to resistance if area doubles?
resistance halves
what happens to resistance if radius doubles?
decreases by 4 times
formula for volts
energy / charge (coulombs)
formula for energy
(voltage x current = power) x time
formula for power (energy)
energy / time
formula for power (voltage)
voltage x current (amps)
unit for energy (power flow with time)
kilowatt-hour (kwh)
what is a variable resistor
helps introduce varying resistance in the circuit
HOW DOES LIGHT DEPENDANT RESISTOR AFFECT RESISTANCE?
LIGHT INTENSITY INCREASES, RESISTANCE DECREASES (DARK = MORE RESISTANCE) (CURVED LINE C ON GRAPH THOUGH)
what do electric circuits do?
transfer energy from a source of electrical energy (electrical cell or mains supply) to the circuit components and then into the surroundings
total resistance of series circuit
R1 + R2 + R3
4 features of series circuit
-voltage is divided
-if one disconnects all don’t work
-current is the same anywhere (ampmeter)
-V1 + V2 + V3 = supply voltage (EMF)
total resistance in parallel circuit
1/R1 + 1/R2
-total resistance is less than smallest resistance (each resistor)
5 features of parallel circuit
-current splits up in proportion do resistance in each branch (larger resistance, larger current)
–> I1 + I2 = I
-each branch gets same voltage as supply
-used in domestic wiring
-if one disconnects, others still work
-current from source is larger than current in each branch
short circuit
huge current goes through (no resistor) and could cause a fire
