Electricity New Flashcards
electrical current
the rate of flow of electric charge
what is current measured in
amperes/amps (A)
1 amp = 1 coulomb flowing in one second
which way does current flow
positive terminal to the negative terminal of a cell
how to measure current
ammeter
what is charge measured in
coulombs (C)
equation linking current, charge and time
Q = I x t
Q = charge
I = current
t = time
voltage
-the energy transferred per unit charge passing between two points
-measured in volts(V)
what is one volt equivalent to
1 joule per coloumb
what happens when electrons flow through a cell
gain energy
how much energy do electrons gain when do flow through a cell
depending on the voltage of the cell, e.g. 12V mean 12Joules per coulomb
how to measure voltage
using a voltmeter
how must a voltmeter be set up
in parallel with the component being measure
why must voltmeters be set up in parallel
they have a very high resistant so it does not alter the flow of current
equation linking energy transferred, charge and voltage
E = Q x V
resistance
-the opposition of a component to the flow of electric current through it
-measured in ohms (Ω)
why are wires made of copper
-has a low electrical resistance, making it a good conductor
equation linking resistance, potential difference and current
V = I x R
how does the current change throughout a series circuit
current is the same throughout
how does current in a parallel circuit change
the current has different values at different points in the circuit, the sum of the current in the individual branches = total current before and after the branches
how does the current flowing into and out of a junction in a parallel circuit change
-it is the same as charge is conserved
how to find the total resistance of a series circuit
R = R1 + R2
how to find total voltage in a series circuit
V = V1 + V2 + V3
what does it mean for the resistance if the relationship between current and voltage is linear
-the resistance is constant
what does it mean if the relationship between current and voltage is non linear
the resistance is non linear
shape on non linear graphs including axes
-s shape(current increases at a proportionally slower rate
-current on y, voltage on x
examples of components with linear IV graphs
-fixed resistors at a constant temperature
-wires at a constant temperature
components with non-linear IV graphs
-filament lamps
-diodes
-LDRs
-thermistors
why does resistance increase as current through a filament lamp increase
-higher current causes the temperature of the filament to increase
-the higher temperature causes the atoms in the metal lattice of the filament to vibrate more
-increase in resistance as it becomes more difficult for free electrons to pass through
-since resistance opposes the current, this causes it to increase at a slower rate
diode
allows current to flow in one direction
forward bias
only allowing the current to flow in one direction
why do diodes only allow current to flow in one direction
it has a very high resistance and no current flow(reverse bias)
IV graph of a diode
flat when x <0, then curves up
ammeter
measures the current through the component
voltmeter
measures the voltage across the component
variable resistor
varies the current through the circuit
power source
to provide a source of potential difference(voltage
wires
connects the components together in a circuit
how can a circuit be set up to vary current to measure IV for a filament lamp or diode
-variable resistor to change the current flowing through the filament lamp/diode
dependent and independent variables for investigation of current and voltage for a filament lamp/diode
current is independent variable
voltage is dependent variable
how do variable resistors vary resistance
change the length of wire that makes up the circuit
what type of wires have higher resistance
-longer and thinner wires have more resistance
how does the resistance of a thermistor change
resistance of a thermistor depends on its temperature
how does resistance of thermistor vary when temperature changed
as temperature increases, resistance of a thermistor decreases
-as temperature decreases, the resistance of a thermistor increases
thermistor graph of resistance and temperature
Curves up(current on y voltage on x)
how does resistance of an LDR vary
depends on the light intensity on it
how does resistance of an LDR change with light intensity
-as light intensity increases the resistance of an LDR decreases
-as the light intensity decreases the resistance of an LDR increases
how to show greater light intensity on an LDR when drawing circuits
more arrows onto the LDR
LDR graph of resistance and light intensity
Inversely proportional
LED/light emitting diodes
only allow current to flow through them in one direction
power
the rate of energy or the amount of energy transferred per second
equation linking power, current and voltage
P = I x V
unit of power
watts(joules per second)
what is a fuse
safety device to cut off the flow of electricity to an appliance if the current becomes too large
what does a fuse consist of
glass cylinder containing thin metal wire that melts easily
what happens if the current in a wire with a fuse becomes too large
-the wire heats up and melts
-causes the wire to break, breaking the circuit and stopping the current
-prevents more damage to the equipment or a fire
how to choose the right rating of a fuse
slightly higher than the current needed by the appliance
what happens if the fuse current rating is too low
-it will break the circuit when an acceptable current is flowing through
what happens if the fuse current is too high
it will not break the circuit in enough time before damage occurs
work done
energy transferred
equation relating energy, power and time
E = P x T
equation relating energy, current, voltage and time
E = I x V x t
examples of electrical safety hazards
-damaged insulation - potential lethal shock of someone touches exposed wire
-overheating cables - too much current through a small wire leads to wire overheating; fire or melts insulation
-damp conditions - moisture in contact with live wires can conduct electricity, causing a short circuit in a device(fire) or electrocution risk
safety features built into domestic appliance
-double insulation
-earthing
-fuses
-circuit breakers
insulation
covering the conducting part of wire with an insulating material to improve electrical safety
double insulation
-appliances have two layers of insulation; one around the wire and a non metallic case as a second layer of insulation
-no earthing required
hazards of metal casing
if live wire comes into contact with the case, the case becomes electrified, electrocuting anyone who touches it
what does earth wire do
additional safety wire to reduce risk of electrocution if live wire touches the metal casing
what happens if live wire comes into contact with metal casing if appliance is earthed and fused
-earth wire provides low resistance path to the earth
-surge of current in the earth wire and in the live wire
-high current through fuse causes it to melt and break
-cuts off supply of electricity to the appliance
circuit breaker
automatic electromagnet switch that breaks the circuit of the current exceeds a certain value
advantage of circuit breaker
-does not melt and break, can be reset and used again
-works faster
how is energy lost from resistacne
energy is dissipated into the surroundings by heating
direct current
a steady current, constantly flowing in the same direction in a circuit, from positive to negative
what appliances produce direct current
cells or batteries
how does current travel in a dc circuit
in one direction from positive terminal to negative terminal
alternating current
current that continuously changes its direction, going back and forth around a circuit
what terminals does an alternating current power supply have
two identical terminals that change from positive to negative and back again
what produces alternating current
electrical generators
main electricity voltage
230V
what type of current is mains electricity
alternating current
what frequency does main electricity current alternate at
50Hz
conductor
a material that allow charge to flow through it easily
why are conductors able to conduct electricity
they are made up of positively charged metal ions and outermost electrons delocalised, so they are free to move
insulator
material that has no free charges, does not allow the flow of charge through them very easily
how can insulators conduct electricity
they can conduct static electricity, charge can build up on their surfaces and if touched the charge would be conducted away
practical to investigate charging by friction
- Take a polythene rod, hold it at its centre and rub both ends with a cloth
- Suspend the rod, without touching the ends, from a stand using a cradle and nylon string
- Take an acrylic rod and rub it with another cloth
- Without touching the ends of the acrylic rod bring each end of the acrylic rod up to, but without touching, each end of the polythene rod (if the ends do touch, the rods will discharge and the forces will no longer be present)
- Record any observations of the polythene rod’s motion
- Repeat, changing out the acrylic rod for rods of different materials
what will happen in the practical where two insulators are charged
-if materials is repelled, then the materials have the same charge
-if material is attracted, then the materials have opposite charges
what happens if two objects have the same charge
the will repel each other
what happens if two objects have opposite charges
they will attract
what happens if a neutral object and a charged object are near each other
they will attract each other
how to increase the force of two charged objects
bring them closer together
what happens if an object loses electrons
it becomes positively charged
what happens if an object gains electron
it becomes negatively charged
what type of force is attraction and repulsion force an example of
non contact force
static electricity
accumulation of charge on an object
what happens when insulating materials are rubbed against each other
-they become electrically charged(charge by friction)
-negatively charged electrons are transferred from one material to the other
why can charge remain on insulators
the charge cannot immediately flow away
photocopiers
- Image of document is projected onto positively charged plate
- The plate loses its charge in the light areas and keeps the positive charge in the dark areas (i.e the text)
- A negatively charged black toner powder (the ink) is applied to the plate and sticks to the part where there is a positive charge
- The toner is then transferred onto a new blank sheet of white paper
- The paper is heated to make sure the powder sticks (hence why photocopied paper feels warm)
bicycle sprays
-as drops of paints emerge from spray gun they are charged
-drops carry the same charge so they repel and spread out forming a thin spray
-metal bicycle frame has a wire attached to an electrical supply giving the frame opposite charge
-paint drops attracted to the surface of the frame, reaching difficult places
inkjet printer
-droplets of ink are forced out of a fine nozzle, and are then given an electrical charge
-droplets are deflected as they pass between two metal plates. A voltage is given to the plates, one is -ve, one is +ve
-droplets are attracted to the plate of the opposite charge and are repelled from the plate with the same charge
-size and direction of the voltage across each plate changes so each droplet is deflected to hit a different place on the paper
electrostatic precipitators
-smoke rises up chimney, passing through mesh of wires that is negatively charged
-ash and dust particles become negatively charged
-charged particles are attracted by and stick to metal earthed plates
-cleaner smoke is released into the atmosphere
dangers of static electricity
-risk of electrocution
-risk of fire or explosion due to a spark close to a flammable gas or liquid
sparking
-when large amounts of charge builds up, producing large potential difference across a gap
-if potential difference is large enough, current can travel through the air between objects, a spark
how is fuelling vehicles dangerous
-fuel runs through pipes at a fast rate
-friction between fuel and pipe causes the fuel to gain charge
-this charge can cause a spark and fuel could ignite and cause an explosion
how to prevent dangers of fuelling vehicles
-connect fuel tank to the Earth with a copper wire called bonding line during refuelling
-this carries charge through to the Earth which stops risk of sparks
-easier for charge to flow down bonding line than spark
