electricity Flashcards
what is current
the rate of flow of charge
ohms law
the current in an ohmic conductor is proportional to the voltage across it, provided that the temperature is kept constant
why temperature affects resistance
as the temperature increases the electrons vibrate faster and therefore more collisions between them occur, making it more difficult for them to flow, increasing the resistance.
AC/DC
alternating current: mains supply, the flow of the current changes direction
direct current: cells and batteries, the flow of the current remains in the same direction
fuses vs circuit breakers
both break the circuit when the current gets too high. Fuses have a very thin wire made of a metal with a low melting point so that if the current gets too high it will melt and the fuse will ‘blow’ so the current can’t flow through. (connected to live wire, most common is 3A, 5A, 13A). Circuit breakers get ‘tripped’ and turn off the circuit.
- fuses are simple and cheaper but have to be replaced each time the fuse wire melts
- circuit breakers can be reset but are more expensive
earthing/earth wire
earthing is when an alternative pathway for the current to escape is provided so that the electricity is diverted through the earth wire rather than coming through us. This prevents electric shocks if the live wire comes loose in an appliance and touches the metal casing.
double insulation
when the entire appliance is covered in a plastic casing so that there is no exposed metal parts that we can touch. plastic is an insulator, doesn’t conduct electricity so we cannot get an electric shock. no need for earth wire if an appliance is double insulated
current in resistors
when a current flows through a resistor there is an electrical energy transfer which increases the temperature of the resistor.
this is because the electrons collide with the ions in the lattice that make up the resistor as they flow through it. This gives the ions energy which makes them vibrate and heat up.
This increase in temperature increases the resistor’s resistance, so less current will flow.
toasters and resistors
toasters contain a coil of wire with a really high resistance. when a current passes through the coil, its temperature increases so much that it causes it to glow and give off infrared radiation which cooks the bread.
power
the rate of energy transferred
power formula
power= current x voltage
P= I x V
energy transferred formula
E= V x I x t
or
E= Q x V (since Q= I x t)
adding more resistors to a parallel circuit
decreases the total resistance because it is easier for the electrons to flow through the circuits, there are more paths
LDR’s
light dependent resistors, high resistance in the dark, low resistance in the light.
e.g automatic lighting, burglar alarms, photographic exposure films
thermistors
high resistance in the cold, low resistance in the heat
e.g fire alarms
electric current in solid metallic conductors
electric current in solid metallic conductors is a flow of negatively charged electrons
why is current conserved at a junction
electrons are a physical matter that cannot be created or destroyed, charge is conserved. the total amount of current flowing through the circuit must be constant
voltage is
energy transferred per unit charge passed
the volt is a joule per coulomb
charge formula
Q= I x t
static electricity practical
two insulating materials like an uncharged polythene rod and an uncharged cloth are rubbed together. the electrons from the cloth move onto the rod leaving the cloth with a positive charge. The rod becomes negatively charged. You can test this by holding the rod over tiny pieces of paper, if the rod is charged the electrons in the rod will attract the protons in the paper and it will stick to the rod.
dangers of static electricity
fuelling: as fuel flows out of a filler pipe, static can build up. this can easily lead to a spark which in a dusty or fumy place can cause an explosion.
lightning: rain drops and ice bump together inside storm clouds, knocking off electrons and leaving the top of the cloud positively charged and the bottom negative. this creates a huge voltage and a big spark
inkjet printer
tiny droplets of ink are forced out of a fine nozzle, making them electrically charged. the droplets are deflected as they pass between two metal plate. a voltage is applied to the plates, one is negative and the other is positive. the droplets are attracted to the plate of the opposite charge and repelled from the plate with the same charge. the size and direction of the voltage across each plate changes so each droplet is deflected to hit a different position on the paper.
photocopier
positive charges are sprayed onto a turning drum whose surface is covered in selenium. a bright light is shone onto the paper to be copied. the white parts of the paper reflect the light onto the drum, the dark part don’t. the places where the light has been reflected onto the drum, the selenium loses its charge but where no light was reflected the charge remains. a negatively charged powder called toner is dropped onto the drum where it sticks to the charged areas (opposite charges attract). a piece of paper is pressed against the drum and picks up the pattern of the carbon powder, the powder is then fixed in place by a heater.
