Topic 5 Flashcards
What happens to resistance with temperature
RESISTANCE INCREASED WITH TEMPERATURE
-when current flows through a resistor, it heats up
-this happens when electrons collide with the ions in the lattice, giving the ions energy, causing them to heat up
-the more the ions vibrate, the harder it is for electrons to get through, so for a given p.d the current decreases as the resistor heats up
-if the resistor gets too hot, no current can pass through (apart from in a thermistor
Current-voltage graphs
Current on y axis, voltage on x axis
-Resistors and wires - straight line through the origin (like x = y graph shape)
-filament lamp - s shape graph through origin (like y = x*3)
-diode - current flows in one direction so it doesn’t go into negatives on the y axis
-curves upwards from x axis, not necessarily through the origin
LDR
-light dependent resistor
-in bright light the resistance falls
-in darkness, resistance is highest
-used for things like automatic night lights and burglar detectors
Thermistors
-temperature dependant resistor
-in hot conditions, resistance is low
-in cold conditions, resistance is high
-used for things like temperature detectors in car engines and electronic thermostats
Adding resistors in series vs parallel
Series:
-adding resistors increases total resistance, as it is just the sum of resistance
-all resistors have to share the p.d, so p.d is lower, so current is also lower
Parallel:
-adding resistors decreases total resistance
-both resistors have the same p.d, and the force causing current to flow is the same for each
-by adding another loop, there is another way for current to flow, causing an increase in current and therefore a decrease in total resistance
Wires in a plug
-live wire (brown) carries the voltage (230V)
-neutral wire (blue) completes the circuit (0V)
-earth wire (green and yellow) is for safety as it carries the current away if something goes wrong, stopping the appliance from becoming live (0V)
Mains electricity
-a.c at around 230V
-frequency of about 50 cycles per second (50Hz)
How do fuses work
-surges in current can occur because of changes in a circuit or faults and can cause fires or electric shocks
-this surge melts the fuse which is connected to the live wire, breaking the circuit, cutting off the live supply
-fuses should be rated to just above the normal operating current
Circuit breakers
-these are used in place of fuses
-instead of melting a fuse, a large current will ‘trip’ a circuit breaker
-circuit breakers work quicker than fuses and can be reset, which is easier but more expensive than replacing a fuse
How does static cause sparks
-as electric charge builds on an object, the p.d between the object and the earth (which is 0V) increases
-if the p.d gets large enough, electrons can jump across the gap between the charged object and the earth (this is the spark)
-they can also jump to any earthed conductor that is nearby, which is why you get static shocks from clothes or getting out of a car
-this usually happens when the gap is small, but not always as lightning is this same process on a larger scale
Uses of static electricity
-electrostatic sprayers like paint or insecticide sprayers
How an static electricity be dangerous
-refuelling cars - as fuel flows through the filler pipe, static can build up, which can lead to a spark, being potentially very dangerous
-static on aeroplanes - as planes fly though the air, the friction between the aircraft and the air causes the plane to become charged. This buildup can interfere with with communication equipment
-lightning - raindrops and ice bump together inside storm clouds, leaving the top of the cloud positively charged and the bottom negative. This creates a huge voltage and a big spark, which can damage homes or start fires as it strikes the ground
-some of these dangers can be reduced by earthing objects
Earthing objects
-dangerous sparks can be prevented by connecting a charged object to the ground using a conductor (eg. Copper wire), this is called earthing
-earthing provides an easy route for static charges to travel to the ground
-the electrons flow down the conductor if they’re negative and up if they’re positive
-fuel tankers must be earthed to prevent any sparks that cause the fuel to explode
Main magnetic elements
Iron nickel and cobalt
Compounds of these metals are also magnetic eg. Steel, as it contains iron
Permanent vs induced magnets
-permanent magnets produce their own magnetic field all the time
-induced magnets only have a magnetic field when they are in another object’s magnetic field
-if you put any magnetic material into a magnetic field, it becomes and induced magnet
-when that magnetic field is taken away, it stopped producing its own magnetic field. Hard materials like steel lose their magnetism slowly so permanent magnets are usually made form these materials
