physics topic 2 - electricity Flashcards
what is electric current?
the flow of electric charge
how will current only flow around a complete circuit?
if there is a potential difference
what is the unit of current?
amperes, a
what is potential difference and what does it do?
voltage, its the driving force that pushes around the charge
what is the unit for potential difference?
volts, v
what is resistance?
anything in a circuit which slows the flow of charge down
what is the unit of resistance?
ohms
what does the current flowing through a component depend on?
the potential difference across it and the resistance of the component
the greater the resistance across a component=
the smaller the current that flows through it
when current flows past a point in a circuit for a length of time then the charge that has passed is given in what formula ?
I(current)=Q(charge)/ T (time)
what is charge measured in?
coulombs
what is time measured in?
seconds
what passes through a circuit more when a bigger current flows?
more charge
what does the symbol for a fuse look like?
a rectangle with a line going all the way through it
what does the symbol for an LED look like?
a play button with two right upward diagonal arrows
what does the symbol for a resistor look like?
a rectangle with no line going all the way through it
what does the symbol for a variable resistor look like?
a rectangle with an upward right facing arrow going diagonally
what does the symbol for a diode look like?
a play button
what does the symbol for an LDR look like?
a circle with a rectangle in it with two diagonal arrows coming down on it from the left
what does the symbol for a thermistor look like?
rectangle with a line that goes through it diagonally (it starts straight underneath)
what is the equation for resistance?
R=V/I
what is the resistance of an ohmic conductor like?
it is constant and the current flowing the ohmic conductor is directly proportional to the potential difference across it
in what components does resistance change?
filament lamp, diode
what happens when an electrical charge flows through a filament lamp?
it transfers some thermal energy to the lamp which heats up
with the filament lamp what increases with the current ?
resistance
what does resistance depend on with diodes?
the direction of the current
what happens with diodes?
they will happily let current flow in one direction but if its reversed the resistance is very high
what does the term IV characteristics refer to?
a graph which shows the current flowing through a component as the potential difference across it increases
what do linear (ohmic) components have?
have an IV characteristic that’s a straight line
what do non linear (non ohmic) components have?
have a curved IV characteristic
how do you set up an experiment to find a components IV characteristic?
set up a test circuit and use a variable resistor to alter the pd and current across the component. take several reading then swap the wires so the direction is reversed and repeat steps
what is the IV graph for an ohmic conductor like?
current directly proportional to p.d., so you get a straight line
what is the IV graph like for a filament lamp?
when the current makes the temperature of the filament increase, the resistance increases. less current can flow through as the resistance increases so the graph gets shallower
what is the IV graph for a diode like?
current only flows through a diode in one direction so the diode has high resistance in the reverse direction
what is an LDR?
a resistor that is dependant on the intensity of light
in bright light what happens to an LDR?
resistance falls
in darkness what happens to an LDR?
the resistance goes up
give an example of an LDR component?
street lights/ burglar detectors
what does the graph for an LDR look like?
as the light intensity gets higher the resistance gets lower
what is a thermistor?
a temperature dependant resistor
what’s a thermistor like in hot conditions?
the resistance drops
what’s a thermistor like in cool conditions?
the resistance goes up
give an example of a thermistor component?
electronic thermostats
what does the graph for a thermistor look like?
as the temperature increases the resistance decreases
what are sensing circuits used for?
to turn on or increase the power of components on the conditions that they are in
how is the thermistor used in the circuit of a fan?
as the heat increases the resistance in the thermistor drops and the fan can go faster to cool it down
how can you connect the LDR across a variable resistor instead of a fixed one?
if you connect a bulb in parallel to an LDR , the p.d.across both the LDR and the bulb will be high when its dark so the bulb will get brighter as the room gets darker
in series circuits, what happens with the p.d. of the cells if there is more than one?
they add up
what happens to the p.d. in a series circuit?
the p.d. of all the components adds up to the p.d. of the power supply
what is the current like in a series circuit?
current in the same everywhere
whats the size of the current determined by?
the total p.d. of the cells and the resistance of the circuit ( I = V/R)
what happens to resistance in a series circuit?
the total resistance of two components is just the sum of their resistance
what is the p.d. like across a parallel circuit ?
every component has the same p.d. and get the full source p.d.
what is current like across a parallel circuit?
the total current is the sum of the current of each component
what happens to resistance in a parallel circuit ?
if you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors
how can you investigate the resistance by adding more resistors in series?
keep adding resistors in series measuring the pd. and current as you go
how can you investigate the resistance by adding more resistors in parallel?
build a parallel circuit and calculate the current and resistance. after add a resistor in parallel and measure the current going through the whole circuit and use this and the o.d. to calculate the resistance across the whole circuit
in series what, should the graph look like for the investigation?
as the number of identical resistors increases, so does the resistance
in parallel what should the graph look like for the investigation?
as you add the number of identical resistors, the overall resistance decreases
what happens in ac supplies?
the current constantly changes direction
the uk mains supply is ac, how much is it at?
230v
what is the frequency of the ac mains supply ?
50 cycles per second or 50Hz
what is dc (direct current)?
supplied by cells and batteries
what direction does direct current go in?
it always flows in the same direction
what colour is neutral wire?
blue( around 0v)
what colour is the live wire?
brown ( provides the 230v from the mains supply)
what colour is the earth wire?
green and yellow (0v and protects the wiring)
what wire gives you an electric shock as current flows through you?
live wire
energy transferred depends on what?
the power
the total energy transferred by an appliance depends on what?
how long the appliance is on for and its power
what is the equation for energy transferred?
energy transferred = power x time
what are appliances given so that you can see the maximum power it can work at before it becomes dangerous?
a power rating
what does the power rating tell you?
the maximum amount of energy transferred
how does the power rating help customers?
it helps them choose between models because the lower the power rating the cheaper it is to run
an appliance may be more powerful but might not be what?
efficient
what is potential difference?
energy transferred per charge passed
when an electrical charge goes through a change in potential difference, what happens ?
energy is transferred
what is the formula for energy transferred ?
energy transferred = charge x voltage
how can you calculate the power of an appliance?
power= p.d. x current
how do you find the power if you don’t know the p.d.?
p= I squared x r
what is the national grid?
a giant system of cables and transformers that covers the UK to connect power stations to consumers
what does the national grid do?
transfers electrical power from power stations anywhere on the grid to anywhere else on the grid where its needed
power stations have to use enough electricity because?
the demand (usage ) for electricity changes throughout the day therefore enough has to be supplied
when does demand increase for electricity?
when people : get up, come home from school/ work , when it starts to get dark/cold and major sports events
power stations often run well below what?
their maximum power output , so there’s spare capacity to cope with the high demand
what is kept at standby in case the demand isn’t enough?
lots of smaller power stations
to transmit a huge amount of power what do you need?
either a high potential difference or a high current
what is the problem with a high current?
you lose loads of energy as the wires heat up and the energy is transferred to the thermal energy store of the surroundings
its must cheaper to boost up what?
to boost up the p.d. really high and keep the current low
what is the p.d. boosted up to in the national grid?
400,000v
how is the national grid efficient ?
the pd increases decreasing the current which decreases energy lost by heating the wires and the surroundings
how does the potential difference change in the national grid?
through transformers and pylons with huge insulators
what do step up and step down transformers do?
step up transformers make the p.d. bigger to go into the pylons and step down transformers bring it back down to safe, stable levels
what is static electricity caused by?
friction
what happens when certain insulating materials rub together?
negatively charged electrons will be scraped off one and dumped on the other
what does it leave when electrons rub off onto other materials in static electricity?
it leaves materials electrically charged, with a positive static charge on one and an equal negative static charge on the other
what never moves in static electricity?
positive charges
a positive static charge is always caused by what?
electrons moving away elsewhere
as electric charge builds up on an object what happens to the p.d.?
the potential difference between the object and the earth increases
if the potential difference in an object gets large enough what happens?
electrons can jump across the gap between the charged object and the earth- this is a spark
how can you get static shocks getting out a car?
electrons can jump to any earthed conductor that is nearby. a charge builds up on the metal doorframe an when you touch it the charge travels through you to earth
opposite charges do what?
attract
when charges get further apart what happens to the forces?
they get weaker
what opposite force is there between opposite charges?
electrostatic attraction
