Flashcards in module 2 Deck (56):
What is electric current?
it is a flow of electric charge. Will only flow round a complete circuit if there is a potential difference so current can only flow if there is a source of potential difference.
Is the current the same or different in 1 loop?
What is voltage?
driving force that pushes charge round
What is resistance?
anything that slows the flow down. Unit is ohm
What does a current flowing through a component depend on?
potential difference across it and the resistance of the component. The greater the resistance across a component the smaller the current (for a given potential difference across the component).
What is current?
The rate of flow of charge, when current flows past point in circuit for length of time then charge has passed in formula, more charge passes around circuit when larger current flows
What is the formula for current?
Q=IT Q=charge flow (coulombs, C) I=current(A) T=time(s)
How do you draw wires in a circuit diagram?
in straight lines and closed circuit, can follow wire from 1 end of the power supply through and components to other end of power supply.
What is the formula for potential difference?
V=IR v=potential difference (V) I=current(A) R=resistance(see revision guide [ohms]).
What does resistance depend on? Where must the ammeter and voltmeter be placed?
Whether components are in series or parallel, the length of wire. The ammeter must be placed in series, the voltmeter must be placed in parallel only around whatever investigating.
How do you test for resistance?
Attach crocodile clip with 0 cm on ruler, attack second clip 10 cm from first clip write length between clips. Close switch and record current across wire and potential difference across it. Open switch move second clip close switch record new length, current and voltage. Repeat experiment
How do you calculate for resistance and how do you draw the graph?
Use V=IR or R=V/I, plot resistance against wire length and draw line of best fit meaning resistance is directly proportional to wire length (if goes through origin coordinates 0,0). If doesn't go through origin because first clip not at 0 cm then all readings will be a bit out (systematic error).
What is a ohmic conductor?
Resistance doesn't change with current, at constant temperature current is directly proportional to current. Some resistance of resistors do change e.g. diode or filament lamp.
What happens with filament lamp or diode resistors?
filament: transfers some energy to thermal store, filament designed to heat up, resistance increases with temperature so current increases it heats up more and resistance increases.
diode: depend on direction of current let current flow in 1 direction has high resistance if it's reserved.
What does the term I-V characteristics refer to?
The graph which shows current flowing through component changes as voltage increases. liner components have straight lines, non-liner have curved lines
How do you test for components I-V characteristic?
set up circuit, begin to vary variable resistor (alters current flowing through circuit and voltage across component), take several readings from voltmeter and ammeter to see how current and potential difference changes. Repeat to get average potential difference at each current, swap wires connected to cell so direction of current changes. plot current against voltage in graph.
How should filament, diode and ohmic conductor graphs look?
ohmic conductor: current directly proportional to potential difference-straight line
filament lamp: temperature increases as resistance increases-less current can flow per unit of voltage-graph gets shallower so curve
diode: only flow in 1 direction diode has high resistance in other direction so straight line then steep gradient.
What is an LDR?
light dependent resistor-depends on light intensity. if bright light-resistance falls, darkness-resistance highest used in automatic night lights and burglar detectors.
What is a thermistor?
a temperature dependent resistor, hot conditions resistance falls, cool conditions resistance goes up. Used in temperature detectors.
How do sensing detectors work?
turn on to increase power to components if component and resistor in parallel have same potential difference. Potential difference shared by resistors-larger components resistance, the more potential difference it takes. resistance changes with e.g. light intensity for LDR
Why are series circuits unreliable?
If 1 component fails, the circuit fails
How does potential difference and current work in series circuits?
Potential difference is shared between components-potential difference across circuit adds up to source. Current is the same everywhere current is determined by total voltage of cells and resistance of circuit I=V/R.
How does resistance act is series?
adds up to sum of resistors. because resistors share voltage. voltage across each resistor is lower so current through each resistor is also lower. current same through circuit so current reduced when resistor added-resistance of circuit increases. larger components resistance-larger share of total potential difference.
How do cells act in series?
larger voltage with more cells if all connected in same way.
Why are parallel circuits more reliable than series circuits?
each component separately to power supply except ammeters which are always connected in series. If remove 1 component hardly affects the others. most things are connected in this way (you can turn each thing off separately). everyday circuit are a mixture of series and parallel.
How does potential difference work in parallel?
all components get full source of voltage. voltage across all components. Identical bulbs in parallel will have same brightness.
How does current work in parallel?
total current in circuit is total current in all components. current splits/rejoins at junctions. current leaving and joining junction is the same if identical components same amount of current through both of them.
How do you investigate adding resistors in a series circuit?
add 4 identical resistors, build circuit not voltage of battery. measure current using ammeter the calculate resistance R=V/I, add another resistor in series, measure current and voltage to find resistance repeat until you have 4 resistors. Plot graph of numbers of resistors to total resistance.
How do you investigate adding resistors in parallel?
use the same equipment, build same initial circuit. measure current and find value of voltage, calculate resistance R=V/I, add another resistor in parallel measure current and voltage to find resistance. repeat until used 4 resistors plot graph numbers of resistor to total resistors.
What should the results for investigating resistance look like?
series: adding resistors increases total resistance adding resistor decreases total current of circuit more resistors=larger resistance of circuit. graph=straight parallel line
parallel:add resistors=total current increase total resistance decrease. more resistors added-lower total resistance. graph=decreasing curve.
What are ac and dc currents?
ac:changes direction produced by alternating voltages, positive and negative keep alternating. UK's mains at ac are at 230V the frequency is at 50 cycles per second 50hz
dc:cells and batteries supply. same direction created by direct voltage.
What are the three colours of wires, what materials?
Three wires inside most appliances, core of copper and coloured plastic coating. colour of insulation shows its purpose, the colours are the same in all appliances.
What are the 3 wires?
neutral:blue completes circuit and carries current around 0V flow in through live and out through neutral.
Live:brown produces alternating voltage at around 230V
Earth:green and yellow for protecting wiring and for safety, stops appliance becoming live only carries current if falt. It's at 0V
Why is the live wire dangerous?
The body is at 0V if live wire touches you large amount of voltage is produced and current will flow-causes electric shock which is deadly. Even if appliance is switched off still danger of electric shock-still voltage in wire if touched could go through your body connection between live and earth wire is dangerous if low resistance huge current will flow-fire.
Why does energy transfer?
Because charge doesn't work against resistance.
What happens with a higher current?
more energy is transferred to thermal store
Why does energy transfer depend on power?
Total energy transferred by appliance depends on how long appliance on for and its power. power of appliance is given energy transferred per second more energy transferred in given time higher power.
What is the equation for energy transfer?
Why else does energy transfer depend on power?
appliance's power rating is maximum safe power they can operate at-maximum operating power. power rating=amount of energy transferred between stores per second when appliance in use. helps customer choose model-lower rating less electricity appliance uses in given time so cheaper to run.
What does a higher power rating mean?
may be more powerful than another but less efficient, might still only transfer same or less amount of energy to useful stores.
How does energy and potential difference act in a circuit?
when charge goes through change in voltage-energy is transferred. energy supplied to charge at power source to 'raise' through a potential, charge gives up when 'falls' through potential drop elsewhere in circuit. battery with larger voltage supplies more energy to circuit every coulomb of charge flows round it-charge raised up 'higher' at start.
What is the equation for energy transferred?
E=QV e=energy transferred(J) Q=charge flow(C) V=voltage(V)
What is the equation for power?
P(W)=potential difference(V)*current(A)/P=I squared R P=power(W) I=current(A) R=resistance(see revision quide)
What is the national grid?
web of wires over the UK
How is electricity distributed across the national grid?
cables and transformers, covers UK connects power station to consumer.
How doe electricity production meet demands?
demand changes daily, power station have to keep up with demand-by prediction, demand increases in the morning, when it gets dark and cold or when there are popular events, power stations often run well below maximum output to keep with demands or other power station shuts down. some smaller stations kept on standby.
How does the national grip use potential difference at a low current?
national grid either needs high voltage or high current. high current needs loads of energy as wires heat up energy transferred to thermal stores and surroundings. Cheaper to boost voltage to 400,000V and keep current as low as possible any given power increasing voltage and decreasing current decreases energy loss by heat or to surroundings makes national grid as efficient as possible.
How do transformer work in the national grid?
to get voltage to 400,000V requires transformers and pylons with huge insulators-still cheaper. There are step up and down transformers for efficient transmission at both ends.
How is static electricity created by friction?
insulating materials are rubbed together, negatively charged particles are scraped of 1 material and put on another. leave materials electrically charged with positive charge on 1 side and negative in the other, way electrons are transferred depends on materials.
Why do only electrons move?
electrons move causing a material gaining electrons to be negatively charged and other material to be positive.
Why does too much static electricity cause sparks?
electric charge builds up, voltage between object and earth increases, if high enough voltage electrons jump across gap between earth and charged object-spark.
How else does static electricity cause sparks?
electrons jump to any earthed conductor nearby- why get static shock getting out car, charge builds up on car frame, when touch car it goes through you usually happens in fairly small gap (not always e.g. lighting is big spark).
Why do opposite charges repel?
When 2 charges get close to each other they exert a force, oppositely charge are attracted, same repel. forces get weaker with distance force move objects-electrostatic attraction/repulsion non-contact force. Test by placing same/differently charged rods near each other.
What is an electric field?
created around any electrically charged object, closer objects get stronger field. show this by drawing field lines (always at right angles, closer lines=stronger field).
Why is there a for in a field of charged objects?
when charged object placed in electric field of other object it feels force, causes attraction/repulsion. Force caused by electric fields of charged objects interacting with each other, force on object linked t strength of electric field.