Current Electricity Flashcards
what is current?
- the rate of flow of charge round a circuit
- I
what carries the charge?
- electrons
- these are negatively charged particles
when will current flow through a component?
current will only flow if there is a voltage across that component
what is current measured in?
Amps / A
what is voltage?
- the force that drives the flow of electrons/current round the circuit.
- it’s provided by the battery/cell
- kind of like “electrical pressure”
- V
what is voltage measured in?
Volts / V
what is resistance?
- anything that resists or opposes the flow of electrons.
- aka slows the flow down
what happens if you add more components to the circuit?
- ## if you add more components (one after the other) there will be a HIGHER OVERALL RESISTANCE.
what is resistance measured in?
Ohms / Ω
what do the relative sizes of voltage and resistance decide?
HOW BIG THE CURRENT WILL BE.
- if you increase the voltage, then more current will flow
- if you increase the resistance, then less current will flow (or more voltage will be needed to keep the same current flowing.)
what happens if you increase the voltage?
more current will flow
what happens if you increase the resistance?
less current will flow (or more voltage will be needed to keep the same current flowing.)
what is an ammeter?
- measures the current (in amps) flowing through the component
- must be placed IN SERIES anywhere in the main circuit, but never in parallel like the voltmeter.
what is a voltmeter?
- measures the voltage (in volts) across the component
- must be placed IN PARALLEL around the component under test, NOT around the variable resistor or the battery!
what are the 5 important points to remember about the standard test circuit?
- this very basic circuit is used for testing components, and for getting I-V graphs for them.
- the component, the ammeter and the variable resistor are all in series, which means they can be put in any order in the main circuit. the voltmeter can only be placed in PARALLEL around the component under test.
- as you vary the variable resistor it alters the current flowing through the circuit
- this allows you to take several pairs of readings from the ammeter and voltmeter.
- you can then plot these values for current and voltage on an I-V graph.
what is the mains supply?
- the UK mains supply is approx. 230 volts
- it is an alternating current (a.c.) supply which means the current is constantly changing direction
what are battery supplies?
- cells and batteries supply direct current (d.c.).
- this means the current keeps flowing in the same direction.
V =
IR
what does the gradient of an I-V graph show you?
- how the resistance of the component behaves
- the steeper the graph, the lower the resistance
what does a straight line I-V graph show?
- has a constant gradient
- shows a constant resistance
what does a curved I-V graph show?
the resistance is changing
for wires and resistors, what happens to the resistance when you increase the temperature?
- it increases
- As temperature increases, all of the ions in the metal vibrate faster, which makes it harder for electrons to pass along the wire (or in other words, the resistance increases)
what does an I-V graph show?
shows you how the current varies as you change the voltage
what does a wire I-V graph show?
https: //keystagewiki.com/images/thumb/2/29/IVGraphResistor.png/800px-IVGraphResistor.png
- the current through a wire (at a constant temperature) is PROPORTIONAL to voltage
what does a filament lamp I-V graph show?
https: //i.stack.imgur.com/5dg9x.gif
- as the temperature of the metal filament increases, the resistance increases, hence the curve.
what do resistor I-V graphs show?
https: //cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/07/2.1.6-Ohms-Law-Graph.png
- the current through a resistor (at constant temperature) is PROPORTIONAL to voltage.
- different resistors have different resistances.
what does a diode I-V graph show?
https: //cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/07/2.1.6-Diode-I-V-graph.png
- current will only flow through a diode in one direction, as shown.
what is a light-emitting diode?
- light emitting diodes (LEDs emit light when a current flows through them in the forward direction. they have lots of practical applications.
- they are used for the numbers on digital clocks, in traffic lights and in remote controls
- unlike a light bulb, they don’t have a filament that can burn out.
- LEDs, like lamps, indicate the presence of current in a circuit. they are often used in appliances that they are switched on.
what is an LDR?
- a light-dependent resistor is a special type of resistor that changes its resistance depending on how much light falls on it.
- in bright light, the resistance falls
- and in darkness, the resistance is highest.
- this makes it a useful device for various electronic circuits e.g. burglar detectors
what is a thermistor?
- a thermistor is a temperature-dependent resistor.
- in hot conditions, the resistance drops
- in cool conditions, the resistance goes up
- thermistors make useful temperature detectors e.g. car engine temperature sensors, thermostats and fire alarms.
what does charge through a circuit depend on?
- current
- time
in solid metallic conductors, what is charge carried by?
negatively charged electrons
what happens when a bigger current flows?
more charge passes around the circuit
why is current conserved at a junction in a circuit?
- Components that are connected on separate loops are connected in parallel.
- The current is shared between each component connected in parallel.
- The total amount of current flowing into the junction, or split, is equal to the total current flowing out.
- The current is described as being conserved.
what happens when a charge drops through a voltage?
IT TRANSFERS ENERGY.
- when an electrical charge (Q) goes through a change in voltage (v) then energy (E) is transferred.
- energy is supplied to the charge at the power source to “raise” it through a voltage.
- the charge gives up this energy when it “falls” through any voltage drop in components elsewhere in the circuit.
- the bigger the change in voltage, the more energy is transferred for a given amount of charge passing through the circuit.
- that means a battery with a bigger voltage will supply more energy to the circuit for every coulomb of charge which flows round it. this is because the charge is raised up “higher” at the start and more energy will be dissipated in the circuit too.
what is voltage?
the energy transferred per unit charge passed
what is volt defined as?
one volt is one joule per coulomb
energy transferred =
charge x voltage
E = Q x V
how are components connected in series circuits?
- the components are connected in a line, end to end, between the +ve and -ve of the power supply
(- except for voltmeters, which are always connected in parallel, but don’t count as part of the circuit)
what happens if you remove/disconnect 1 component in a series circuit?
- the circuit is broken
- they all stop working
- generally not very handy and in practice, only a few things are connected in series
example of series circuit?
fairy lights
for a series circuit:
- there’s a bigger supply voltage when more cells are in series
- e.g. when 2 batteries with a voltage of 1.5V are connected in series, they supply 3V between them.
what happens to current in series?
- the current is the same everywhere.
- the size of the current depends on the total voltage and the total resistance of the circuit
- I = Vtotal / Rtotal
what happens to voltage in series?
- the total voltage of the supply is Shared between components in Series
- the voltage for each component depends on its resistance
what happens to resistance in series?
- the total resistance of the circuit depends on the number and type of components used.
- the total resistance is the sum of the resistance of each component in the circuit
- Rtotal = R1 + R2 + R3…
how is each component connected in a parallel circuit?
- each component is connected SEPERATELY to the +ve and -ve of the power supply
what happens if you remove or disconnect one component in parallel?
it will hardly affect the other components at all.
how are most things connected?
- in parallel
- e.g in cars and household electrics
- each light switch in a house is part of a branch of a parallel circuit, it just turns one light or set of lights on and off.
what are everyday circuits made up of?
- everyday circuits often contain a mixture of parallel and series parts
- when looking at components on the same branch the rules for series apply
what happens to voltage in parallel?
- the voltage is the same across all branches
- V1 = V2 etc
what happens to current in parallel?
- current is SHARED between branches
- the total current flowing around the circuit is equal to the total of all currents through the separate components
- Itotal = I1 + I2
what happens at junctions in a parallel circuit?
- in a parallel circuit, there are junctions where the current either splits or rejoins.
- the total current going into a junction equals the total current leaving it, as charge can’t just disappear or appear
what does the current through a branch depend on?
- the current through a branch depends on the RESISTANCE of the branch
- the higher the resistance, the harder it is for charge to flow, and so the lower the current in that branch.
- if two identical components are connected in parallel then the same current will flow through each component.
what happens to the total resistance of the circuit if you add a second resistor in parallel?
DECREASES
wavelength
distance from one peak to the next
frequency
how many complete waves there are per second passing a certain point. measured in Hz
amplitude
the height of the wave from rest to crest
speed
v, how fast the wave goes
time period
the time it takes (in s) for one complete wave to pass a point.
what are examples of transverse waves?
- light
- EM waves
- waves on strings
- water waves
what is a transverse wave?
the oscillations are perpendicular to the direction of wave travel
what is a longitudinal wave?
the oscillations are parallel to the direction of wave travel
what are examples of longitudinal waves?
- sound
- shock waves (seismic P waves)
what do all waves do?
- transfer energy in the direction they’re travelling WITHOUT transferring matter.
- e.g. microwaves warm things up their energy is TRANSFERRED to the food cooking.
- waves can also be used as signals to transfer information from one place to another, e.g. light in optical fibres, or radio waves travelling through air.
what do two or more waves travelling together have?
- WAVEFRONTS
- imaginary planes that cut across all the waves, connecting the points on adjacent waves which are vibrating together
- the distance between each wavefront is equal to one wavelength, i.e. each wavefront is at the same point in the cycle.
how does the DOPPLER EFFECT work?
- the waves produced by a source which is moving towards or away from an observer will appear to have a different wavelength than they would if they were stationary.
- this is because the wave speed is constant, so if the source is moving, it “catches up” to the waves in front of it. this causes the wavefronts to bunch up in front of the moving source and spread out behind it.
what happens when an object is moving towards you Doppler effect?
- the frequency of a source moving towards you will seem higher and its wavelength will appear shorter.
what happens when an object is moving away from you Doppler effect?
the frequency of a source moving away from you will seem lower and wavelength will seem longer.
EM waves in order
- radio
- micro
- infrared
- visible light
- UV
- Xrays
- Gamma rays
what is visible light?
a transverse wave
what does reflection of visible light allow us to do?
see most objects. light bounces off them into our eyes.
what happens when light reflects off an uneven surface?
- e.g. a piece of paper
- the light reflects off at all different angles and you get a diffuse reflection
what happens when light reflects off a smooth surface?
e. g. a mirror
- its all reflected at the same angle and you get a clear reflection
what is the law of reflection?
angle of incidence = angle of reflection
applies to EVERY reflected ray.
what is the normal?
- an imaginary line that is perpendicular to the surface at the point of incidence.
- shown as a dotted line
what is the angle of incidence?
- the angle between the incoming wave and the normal.
what is the angle of reflection?
- the angle between the reflected wave and the normal
when are virtual images formed?
- when the light rays bouncing off an object onto a mirror are diverging, so the light from the object appears to be coming from a completely different place.
- plane mirrors
how are waves refracted?
- waves travel at different speeds in substances with different densities.
- EM waves travel more slowly in denser mediums
- sound waves travel faster in denser mediums
- when a wave crosses a boundary between two substances, it changes speed.
what happens when a wave hits a boundary face on?
it slows down but carries on in the SAME DIRECTION
what happens when a wave hits a boundary at an angle?
it changes direction and is refracted
what happens when rays pass through a glass block?
- as light passes from the air into the block (a denser medium), it bends towards the normal. this is because it slows down
- when the light reaches the boundary on the other side of the block, its passing into a less dense medium. so it speeds up and bends away from the normal.
- the light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with, its been refracted towards the normal then back again by the same amount.
