Section 6 - Electricity
Current, Potential Difference & Resistance: Cover Me
Define Current Current is the rate of flow of charge (current in a wire is like Water flowing in a Pipe, the Amount of water that flows depends on the Flow Rate & the Time. its the same with electricity - Current is the Rate of Flow of Charge)
Change in Q = It OR I =Q/t
^^ where Change in Q is the Charge in Coulombs, I is the current in amperes & Change in t is the time taken in seconds
(remember that Conventional Current flows from + to -, the opposite way from electron flow)
the Coulomb is the unit of Charge, Define One Coulomb (C) one Coulomb (C) is defined as the Amount of Charge that passes in 1 Second if the Current is 1 Ampere (A)
what can u measure the current flowing thru a part of a circuit using u can measure the current flowing thru a part of a circuit using an Ammeter
Potential Difference is the E.......... per Unit C........ Energy Charge
what do u need to do on it to make electric charge flow thru a conductor to make electric charge flow thru a conductor, u need to do work on it
what is P.d, or Voltage defined as P.d, or Voltage is defined as the Work Done (energy converted) per Unit Charge Moved
V = V=W/Q (W is the work done in joules)
what can u measure the P.d across a component using u can measure the P.d across a component using a Voltmeter
remember, the P.d across components in parallel is The Same, so the Voltmeter shud be connectd in Parallel with the component
Definition of the Volt, when is the P.d across a component 1 Volt Definition of the Volt, the P.d across a component is 1 Volt when u convert 1 Joule of energy moving 1 Coulomb of charge thru the component ( 1 V = 1 J C to the -1 )
Everything has R............. Resistance
what will flow if u put a P.d across an Electrical Component if u put a P.d across an Electrical Component, a Current will flow
^^ what does how much current u get for a particular p.d depend on How Much current u get for a particular P.d depends on the Resistance of the component
^^ what can u think of a components Resistance as a Measure of u can think of a components Resistance as a Measure of how Difficult it is to get a Current to Flow thru
R= Mathematically, Resistance is: R=V/I (this equation defines resistance)
Resistance is measured in O..... (insert said units symbol) Ohms
unless told otherwise, what can u assume Voltmeters are + & ammeters have no what unless told otherwise, u can assume Voltmeters are infinitely resistant, & that ammeters have no resistance
whats the deal with when a component has a Resistance of 1 Ohm a component has a resistance of 1 Ohm if a P.d of 1 V makes a Current of 1 A flow thru it
for an O......... Conductor, R is a C............ - gtt Ohmic Constant
a chap called Ohm did most of the early work on resistance, what did he develop a rule to predict about Current a chap called Ohm did most of the early work on resistance, he developed a rule to Predict how the Current wud Change as the applied Potential Difference Increased, for Certain Types of conductor
the rule is now called Ohms Law & the conductors that Obey it (mostly m........) are called Ohmic Conductors (mostly metals)
provided the Physical Conditions, such as Temperature, remain Constant, whats the deal with the Current thru an ohmic conductor provided the Physical Conditions, such as Temperature, remain Constant, the Current thru an ohmic conductor is Directly Proportional to the Potential Difference across it
1) the graph shows what happens if u plot current against voltage for an ohmic conductor (so Current on y axis & P.d on x axis with a diagonal directly proportional line)
2) as u can see its a Straight-Line graph - Doubling the p.d Doubles the Current
3) what this means is that the Resistance is Constant - V / I is always a fixed value
4) often Factors such as L...... Level or T.............. will have a Significant Effect on resistance (the resistivity changes, see p86), so u need to remember that Ohms law is Only true for Ohmic Conductors under C.......... Physical Conditions, eg temperature Light Temperature Constant
5) Ohms law is a Special Case - lots of components arent ohmic conductors & have characteristic current-voltage (I-V) graphs of their very own
I/V Characteristics:
I/V Graphs show how Resistance Varies -
whats the deal with what the term 'I/V Characteristic' refers to 2 the term 'I/V Characteristic' refers to a Graph of I Against V which shows how the Current (I) flowing thru a Component Changes as the Potential Difference (V) across it is increased
u cud also be asked about a V/I graphs, theyre pretty similar, but with V plotted against I, the resistance at a point on the graph is simply V/I at that point
the I/V graph for an O.......... Conductor is a S........... Line thru the O........ Ohmic Straight Origin (for eg a Resistor)
whats the deal with the current thru an Ohmic Conductor, at Constant Temperature at Constant Temperature, the Current thru an Ohmic Conductor (eg metals) is Directly Proportional to the Voltage (ie their resistance is constant, see p83)
^^ this means that the I/V characteristic graph for an ohmic conductor at a constant temperature is a Straight Line thru through the Origin
higher the resistance = lower the ....... current
the I/V Characteristic for a F......... L..... is a C...... - gtt Filament Lamp Curve
main point: whats the deal with the curve for the I/V characteristic of a Filament Lamp the I/V characteristic for a Filament Lamp is a Curve that starts Steep but gets Shallower as the Voltage Rises (decreasing gradient) cos its resistance increases as it becomes hotter
why does the resistor heat up cos the Es collide with the ions in the lattice that make up the resistor as they flow thru it, this gives the ions En which causes them to Vibrate & Heat Up, collissions between the Es & ions in lattice lead to KE of Es being dissipated as thermal En
whats the deal with the more the Ions Vibrate the more the ions vibrate the harder it is for Es to get thru the resistor cos theres more collissions
what happens to the current as the resistor heats up current decreases cos resistance increases due to the heating effect making it harder for Es to pass thru (cos ions vibrate after collissions & heat up, meaning resistance increases)
what is the Filament in a lamp the Filament in a lamp is just a Coiled-Up length of Metal Wire
^^ so u might think it shud have the Same Characteristic Graph as a Metallic Conductor, but why doesnt it it doesnt because it Gets Hot
^^ Current flowing thru the lamp Increases its Temperature
^^ the Resistance of a Metal Increases as the Temperature Increases
(the V/I graph for a filament lamp is a curve that starts shallow & gets steeper as the current & voltage increases)
Semiconductors are used in S........ Sensors
what are Semiconductors nowhere near as good at as Metals Semiconductors are Nowhere Near as good at Conducting elecrticity as Metals
^^ why are Semiconductors are Nowhere Near as good at Conducting elecrticity as Metals this is cos there are far, far Fewer Charge Carriers available
^^ however, whats the deal with if Energy is supplied to the semiconductor however, if Energy is supplied to the semiconductor, More Charge Carriers can be Released
^^ what does this mean they make Excellent Sensors for this means that they make Excellent Sensors for detecting Changes in their Environment
whats the deal with a Thermistor a Thermistor is a Resistor with a Resistance that depends on its Temperature
u only need to know about NTC thermistors - NTC stands for 'Negative Temperature Coefficient'
^^ what does this mean this means that the Resistance Decreases as the Temperature Goes Up
the I/V characteristic graph for an NTC thermistor curves upwards
Increasing the current thru the thermistor increases its temperature. the increasing gradient of this characteristic graph tells u that the resistance is decreasing. as usual, the gradient of the V/I graph does the opposite
whats the deal with warming the thermistor warming the thermistor gives more Es enuf Energy to Escape from their atoms
^^ this means that there are More Charge Carriers available, so the resistance is lower cos its easier for electricity to flow
what sort of conductor is a thermistor a thermistor is a Semiconductor
whats the deal with a semiconductor + why its difficult for electricity to pass thru the semiconductor = makes resistance high + cos its made out of a semiconducting material
whats the deal with a semiconducting material semi conducting material = small no. of atoms are ionised so theres few Es available for conduction (hence why semiconductors have high resistance)
whats the deal with the hotter we make it the hotter we make it the more free Es we have = easier for electricity to flow
so whats the relation between resistance & temperature as temperature increases, resistance decreases
D....... only let C......... Flow in One D.......... Diodes Current Direction
what are Diodes (including light emitting diodes (LEDs)) designed to do Diodes (including light emitting diodes (LEDs)) are designed to let Current Flow in One Direction only
(u dont need to be able to explain how they work, just what they do)
whats Forward Bias Forward Bias is the Direction in which the Current is Allowed To Flow
whats the deal with what most Diodes require before they will conduct Most diodes require a Threshold Voltage of about 0.6 V in the Forward Direction before they will conduct
whats the deal with the Resistance in the Reverse Bias + wb the Current in the Reverse Bias in Reverse Bias, the Resistance of the diode is Very High + & the current that flows is Very Tiny
(diodes let current flow in the direction that the triangle in the circuit symbol points)
Resistivity & Superconductivity:
How many things determine Resistance Three things determine Resistance
if u think about a nice, Simple Electrical Component, like a Length of Wire, its Resistance depends on:
1) L......... (I) Length
^^ whats the deal with the Longer the wire the Longer the wire, the More Difficult it is to make a Current Flow
2) A..... (A) Area
^^ whats the deal with the Wider the wire the Wider the wire, the Easier it will be for the Es to pass along it
3) R............ (p) Resistivity the (p) is like some lower case slanted ish p which is the greek letter rho
^^ what does the Resistivity depend on Resistivity (p), which Depends on the Material
^^ the S............ may make it easy or difficult for c........ to flow Structure charge
^^ in general, what does Resistivity depend on as well, like T............ & Light I......... in general, Resistivity depends on Environmental Factors as well, like Temperature & Light Intensity
what is the Resistivity of a material defined as the Resistivity of a material is defined as the Resistance of a 1 m Length with a 1 mSquared Cross-Sectional Area
^^ what is it measured in it is measured in Ohm-Metres
p= p = RA / I ( p (greek letter 'rho') = Resistivity, A = cross-sectional area in mSquared, I = length in m )
(Typical Values for the Resistivity of Conductors are Really Small, eg for Copper (at 25 degCelsius) p = 1.72 x 10 to the -8 Ohm-Metres)
S................... have Z...... Resistivity Superconductors Zero
normally, what do all material have normally, all materials have Some Resisitivity - even really good conductors like silver & copper
whats the deal with what that Resistance means whenever electricity flows thru them 2 that resistance means that whenever electricity flows thru them, they Heat Up, & some of the electrical energy is Wasted as thermal energy (heat)
how can u lower the resistivity of many materials like metals u can Lower the resistivity of many materials by Cooling Them Down
whats the deal with if u Cool some materials (eg mercury) down to below a 'Transition Temperature' 2 if u Cool some materials (eg mercury) down to below a 'Critical Temperature' their Resistivity Disappears Entirely & they become a Superconductor
whats the deal with the electrical energy without any Resistance + so none of its W........ without any Resistance, None of the electrical energy is turned into heat, + so None of its wasted
^^ that means u can start a current flowing in a circuit using a magnetic field, take away the magnet & the current wud carry on flowing F........ forever
theres a catch tho, whats the deal with most 'normal' conductors, eg metals, most 'normal' conductors, eg metals, have transition temperatures below 10 Kelvin (- 263 degCelsius)
^^ Getting things that cold is H....., & Really E........... Hard Expensive
what are solid-state physicists all over the world trying to develop solid-state physicists all over the world are trying to develop Room-Temperature Superconductors
^^ so far, theyve managed to get some weird Metal Oxide things to superconduct at about 140 K (-133 DegCels), which is a much easier temperature to get down to. theyve still got a long way to go though
Uses of Superconductors
Using superconducting wire u cud make:
1) whats the deal with the Power Cables u cud make Power Cables that transmit electricity without any Loss of power
2) whats the deal witih Really Strong Electromagnets Really Strong Electromagnets that Dont need a constant power source (for use in medical applications & Maglev trains)
3) whats the deal with Electronic Circuits 2 Electronic Circuits that work really Fast, cos theres no resistance to slow them down
Electrical Energy & Power:
Define Power Power (P) is Defined as the Rate of Transfer of Energy
^^ what is Power measured in Power is measured in Watts (W),
^^ where 1 Watt is equivalent to 1 Joule per Second
^^ or P = E / t
theres a really simple formula for Power in Electrical Circuits:, what is it P = VI
this makes sense, since:
1) what is P.d (V) defined as P.d (V) is defined as the Energy Transferred per Coulomb
2) what is Current (I) defined as Current (I) is defined as the Number of Coulombs transferred per Second
3) so p.d x current is Energy Transferred per Second, ie Power
u know from the definition of Resistance that: V = IR
Combining the 2 Equations give u loads of Different Ways to Calculate Power
P = IV
P = Vsquared / R
P = Isquared x R
obviously, which equation u shud use the depends on what Quantities ur given in the Question
E........ is easy to Calculate if u know the P....... Energy Power
Sometimes its the Total Energy transferred that ur interested in. in this case u simply need to Multiply the Power by the Time. so:
E = VIt [ or E = (Vsquared / R) x t, or E = IsquaredRt ]
E.m.f & Internal Resistance:
B......... have Resistance Batteries
where does Resistance come from Resistance comes from Es Colliding with Atoms & Losing Energy to other forms (GCSE explanation as well in other notes)
in a Battery, which type of Energy is used to make Es Move in a Battery, Chemical Energy is used to make Es
^^ as they move, what do they collide with - so batteries M...... have resistance as they move, they collide with atoms inside the battery - so batteries Must have resistance
^^ what is this Resistance called this is called Internal Resistance
whats the deal with Internal Resistance and Batteries & Cells Internal Resistance is what makes Batteries & Cells Warm Up when theyre used
^^^ what do chemical reactions in the battery produce chemical reactions in the battery produce electrical energy
what is Load Resistance Load Resistance is the total resistance of all the components in the external circuit (u might see it called 'external resistance')
whats called the Batteries Electromotive Force or e.m.f (some weird e like symbol) the amount of Elecrtrical Energy the battery produces for each Coulomb of charge is called its Electromotive Force or e.m.f
^^ be careful - e.m.f Isnt actually a force. its measured in Volts
weird e symbol = E / Q
the Potential Difference across the Load Resistance (R) is the Energy Transferred when One Coulomb of charge flows thru the Load Resistance
^^ what is this P.d called this P.d is called the Terminal P.d (V)
if there was No Internal Resistance, what wud the Terminal P.d be the same as if there was No Internal Resistance, the Terminal P.d wud be the Same as the e.m.f
^^ However, in Real power supplies, theres Always Some Energy Lost overcoming the internal resistance
what is the Energy Wasted per Coulomb overcoming the internal resistance called the Energy Wasted per Coulomb overcoming the internal resistance is called the Lost volts (v)
Conservation of Energy Tells us:
Energy per Coulomb supplied by the Source = Energy per Coulomb transferred in Load Resistance + Energy per Coulomb wasted in Internal Resistance
Conservation of Energy & Change:
Charge Doesn't 'Leak Away' anywhere - its Conserved
whats doesnt happen to Charge as it Flows thru a circuit 2 as Charge Flows thru a circuit, it Doesnt get Used Up or Lost
this means that whatever Charge Flows Into a junction will Flow Out again
since Current is Rate of Flow of Charge, it follows that whatever Current Flows Into a junction is the same as the current Flowing Out of it
what does Kirchhoff's First Law says the total Current Entering a Junction = the total Current Leaving It
Energy is Conserved too
Energy is Conserved. u already know that. in Electrical Circuits, Energy is Transferred Round the circuit
Energy Transferred To a charge is e.m.f, & energy Transferred From a charge is P.d
in a Closed Loop, these 2 quantities must be Equal if energy is conserved (which it is)
what does Kirchhoff's Second Law says the Total e.m.f around a Series Circuit = the Sum of the P.d.s across each component
Exam Qs get u to Combine Resistors in Series & Parallel
( a typical exam Q. will give u a Circuit with bits of information missing, leaving u to fill in the gaps. not the most fun....but on the plus side u get to ignore any internal resistance stuff
^^ u need to remember the Following Rules:
Series Circuits: (diagram for both series & parallel u cud copy)
1) whats the deal with the Current + why Same Current at All Points of the circuit + since there are no junction
2) e.m.f Split between Components (by Kirchoffs 2nd law), so: ews = Vone + Vtwo + Vthree
3) V = IR, so if I is constant: IRtotal = IRone + IRtwo + IRthree
4) cancellling the Is gives: Rtotal = Rone + Rtwo + Rthree
Parallel Circuits:
1) whats the deal with Current Current is Split at each Junction
^^ Current is Split at each Junction, so: I = Ione + Itwo + Ithree
2) Same P.d across All Components (three seperate loops - within each loop the e.m.f equals sum of indivual p.d.s)
3) so, V/Rtotal = V/Rone + V/Rtwo + V/Rthree
4) cancelling the Vs gives: 1/Rtotal = 1/Rone + 1/Rtwo + 1/Rthree
The Potential Divider:
use a Potential Divider to get a F......... of a Source Voltage Fraction
at its simplest, what is a Potential Divider at its simplest, a Potential Divider is a circuit with a Voltage Source & a couple of Resistors in series
whats the deal with how the Potential Difference across the voltage source (eg a battery) is Split the P.d across the Voltage Source (eg a battery) is Split in the Ratio of the Resistances
so, if u had a 2 Ohm resistor and a 3 Ohm resistor, youd get 2/5 of the p.d across the 2 Ohm resistor & 3/5 across the 3 Ohm
u can use Potential dividers to supply a potential difference, Vout, between Zero & the potential difference across the voltage source
^^ this can be useful, eg if u need a Varying p.d supply or one that is at a Lower p.d than the voltage source
this circuit is mainly used for Calibrating Voltmeters, which have a Very High Resistance
if u put something with a Relatively Low Resistance across R2 though, u start to run into Problems
^^ youve Effectively got 2 Resistors in Parallel, which will always have a Total resistance Less than R2
^^ that means that Vout will be Less than uve calculated, & will depend on whats connected across R2
use a Variable Resistor to vary the Voltage
if u replace Rone with a Variable Resistor, u can change Vout
when Rone = 0, Vout = Vs
as u increase Rone, Vout gets smaller
Add an LDR or Thermistor for a Light or Temperature Sensor
whats the deal with a LDR a LDR has a very High Resistance in the Dark, but a Lower Resistance in the Light
whats the deal with an NTC Thermistor an NTC Thermistor has a High Resistance at Low Temperatures, but a much Lower Resistance at High Temperatures (it varies in the opposite way to a normal reisistor, only mich more so)
Either of these can be used as one of the Resistors in a Potential Divider, giving an Output Voltage that Varies with the Light Level or Temperature
(theres yet another diagram circuit picture with notes next to it i cant present here)
a Potentiometer uses a Variable Resistor to give a Variable Voltage - gtt
a Potentiometer has a variable resistor replacing Rone & Rtwo of the potential divider, but it uses the Same Idea (its even sometimes Called a potential divider just to confuse things)
u move a Slider or turn a knob to Adjust the Relative Sizes of Rone & Rtwo
^^ that way u can vary Vout from 0 V up to the source voltage
this is dead handy when u want to be able to Change a Voltage Continuously, like in the Volume Control of a stereo
Current, Potential Difference & Resistance: Cover Me
Define Current Current is the rate of flow of charge (current in a wire is like Water flowing in a Pipe, the Amount of water that flows depends on the Flow Rate & the Time. its the same with electricity - Current is the Rate of Flow of Charge)
Change in Q = It OR I =Q/t
^^ where Change in Q is the Charge in Coulombs, I is the current in amperes & Change in t is the time taken in seconds
(remember that Conventional Current flows from + to -, the opposite way from electron flow)
the Coulomb is the unit of Charge, Define One Coulomb (C) one Coulomb (C) is defined as the Amount of Charge that passes in 1 Second if the Current is 1 Ampere (A)
what can u measure the current flowing thru a part of a circuit using u can measure the current flowing thru a part of a circuit using an Ammeter
Potential Difference is the E.......... per Unit C........ Energy Charge
what do u need to do on it to make electric charge flow thru a conductor to make electric charge flow thru a conductor, u need to do work on it
what is P.d, or Voltage defined as P.d, or Voltage is defined as the Work Done (energy converted) per Unit Charge Moved
V = V=W/Q (W is the work done in joules)
what can u measure the P.d across a component using u can measure the P.d across a component using a Voltmeter
remember, the P.d across components in parallel is The Same, so the Voltmeter shud be connectd in Parallel with the component
Definition of the Volt, when is the P.d across a component 1 Volt Definition of the Volt, the P.d across a component is 1 Volt when u convert 1 Joule of energy moving 1 Coulomb of charge thru the component ( 1 V = 1 J C to the -1 )
Everything has R............. Resistance
what will flow if u put a P.d across an Electrical Component if u put a P.d across an Electrical Component, a Current will flow
^^ what does how much current u get for a particular p.d depend on How Much current u get for a particular P.d depends on the Resistance of the component
^^ what can u think of a components Resistance as a Measure of u can think of a components Resistance as a Measure of how Difficult it is to get a Current to Flow thru
R= Mathematically, Resistance is: R=V/I (this equation defines resistance)
Resistance is measured in O..... (insert said units symbol) Ohms
unless told otherwise, what can u assume Voltmeters are + & ammeters have no what unless told otherwise, u can assume Voltmeters are infinitely resistant, & that ammeters have no resistance
whats the deal with when a component has a Resistance of 1 Ohm a component has a resistance of 1 Ohm if a P.d of 1 V makes a Current of 1 A flow thru it
for an O......... Conductor, R is a C............ - gtt Ohmic Constant
a chap called Ohm did most of the early work on resistance, what did he develop a rule to predict about Current a chap called Ohm did most of the early work on resistance, he developed a rule to Predict how the Current wud Change as the applied Potential Difference Increased, for Certain Types of conductor
the rule is now called Ohms Law & the conductors that Obey it (mostly m........) are called Ohmic Conductors (mostly metals)
provided the Physical Conditions, such as Temperature, remain Constant, whats the deal with the Current thru an ohmic conductor provided the Physical Conditions, such as Temperature, remain Constant, the Current thru an ohmic conductor is Directly Proportional to the Potential Difference across it
1) the graph shows what happens if u plot current against voltage for an ohmic conductor (so Current on y axis & P.d on x axis with a diagonal directly proportional line)
2) as u can see its a Straight-Line graph - Doubling the p.d Doubles the Current
3) what this means is that the Resistance is Constant - V / I is always a fixed value
4) often Factors such as L...... Level or T.............. will have a Significant Effect on resistance (the resistivity changes, see p86), so u need to remember that Ohms law is Only true for Ohmic Conductors under C.......... Physical Conditions, eg temperature Light Temperature Constant
5) Ohms law is a Special Case - lots of components arent ohmic conductors & have characteristic current-voltage (I-V) graphs of their very own
I/V Characteristics:
I/V Graphs show how Resistance Varies -
whats the deal with what the term 'I/V Characteristic' refers to 2 the term 'I/V Characteristic' refers to a Graph of I Against V which shows how the Current (I) flowing thru a Component Changes as the Potential Difference (V) across it is increased
u cud also be asked about a V/I graphs, theyre pretty similar, but with V plotted against I, the resistance at a point on the graph is simply V/I at that point
the I/V graph for an O.......... Conductor is a S........... Line thru the O........ Ohmic Straight Origin (for eg a Resistor)
whats the deal with the current thru an Ohmic Conductor, at Constant Temperature at Constant Temperature, the Current thru an Ohmic Conductor (eg metals) is Directly Proportional to the Voltage (ie their resistance is constant, see p83)
^^ this means that the I/V characteristic graph for an ohmic conductor at a constant temperature is a Straight Line thru through the Origin
higher the resistance = lower the ....... current
the I/V Characteristic for a F......... L..... is a C...... - gtt Filament Lamp Curve
main point: whats the deal with the curve for the I/V characteristic of a Filament Lamp the I/V characteristic for a Filament Lamp is a Curve that starts Steep but gets Shallower as the Voltage Rises (decreasing gradient) cos its resistance increases as it becomes hotter
why does the resistor heat up cos the Es collide with the ions in the lattice that make up the resistor as they flow thru it, this gives the ions En which causes them to Vibrate & Heat Up, collissions between the Es & ions in lattice lead to KE of Es being dissipated as thermal En
whats the deal with the more the Ions Vibrate the more the ions vibrate the harder it is for Es to get thru the resistor cos theres more collissions
what happens to the current as the resistor heats up current decreases cos resistance increases due to the heating effect making it harder for Es to pass thru (cos ions vibrate after collissions & heat up, meaning resistance increases)
what is the Filament in a lamp the Filament in a lamp is just a Coiled-Up length of Metal Wire
^^ so u might think it shud have the Same Characteristic Graph as a Metallic Conductor, but why doesnt it it doesnt because it Gets Hot
^^ Current flowing thru the lamp Increases its Temperature
^^ the Resistance of a Metal Increases as the Temperature Increases
(the V/I graph for a filament lamp is a curve that starts shallow & gets steeper as the current & voltage increases)
Semiconductors are used in S........ Sensors
what are Semiconductors nowhere near as good at as Metals Semiconductors are Nowhere Near as good at Conducting elecrticity as Metals
^^ why are Semiconductors are Nowhere Near as good at Conducting elecrticity as Metals this is cos there are far, far Fewer Charge Carriers available
^^ however, whats the deal with if Energy is supplied to the semiconductor however, if Energy is supplied to the semiconductor, More Charge Carriers can be Released
^^ what does this mean they make Excellent Sensors for this means that they make Excellent Sensors for detecting Changes in their Environment
whats the deal with a Thermistor a Thermistor is a Resistor with a Resistance that depends on its Temperature
u only need to know about NTC thermistors - NTC stands for 'Negative Temperature Coefficient'
^^ what does this mean this means that the Resistance Decreases as the Temperature Goes Up
the I/V characteristic graph for an NTC thermistor curves upwards
Increasing the current thru the thermistor increases its temperature. the increasing gradient of this characteristic graph tells u that the resistance is decreasing. as usual, the gradient of the V/I graph does the opposite
whats the deal with warming the thermistor warming the thermistor gives more Es enuf Energy to Escape from their atoms
^^ this means that there are More Charge Carriers available, so the resistance is lower cos its easier for electricity to flow
what sort of conductor is a thermistor a thermistor is a Semiconductor
whats the deal with a semiconductor + why its difficult for electricity to pass thru the semiconductor = makes resistance high + cos its made out of a semiconducting material
whats the deal with a semiconducting material semi conducting material = small no. of atoms are ionised so theres few Es available for conduction (hence why semiconductors have high resistance)
whats the deal with the hotter we make it the hotter we make it the more free Es we have = easier for electricity to flow
so whats the relation between resistance & temperature as temperature increases, resistance decreases
D....... only let C......... Flow in One D.......... Diodes Current Direction
what are Diodes (including light emitting diodes (LEDs)) designed to do Diodes (including light emitting diodes (LEDs)) are designed to let Current Flow in One Direction only
(u dont need to be able to explain how they work, just what they do)
whats Forward Bias Forward Bias is the Direction in which the Current is Allowed To Flow
whats the deal with what most Diodes require before they will conduct Most diodes require a Threshold Voltage of about 0.6 V in the Forward Direction before they will conduct
whats the deal with the Resistance in the Reverse Bias + wb the Current in the Reverse Bias in Reverse Bias, the Resistance of the diode is Very High + & the current that flows is Very Tiny
(diodes let current flow in the direction that the triangle in the circuit symbol points)
Resistivity & Superconductivity:
How many things determine Resistance Three things determine Resistance
if u think about a nice, Simple Electrical Component, like a Length of Wire, its Resistance depends on:
1) L......... (I) Length
^^ whats the deal with the Longer the wire the Longer the wire, the More Difficult it is to make a Current Flow
2) A..... (A) Area
^^ whats the deal with the Wider the wire the Wider the wire, the Easier it will be for the Es to pass along it
3) R............ (p) Resistivity the (p) is like some lower case slanted ish p which is the greek letter rho
^^ what does the Resistivity depend on Resistivity (p), which Depends on the Material
^^ the S............ may make it easy or difficult for c........ to flow Structure charge
^^ in general, what does Resistivity depend on as well, like T............ & Light I......... in general, Resistivity depends on Environmental Factors as well, like Temperature & Light Intensity
what is the Resistivity of a material defined as the Resistivity of a material is defined as the Resistance of a 1 m Length with a 1 mSquared Cross-Sectional Area
^^ what is it measured in it is measured in Ohm-Metres
p= p = RA / I ( p (greek letter 'rho') = Resistivity, A = cross-sectional area in mSquared, I = length in m )
(Typical Values for the Resistivity of Conductors are Really Small, eg for Copper (at 25 degCelsius) p = 1.72 x 10 to the -8 Ohm-Metres)
S................... have Z...... Resistivity Superconductors Zero
normally, what do all material have normally, all materials have Some Resisitivity - even really good conductors like silver & copper
whats the deal with what that Resistance means whenever electricity flows thru them 2 that resistance means that whenever electricity flows thru them, they Heat Up, & some of the electrical energy is Wasted as thermal energy (heat)
how can u lower the resistivity of many materials like metals u can Lower the resistivity of many materials by Cooling Them Down
whats the deal with if u Cool some materials (eg mercury) down to below a 'Transition Temperature' 2 if u Cool some materials (eg mercury) down to below a 'Critical Temperature' their Resistivity Disappears Entirely & they become a Superconductor
whats the deal with the electrical energy without any Resistance + so none of its W........ without any Resistance, None of the electrical energy is turned into heat, + so None of its wasted
^^ that means u can start a current flowing in a circuit using a magnetic field, take away the magnet & the current wud carry on flowing F........ forever
theres a catch tho, whats the deal with most 'normal' conductors, eg metals, most 'normal' conductors, eg metals, have transition temperatures below 10 Kelvin (- 263 degCelsius)
^^ Getting things that cold is H....., & Really E........... Hard Expensive
what are solid-state physicists all over the world trying to develop solid-state physicists all over the world are trying to develop Room-Temperature Superconductors
^^ so far, theyve managed to get some weird Metal Oxide things to superconduct at about 140 K (-133 DegCels), which is a much easier temperature to get down to. theyve still got a long way to go though
Uses of Superconductors
Using superconducting wire u cud make:
1) whats the deal with the Power Cables u cud make Power Cables that transmit electricity without any Loss of power
2) whats the deal witih Really Strong Electromagnets Really Strong Electromagnets that Dont need a constant power source (for use in medical applications & Maglev trains)
3) whats the deal with Electronic Circuits 2 Electronic Circuits that work really Fast, cos theres no resistance to slow them down
Electrical Energy & Power:
Define Power Power (P) is Defined as the Rate of Transfer of Energy
^^ what is Power measured in Power is measured in Watts (W),
^^ where 1 Watt is equivalent to 1 Joule per Second
^^ or P = E / t
theres a really simple formula for Power in Electrical Circuits:, what is it P = VI
this makes sense, since:
1) what is P.d (V) defined as P.d (V) is defined as the Energy Transferred per Coulomb
2) what is Current (I) defined as Current (I) is defined as the Number of Coulombs transferred per Second
3) so p.d x current is Energy Transferred per Second, ie Power
u know from the definition of Resistance that: V = IR
Combining the 2 Equations give u loads of Different Ways to Calculate Power
P = IV
P = Vsquared / R
P = Isquared x R
obviously, which equation u shud use the depends on what Quantities ur given in the Question
E........ is easy to Calculate if u know the P....... Energy Power
Sometimes its the Total Energy transferred that ur interested in. in this case u simply need to Multiply the Power by the Time. so:
E = VIt [ or E = (Vsquared / R) x t, or E = IsquaredRt ]
E.m.f & Internal Resistance:
B......... have Resistance Batteries
where does Resistance come from Resistance comes from Es Colliding with Atoms & Losing Energy to other forms (GCSE explanation as well in other notes)
in a Battery, which type of Energy is used to make Es Move in a Battery, Chemical Energy is used to make Es
^^ as they move, what do they collide with - so batteries M...... have resistance as they move, they collide with atoms inside the battery - so batteries Must have resistance
^^ what is this Resistance called this is called Internal Resistance
whats the deal with Internal Resistance and Batteries & Cells Internal Resistance is what makes Batteries & Cells Warm Up when theyre used
^^^ what do chemical reactions in the battery produce chemical reactions in the battery produce electrical energy
what is Load Resistance Load Resistance is the total resistance of all the components in the external circuit (u might see it called 'external resistance')
whats called the Batteries Electromotive Force or e.m.f (some weird e like symbol) the amount of Elecrtrical Energy the battery produces for each Coulomb of charge is called its Electromotive Force or e.m.f
^^ be careful - e.m.f Isnt actually a force. its measured in Volts
weird e symbol = E / Q
the Potential Difference across the Load Resistance (R) is the Energy Transferred when One Coulomb of charge flows thru the Load Resistance
^^ what is this P.d called this P.d is called the Terminal P.d (V)
if there was No Internal Resistance, what wud the Terminal P.d be the same as if there was No Internal Resistance, the Terminal P.d wud be the Same as the e.m.f
^^ However, in Real power supplies, theres Always Some Energy Lost overcoming the internal resistance
what is the Energy Wasted per Coulomb overcoming the internal resistance called the Energy Wasted per Coulomb overcoming the internal resistance is called the Lost volts (v)
Conservation of Energy Tells us:
Energy per Coulomb supplied by the Source = Energy per Coulomb transferred in Load Resistance + Energy per Coulomb wasted in Internal Resistance
Conservation of Energy & Change:
Charge Doesn't 'Leak Away' anywhere - its Conserved
whats doesnt happen to Charge as it Flows thru a circuit 2 as Charge Flows thru a circuit, it Doesnt get Used Up or Lost
this means that whatever Charge Flows Into a junction will Flow Out again
since Current is Rate of Flow of Charge, it follows that whatever Current Flows Into a junction is the same as the current Flowing Out of it
what does Kirchhoff's First Law says the total Current Entering a Junction = the total Current Leaving It
Energy is Conserved too
Energy is Conserved. u already know that. in Electrical Circuits, Energy is Transferred Round the circuit
Energy Transferred To a charge is e.m.f, & energy Transferred From a charge is P.d
in a Closed Loop, these 2 quantities must be Equal if energy is conserved (which it is)
what does Kirchhoff's Second Law says the Total e.m.f around a Series Circuit = the Sum of the P.d.s across each component
Exam Qs get u to Combine Resistors in Series & Parallel
( a typical exam Q. will give u a Circuit with bits of information missing, leaving u to fill in the gaps. not the most fun....but on the plus side u get to ignore any internal resistance stuff
^^ u need to remember the Following Rules:
Series Circuits: (diagram for both series & parallel u cud copy)
1) whats the deal with the Current + why Same Current at All Points of the circuit + since there are no junction
2) e.m.f Split between Components (by Kirchoffs 2nd law), so: ews = Vone + Vtwo + Vthree
3) V = IR, so if I is constant: IRtotal = IRone + IRtwo + IRthree
4) cancellling the Is gives: Rtotal = Rone + Rtwo + Rthree
Parallel Circuits:
1) whats the deal with Current Current is Split at each Junction
^^ Current is Split at each Junction, so: I = Ione + Itwo + Ithree
2) Same P.d across All Components (three seperate loops - within each loop the e.m.f equals sum of indivual p.d.s)
3) so, V/Rtotal = V/Rone + V/Rtwo + V/Rthree
4) cancelling the Vs gives: 1/Rtotal = 1/Rone + 1/Rtwo + 1/Rthree
The Potential Divider:
use a Potential Divider to get a F......... of a Source Voltage Fraction
at its simplest, what is a Potential Divider at its simplest, a Potential Divider is a circuit with a Voltage Source & a couple of Resistors in series
whats the deal with how the Potential Difference across the voltage source (eg a battery) is Split the P.d across the Voltage Source (eg a battery) is Split in the Ratio of the Resistances
so, if u had a 2 Ohm resistor and a 3 Ohm resistor, youd get 2/5 of the p.d across the 2 Ohm resistor & 3/5 across the 3 Ohm
u can use Potential dividers to supply a potential difference, Vout, between Zero & the potential difference across the voltage source
^^ this can be useful, eg if u need a Varying p.d supply or one that is at a Lower p.d than the voltage source
this circuit is mainly used for Calibrating Voltmeters, which have a Very High Resistance
if u put something with a Relatively Low Resistance across R2 though, u start to run into Problems
^^ youve Effectively got 2 Resistors in Parallel, which will always have a Total resistance Less than R2
^^ that means that Vout will be Less than uve calculated, & will depend on whats connected across R2
use a Variable Resistor to vary the Voltage
if u replace Rone with a Variable Resistor, u can change Vout
when Rone = 0, Vout = Vs
as u increase Rone, Vout gets smaller
Add an LDR or Thermistor for a Light or Temperature Sensor
whats the deal with a LDR a LDR has a very High Resistance in the Dark, but a Lower Resistance in the Light
whats the deal with an NTC Thermistor an NTC Thermistor has a High Resistance at Low Temperatures, but a much Lower Resistance at High Temperatures (it varies in the opposite way to a normal reisistor, only mich more so)
Either of these can be used as one of the Resistors in a Potential Divider, giving an Output Voltage that Varies with the Light Level or Temperature
(theres yet another diagram circuit picture with notes next to it i cant present here)
a Potentiometer uses a Variable Resistor to give a Variable Voltage - gtt
a Potentiometer has a variable resistor replacing Rone & Rtwo of the potential divider, but it uses the Same Idea (its even sometimes Called a potential divider just to confuse things)
u move a Slider or turn a knob to Adjust the Relative Sizes of Rone & Rtwo
^^ that way u can vary Vout from 0 V up to the source voltage
this is dead handy when u want to be able to Change a Voltage Continuously, like in the Volume Control of a stereo
