Electricity Flashcards
What is electric current measured in
Amperes (A)
Define electric current
Rate of flow of charge
Electric Current equation
I = Change in Q / Change in t
I = Current (A)
Q = Charge Transferred (C)
t = Time (s)
Describe current
Amount of charge passing a given point in a given circuit per unit time
What is 1 A the same as
One coulomb of charge passing a given point per second (1 C s^-1)
What is electrical charge
A physical property measured in coulombs (C)
Define Coulomb
The electric charge flowing past a point in one second when there is an electric current of one ampere
What is one coulomb equivalent to
One ampere second (A s)
What is e
Elementary charge
1.6 x 10^-19 C
Equation for net change on an object
Q = +-ne
Q = net charge on the object in coulombs
n = number of electrons
e = elementary charge
Describe charge on an object
Quantised - can only have certain values (integer multiples of e)
When is an ammeter used and how
Measure the electric current at any point in a circuit
Placed directly in series and at the point where you want to measure the current
Why are ammeters placed in series
They should have the lowest possible resistance - reduce the effect resistance has on current
Ideal ammeter has zero resistance- no effect on current it measures
Structure of a metal
Lattice of positive ions surrounded by delocalised electrons
Positive ions are not free to move - vibrate around fixed points - vibrate more vigorously when temperature increases
Why may current get larger
Greater number of electrons moving past a given point each second
Same number of electrons moving faster through metal
What does the conservation of charge state
Electric charge can neither be created nor destroyed
Kirchhoffs first law
For any point in a circuit - the sum of current into that point is equal to the sum of current out of that point
What is number density
Number of free electrons per cubic metre of material
Conductors number density
Order of 10^28 m^-3
Insulators number density
Much lower than conductors
Semiconductors number density
In between insulators and semiconductors
10^17 m^-3
What do semiconductors need to do carry the same amount of electrons as conductors and why
They need to move much faster
Lower number density
Equation for current with v
I = Anev
I = electric current (A)
A = cross sectional area (m^2)
e = elementary charge (C)
v = mean drift velocity (ms^-1]
Derive I = Anev
I = Change in Q / Change in T
I = neV / Change in T
V/ Change in T = Av
I = neV / Change in T = neAv
What is Potential Difference
Measure of the transfer of energy by charge carriers
Measured in Volts
What is one Volt
P.D across a component when 1 J of energy is transferred per unit charge passing through the component
1 V = J C^-1
Define Potential Difference
Energy transferred from electrical energy to other forms per unit charge
Potential Difference equation
V = W/Q
V = P.D (Volts)
W = Energy transferred by Q
Q = Charge (C)
What is a Voltmeter
Component used to measure P.D which is always connected in parallel
Conditions for ideal voltmeter
In parallel and infinite resistance - no current will pass through the voltmeter
Define EMF
Energy transferred from chemical energy to electrical energy per unit charge
EMF equation
W / Q = Weird E
Weird E (V)
Q = Charge (C)
W = Energy transferred by Q
Energy transfer equation (charges)
W = VQ
W = Weird E x Q
What is an electron gun
An electrical device used to produce a narrow beam of electrons
What is thermionic emission
Emission of electrons through heat
How does an electron gun work in most cases
A small metal filament is heated by an electric current - electrons in the wire gain kinetic energy - some of them gain enough KE to escape from the surface of the metal
Heated filament in a vacuum - high p.d applied between filament and an anode - explain what happens
Filament acts as a cathode - free electrons accelerate towards the anode - gaining KE
If anode has a small hole - electrons in line with the hole can pass through it - gives rise to a beam of electrons with a specific KE
Energy transfers in Electron Gun
Electrons accelerate towards anode - gain KE - Work done on single electron from cathode to anode is eV
Work done on electron = 1/2mv^2 (assumes electrons have negligible kinetic energy at cathode)
What is resistance
The opposition to a flow of electric current
How to determine resistance in a circuit
Measuring the current in a component and the p.d in a component
Using R = V/I
What is the unit of resistance
Ohm
Define the ohm
Resistance of a component when a P.d of 1V is produced per ampere of current
1 Ohm = 1 V A^-1
Deduction made from IV graph for resistors
Straight line through O
P.D across resistor is directly propertional to the current in the resistor
Resistor obeys Ohms Law
Resistance of resistor is constant
Resistor behaves same way regardless of polarity
Deductions made from IV graph of a filament lamp
Passes through O - straight through the middle - curves off
P.d is not directly proportional to current
Does not obey V = IR - non ohmic component
Resistance is not constant
What is a diode
A component which allows a current in one particular direction
What do LEDs emit
Light of a certain wavelength
IV characteristic for a diode
P.D across a diode is not directly proportional to the current
Non-ohmic component
Resistance is not constant
Behaviour depends on polarity
Gradient stays flat until threshold p.d and slowly increases
What factors affect resistance
Temperature
Material
Length
Cross sectional area
Define resistivity
Electrical property of a material
Relationship between resistance of a wire and it’s length
Directly proportional
Relationship between resistance of a wire and cross sectional area
Cross sectional area increases - resistance decreases
R is directly proportional to 1 / A
Calculate resistivity from resistance
R is directly proportional to L / A
Resistivity is the constant
R = (Resistivity x length) / Area
Resistivity unit
Ohm meter
Define Resistivity
Resistivity of a material at a given temperature is the product of the (resistance of the material and its cross sectional area) divided by length
Resistivity equation
Weird p = RA/L
Resistivity relationship with temperature
As temperature increases - resistivity increases
Deduce resistivity from a graph
Multiplying gradient by cross sectional area
Gradient = resistivity / A
What does negative temperature coefficient do to a component
Resistance drops as the temperature increases
What is a thermistor
A semiconductor with a negative temperature coefficient
Change in resistance is often dramatic
Thermistor experiment
Use a water bath to control the temperature of a thermistor
An ohmmeter for a quick and simple recording of the resistance
Ammeters and voltmeter CAN be used to measure current and P.D
Resistance calculated through R = V/I
IV Characteristics of a thermistor
Temperature increases as current increases
Temperature increase leads to a drop in resistance - number density of charge carriers increase
Straight gradient curves vertically (graph)
What is a LDR
A Semiconductor in which the number density of charge carriers changes depending on light intensity of the incident light
When do LDRs have high and low resistance and why
Dark conditions - high resistance
Number density of free electrons is low
Bright conditions - low resistance
Number density of free electrons is high
Investigate LDR
Resistance of LDR varies with distance form a constant light source
Narrow tube made of black cardboard placed around LDR greatly reduces the effect of other background sources of light
Will see a curved L graph
Equations for electrical power
P = VI
Leads to (P = IR x I) = (P = I^2R)
P = VI and I = V/R
Leads to (P = V x V/R) = (P = V^2 / R)
Derive P = IV
P = W/t
V = W/Q W = VQ
P = VQ/t Q/t = I
P = VI
Calculate energy transferred in a circuit
P = W/t W = Pt
P= VI
W = VIt
What is a kilowatt hour
Energy transferred by a device with power 1kW for 1h
1kWh = 3.6 MJ
J = Ws OR kWh = kW x h
Kirchhoff’s Second Law
Sum of EMF = Sum of P.Ds
Total energy transferred to the charges = total energy transferred from the charges
Current in a series circuit
Same in every position
Charge isn’t used up - just flows around
EMF in a series circuit
EMF is shared between components
Components with greater resistances take up a greater proportion of EMF
Same rule applies for circuits with more than one source of EMF
If sources of EMF are connected with opposing polarities - sum EMF is the difference
Resistance in a parallel circuit
Greater resistance of the branch - lower the current that passes through it
Changes made to one branch does not affect the other branches
EMF and P.D in a parallel circuit
Total p.d in each branch = total emf from the power supply
Resistors in series
Total resistance = sum of individual resistances
Resistors in parallel
1 / Total Resistance = 1 / R1 + 1 / R2…
Four key electrical relationships
I = change in Q / change in t
V = W / Q
P = VI
V = IR
Size of internal resistance required for large current
Small internal resistance
What is lost volts
When Terminal P.D is less than actual EMF
Relationship between EMF, Terminal P.D and lost volts
EMF = Terminal P.D + Lost volts
How does changing current affect terminal p.d and lost volts
More charges travel though the cell per second - more work done is done by charges - increasing lost volts - lowers terminal p.d
Equation for lost volts applying V=IR to the internal resi
I x r (TIR)
If r is fixed - current is directly proportional to the lost volts
EMF compared to terminal p.d
EMF always more than terminal p.d unless there is no current
When current is very small, EMF = P.d
Potential Divider Equation
V out = (R2 / R1 + R2) x V in
Simplest way to vary V out
Replace one of the fixed resistors with a variable resistor
Low voltage electric circuits use a potentiometer
