P2 - Electrons, waves and photons

variable resistor circuit symbol

Fuse circuit symbol

heater symbol

thermistor symbol

diode symbol

LED symbol

Light dependent resistor symbol

direction of conventional current

positive to negative terminal

direction of electron flow in a circuit

negative to positive terminal

what causes the flow of charge in metals

the movement of delocalised electrons

charge carriers in electrolytes

positive and negative ions

which is an SI base unit: A or V

Ampere

conservation of charge and how it relates to circuits

Charges cannot be created or destroyed, charge is always conserved. Therefore the sum of the charges flowing into a circuit must equal the sum of the charges leaving them.

charge definition

a physical quantity that can be either positive or negative

what is the structure of metals

crystalline

current definition

Rate of flow of charge (Q=It)

define all symbols in I=nAve

• I=current

• n=Number of electrons per unit volume

• A = cross sectional area

• v = mean drift velocity

• e= electron charge

what property determines the conductivity of a material

the number of free electrons per unit volume

e.m.f defenition

Energy gained per unit charge

P.D definition

The energy transfered per unit charge (by charges passing through a component)

1eV defintion

The energy transfered when one electron travels through a potential difference of one volt.

Ohms law

The current through a conductor is directly proportional to the potential difference across it, provided temperature is constant

defining equation for resistance

R=V/I

four factors affecting electrical resistance

• The material of the wire

• The length of the wire

• The cross sectional area of the wire

• The temperature of the wire

define and explain relationship between length of wire and resistance

length of wire and resistance are directly proportional. Because the electrons experience a smaller potential gradient as the wire gets longer. the increased length of the wire means that the voltage per metre value is lower, causing a lower drift velocity and smaller current.

define and explain relationship between cross-sectional area and resistance

Resistance and cross sectional area are incersely proportional. Because there is a bigger volume of electrons that are able to flow at the same time

define and explain relationship between temperature and resistance

as temperature increases so does cross-sectional area. The KE of the ions increases, effectively increasing there cross sectional area an making it more difficult for electrons to pass them.

description/drawing of I-V characteristics of a resistor at constant temperature

Acts according to ohms law, directly proportional. Acts in the same way regarldess of the direction of current

Description/drawing of I-V characteristics of a filament lamp

At low temperatures acts in accordance with ohms law. At higher potential differences, the gradient decreases as current increases due to an increased resistance.

Description/drawing of I-V characteristics of a diode or LED

The diode allows current to flow with a positive P.D, from positive to negative terminal. In the reverse bias direction effectively no current can pass through and it’s said to have almost infinite resistance.

useful characteristics of LEDs

• Switch on instantly

• Are very robust

• Are very versatile

• Operate with low potentials

• Have a long working life

resistance of a thermisitor - draw/describe

the resistance of a negative coefficient thermistor decreases as its temperature increases

resistance of LDR - draw/describe

the resistance of an LDR decreases as the intensity of light falling on it increases

factors in a wire affecting resistivity

none, it is an intrinsic property

define symbols in resistivity

R=resistance

rho=resistivity

l=length

A = cross sectional area

the effect of temperature on resistivity

Resistivity increases as temperature increases (for most materials, especially metals)

power definition

rate of energy transfer

voltage and current of power being transferred through the national grid

Low current and high voltage, as a higher current means more energy is wasted as thermal energy.

1 kilowatt-hour (kWh)

the energy used by a 1kW device in one hour

Kirchhoffs first law

The sum of currents entering a junction in a circuit is equal to the sum of currents leaving it

where does kirchhoffs first law come from

the conservation of charge

kirchhoffs second law

In any closed loop, the sum of the e.m.f is equal to the sum of the products of the current and the resistance (sum of P.Ds

where does kirchoffs first law come from

conservation of energy

current in a series circuit

current has the same value at any point in the circuit

e.m.f. in a series circuit

the e.m.f. in the cell is equal to the sum of the potential differences across the resistors

resistance in a series circuit

The total resistance of a circuit is equal to the sum of the individual resistors

how do we derive the equation for resistors in a series circuit

we know the sum of the P.Ds across all the resistors must equal the e.m.f., and the current flowing through each resistor is equal. Using ohms law (V=IR) and given V=V1+V2+V3, therefore IRt=IR1+IR2+IR3, factorising I gives Rt=R1+R2+R3

how are ammeters connected in a circuit

in series with components

how are voltmeters connected in circuits

IN parallel with circuit components

Voltage in parallel circuits

The sum of the components P.D in each branch of a circuit is equal to the E.m.f of the cell. P.Ds in a single loop equal cells e.m.f. (Vtotal=V1=V2)

Current in a parallel circuit

the total current in a parallel circuit is the sum of the individual branch currents (Itotal=I1+I2)

the equation for resistance in parallel circuits

1/R=1/R1+1/R2+1/R3

deriving the equation for resistance in parallel circuits

we know current divides at junctions, and voltage in each branch is the same as the cell voltage

I=I1+I2+I3

V=V1=V2=V3

applying ohms law (I=V/R) to the first equation and rearranging gets V/R=V1/R1+V2/R2+V3/R3

as the Vs are equal they can be facotrised and cancelled to get 1/R=1/R1+1/R2+1/R3

potential divider circuit

uses two resistors in series to split or divide the voltage of the supply in a chosen ratio so that a chosen voltage can be provided to another device or circuit

potential divider equation

effect when output resistor in a potential divider circuit is a variable resistor (R2 is variable)

Vout will be zero when the variable resistor has a value of o and will be maximum when the variable resistor is at its maximum

effect when output resistor in a potendial divider circuit is a variable resistor

as resistance decreases as temperature increases, Vout will be lowest at higher temperatures

effect when using an LDr IN place of the output resistor in a potential divider circuit

The LDR has a high resistance when light intensity is low, this means output voltage increases in dark conditions

terminal P.D.

The potential difference recorded across the terminals of a cell

internal resistance of a source of e.m.f.

The resistance to electrical current of the materials inside. When current flows, energy is transfered to these materials, resulting in the terminal P.D dropping.

E.m.f. equation

progressive waves

waves that transfer energy away from a source

longitudinal waves

oscillations are parallel to the direction of energy transfer

transverse waves

oscillations are perpendicular to the direction of energy transfer

wavelength

The distance between two successive identical points that have the same pattern of oscillation. It is also the distance the wave travels before it repeats itself.

Period

The time taken for one complete oscillation to take place at any point

Frequency

the number of oscillations per unit time

displacement (of a wave)

the distance any part of the wave has moved from its equilibrium position

Amplitude (of a wave)

the maximum displacement

what do the axis of an oscilliscope show

Time -xaxis

Voltage - yaxis

what is the time base setting on an oscilliscope

the time taken for the dot produced to travel 1cm horizontally across the screen

how to calculate period from an oscilliscope

period=distance between peaks x time base setting

deriving v=f(lambda)

speed=distance/time

v=(lambda)/T

we know f=1/T

so v=f(lambda)

intensity word defintion

the rate at which energy is transferred from one location to another as the wave travels through space, perpendicular to the direction of travel

Intensity formula

intensity = power/area

relationship between intensity and amplitude

Intesity is proportional to amplitude squared

wavefront definition

lines of constant phase, the distance between them represents wavelength

when does refraction occur refraction

occurs when a wave moves from one material to another

what happens during refraction

the wave changes speed, and changes direction if it is not already traveling along the normal

diffraction

the spreading out of a wave after passing around an obstacle or through a gap

when is diffraction most obvious

when the wavelength of the wave is the same size as the gap its travelling through

interference

the addition of two or more waves that results in a new wave pattern

which type of em wave has the greatest frequency

gamma (most dangerous)

general wavelengths of the e.m. spectrum

which e.m wave has the longest wavelength

radio

plane-polarised wave

A wave in which the oscillations of the field and the direction of travel are confined to a single plane

polarising filter

produces plane-opolarised light by selective absorbtion of one component of the incident oscillations

Malus’ law (transmitted intensity through a polarising filter)

I=Imax cos²(theta)

what happens to the speed of a wave when it moves from a material of lower refractive index to one of higher refractive index

speed will decrease

refractive index formula

refractive index, n = (speed of light in a vacuum)/ (speed of light in the material)

snells law

n sin(theta) = constant

when can snells law be used

for a wave travelling between two different materials with different refractive indices. To find the angle of the incident or refractive rays to the normal, or the refractive index of either material

what is the refractive index of air

very close to 1

total internal reflection definition

total internal reflection occurs when the angle of refraction is greater than 90 degrees and the light is refracted along the boundary

when does total internal reflection occur

When The angle of incidence is greater than the critical angle and the incident refractive index n1 is greater than the refractive index of the material at the boundary n2

critical angle

When the angle of refraction is exactly 90° the light is refracted along the boundary

critical angle formula in booklet

sinC=1/n (when wave is travelling to boundary from air)

critical angle between two mateirals not involving air

sinC =n2/n1 where n2 is less than n1

principle of superposition

when two or more waves of the same type meet, the resultant wave is the sum of the displacement of the individual waves

interference

Interference occurs when waves overlap and their resultant displacement is the sum of the displacement of each wave