P5 - Electric Circuits

Página 1

Static Electricity

2 insulating materials -> rubbed together

electrons scraped one -> dumped on the other

leaves positive charge on 1 -> negative static charge on other

electrons removed -> particles left -> positive

ions -> charged particle

+ve & -ve -> only produced by movement of electrons

2 opposite electric charges -> attracted

2 same electric charges -> repel

Attracting Dust

Dust particles -> tiny & lightweight -> attracted to charge

objects -> insulating materials -> easily charged -> attract dust

Clinging clothes & crackles

Synthetic clothes -> dragged over one another

electrons scraped off -> static charges on both

attraction -> sparks/shocks -> charges rearrange themselves

Bad Hair Days

Static builds up on hair

each strand same charge -> repel

Electric Current

Electric current -> flow of charge

electrical circuit -> metal conductors -> full of charges -> free to move

current can't flow in insulator -> few charges free to move

complete circuit -> battery pushes free charges -> through wires

charge flows all way round

Current (I) -> only flow if there's voltage across component -> units: amps (A)

Voltage (V) -> pushes current round -> unit: volts (V)

Resistance (R) -> caused by things in circuit -> resist flow of charge -> units: ohms ( )

Voltage push -> resistance opposing it -> relative sizes decide how big current will be

Increase voltage -> more current

Increase resistance -> less current

Supply electricity -> supply energy

Power supplies -> transfer energy to the charge -> transfers to components

Work done -> energy is transferred

Power -> rate electrical power supply transfers energy -> Watts (W) -> Kilowatts (kW) -> 1000 W = 1kW

Appliance - high power -> transfers lots of energy in short time

Energy comes from current

Electrical power: Power (W) = Voltage (V) x Current (A)

Página 2

Electric Circuits

basic circuit -> used testing components

component, ammeter & variable resistor -> in series -> put in any order

ammeter -> in series

voltmeter -> in parallel

Varying variable resistor -> current flowing through circuit

Potential difference -> how much energy transferred to/from each unit of charge as it moves between two points

battery transfers energy to charge

components transfer away from charge

voltage -> how much work battery does

voltmeter -> measure potential difference between two points

Resistance

Voltage-current (V-I) graphs -> how current in circuit varies as voltage changes

current through component -> proportional to voltage across it -> resistance stays consistent

constant temperature -> resistance of component -> steady and equal to inverse of gradient (1/gradient)

steeper graph -> lower resistance

Resistance ( ) = Voltage (V) / Current (A)

electrons move through resistor -> collide w/ positive ion

collisions -> ions vibrate more -> increase in temp.

filament lamp -> piece of wire -> really high resistance -> current passes through: temp. increases -> glows

light dependant resister (LDR) -> changes resistance depending on how much light there is

bright light -> resistance falls

darkness -> resistance higher

e.g automatic light night

thermistor -> resistance depends on temp

hot -> resistance drops

cool -> resistance increases

e.g electronic thermostats

Página 3

Series Circuits

series circuits -> components connected in a line -> end to end

total potential difference (P.D.) -> supply is shared between various components

V = V1 + V2

series circuit -> same current -> flows through all parts

size of current determined by P.D of cells & total resistance

all components get same current

A1 = A2 + A3

total resistance -> sum of individual resistances -> R = R1 + R2 + R3

resistance of two resistor -> bigger than one -> battery push charge through all of them

bigger resistance -> bigger share of total P.D -> more work done

resistance one component changes -> potential difference across all changes

connect several cells in series -> bigger total voltage

adding cells -> series increases voltage w/out increasing resistance -> total current increases

each cell the same as total current

Parallel Circuits

parallel circuits -> separately connected to +ve & -ve of supply

remove/disconnect one -> hardly affects others

parallel -> P.D across each component equal to P.D of battery > same across all components

V1 = V2 = V3

current -> shared between branches -> total current leaving battery -> equal to total of currents in separate branches

A = A1 + A2

Total resistance -> always less than branch w/ smallest

resistance is lower -> charge more than one branch to take

two resistors in parallel -> lower resistance than circuit with resistors by themselves -> parallel circuits -> higher currents

Each component -> separately connected to battery -> current in each component is same

resistance -> controls how much current voltage push through it

component w/ least resistance -> largest current

all components have same P.D -> same P.D causes larger current flow through smaller resistance

Página 4

Mains Electricity

mains domestic electricity supply -> 230 volts

produced by generators -> electromagnetic induction

AC -> mains electricity -> easier to generate - > simpler to distribute over long distance

AC Generator

AC supply (alternating supply) -> current constantly changing direction

AC -> mains electricity -> easier to generate -> simpler to distribute over long

Generator -> kinetic energy -> turns to electrical energy

Coil rotates inside magnetic field

Produces current -> process of electromagnetic induction

Coil spinning -> direction sides of coil moving -> continually changing -> current continually change

Direction motion reverses -> current reverses -> alternating current (AC)

Side of coil moving perpendicular -> magnetic field lines -> current at maximum -> (coil cutting through most magnetic field lines)

Sides of coil moving parallel -> current at zero

Coil passes -> movement reversed & direction of the current

DC Motor

Batteries -> DC supply (direct current) -> current flows in same direction

Current flows around coil in magnetic field

2 sides coil -> perpendicular to magnetic field lines

Current -> opposite directions -> opposite sides -> forces either side in opposite directions

Turning motion

Sides of coil -> right angles to magnetic field -> force at maximum

coil turned 90 degrees -> sides moving parallel w/ magnetic field -> force cancel each other

Momentum of coil -> carry past stage ^^^

Forces now opposite direction -> motor turning

Because current -> flow in opposite direction -> 2 parts of split ring commutator touch other brushes

5 factors affect size of induced voltage

Add iron core inside coil

Increase strength of magnetic field

Increase speed of rotation

Increase number of turns on coil

Reduce - take out iron core/decrease one of above

Página 5

Transformers

Change size of voltage -> increase/decrease -> step-up/step-down transformer -> e.g mobile phone, chargers, laptop, speakers, national grid

Primary & secondary not connected -> wound around iron core

Step-up transformer -> more turns on secondary side

Step-down transformer -> more turns on primary side

Made -> 2 coils wire -> from separate circuits -> wrapped around iron core

Used change size of alternating voltage

Changing current -> primary coil -> changing magnetic field

Changing magnetic field -> change in current -> induced in secondary coil

Difference in no. coils -> primary & secondary size -> determines voltage increases/decreases & how much

primary voltage primary turns

-------------------------- = -------------------------

secondary voltage secondary turns

Vp Np Np Vp Vp

----- = ----- Vs = ------ x Vp Np = ----- x Ns Ns = ------ x Np

Vs Ns Ns Vs Vs

Generator

Generator -> kinetic energy -> turns into electrical energy

Magnet rotates inside coil of wire

Produces current -> process of electromagnetic induction

Magnet spinning -> magnetic field continually changing -> current continually change

Magnetic field reverses -> current reverses -> alternating current (AC)

Magnetic Fields

Region where magnetic materials (e.g iron & steel) & wires carrying currents experience a force acting on them

Magnetic field around straight current-carrying wire

Made of concentric circles

Bend current-carrying wire round into coil -> magnetic fields round side of loop reinforce at centre

Coil -> lots of turns -> more reinforcement at centre

Current-carrying wire/coil -> exert force on another current-carrying wire/coil/permanent magnet

Current-carrying wire -> diff. magnetic field -> 2 magnetic field affect one another -> result: force on wire

Full force -> wire at right-angle to line of force of magnetic field

Wire runs parallel -> won't experience force -> angles between 0 - 90 -> some force

Force always acts right-angle to both lines of force of magnetic field & direction of current

Fleming's rule of thumb -> thumb: motion -> index: field -> middle finger -> current

The Motor Effect

Rectangular coil -> uniform magnetic field (same strength everywhere in field) -> force -> turn -> motor effect

Force act on two side arms of coil -> force act one up & one down -> rotate

Split-ring commutator -> swapping contacts every half term

Reverses direction of current every half turn -> keep coil rotating continuously -> same direction

Direction of force -> reverse every half turn -> coil change direction every half turn instead of fully rotating

E.g. DVD player, electric cars, trains, hard disc drive