Electricity
Combined Science
- Created by: abbienoice
- Created on: 21-11-20 15:22
View mindmap
- Electricity
- Circuits
- Parallel Circuits
- each component is separately connected to the battery - except the ammeter as its always connected in a series
- if you remove one of them it will hardly affect the others
- how most things are connected - in houses and cars you have to be able to switch things off separately
- potential difference is the same across all components - V1 = V2 = ....
- the total current is the total amount of current through different components - total I = I1 + I2 + ...
- adding a resistor reduces the total resistance
- Series Circuit
- the different components are connected in a line end to end
- if you remove one component the circuit is broken
- the potential difference is shared - total V = V1 + V2 + ...
- current is the same everywhere - I1 = I2 = ......
- the total resistance is the sum of the components resistances - total R = R1 + R2
- Resistance
- anything that slows the flow down
- for some components, as the current through them is changed the resistance changes aswell
- the resistance of ohmic conductors doesnt change with the current - at a constant temp the current flowing through is directly proportional to the potential difference
- eg. diodes, a filament lamp
- filament lamp - transfers some energy to the thermal energy store of the filament - as the current increases the lamp heats up and resistance increases
- diode - resistance depends on the direction of the current - will happily let current flow in one direction but have a very high resistance if reversed
- resistance increases temperature
- Current
- electric current is a flow of electrical charge
- electrical charge will only flow round a complete (closed) circuit if theres a potential difference
- potential difference is the driving force that pushes the charge around - unit is the volt
- measured in amps
- in a single, closed loop the current has the same value everywhere in the circuit
- the greater the resistance across a component, the smaller the current that flows
- Symbols table*
- circuit devices
- LDR
- Light Dependent Resistor
- dependent on the light intensity
- bright light = resistance falls
- darkness = resistance is high
- lots of applications - automatic night nights, outdoor lighting and burglar detectors
- thermistor
- temperature dependent resistor
- hot conditions - resistance drops
- cool conditions - resistance goes up
- make useful temperature detectors
- you can use them in sensing circuits
- sensing circuits can be used to turn on or increase the power to components depending on the conditions
- LDR
- everyday circuits often include a mixture of series and parallel circuits
- Parallel Circuits
- Electricity in the Home
- electricity supplies
- alternating current (ac)
- current constantly changing direction
- produced by alternating voltages in which the positive and negative ends keep alternating
- UK mains supply is ac at around 230V
- frequency = 50 cycles per second or 50 Hz
- direct current (dc)
- cells and batteries
- current always flowing in the same direction
- created by direct voltage
- alternating current (ac)
- cable wires
- LIVE WIRE
- brown
- provides alternating potential difference from the mains supply
- can give you an electric showck
- EARTH WIRE
- green and yellow
- protects the wiring and for safety
- stops appliance casting from becoming live
- usually holds no current
- NEUTRAL WIRE
- blue
- completes the circuit
- when the appliance is operating the current flows through the live and neutral wires
- around 0 V
- any connection between the live and earth wire can be dangerous
- LIVE WIRE
- electricity supplies
- Power
- the power of an appliance is the energy that it transfers per second
- power ratings
- appliances are labelled with the maximum safe power they can operate at
- tells you the maximum amount of energy transferred between stores per second when in use
- helps customers choose between models
- the lower the rating, the less electricity an appliance uses in a given time - cheaper to run
- a higher power doesnt mean that it transfers more energy usefully - less effiecient
- The National Grid
- a giant web of wires that cover the whole of Britain - gets electricity from power stations to homes
- demand
- energy demands changes at different times in day
- power stations can predict when the most electricity will be used - when people get up, come home and when it starts to get cold and dark
- power stations often run at well below their maximum power output - spare capacity to cope with high demand
- small power stations are kept in standby just incase
- energy demands changes at different times in day
- to transmit huge amounts of power you need high potential difference or high current
- high current - lose loads of energy as wires heat up - energy is transferred to thermal energy store of surroundings
- cheaper to boost potential difference high and keep current low
- increasing the potential difference decreases the current - makes the national grid efficient
- transformers
- changes the potential difference
- have two coils - a primary and secondary coil joined with an iron core
- potential difference is increased using a step-up transformer
- they have more turns on the secondary coil
- the potential difference is then reduced at the local consumer end using a step-down transformer
- they have more turns on the primary coil
- power of primary coil = potential difference x current
- Circuits
Comments
No comments have yet been made