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P2 - Electricity (The National Grid (Electricity Productions has to Meet…
P2 - Electricity
The National Grid
Electricity is Distributed via the National Grid
The national grid is a system of cables and transformers
The national grid transfers power from power stations to anywhere it's needed
Electricity Productions has to Meet Demand
Electricity usage change
They can predict when most electricity is needed
Power stations run below their maximum power output
Lots of smaller stations are kept on standby
The National Grid Uses a High Pd and a Low Current
You need a high potential difference or current
The problem with high current is that the wire heats up
Much cheaper to boost pd really high
Increasing pd decreases current
Potential Difference is Changed by a Transformer
Needs transformers and big pylons
Have to step pd up at one end and down at the other
Increased using a step-up transformer
Reduced using step-down transformer
Circuit Devices
LDR is Short for Light Dependent Resistor
Dependent on the light intensity
Resistance falls in bright light
Resistance is highest in darkness
Can be used for night lights and burglar detectors
The Resistance of a Thermistor Depends on Temperature
Temperature dependent resistor
Resistance drops in heat
Resistance increases in the cold
Make useful temperature detectors
You Can Use LDRs and Thermistors in Sensing Circuits
Used to turn on or increase power to a component
Used to operate fans
Resistor and fan always have the same potential difference
Shared between thermistor and the fixed resistor and fan
With heat, the resistance decreases and it takes less of the pd
Investigating Resistance :star:
You Can Investigate Adding Resistors in Series
Four identical resistors
Basic circuit - take note of pd
Measure current. Calculate resistance
Add another resistor
Measure current and calculate resistance
Repeat step 4 and 5 until all resistors have been added
Plot a graph of number of resistors against resistance
Or in Parallel
Build same initial circuit
Measure current and calculate resistance
Add resistor, in parallel
Measure the current and calculate resistance
Repeat 3 and 4 until all resistors are in
Plot a similar graph
Your Results Should Match the Resistance Rules
Adding resistors in series increases total resistance
The more resistors you add, the larger the resistance
Parallel, total current increases so resistance decreases
The more resistors you add, the smaller the overall resistance
Electricity in the Home
Mains Supply is ac, Battery Supply is dc
Alternating current or direct current
ac - current constantly changes direction
Mains supply is around 230V
Frequency is 50Hz
Cells and batteries are dc
Direct current always flows in the same direction
Most Cables Have Three Separate Wires
Connected by a three-core cable
The colour of the wire shows the purpose
Always the same for every appliance
Wires
Live wire - brown
Neutral wire - blue
Earth wire - green and yellow
The Live Wire Can Give You an Electric Shock
Your body is at 0 V
Large potential difference through body - large electric shock
Even if its off, still electric shock risk
Connection between live and earth is dangerous
Current
Current is the Flow of Electrical Charge
Electric current is the flow of electric charge
In a single, closed loop, the potential difference is the same everywhere
Potential difference is measured in volts
Resistance is measured in ohms
The current is dependent on the potential difference and resistance
The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component) :warning:
Total Charge Through a Circuit Depends on Current and Time
Q = I x t :warning:
Resistance and V = IR
There's a Formula Linking Potential Difference and Current
V = I x R
You Can Investigate the Factors Affecting Resistance :star:
The Ammeter
Measures the current
Must be placed in series
The Voltmeter
Measures potential difference
Must be placed in parallel
Attach a crocodile clip at 0cm
Attach second clip and note the distance between
Close switch and measure current and potential difference
Open switch, move crocodile clip and repeat
Repeat multiple times
Use measurements to calculate resistance
Plot a graph
Straight line through origin (directly proportional)
Resistance and I-V Characteristics
Ohmic Conductors Have a Constant Resistance
The resistance doesn't change with the current
The resistance of some resistors and components do change
Resistance increases with temperature, so current increases, filament lamp temp increases, resistance increases
Diodes - resistance depends on direction of current
Three Very Important I-V Characteristics : :star:
Set up a test circuit
Vary the resistor to alter the current
Take several readings of current and pd to see how it changes
Change the current direction
Plot a graph of current against voltage
Graphs
Ohmic conductor - directly proportional
Filament lamp - slight curve at start and end
Diode - Takes off really quickly
Parallel Circuits
Parallel Circuits - Independence and Isolation
Each component is separately connected
Removing one doesn't affect others
How most things are connected
Everyday circuits are a mixture of series and parallel
Current is Shared Between Branches
Total current is the total through all separate components
Junctions where the current splits and rejoins
If two identical components are connected they have the same curent
Potential Difference is the Same Across All Components
All components get the full pd
Identical bulbs are at the same brightness
Adding a Resistor in Parallel Reduces the Total Resistance
If two resistors are next to each other, the resistance is less than the resistance of the smallest one
Power of Electrical Appliances
Energy is Transferred from Cells and Other Sources
Charge does work against resistance
Electrical appliances are designed to transfer energy when a current flows
No appliance transfers all energy usefully
Energy Transferred Depends on the Power
The total energy depends on time and power
The power is the energy transferred per second
E = P x t :warning:
Appliances give a power rating
Power rating is the maximum amount of energy transferred per second
Helps the customer choose between models
Higher power doesn't mean more energy is usefully transferred
Power
Potential Difference is Energy Transferred per Charge Passed
When charge goes through a change in potential difference, energy is transferred
Energy supplied to charge at the source
Charge gives up energy if there's a potential drop
E = Q x V :warning:
Power Also Depends on Current and Potential Difference
P = VI :warning:
P = I^2 x R :warning:
Series Circuits
Series Circuits - All or Nothing
Different components are connected in a line
If you remove one component, the whole circuit is broken
Potential Difference is Shared
The total pd is spread across all components
Current is the Same Everywhere
The same current flows through all components
Size of current is determined by I = V / R
Resistance Adds Up
The total resistance is the sum of all resistances
By adding a resistor, they just have to share the resistance
Current decreases when the resistance is split
The bigger the resistance, the bigger the share of potential differences
Cell Potential Differences Add Up
Bigger pd when more cells are in series