P2 - Electricity Flashcards
What is electric current
Flow of an electrical charge which only flows around a closed circuit
Current is in ampere A
What is potential difference
The driving force that pushes the charge round
In volts V
Bigger pd = bigger current
What is resistance
Anything that slows the current down
Resistance is in ohm
Big resistance = slow current
The size of the current is….
The rate of flow of charge
More charge = more current
Equation for charge
Charge = current x time Q = I x t
Circuit symbols - look at notes
Beep hoop
What is conventional current direction
Goes from positive terminal to the negative terminal of the battery /cell
Equation for potential different
Potential difference = current x resistance
V = I x R
Factors affecting resistance
Length of wire (in either parallel or series circuit)
Why do some wires need a larger potential difference
So they can produce more current than others as they have a larger electrical resistance
What do ohmic conductors have
Have constant resistors
The resistance of an ohmic conductor doesn’t change with current
At a constant temperature, the current through and ohmic conductor is directly proportional to….
The potential difference across it
Key information of filament lamp
Charge flows through a lamp and the Lamp heats up, resistance increases to decrease the current so lamp doesn’t overheat
Key information of diode
Resistance depends on direction. It will let current flow, but high resistance if direction is reversed
What are current to potential difference graphs
Graph to show how current flowing through a component changes as the potential difference across it increases
Current - potential difference graph of ohmic conductor
Linear component - both increase at same rate
Current - potential difference graph of a filament lamp and diode
Both Non linear (look at notes)
Key information of a light dependent resistor
Resistance depends on light intensity
Bright light = resistance falls
Dim light = high resistance
Used for night lights, outdoor lighting, burglar detectors
Key information of a thermistor
Temperature dependent resistor
Hot = resistance drops
Cold = resistance increases
Both can be used in sensing circuits to turn on or increase power to components depending on conditions
Key information of series circuits
Different components connected in a line between +ve and -ve of power (voltmeters always in parallel)
If you remove one component circuit breaks
Potential difference in series circuit
Total potential difference is shared between the components. Always adds up
V1 + V2 = Vtotal etc.
Current in a series circuit
Same current flows through all circuits I1=I2=I3
Resistance in a series circuit
2 components resistance will add up Rtotal =R1 + R2
Two resistors share total potential difference
PD lower, current lower in resistor
Current same everywhere. New resistor = lower current everywhere
Bigger resistance = bigger share of PD
Key information of parallel circuit
Each component is seperately connected (apart from ammeters)
Remove 1 component = little to no consequence
Resistor in parallel circuit
Adding a resistor decreases total resistance
Both resistors have same potential difference meaning current flow is the same as the potential difference source for each added resistor
This increases current as another loop gives more directions for current to go in
2 resistors = total resistance = less than the resistance of the smallest of the two resistors
Potential difference in a parallel circuit
All components get full source of potential difference - equal all around so bulbs all same brightness
Current in parallel circuit
Total current = total of all currents through separate components
Total current into a junction is the same as leaving
If two identical components are parallel then the same current will flow through them
Difference of resistors in the circuits
Series - new resistor will increase resistance, decreasing current
Parallel - new resistor will increase current, decreasing resistance
Name the two main current supplies
Alternate current and direct current
How are alternate currents produced
AC produced by alternate voltages in which positive and negative ends always changing and electrons pass on energy
AC supplies constantly changing direction
What is direct current
Current always flowing in same direction
Supplied by cells and batteries
Goes from high charge to low charge through series circuit
What Are the 3 wires of a cable
Neutral wire -blue
Live wire -brown
Earth wire - green and yellow
Role of the neutral wire
Completed the circuit and carried away current
Electricity normally flows I’m through the live wire and out the neutral. Around 0v
Role of the live wire
Provides alternating potential different around 230V
Role of earth wire
Protecting the wiring. Stops appliance casing from becoming live no current unless fault so 0v
What’s a fuse
A safety device marked with the current it can carry. Normally 3A 5A or 13A
What happens if you touch the live wire
As your body is at 0V, a large potential difference is produced across your body and a current flows through you
Safety features of a cable
Switches connected to a live wire - when off, no current through appliance
Fuse is a tube with a wire in it. Current passes through the wire and the wire heats up, if the current exceeds the limit the wire melts and breaks the circuit
If appliance develops fault, mets, part with high voltage will go to the earth wire instead of you
Circuit breakers like fuses - detect change in current and switch off supply (quicker and no replacement unlike fuse)
Energy is transferred between stores electrically by
Electrical appliances
Why does a moving charge transfer energy
Because the charge does work against the resistance of the circuit
Electrical appliances are designed to…
Transfer energy to components in the circuit when a current flows
What happens when current passes through a resistor
Energy is transferred because electrical work is done against the resistance. Energy transferred by heating and resistor becomes warm -> why circuits always warm as they all have some form of resistor
How can resistance in circuits be reduced
Using wires made from metals with low resistance like copper
Or thicker wires = lower resistance
Decreased by coding metals so lattice ions aren’t vibrating as much
What happens inside a resistor
Electrons(the current) flow through the lattice of vibrating ions, and they collide with the ions - more collisions they make, harder it is for them to pass through.
When electrons collide with ions they transfer energy to the,
Transfer of energy examples
Kettle
Handheld fan
Kettle transfers energy electrically from the mains AC supply to the thermal energy stores inside the kettle
Energy transferred from battery of a handheld fan to the kinetic energy store of a fans kotor
Equation for efficiency
Efficiency = useful energy output/total energy output
No appliance transfers all energy usefully
Higher current = more wasted energy
Total energy transferred depends on
How long the appliance is on for, and it’s powerful
What’s the power of an appliance
It’s the energy transferred per second
Equation for energy transferred
Energy transferred = power(W) x time(s)
E = Pt
What’s a power rating
Maximum safe power that an appliance can operate at
Tells you maximum amount of energy transferred between stores per second
Energy is supplied to the….
Charge at the lower source
Charge gives up this energy when it ‘falls’ through any potential drops in components elsewhere in the circuit
Equation for energy transferred
Energy transferred = charge flow (C) x potential difference
What happens if a battery has a large potential difference
It will supply more energy to the circuit for every coulomb of charge which flows round it, as the charge is raised up ‘higher’ at the start
Equation for power
Power (W) = Potential difference x current (A)
P = VI
Or power = current squared x resistance
What is the national grid
A giant web of wires that covers all of Britain
Supplies electricity from power stations to homes
Giant system of cables and transformers that connect power stations to consumers
When are there Peak in demands of electricity
Morning, when it’s dark or cold and when people come home from work/school
Why does the national grid use high PD and low current
To transmit lots of power, you need either a high pd or current.
However, high current = lose loads of energy through waste
Cheaper to boost pd
Increasing pd decreases current, decreasing energy lost > more efficient way
What do transformers do
Step up the pd as it leaves the power stations, but step down the pd to a safe level when given to consumers
