Current and Mains Electricity Flashcards
How are voltmeters always connected?
In parallel
If an insulating material gains electrons what happens? Eg?
Negatively charged
Eg polythene rod
If an insulating material loses electrons what happens? Example?
It becomes positively charged eg Perspex rod
When do insulting materials come charged?
When rubbed together because electrons are transferred from one material to another
Features of a series circuit?
Dame current across each component
Adding the potential differenc gives the total
Sum of resistance = total resistance
Features of a parallel circuit?
Sum of the currents = total current
Potential difference is the same
The bigger the resistor the smaller the current
When would an appliance be doubly insulated?
When it has a plastic case
How many milliamps are equal to one amp?
0.001 amp
What does reversing the current do?
Reverse the voltage
When does the resistance of a filament lamp increase?
As the temperature increases
Why are socks and plug cases made of stiff plastic materials?
Because it is a good electrical insulator
Why does a resistor become hot when an electrical charge flows through it?
Energy trad ferret to the resistor makes it hot
Why are electrical faults dangerous?
Can cause electric shocks and fires
How are ammeters always connected?
In series
What unit is potential difference?
VoLTs
What is the frequency of the mains electricity supply in the UK?
50 Hz
Unit of charge?
Coulombs
Advantage of putting an insulating sleeve on a water tank?
Water stays hotter for longer
Heat for less time
Cost soon recovered
Advantage of hydroelectric power?
Can store energy for later use
With the same amount of fossil fuels, how can you reduce the amount of negative effects of burning fossil fuels?
Plant trees
Carbon capture
What happens when you run a perspex rod with a dry cloth?
Transfers electrons from the surface atom of the rod on to the cloth
Perspex rod becomes positively charged as it loses electrons
What happens when you rub a polythene rod with a dry cloth?
Transfers electrons to the surface atoms of the rod on to the cloth
So the polythene rod becomes negatively charged (as the rod gains electrons )
What is charge measured in?
Coulombs C
What’s currents measured in?
Amps (A)
What does a battery consist of?
Two or more cells connected together
What is resistance in?
Ohms
In a series circuit what is the current?
The same in each component
For a series circuit what is the potential difference?
It is found when adding up all of the potential differences
What is the current in a parallel circuit?
The sum of all the currents through the separate components
In a parallel circuit what is the pd?
The same across each component
What is peak voltage of an alternation potential difference?
Maximum voltage measured from 0 volts
What is a neutral wire at?
0 volts
What does the live wire do?
Alternates in every cycle between positive and negative
What happens when insulating materials are rubbed against each other?
They become electrically charged
What happens when two electrically charged objects are brought together?
They exert force on each other
What is electric current?
The flow of electric charge
What is the SIZE of the electric current?
The RATE of the flow of electric charge
What is the definition of potential difference between two points in an electrical circuit?
The energy transferred per coulomb of charge that passes between the points
What do current- potential difference graphs show?
How the current through a component varies with the potential difference across it
When does the resistance of a filament bulb increase?
As the temperature of the filament increase
Why is there increased use of LEDs?
Much smaller current than other forms of lighting
When does the resistance of a light dependent resistor decease?
As light intensity increase
When does an LED emit light?
When a current flows through it in a forward direction
What direction does the current in a diode flow?
On only one direction
The diode has very high resistance in th reverse direction
How can you find the resistance of a components?
By measuring the current through
And the potential difference across
The component
What is current through a resistor directly proportional to?
At a constant temperature
The potential difference across the resistor
What does current depend on?
Resistance
The greater the resistance the ____ the current?
For a given potential difference
Smaller the current
How can you work out the potential difference for cells in series?
The sum of the potential difference of each cell
Depending on direction
What is the rule for: resistance, current and potential difference
In SERIES
- resistance is the sum of the resistance of each component
- same current though each component
- total potential difference is shared between the component
What is the rule for: current and potential difference
In PARALLEL
- same potential difference
- total current is the sum of the currents across each component
What is an alternating current?
A current that is constantly changing direction
What is the frequency of the U.K. mains supply?
50 Hz
What is the voltage of the UK mains supply?
230 V
How are most electrical appliances connected to the mains?
Using a cable
And a three pin plug
What is the structure of an electrical cable?
Live wire - alternating voltage makes it dangerous
Neutral wire - earthed at the local substation
What do oscilloscopes measure?
The peak potential difference and the frequency of a low voltage supply
What is the brown wire connected to?
The live pin
What is the blue wire connected to?
The neutral pin
What is the green and yellow wire connected to?
The earth pin
What does a three core cable contain?
Brown - live pin
Blue - neutral pin
Green and yellow - earth pin
What does a two core able consist of?
Brown wire - live pin
Blue wire - neutral pin
What happens if the current in a fuse wire exceeds the rating of the fuse?
It will melt
Breaking the circuit
If an electrical fault causes too great a current, what happens?
The circuit is disconnected by a dude
Or w circuit breaker in the live wire
Describe what materials are used for cables?
Copper - the wire because it is a good electrical conductor and easily bends
Covered in an outer layer of:
Rubber/plastic - good electrical insulator and so prevents anyone touching the cable recovering an electric shock
When are two core cables used?
For appliances that have plastic cases
What does the longest pin of a three pin plug do?
Makes contact with the Earth wire of a wall socket circuit
So when you plug an appliance with a metal case to a wall socket
The case is automatically earthed
What are pins made of and why?
Brass
Because brass is a good conductor
And does not rust or oxidise
Why aren’t pins made of copper?
Copper isn’t as hard as brass
What is the case material of a plug?
An electrical insulator
The inside of the case is shaped so the wires and pins CAN NOT touch eachother when the plug is sealed
How does a fuse work?
Is between the live pin and the live wire
If too much current passes through the wire in the fuse
It melts an cute the live wire off
When are are thicker cables used?
When more current is needed
So wired need to be thicker
To stop th heating effect of the current making the wires too hot
What is a fuse rating?
The maximum current that can pass through it without melting the fuse wire
How is the fuse connected?
In series with the live wire between the live wire and the appliance
What is a circuit breaker?
An electromagnetic switch that opens when there is a fault
How does a circuit breaker work?
If the current in the live wire is too large
The magnetic field or th electromagnet is strong enough to pull the switch contacts apart
Which stops the current in the live wire flowing
And the switch stays open
How is the electromagnet connected to the live wire?
In series
Advantages of circuit breakers?
Work faster
Reset more quickly
How does a residual current circuit breaker work?
A RCCB cute off the current in the live wore when it is different from the current in the neutral wire
When can an RCCB be used?
When there is no earth connection
Advantages of Residual current circuit breakers?
More sensitive than a fuse or an ordinary circuit breaker
Works even faster
When would an appliance have no earth wire?
when it is double insulated
When use a two core cable?
When the appliance is double insulated
And so no earth wire is needed
When use a three core cable?
If an earth wire is needed because the appliance had a metal case
How do you chose the thickness of a cable?
So the heating effect of the current in the cable is insignificant
Advantages of LED?
Used for spotlight Operate at low voltage And low power There are much more efficient than filament bulbs And last longer
Disadvantage of filament bulbs?
Very inefficient
The energy from the hot bulb gradually makes the plastic party’s of the bulb socket brittle and they crack
Why are electrical faults dangerous?
They can cause electric shocks and fires
When use a two core cable?
When the appliance is double insulated
And so no earth wire is needed
When use a three core cable?
If an earth wire is needed because the appliance had a metal case
How do you chose the thickness of a cable?
So the heating effect of the current in the cable is insignificant
What are low energy compact fluorescent bulbs used for?
Took lighting
Advantages of LED?
Used for spotlight Operate at low voltage And low power There are much more efficient than filament bulbs And last longer
Disadvantage of filament bulbs?
Very inefficient
The energy from the hot bulb gradually makes the plastic party’s of the bulb socket brittle and they crack
Why are electrical faults dangerous?
They can cause electric shocks and fires
What is the current for an electrical appliance?
The charge that flows through every second
What is the pd across any electrical appliance?
The energy transferred by each coulomb of charge that passes through it
What is the power of any electrical appliance?
The energy transferred every second
How does a resistor become hotter?
Electrons are made to pass through resistor by battery
Each electron collided with vibrating metal ions in resistor transferring energy to them
The ions therefore gain kinetic energy and vibrate more
The resistor becomes hotter
When does an electrician use a two core able?
If the appliance is doubly insulated
No earth wire needed
When does an electrician use a three core cable?
If an earth wire is needed because the appliance had a metal case
How does an electrician select a cable for use?
Suitable thickness so the heating effect of the current in the cable is insufficient
What materials does static electricity not work with?
Conductors
What materials does static electricity work with?
Insulators
What happens when a child moves down a plastic slide?
Electrons are transferred from the child to the plastic
DUE TO FRICTION
The child has a positive charge
The hairs on his head now all have a positive charge so repeal eachother
What do you know if a child’s hair stands on end?
The charges have the same polarity
So they repeat
What force causes static electricity?
Friction
Why does static electricity not happen with a metal?
Charge would pass through
Metal is not an insulator (is a conductor)
When would you fit a two core cable not a three core cable?
When it is double insulated
How does an RCCB operate?
Detects a very small difference between the current in the live wire and neutral wire
How does a diode work?
Large resistance in one way
Stops current flowing in the other way
How does an RCCB workV
Detects difference in current between the live and neutral wire
Current flows to earth
Definition of potential difference?
Energy transferred per coulomb of charge
Difference between protostar and main sequence star?
Protostar is at a lower temperature
And dosent emit radiation
As nuclear fusion hasent started
When does resistance happen?
Electrons passing through a torch bulb have to push their way through lots of vibrating ions in the metal filament
The ions resist the passage of electrons
When does the resistance of a metal filament increase?
When the temperature increases
The ions in the metal filament vibrate more as temperature increases
They resist the passage of electrons
What do oscilloscopes measure?
The peak potential difference and the frequency of low voltage ac supply p
What happens when charge flows through a resistor?
Energy is transferred to the resistor which makes it get hotter
