Electricity Flashcards
What current does the mains supply?
Alternating Current
What current do batteries/cells supply?
Direct Current
In ac supplies, what is happening to the current?
Current is constantly changing direction
How alternating currents produced?
By alternating potential difference in which the positive and negative ends keep alternating
What do the UK mains supply?
ac around 230 V
What is frequency of the ac mains supply in the UK?
50 cycles per second or 50 Hz
What does direct current mean?
Is current that is always flowing in the same direction
How is direct current created?
By a direct potential difference
How most electrical appliances connected to mains supplies?
By three-core cables
What are three-core cables?
Means they have 3 wires inside them - each with core of copper & coloured plastic coating
What is the name of the blue coloured wire?
Neutral wire
Where is neutral wire found?
On the left of the plug
What does the neutral wire do?
Completes the circuit and carries away current (electricity normally flows out through it)
How many volts is the neutral wire at?
0 V
What is the name of the brown coloured wire?
Live wire
Where is the live wire found?
On the right of the plug
What does the live wire do at?
Provides the alternating pd from the mains supply (normally electricity flows in through it)
How many volts is the live wire?
about 230 V
What is the name of the green and yellow coloured wire?
Earth wire
What does the earth wire do?
Protects the wiring, and for safety - it stops the appliance casing from becoming live
Does the earth wire carry current?
No, doesn’t usually unless there’s a fault1
How many volts is the earth wire at?
0 V
Why does touching the live wire give you an electric shock?
- Your body (just like Earth) is 0 V
- If you touch live wire = large potential difference is produced across your body & current flows through you
- Causes large electrical shock = injure/kill you
Why is it even when the plug or light switch is turned off, there is danger of getting an electrical shock?
- Current isn’t flowing but still pd in live wire
- If contact with live wire = body would provide a link between supply & earth = current would flow through you
Why is any connection between live and earth is dangerous?
If link creates low resistance path to earth = huge current will flow - result in fire
Describe briefly how electricity is generated from fossil fuels
- Heat is released from fuel and boils the water to make steam
- The steam turns the turbine
- The turbine turns a generator and electricity is produced
- The electricity goes to the transformers to produce the correct voltage
Why is steam used to turn the turbine and not water?
Steam has more energy than water
What are electrical applicances are designed to do?
Transfer energy to components in the circuit when a current flows
What does it mean an electrical appliance has a higher current?
Higher current = more energy transferred to thermal energy stores of components (& surroundings)
What does it mean if an electrical appliance transfers more energy than another one in the same given time?
It has a higher power
What does a power rating of an electrical appliance tell you?
Tells you maximum amount of energy transferred between stores per second when appliance is in use
If an electrical appliance has a higher power, does this mean it transfers energy more usefully?
No
How does giving electrical appliances power ratings help customers choose between models?
e.g. Lower power rating = less electricity an appliance uses in given time = cheaper to run
What are potential differences?
Energy Transferred per Charge Passed
When an electrical charge goes through a change in p.d., what happens?
Energy is transferred
Why is energy is supplied to the charge at the power source?
To ‘raise’ it though a potential
When does this charge give up this energy?
When it ‘falls’ through any potential drop in components elsewhere in circuit
What will a battery with bigger pd supply and why?
It’ll supply more energy to circuit for every coulomb of
charge which flows round it because the charge is raised
up “higher” at the start
What is the national gird?
Giant system of cables and transformers that covers UK & connects power stations to consumers
What does the national gird do?
Transfers electrical power from power stations anywhere on gird (the supply) to anywhere else on the gird where it’s needed (the demand) i.e. distributes electricity
When does the demand for electricity increase?
- People get up in the morning
- Come home from school or work
- It starts to get dark or cold
- Popular events like sporting final = peak in demand
Why do power stations often run at well below their maximum power output?
So there’s spare capacity to cope with high demand (even if there’s an unexpected shut-down of another station)
To transmit a huge amount of power, what is needed?
A high potential difference or high current is needed
What is the problem with using a high current?
Problem is that loads of energy is lost as wires heat up so energy is transferred to thermal energy store of surroundings
Why does the national grid uses high pd and low current?
Much cheaper to boost the pd up really high (to 400,000 V) and keep current relatively low
For a given power, what does increasing the pd do to the current?
Increasing the pd decreases the current which decreases energy lost by heating wire and surroundings - makes the national grid an efficient way of transferring energy
What does the national grid require to get pd to 400 000 V to transmit power?
Requires transformers & big pylons with huge insulators
What do transformers have to do to the pd?
Have to step pd up at 1 end, for efficient transmission, & then bring it back down to safe, usable levels at other end
How is pd increased?
It’s (‘stepped up’) using a step-up transformer
How is pd decreased?
It’s (‘stepped down’) using a step-down transformer
What is the build-up of static caused by?
Caused by friction
What happens when certain insulating materials are rubbed together?
- Negatively charged electrons will be scraped off one and dumped on other
- This will leave materials electrically charged, with a positive static charge on one & an equal negative static charge on the other
What happens when a polythene rod is rubbed with a cloth duster?
Electrons move from the duster to the rod
What happens when an acetate rod is rubbed with a cloth duster?
Electrons move from the rod to the duster
How are both positive (+ve) and negative (-ve) electrostatic charges produced?
By the movement of electrons
Do positive charges move?
NO
What is a positive charge caused by?
Caused by electrons moving away elsewhere
What happens the material loses electrons?
Material loses the electrons, loses some negative charge = left with an equal positive charge
How are sparks created?
- As electric charge builds on an object = pd between object and earth (which is at 0 V) increases
- If pd gets large enough = electrons can jump across gap between charged objects and earth = this is the spark
Why can you get a static shock when getting out of car?
- Sparks can jump to any earthed conductor that’s nearby
- A charge builds up on car’s metal frame, and when you touch car, charge travels through you to earth (usually happens when gap if fairly small)
What is lightning essentially?
A really big spark
When 2 electrically charged objects are brought close together what happens? What are these forces do and what are they known as?
- They exert a force on one another
- These forces will cause objects to move if they’re able to = known as electrostatic attraction/repulsion & is non-contact force
If you suspend a rod with known charge from piece of
string, and place an object with same charge nearby it, what will happen?
It will repel the rod
If you suspend a rod with known charge from piece of
string, and place an oppositely charged object nearby it, what will happen?
Will cause rod to move towards the object
What does an electrically charged object create?
An electric field
When a charged object is placed in electrical field of another object, what happens and what is caused by?
It feels a force (that causes attraction or repulsion) caused by electric fields of each charged object interacting with each other
What does isolated mean?
It’s not interacting with anything
Describe how sparking can be explained by electric fields
- Sparks are caused when there’s a high enough pd between a charged object and the earth (or earthed object)
- A high pd = strong electric field between charged object & earthed object
- Strong electric field = electrons in air particles to be removed (known as ionisation)
- Air is normally an insulator, but when ionised it much more conductive = so current can flow through it → this is the spark
Name 3 things to bare in mind when drawing electric fields?
- Electric field lines go from positive to negative
- They’re always at a right angle to surface
- The closer together the lines are = stronger the field is
State the units for equation E = QV (Energy transferred = Charge flow x Potential Difference)
Energy transferred = J
Charge flow = C
Potential Difference = V
State the units for equation P = VI (Power = Potential Difference x Current)
Power = W
Potential Difference = V
Current = A
