Electricity Flashcards
What are the two types of electrical supplies?
alternating current (ac) and direct current (dc)
What happens to the current in an ac supply?
it is constantly changing direction
How are alternating currents produced?
by alternating voltages in which the positive and negative ends keep alternating
What type of electricity supple is the the UK domestic mains supply (the electricity in your home) ?
ac supply
What voltage is the UK domestic mains supply?
around 230V
What is the frequency (how often the current changes direction) of the UK domestic mains supply?
50 Hz (hertz)
or 50 cycles per second
What type of current do cells and batteries supply?
direct current
What is a direct current?
a current that is always flowing in the same direction
How is a direct current created?
by a direct voltage in which the positive and negative ends of the source are fixed
How are most electrical appliances connected the the mains supply?
by three-core cables
What does a three-core cable mean?
they have three wires inside of them, each with a core of copper and a coloured plastic coating
What does the colour of the plastic insulation on each cable show?
its purpose, the colours are always the same for every appliance so this is easy to tell
What colour is the live wire?
brown
What colour is the neutral wire?
blue
What colour is the earth wire?
green and yellow
What is the purpose of the live wire?
provides the alternating potential difference from the mains supply
What is the purpose of the neutral wire?
to complete the circuit and carry away the current
What is the potential difference of the live wire?
about 230V
What is the potential difference of the neutral wire?
around 0V
What is the potential difference of the earth wire?
0V
What is the purpose of the earth wire?
for protecting the wiring and for safety as it stops the appliance casing becoming live, it doesn’t usually carry a current, it only does when there is a fault
What is the potential difference of your body (and the earth)?
0V
What would happen if you touch the live wire?
as your body and the earth are both at 0V a large potential difference is produced across your body and a current flows through you
this causes a large electric shock which could injure or even kill you
How is there still a danger of electric shock even even if a pulg socket or light switch is turned off?
although a current isn’t flowing there is still a potential difference in the live wire so if you made contact with it your body would provide a link between the supply and the earth so a current would flow through you
How can the connection between the live wire and the earth be particularly dangerous?
if the link provides a low resistance path to the earth a huge current (because of V=IR) will flow which could result in a fire
What is a material called if electrical charges can easily move through it?
a conductor
What is an example of a good conductor?
metals
What is a material called if electrical charges cannot easily move through?
an insulator
What are 2 examples of good insulators?
plastics and rubbers
What is a static charge?
an electric charge which cannot move
Where are static charges usually found?
on electrical insulators
What charge can static charges have?
positive or negative
How can a static charge build up on a conductor?
if it’s isolated (nowhere for the charge to flow to)
When certain insulating materials are rubbed together what can happen?
friction causes negatively charged electrons to be transferred from one to the other
How are electrostatic charges produced?
by the movement of electrons
What happens when an electrical charge builds up on an object?
the potential difference between the object and the earth increases
What can happen is the potential difference between an object and the earth gets large enough?
electrons can jump across the gap between the charged object and the earth (called a spark)
What else can electrons jump between other than a charged object and the earth?
a charged object and any earthed conductor that is nearby
When two elecrically charged objects are brought close together what do they exert on each other?
a non-contact force
opposite charges _______
attract
like charges ______
repel
What is the attraction/repulsion between charges known as?
electrostatic attraction/repulsion
What will electrostatic attraction/repulsion forces cause objects to do?
move if they can
What happens to the electrostatic attraction/repulsion forces the further apart the objects are?
they get weaker
What are 2 examples of electrotatic attraction/repulsion force?
two charged rods, one suspended by a piece of string so free to move
rubbing a ballon on your hair
electrons: hair to balloon
What is an electric field created around?
any electrically charged object
The closer you get to the object the __________ the electric field is.
stronger
How can you show an electric field around an object?
by using field lines
Where do electric field lines point from and to?
from positive charge towards negative charge
Where are field lines drawn?
at right angles to a charged object’s surface
The closer field lines are the __________ the field is.
stronger
What does a charged object placed inside the electric field of another charged object experience?
a non-contact force
What is a charged object placed inside the electric field of another charged object experiencing a non-contact force caused by?
the electric fields of each object interacting with each other
What does a charged object placed inside the electric field of another charged object cause?
the attraction or repulsion between two objects
What can a high potential difference cause?
a strong electric field between a charged object and an earthed object
What can a strong electric field between a charged object and an earthed object caused by a high potential difference cause in terms of electrons?
electrons in the air particles are removed (ionisation)
What is air normally?
an insulator
What happens when the air is ionised?
it becomes much more conductive so a charge can flow through it (a spark)
What does a voltmeter measure?
potential difference
How are voltmeters connected?
across the component, in parallel
What do ammeters measure?
current
How are ammeters connected?
in line with a component, in series
What is an electric current?
a flow of electric charge
What are the positive and negative ends of cells?
positive terminal is the longer line
negative terminal is the shorter line
Which direction does current flow around a circuit?
flows from positive to negative
What is current measured in?
amperes, A
What is the current equivalent to?
the rate of flow of charge
What is the unit of charge?
coulombs (C)
What does a circuit need in order for a charge to flow?
it needs to be a complete (closed) circuit and needs to have a source of potential difference
What is potential difference?
the driving force that pushes the charge around
What is the unit for potential difference?
volts, V
What is resistance?
anything in the circuit which reduces the flow of current
What is the unit for resistance?
ohms, Ω
What determines the current flowing through a component?
the resistance and potential difference across the component
What is an ohmic conductor?
a conductor that has a constant resistance for any current through it
