2 Electricity Flashcards
whats an amp (A)
the number of coulombs per second (the number of charge flowing per second)
eg 1 amp = 1 coulomb per second
whats a coulomb (C)
a unit of charge
whats a joule (J)
a unit of energy
whats a ohm (Ω)
a unit of resistance
what is a second (s)
a unit of time
what is a volt (V)
the amount of energy per each unit of charge
eg 1 volt = 1 joule per coulomb
what is a watt (W)
the amount of energy supplied per second
eg 1 watt = 1 joule per second
why are electrical appliances insulated
the part of the appliance where the current flows is made of metal so if it comes into conduct with a person it could give an electric shock
what is insulation and how does it protect the user
covering a wire with an insulating material and so it will prevent the current entering the body by being a physical barrier from the live wire carrying the current
what is double insulation
an appliance which does not have a metal case is ‘double insulated’
example:
Insulation around the wires themselves
A non-metallic case that acts as a second layer of insulation
the case being made out of an insulator means that even if there is a fault and the live wire touches the case then the user is still protected because the case is an insulator so it won’t conduct
why does a double insulated appliance not need a earth wire
because the case will never become live (as its made of an insulator) so there will never be need to remove the charge from the case
what is an earth wire
it is connected to the metal case of a appliance and it provides a low resistance (high current) path to the earth
how does the earth wire protect a user
if there is a fault and current is flowing in the case of an appliance (dangerous) then:
the earth wire provides a low resistance path to the earth
It causes a surge of current (because the earth wire is low resistance high current) in the earth wire and hence also in the live wire
The high current through the fuse causes it to melt and break
This cuts off the supply of electricity to the appliance, making it safe
what is a fuse
a piece of thin metal wire that is part of a circuit so if there is a rise in current it will melt breaking the circuit and stopping the current
what is a circuit breaker
an automatic electromagnet switch that switches off if the circuit is the current exceeds a certain value
how does a fuse or circuit breaker protect a user
if there is too much current flowing in a circuit this is dangerous because there will be more heat which means there could be a fire.
the fuse or circuit breaker stops this from happening because they will break the circuit if too much current is flowing preventing this
equation between Current, power, voltage
power = voltage x current
p = VI
equation between current, energy transferred, voltage, time
energy transferred = current x voltage x time
E = IVt
what happens when a current flows through a resistor
During their journey through the resistor, the electrons collide with ions in the resistor material.
These collisions result in friction, converting electrical energy into thermal energy (heat).
so when the current flows through the resistor the energy is changed into thermal energy which leads to an increase in temperature
how can a resistor be used in domestic contexts
Joule heating or Ohmic heating
electric heaters
electric ovens
electric stoves
toasters
what is resistance
As the electrons pass through the metal lattice (wire) they collide with ions
The ions resist the flow of the electrons
what is AC
A current that continuously changes its direction, going back and forth around a circuit (mains electricity)
what is DC
A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative (a cell or battery)
advantages of a series circuit
simple to construct
less likely to overheat
disadvantages of a series circuit
if one component fails then the whole circuit won’t work
advantages of a parallel circuit
if one component fails the rest will still work
components have a higher voltage
disadvantages of a parallel circuit
more likely to overheat
is a series or parallel circuit better for domestic lighting
parallel
Each light operates independently, ensuring that a single bulb failure does not darken the entire room.
You can add or remove lights without affecting the others.
can also switch off one light and leave the rest on
current in series
the current is the same everywhere in the circuit
current will increase if there more voltage or less components
voltage in series
The voltage across all of the components adds up to the supply voltage from the cell
current in parallel
the current splits at junctions
the current in each of the branches adds up to the total current leaving the cell
voltage in parallel
the total voltage of the cell applies to each section of the circuit
the voltage across two components connected in parallel is the same
whats the relationship between the current and voltage and resistance
as voltage increases so does current
as resistance increases current decreases
what is the relationship between current and voltage in wires
if you increase the voltage across a wire, the current through the wire will increase, provided the resistance remains constant. Similarly, if you decrease the voltage, the current will decrease. This relationship is directly proportional for a given resistance
what is the relationship between current and voltage in resistors
if you increase the voltage across a resistor, the current through the wire will increase, provided the resistance remains constant. Similarly, if you decrease the voltage, the current will decrease. This relationship is directly proportional for a given resistance
what is the relationship between current and voltage in filament lamps
for a filament lamp, current and voltage are not directly proportional
This is because the resistance of the filament lamp increases as the temperature of the filament increases
As the current increases, the temperature of the filament in the lamp increases
The higher temperature causes the ions in the metal lattice of the filament to vibrate more
This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through
Resistance opposes the current, causing the current to increase at a slower rate
what is the relationship between current and voltage in diodes
When the current is in the direction of the arrowhead symbol, this is forward bias (allows the current to flow only in one direction)
This is shown by the sharp increase in potential difference and current on the right side of the graph
When the diode is switched around, this is reverse bias (in reverse direction there is very high resistance and so no current flows)
This is shown by a zero reading of current or potential difference on the left side of the graph
how to investigate how the current changes with voltage
create a circuit with
- a component (eg lamp)
- a ammeter to measure current (added in series)
- a voltmeter to measure voltage (added in parallel)
- a variable resistor to change the current (added in series)
- a cell
how does resistance change with Light dependent resistor
as light intensity increases, resistance decreases non linearly
how does resistance change with thermisters
as temperature increases, resistance decreases non linearly
what can a lamp or LED indicate
if it turns on then it indicates a current in the circuit
formula with current, voltage, resistance
voltage = current x resistance
V=IR
what is the current
the rate of flow of charge
formula with current, charge, time
charge = current x time
Q=It
what is current measured in
amps (A)
what is charge measured in
coulombs (C)
what is energy measured in
joules (J)
what is resistance measured in
ohms (Ω)
what is voltage measured in
volts (V)
what is power measured in
watts (W)
what is electric current in solid metallic conductors
a flow of negatively charged electrons
why is current conserved at a junction in a circuit
current in electrical circuits is made up of charge and charge must be conserved (can’t add or take away)
what is voltage
the amount of energy transferred per unit of charge
a volt is a joule per coulomb
formula with charge, energy transferred, voltage
energy transferred = charge x voltage
E=QV
what are common materials that are electrical conductors or insulators
conductors:
Silver Copper Aluminium Steel
insulators:
Rubber Plastic Glass Wood
practical: investigate how insulating materials can be charged by friction
Take a polythene rod, hold it at its centre and rub both ends with a cloth
Suspend the rod, without touching the ends, from a stand using a cradle and nylon string
Take an acrylic rod and rub it with another cloth
Without touching the ends of the acrylic rod bring each end of the acrylic rod up to, but without touching, each end of the polythene rod (if the ends do touch, the rods will discharge and the forces will no longer be present)
Record any observations of the polythene rod’s motion
Repeat, changing out the acrylic rod for rods of different materials
When two insulating materials are rubbed together, electrons will transfer from one insulator onto the other insulator
A polythene rod is given a negative charge by rubbing it with the cloth
This is because electrons are transferred to the polythene from the cloth
Electrons are negatively charged, hence the polythene rod becomes negatively charged
Conversely, an acetate rod is given a positive when rubbed with a cloth
This is because electrons are transferred away from the acetate to the cloth
Electrons are negatively charged, so when the acetate loses negative charge, it becomes positively charged
If the material is repelled by (rotates away from) the polythene rod, then the materials have the same charge
If the material is attracted to (moves towards) the polythene rod, then they have opposite charges
explain how positive and negative electrostatic charges are produced
a positive charge is produced by loss of electrons
a negative charge is produced by gain of electrons
how is there attraction and repulsion
like charges repel and unlike charges attract
dangers of electrostatic charges
when a large amount of charge builds up, it produces a large potential difference across a gap
If the potential difference is large enough, current can travel through the air between objects – this is a spark
There are dangers of sparking in everyday situations such as fuelling vehicles such as cars and planes and the spark can cause a fire as the fuel is very flammable. (an earth wire stops this)
how could a spark be produced by fuelling a vehicle
A build-up of static charge is a potential danger when refuelling aeroplanes
Fuel runs through pipes at a fast rate
This fuel is very flammable
The friction between the fuel (a liquid insulator) and the pipe causes the fuel to gain charge
If this charge were to cause a spark, the fuel could ignite and cause an explosion
This is prevented by the fuel tank being connected to the Earth with a copper wire called the bonding line during the refuelling
The conductor earths the plane by carrying the charge through to the Earth which removes the risk of any sparks
It is easier for charge to flow down the bonding line than to spark, so sparks are very unlikely to occur
uses of static electricity
photocopiers + inkjet printers
Photocopiers use static electricity to copy paper documents, most commonly in black and white
An image of the document is projected onto a positively charged copying plate
The plate loses its charge in the light areas and keeps the positive charge in the dark areas (i.e the text)
A negatively charged black toner powder (the ink) is applied to the plate and sticks to the part where there is a positive charge
The toner is then transferred onto a new blank sheet of white paper
The paper is heated to make sure the powder sticks (hence why photocopied paper feels warm)
The photocopy of the document is now made
Inkjet printers work in a similar way, but instead of the black toner powder, a small jet of coloured ink is negatively charged and attracted to the correct place on the page
Insecticides are chemicals used to kill pests in order to protect crops
In order to spray crops effectively whilst using a minimal amount of chemicals, the sprayer has to deliver the chemicals as a fine mist and cover a large area
To achieve this, the insecticide is given an electrostatic charge (e.g. positive) as it leaves the sprayer
The droplets of insecticide then repel each other since they are the same charge
This ensures that the spray remains fine and covers a large area
They are also attracted to the negative charges on Earth, so they will fall quickly and are less likely to be blown away
A similar technique is used in the spray painting of cars
what is the effect of changing the resistance on the current in a circuit
If the resistance increases while the voltage stays the same, the current decreases. This is because the current (I) is inversely proportional to the resistance (R).
