Electricity Flashcards
What is current defined by ?
The rate of flow of electric charge
What is current measured in ?
Current is measured in units of amperes or amps (A)
1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C/s
This means the size of an electric current is the amount of charge passing through a component each second
How does a current flow ?
Current flows
when a circuit is formed e.g. when a wire connects the two oppositely charged terminals of a cell
from the positive terminal to the negative terminal of a cell
How do you measure current ?
Current can be measured using an ammeter
Ammeters must be connected in series with the component being measured
What is a charge ?
The wires in an electric circuit are made of metal because it is a good conductor of electric current
In the wires, the current is a flow of negatively charged electrons
What is the conventional current?
Exam Tip
You should always consider current to be the flow of positive charge i.e. from the positive terminal to the negative terminal of a cell. This is known as conventional current.
This is in the opposite direction to electron flow, which is the flow of negatively charged electrons from the negative terminal to the positive terminal of a cell.
This is the convention we use because scientists defined conventional current before they discovered the electron
How do you calculate electric charge?
Current, charge and time are related by the equation:
charge = current × time
Q = I x t
Where:
Q = charge, measured in coulombs (C)
I = current, measured in amps (A)
t = time, measured in seconds (s)
The current, charge and time equation can be rearranged with the help of the following formula triangle:
What is voltage defined as ?
The energy transferred per unit charge passing between two points
Voltage is measured in units of volts (V)
1 volt is equivalent to the transfer of 1 joule of energy by 1 coulomb of charge, or 1 V = 1 J/C
The terminals of a cell make one end of the circuit positive and the other negative
As electrons flow through a cell, they gain energy
For example, in a 12 V cell, every coulomb of charge passing through gains 12 J of energy
As electrons flow through a circuit, they lose energy
For example, after leaving the 12 V cell, each coulomb of charge will transfer 12 J of energy to the wires and components in the circuit
How to measure voltage ?
Voltage can be measured using a voltmeter
Voltmeters must be set up in parallel with the component being measured
Exam Tip
When you are building a circuit in class, always connect the voltmeter last. Make the whole circuit first and check it works, and then connect the voltmeter so that the leads are on each side of the component you are measuring. This will save you a lot of time waiting for your teacher to troubleshoot your circuit!
You might sometimes see voltage called potential difference. This term can be useful when thinking about voltmeters as the potential difference describes a difference between two points, therefore the voltmeter has to be connected between two points in the circuit.
How do you calculate voltage ?
The equation linking the energy transferred, voltage and charge is given below:
energy transferred = charge × voltage
E = Q X V
Where:
E = energy transferred, measured in joules (J)
Q = charge moved, measured in coulombs (C)
V = voltage, measured in volts (V)
This can be rearranged using the formula triangle below:
How is resistance defined as ?
The opposition of a component to the flow of electric current through it
What is resistance measured in ?
Resistance is measured in units of ohms (Ω)
A resistance of 1 Ω is equivalent to a voltage across a component of 1 V which produces a current of 1 A through it
The resistance of a component controls the size of the current in a circuit
For a given voltage across a component:
The higher the resistance, the lower the current that can flow
The lower the resistance, the higher the current that can flow
All electrical components, including wires, have some value of resistance
Wires are often made from copper because it has a low electrical resistance
This is why it is known as a good conductor
How do you compare current and resistor ?
The current, resistance and potential difference of a component in a circuit are calculated using the equation:
voltage = current × resistance
V = I x R
Where:
V = voltage, measured in volts (V)
I = current, measured in amps (A)
R = resistance, measured in ohms (Ω)
This equation can be rearranged with the help of the following formula triangle:
How do you calculate voltage ?
Exam Tip
In exam questions, the resistance of the wires, batteries, ammeters and voltmeters are always assumed to be zero (in the case of voltmeters, they have extremely high resistances so that current does not flow through them, and this has a negligible effect on the overall resistance of the circuit)
What are the two ways to join electrical components ?
There are two ways of joining electrical components:
in series
in parallel
What Is a series circuit ?
A series circuit is a circuit that has only one loop, or one path that the electrons can take
In a series circuit, the current has the same value at any point
This is because the electrons have only one path they can take
Therefore, the number of electrons passing a fixed point per unit time is the same at all locations
This means that all components
in a series circuit have the same current
The amount of current flowing in a series circuit depends on:
the voltage of the power source
the number (and type) of components
Increasing the voltage of the power source drives more current around the circuit
So, decreasing the voltage of the power source reduces the current
Increasing the number of components in the circuit increases the total resistance
Hence less current flows through the circuit
What is a parallel circuit ?
A parallel circuit is a circuit that has two or more loops, or more than one path that electrons can take
Parallel circuits contain junctions and branches
Junctions are points where two or more wires meet to form a new branch
Branches are the sections of wire between junctions
What is a current in a parallel circuit ?
In a parallel circuit, the current has different values at different points in the circuit
This is because the current splits at a junction
Therefore, the electrons have different paths they can take
The sum of the current in the individual branches is equal to the total current before (and after) the branches
Why is a current conserved at a junction in a circuit?
At a junction, the current is always conserved
This means the amount of current flowing into the junction is equal to the amount of current flowing out of it
This is because the charge is conserved
Current does not always split equally – often there will be more current in some branches than in others
The current in each branch will only be identical if the resistance of the components along each branch is identical
Current behaves in this way because it is the flow of electrons:
Electrons, or any charge, cannot be created or destroyed
This means the total number of electrons (and hence current) going around a circuit must remain the same
When the electrons reach a junction, however, some of them will go one way and the rest will go the other
What is voltage in a series ?
In a series circuit, the total voltage of a power supply is shared between the components.
What will be the voltage for two identical components ( with equal resistance) ?
For two identical components (with equal resistance), the voltage across them will be:
the same
equal to half the total voltage of the power supply
What will be the voltage for two non identical components (with different values of resistance)?
For two non-identical components (with different values of resistance), the voltage will be:
higher across the component with the higher resistance
lower across the component with lower resistance
What amount of voltage is in a parallel circuit ?
In a parallel circuit, the total voltage across each branch is the same as the voltage of the power supply
What are the advantages and disadvantages of a series circuit?
A series circuit consists of a string of two or more components connected in a loop
Advantages of a series circuit
All of the components are controlled by a single switch
Fewer wires are required
Disadvantages of a series circuit
The components cannot be controlled separately
If one component breaks, all other components stop working
What are the advantages and disadvantages of parallel circuits?
A parallel circuit consists of two or more components attached across different branches of the circuit
Advantages of a parallel circuit
The components can be individually controlled using their own switches
If one component breaks, then the others will continue to function
Disadvantages of a parallel circuit
Many more wires are involved which can be more complicated to set up
All branches have the same voltage as the supply making it more difficult to control the voltage across individual components
Exam Tip
You may have noticed that for a parallel circuit, all of the components can be controlled by a single switch - like a series circuit. Nevertheless, the exam board still considers this an advantage of series circuits.
Note that the current does not always split equally in a parallel circuit – often there will be more current in some branches than in others. The current in each branch will only be identical if the resistance of the components along each branch are identical. However, the voltage across two components connected in parallel is always the same
What are resistors in a series ?
When two or more resistors are connected in series, the total resistance is equal to the sum of their individual resistances
For two resistors of resistance R1 and R2, the total resistance can be calculated using the equation:
R = R1 + R2
Where:
R is the total resistance, in ohms (Ω)
Increasing the number of resistors increases the overall resistance
The charge now has more resistors to pass through
The total voltage is also the sum of the voltages across each of the ind
Summarise series and parallel circuits .
For components connected in series:
the current is the same at all points and in each component
the voltage of the power supply is shared between the components
the total resistance is the sum of the resistances of each component
For components connected in parallel:
the current from the supply splits in the branches
the voltage across each branch is the same
the total resistance is less than that of each component
What are IV graphs ?
When the voltage V across a component is varied, the current I flowing through it may vary linearly or non-linearly
The relationship between current and voltage of a component can be shown on an IV graph
When the relationship between current and voltage is linear:
the IV graph is a straight line which passes through the origin
the resistance is constant
When the relationship between current and voltage is non-linear:
the IV graph that is not a straight line
the resistance is not constant
What is the IV graph for a wire or a resistor?
The relationship between current and voltage for a wire or fixed resistor is linear, or directly proportional, which means
the IV graph is a straight line, so voltage and current increase (or decrease) by the same amount
the slope of the graph is constant, so resistance is constant
What is the IV graph for a filament bulb?
The relationship between current and voltage for a filament lamp is non-linear, or not directly proportional, which means
the IV graph is not a straight line, so voltage and current do not increase (or decrease) by the same amount
the slope of the graph is not constant, so resistance changes
The IV graph for a filament lamp shows as voltage increases
the current increases at a proportionally slower rate
the resistance increases; the flatter the slope, the higher the resistance
As current through a filament lamp increases, the resistance increases because:
the higher current causes the temperature of the filament to increase
the higher temperature causes the atoms 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
since resistance opposes the current, this causes it to increase at a slower rate
What is a IV graph for a diode?
A diode allows current to flow in one direction only
This is called forward bias
In the reverse direction, the diode has very high resistance, and therefore no current flows
This is called reverse bias
When the current is in the direction of the arrowhead symbol, this is forward bias
On the IV graph, this is shown by a sharp increase in voltage and current on the right side of the graph
This shows the resistance is very low
When the diode is switched around, this is reverse bias
On the IV graph, this is shown by a zero reading of current or voltage on the left side of the graph
This shows the resistance is very high
Investigate the relationship between current and voltage.
In order to investigate the relationship between current and voltage different components, the following equipment is required:
an ammeter - to measure the current through the component
a voltmeter - to measure the voltage across the component
a variable resistor - to vary the current through the circuit
a power source - to provide a source of potential difference (voltage)
wires - to connect the components together in a circuit
The image below shows the circuits set up to obtain IV graphs
for a filament lamp and a diode
The current is the independent variable
The variable resistor is used to change the current flowing through the filament lamp / diode
The voltage is the dependent variable
The voltmeter is used to measure the voltage across the filament lamp / diode
Recording measurements of current and voltage as the current increases enables an IV graph to be plotted for each component
What is resistance ?
Resistance is the opposition to the flow of current
The higher the resistance of a circuit the lower the current
Resistors come in two types:
Fixed resistors
Variable resistors
Fixed resistors have a resistance that remains constant
Variable resistors can change the resistance by changing the length of wire that makes up the circuit
A longer length of wire has more resistance than a shorter length of wire
What are thermistors ?
The resistance of a thermistor depends on its temperature
The resistance of a thermistor is high in cold conditions and low in hot conditions
As the temperature increases the resistance of a thermistor decreases
As the temperature decreases the resistance of a thermistor increases
What are Light-dependent resistors (LDRs)?
The resistance of a light-dependent resistor (LDR) depends on the light intensity on it
The resistance of an LDR is high in dark conditions and low in bright conditions
As the light intensity increases the resistance of an LDR decreases
As the light intensity decreases the resistance of an LDR increases
How can you show the relationship between resistance and light intensity for an LDR ?
What are Lamps & LEDs?
Lamps and light-emitting diodes (LEDs) illuminate (light up) when a current flows through them
This makes them useful for indicating the presence of a current in a circuit
Light-emitting diodes (LEDs)
LEDs are a type of diode
This means they only allow current to flow through them in one direction
Therefore, in a circuit, an LED will only light up if it is placed in the correct direction
The circuit symbol for an LED is as follows:
Exam Tip
Make sure you learn the various symbols mentioned on this page. Many of them are very similar with small differences denoting what they do:
Two arrows pointing towards a symbol mean that it is light-dependent
Two arrows pointing away mean that it is light-emitting
Symbols are sometimes drawn with circles around them (e.g. the LDR). These circles are often optional (although not in the case of meters and bulbs).
What is the electrical power equation ?
Exam Tip
Remember: Power is just energy per second. Think of it this way will help you to remember the relationship between power and energy. You can remember the unit by the phrase: “Watt is the unit of power?”
What are Selecting fuses?
A fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)
Fuses usually consist of a glass cylinder containing a thin metal wire
If the current in the wire becomes too large:
The wire heats up and melts
This causes the wire to break, breaking the circuit and stopping the current
This makes sure that more current doesn’t keep flowing through the circuit and causing more damage to the equipment, or, causing a fire
What is electrical safety ?
Mains electricity is potentially lethal
Potential differences as small as 50 V can pose a serious hazard to individuals
What is insulation and double insulation ?
The conducting part of a wire is usually made of copper or some other metal
If this comes into contact with a person, this poses a risk of electrocution
To improve electrical safety wires are covered with an insulating material, such as rubber
Some appliances do not have metal cases, so there is no risk of them becoming electrified
Such appliances are said to be double insulated, as they have two layers of insulation:
Insulation around the wires themselves
A non-metallic case that acts as a second layer of insulation
Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing
What is the earth wire ?
Many electrical appliances have metal cases
This poses a potential electrical safety hazard:
If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted
The earth wire is an additional safety wire that can reduce this risk
If this happens:
The earth wire provides a low resistance path to the earth
It causes a surge of 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 are Fuses & circuit breakers?
Fuses and circuit breakers are electrical safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)
As explained in the Selecting fuses revision note a fuse consists of a glass cylinder containing a metal wire
A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value
A circuit breaker has a major advantage over a fuse as an electrical safety device because:
It doesn’t melt and break, hence it can be reset and used again
It works much faster
For these reasons, circuit breakers are used in mains electricity in homes as the most important electrical safety device
Sometimes they are misleadingly named “Fuse boxes”
Exam Tip
For your exam, you must explain how insulation, double insulation, earthing, fuses and circuit breakers protect the device or user in different domestic appliances.
What is Electricity & heat?
A current passing through a resistor (or wire) results in the electrical transfer of energy
As explained in Charge & current, current is the rate of flow of charge
The temperature of a resistor increases due to the collisions of the free electrons within the wire
Some of the energy is dissipated into the surroundings by heating
This heating effect is utilised in many domestic contexts, including:
Electric heaters
Electric ovens
Electric hob
Toasters
Kettles
Exam Tip
Remember that a charge moving around an electrical circuit are an example of an electrical transfer pathway. If you are unsure of how to explain energy stores and transfers use the Energy stores & transfers revision note to help.
What is the direct current ?
A direct current (d.c.) is defined as
A steady current, constantly flowing in the same direction in a circuit, from positive to negative
The potential difference across a cell in a d.c. circuit travels in one direction only
The current travels from the positive terminal to the negative terminal
A d.c. power supply has a fixed positive terminal and a fixed negative terminal
Electric cells, or batteries, produce direct current (d.c.)
What is a alternating current ?
An alternating current (a.c.) is defined as
A current that continuously changes its direction, going back and forth around a circuit
An alternating current power supply has two identical terminals that change from positive to negative and back again
The alternating current always travels from the positive terminal to the negative terminal
Therefore, the current changes direction as the polarity of the terminals changes
The frequency of an alternating current is the number of times the current changes direction back and forth each second
In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V
Compare alternating current & direct current.
Exam Tip
If asked to explain the difference between alternating and direct current, sketching and labelling the graphs above can earn you full marks. All the circuits you have studied so far are d.c. circuits. Don’t be put off by an exam question if you are asked to calculate the current, potential difference or resistance in d.c. series circuits, you don’t have to do anything different from what you have already learned!
What are conductors ?
A conductor is a material that allows charge (usually electrons) to flow through it easily
Examples of conductors are:
Silver
Copper
Aluminium
Steel
Conductors tend to be metals
Why do metals conduct electricity very well ?
On the atomic scale, conductors are made up of positively charged metal ions with their outermost electrons delocalised
This means the electrons are free to move
Metals conduct electricity very well because:
Current is the rate of flow of electrons
So, the more easily electrons are able to flow, the better the conductor
Core practical 3: investigating charging by friction
Aim of the experiment
The aim of this experiment is to investigate how insulating materials can be charged by friction
Variables
Independent variable = Rods of different material
Dependent variable = Charge on the rod
Control variables:
Time spent rubbing the rod
Using the same type of cloth
Using the same length of rod
What is the method if this investigation ?
- 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
What is the analysis of the results ?
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
In the example from the diagram above, the acetate rod would be attracted to the polythene rod, as they have opposite charges
What is the electric forces between charges?
he charge of a particle can be:
positive
negative
neutral (no charge)
Electrons are negatively charged particles, whilst protons are positive and neutrons are neutral
This is why in a neutral atom, the number of electrons is equal to the number of protons
This is so the equal (but opposite) charges cancel out to make the overall charge of the atom zero
Therefore, an object becomes negatively charged when it gains electrons and positively charged when it loses electrons
When two charged particles or objects are close together, they also exert a force on each other
This force could be:
Attractive (the objects get closer together)
Repulsive (the objects move further apart)
Opposite charges attract, like charges repel
How do charges attract / repel ?
Whether two objects attract or repel depends on their charge
If the charges are the opposite, they will attract
If the charges are the same, they will repel – charges which are the same (e.g. both positive) are often referred to as like charges
The force is stronger if the objects are closer together
Exam Tip
Remember the saying “opposites attract” when answering questions about forces between charged particles.
What is the production of static?
Static electricity refers to the accumulation of charge on an object, which then attracts other objects or can even produce sparks
When certain insulating materials are rubbed against each other they become electrically charged
This is called charging by friction
The charges remain on the insulators and cannot immediately flow away
One becomes positive and the other negative
An example of this is a plastic or polythene rod being charged by rubbing it with a cloth
Both the rod and cloth are insulating materials
This occurs because negatively charged electrons are transferred from one material to the other
The material, in this case, the rod, gains electrons
Since electrons are negatively charged, the rod becomes negatively charged
As a result, the cloth has lost electrons and therefore is left with an equal positive charge
Exam Tip
At this level, if asked to explain how things gain or lose charge, you must discuss electrons and explain whether something has gained or lost them. Remember when charging by friction, it is only the electrons that can move, not any ‘positive’ charge, therefore if an object gains a negative charge, something else must have gained a positive charge by losing negative electrons.
What is the movement of electrons ?
All objects are initially electrically neutral, meaning the negative and positive charges are evenly distributed
However, when the electrons are transferred through friction, one object becomes negatively charged and the other positively charged
The object the electrons are transferred to becomes negatively charged
The object the electrons transfer from becomes positively charged
This difference in charges leads to a force of attraction between itself and other objects which are also electrically neutral
This is done by attracting the opposite charge to the surface of the objects they are attracted to
In the example below, when the cloth and acetate rod are rubbed together, the electrons are transferred from the rod to the cloth
This results in an attractive force between the two objects once separated
What is the use of Photocopiers?
Photocopiers use static electricity to copy paper documents, most commonly in black and white
1. An image of the document is projected onto a positively charged copying plate
2. The plate loses its charge in the light areas and keeps the positive charge in the dark areas (i.e the text)
3. A negatively charged black toner powder (the ink) is applied to the plate and sticks to the part where there is a positive charge
4. The toner is then transferred onto a new blank sheet of white paper
5. 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
What is the use of Insecticide Sprayers?
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 use of Fuelling Vehicles?
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
Exam Tip
You could be asked to explain other dangers and uses in your exams
They may ask you to explain the movement of charge in terms of electrons
If asked to explain a danger:
State what the danger is (electrocution? fire?)
Explain how the charge can be removed to get rid of the risk i.e earthing (think about which way the electrons have to move)
If asked to explain a use, think carefully about the forces exerted due to static electricity and what they will do
How is Electrostatic Charge used in an Inkjet Printer?
An inkjet printer uses electrostatic charge to direct
the tiny ink droplets to the correct place on the page.
Coloured ink is passed through a very small hole called
a nozzle which separates the ink into many tiny droplets.
The tiny droplets are given an electrostatic charge.
The direction in which the charged ink droplets move can
be controlled by electrically charged metal plates. A
voltage on the plates means that the charged ink droplets
will be attracted to one plate and repelled by the other.
This is very similar to a cathode ray oscilloscope where an
electron beam is directed to a particular place on a screen.
In the picture below, the ink droplets have a positive charge.
The ink droplets are attracted to
the negative plate and repelled by the positive plate.
By controlling the voltage on the plates a particular ink drop
can be precisely positioned on the paper. There are many nozzles,
and the final picture is made up from a very large number of
coloured ink drops, each in exactly the right place for the image.
