electricity Flashcards
What is the condition for electrical charge to flow?
- circuit must be closed
- there must be a source of potential difference
Define current
- the rate of flow of electrical charge
Does current have different values at different points in a single closed loop?
- no, a current has the same value at any point in a single closed loop
What does the current through a component depend on?
- the resistance of the component
- the potential difference across the component
Describe the relationship between the resistance of a component and current
- the greater the resistance of the component, the smaller the current for a given potential difference across the component
What is the function of a thermistor?
- a component whose resistance is related to temperature
- in cool conditions, resistance is high
- make useful temperature detectors
What is the function of an LDR?
- a component whose resistance is related to to the amount of light falling in it
- in darkness, resistance is the highest
- applications include burglar detectors and automatic night lights
What is the function of a diode?
- they only allow current to flow through them in one direction
What is the function of an LED?
- LEDs emit light when they conduct electricity; current can only flow through them in one direction.
What is the function of an on/off switch?
- an on-off switch allows current to flow when closed
What is the function of a cell?
- a cell supplies electrical energy
What is the function of a battery?
- batteries supply electrical energy and are made up of multiple cells
What is the function of a resistor?
- it restricts the flow of current in a circuit
What is the function of a variable resistor?
- a component used to vary and control the current in a circuit
What is the function of a lamp?
- a component that transforms electrical energy into light
What is the function of a fuse?
- a safety device that will break the circuit and stop current flowing
What is the function of a voltmeter?
- used to measure potential difference between two points in a circuit
- connected in parallel
What is the function of an ammeter?
- used to measure current in a circuit
- connected in series
Define an ohmic conductor
- when the resistance is constant
What happens on a graph if the resistance is constant?
- current is directionally proportional to potential difference causing a linear graph
What is the relationship between resistance and ohmic conductors?
What would this look like on a graph?
- the current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor.
- this means that the resistance remains constant as the current changes
- on a graph plot potential difference against current
- the resistance is a straight line through the origin (extends on both negative and positive values)
What happens on a graph if the resistance is not constant?
- resistance changes with the current through the the component so the graph is non liner
- e.g. resistance of a filament lamp increase as the temp of filament increases
- e.g. current flows through one direction so it has a higher resistance in the reverse direction
What is the relationship between resistance and components?
What would this look like on a graph?
- the resistance of components, such as lamps, diodes, thermistors and LDRs is not constant
- it changes with the current through the component.
- the resistance of a filament lamp increases as the temperature of the filament increases
- on a graph plot potential difference against current
- the resistance is a curved line from the negative values that goes through the origin and curves in the positive values (S-shaped)
What is the relationship between resistance and diodes?
What would this look like on a graph?
- the current through a dioxide flows in one direction only
- the diode has a very high resistance in the reverse direction
- on a graph it starts of flat then increases (like a hockey stick)
Describe the relationship between the gradient and resistance
- The gradient of each is 1/resistance, so a sharper gradient means a lower resistance
How does resistance change in an LDR?
- the resistance of an LDR decreases as light intensity increases
- greater the intensity of light, lower resistance - greatest when its dark
- used in automatic night lights
How does resistance change in a thermistor?
- the resistance of a thermistor decreases as the temperature increases
- In hotter temperatures resistance is lower
- often used in temperature detectors / thermostats
How does resistance change with current?
- As current increases, electrons (charge) has more energy
- When electrons flow through a resistor, they collide with the atoms in the resistor
- This transfers energy to the atoms, causing them to vibrate more
- This makes it more difficult for electrons to flow through the resistor
- So resistance increases, and current decreases
How does resistance change with length?
- greater the length, the more resistance, lower the current
- electrons have to make their way through more resistor atoms, so its harder using a shorter wire
How does resistance change with voltage?
- diode allows current to flow freely in one direction
- so in the opposite direction, it has a very high resistance so no current can flow
Describe a series circuit
- closed circuit where current only follows a single path
What are the rules for components connected in series?
- there is the same current through each component
- the size of the current is determined by the total potential difference of the cells and the total resistance of the circuit
- the total p.d. of the power supply is shared between the components
- the total resistance of two components is the sum of the resistance of each component e.g. Rₜₒₜₐₗ = R₁ + R₂
- can only switch them off all at once
Describe a parallel circuit
- branched circuit where current splits into multiple paths
What are the rules for components connected in parallel?
- the p.d. across each component is the same (PD power supply = PD of each branch)
- the total current through the whole circuit is the sum of the currents through the separate components
- the total resistance of two resistors is less than the resistance of the smallest individual resistor
- total resistance is the reciprocal of all of the resistances added together
- components connected separately to power supply so current flows through each one separately meaning you can switch each component off individually
Why does adding resistors in parallel decrease total resistance?
- in a parallel circuit, the net resistance decreases as more components are added,
- there are more paths for the current to pass through
Why does adding resistors in series increase total resistance?
- in a series circuit, the net resistance increases as more components are added
- the current is the same in every single component in the circuit
- so the more resistors we add, the harder it is for current to flow
Why are parallel circuits more advantageous than series circuits?
- if a wire were to be faulty in the series circuit, the whole circuit would be broken
- if a wire were to be faulty in a parallel circuit, only the junction(s) affected would be broken, and the other lamps would still function
What is the mains electricity?
- an ac supply
Describe the domestic energy supply in the UK
- in the UK, the domestic electricity supply has a frequency of 50Hz and is about 230V
What is the difference between direct and alternating current?
- direct current is the movement of charge in only one direction, e.g. cells & batteries
- whereas alternating current is the backward and forward movement of charge, where current continuously varies, from positive to negative
Describe alternating current
- the p.d. constantly changes
- reaching a peak of 230V in one direction, reverting to 0, then reaching a peak of 230V in the opposite direction and repeating.
- there are 50 of these cycles per second, so the frequency is 50Hz
What is the function and colour of the live wire?
- brown, 230V
- carries the alternating potential difference from the supply
- may be dangerous even if mains circuit is off, as current may still be flowing through it
What is the function and colour of the neutral wire?
- blue, 0V
- completes the circuit
What is the function and colour of the earth wire?
- green and yellow stripes, 0V
- a safety wire to stop the appliance becoming live - connected to the earth and casing
What will happen if the live wore touches the metal casing of the appliance?
- it will become live and you will get an electric shock if you touch it
- because current flows through you to the ground
Why is the insulation covering each wire colour coded?
- for easy identification
How does a fuse work?
- fuses have different capacities for how much current can pass through
- if it surpasses the amount, then the fuse overheats and melts which breaks the circuit, to ensure that other components which could be dangerous with excessive amounts of current do not receive it
What happens when charge flows?
- energy is transferred as it does work against the resistance of the circuit (work is done)
What do all domestic appliances transfer?
- transfer electrical energy into other forms of energy, typically kinetic and thermal energy
What are the two main categories of domestic appliances?
- those that transfer electrical energy to the kinetic energy of motors
- those that transfer electrical energy to the thermal energy of heating devices
Describe the energy transfer to the mains in a blender
- they are powered by electrical energy
- which is then transferred to the kinetic energy of electric motors
- to allow for movement (some energy will be dissipated as thermal energy due to friction)
Describe the energy transfer to the mains in a kettle
- they are powered by electrical energy
- which is transferred to thermal energy in the heating element
Give examples of appliances that fit under transferring electrical energy into both kinetic and thermal energy categories
- hair dryers, washing machines
What are everyday appliances designed to do?
- bring about energy transfers
Define power rating
- the maximum power that appliances can safely operate at / the maximum rate of energy transfer
Describe the difference in power rating between appliances designed to transfer electrical energy to thermal energy vs kinetic energy
- usually, appliances designed to transfer electrical energy to thermal energy have a much higher power rating than those designed to transfer to kinetic energy,
- as it takes much more energy per second to heat than to move
What is a misconception about power?
- a higher power does not necessarily mean that an appliance transfers more energy usefully
- i.e. appliance 1 might have a very high power but a very low efficiency
Define the national grid
- a system of cables and transformers linking power stations to consumers
What is transferred using the national grid?
- electrical power is transferred from power stations to consumers
What is the issue with getting electricity to homes? How can this issue be reduced; what is the problem with this resolution?
- energy is lost in power cables due to the resistance of the wires
- we can reduce energy loss by building power stations near to homes,
- but this isn’t feasible, as people don’t want to live near power stations
How are unwanted energy transfers reduced in the national grid?
- by using step-up and step-down transformers which change the p.d.
What do step-up transformers do?
- step-up transformers are used to increase the potential difference from the power station to the transmission cables
- so as the power is constant, current decreases so less energy is lost
What do step-up transformers do?
- step-down transformers are used to decrease
- the potential difference for domestic use & consumer safety
Why is electrical power transferred at a very high p.d. across step-up transformers?
- P=IV equation
- if we increase the potential difference keep the power (rate of energy transfer) the same, then we have a lower current
- a lower current results in less energy being lost
Define insulators and give examples
- insulators are materials that do not conduct electricity, as electrons cannot move through them (they are fixed)
- e.g. cloths and plastic rods
Define a conductor
- materials that can conduct electricity as their electrons can flow (they are not fixed) and carry a charge
What happens when you rub two insulating materials together?
- when they are rubbed against each other (e.g. a cloth and a plastic rod), they become electrically charged as electrons are transferred from one object to the other
- negatively charged electrons are rubbed off one material and on to the other.
- the material that gains electrons becomes negatively charged and the material that loses electrons becomes positively charged.
- these positive and negative charges are equal and opposite
What would happen if conductors were rubbed together?
- electrons will flow in /out of them, cancelling out any effect so they stay neutral
Why can insulators become charged?
- the electrons cannot flow
When does a positive static charge form?
- when an object loses electrons
When doe a negative static charge form?
- when the object gains electrons
When does sparking occur?
- when enough charge builds up, and the objects are close but not touching
- commonly occurs between an object with a static charge and an earthed object
What is the “spark”?
- when the charge jumps through the air from the highly negative object to the highly positive object to balance out the charges
Why is it called static electricity?
- because the two insulators now become electrostatically charged
What happens when two electrically charged objects are brought close together?
- they exert a force on each other (electrostatic force of attraction/repulsion)
What happens when same charged objects are brought together?
- two objects that carry the same type of charge repel
What happens when two differently charged objects are brought together?
- two objects that carry different types of charge attract.
What is attraction and repulsion an example of?
- non-contact force, as force can be felt even when the objects are not touching
How can you increase the force between two objects?
- greater charge - e.g. more positive and more negative objects
- closer together - force is proportional to the inverse square of the distance
How can static electricity cause a spark under certain conditions?
- when we rub two insulators and electrons are transferred from insulator A to insulator B
- insulator A becomes positively charged and B becomes negatively charged.
- the electrons from insulator B are therefore strongly attracted to insulator A’s positive charge
- so sometimes, these electrons can jump across the gap when they are brought close to each other, causing a spark
How are sparks caused?
- sparks are caused when there is a high enough p.d. between a charged object and an earthed object
- a high p.d. causes a strong electric field between the two objects
- causes electrons in the air particles to be removed (ionisation)
- air is normally an insulator but when ionised it is conductive
- current can flow through, this is the spark
How do you prevent sparks leading to fires? e.g. on aircraft fuelling
- the refuelling truck and the aeroplane are earthed to prevent any charged building up and leading to a spark
Describe how sparks take place between static charged objects and earthed objects - what assumptions do you make?
- as electric charge builds up on an object, the p.d. between the charged object and the earthed object increases.
- with a high enough p.d., the electrons can jump across the gap and be transferred from the charged object to the earthed object.
- you must assume that the uncharged object is earthed, not positive
When is an electric field created?
- a charged object creates an electric field around itself
Determine the strength of an electric field
- the electric field is strongest close to the charged object; the further away from the charged object, the weaker the field
What happens when another charged object is placed in the electric field?
- it experiences a non-contact force
- this force causes attraction or repulsion caused by the electric fields of each charge object interacting with each other
- the force gets stronger as the distance between the objects decreases
How can we draw electric field lines?
- draw lines perpendicular to the surface with arrows that point in the direction of a positive particle that is placed within the field
- i.e. field lines for a positive object will go away from the object (if you place a positive object in the field it will be repelled) and field lines for a negative object go towards the object (if you place a negative object in the field it will be attracted)
- stronger the charge the more filed lines present
Describe the interaction between two oppositely charged particles
- the electric field of Q interacts with the electric field of q
- this causes forces to act on both Q and q
- these forces move q and Q closer together
