Topic 10 - Electricity and Circuits Flashcards
What are most electrical appliances connected to?
+The mains supply by three core cables
+This means that they have three wires inside them, each with a core of copper and a coloured plastic coating
+The colour of the insulation on each cable shows its purpose
What are parallel circuits?
+Circuits where electrical components are connected alongside one another forming extra loops
+If a bulb is broken, the current can still flow through other components in the circuit through the other loop.
+When a bulb goes out, other components can still work
How does resistance vary within a thermistor?
+As the temperature increases, the current through the thermistor increases as the resistance decreases
+Keep the potential difference supply constant, gradually heat the thermistor [by placing thermistor against a beaker of hot water]
+You will find…
What prevents current surges and its effects?
The earth wire and a fuse are included in electrical appliances to prevent this from happening
What does the power rating tell you?
+This tells you the maxium amount of energy transferred between stores per second when the appliance is in use
Why does a resistor heat up when a current flows through it?
+Electrons collide with the ions in the lattice that make up the resistor as they flow through it.
+This gives the ions energy, which causes them to vibrate and heat up.
+The more the ions vibrate, the harder it is for electrons to get through the resistor [because there are more collisions]. This means that for a given p.d the current decreases as the resistor heats up.
+If the resistor gets too hot, no current will be able to flow. [Exception: The resistance of a thermistor decreases with an increase in temperature.
What is the process of the standard test circuit?
Non-linear graph
Show how the I-V graph for a filament lamp looks like:
What happens as current flows round a circuit?
+ The charges transfer energy as they struggle against resistance
What happens when an electrical charge flows through a component
- It has to do work against resistance
- This causes an electrical transfer of energy [work done = energy transferred]
What are I-V graphs
+Current - potential difference graphs
+This shows how the current varies as you change the potential difference
What is potential difference?
+The driving force that pushes the charge round
+The energy change per unit of charge between two points
+Unit, volt [V]
What is the Battery supply?
D.C
What is one volt also known as?
One joule per coulomb
What is the formula for power?
What colour is the earth wire
Green and yellow
Where is energy supplied in a circuit?
+Energy is supplied to the charge at the power source to ‘raise’ it through a potential
+The charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit [eg. resistors]
What does it mean when atoms are neutral
Number of protons = Number of electrons
What is mains supply?
A.C
What does total charge depend on?
Current and time
Where is all the mass of an atom located?
In the nucleus +The nucleus is tiny [about 10,000 times smaller than the whole atom]
What is current caused by in metals?
+By a flow of electrons
What does the earth wire do?
+This wire is for safety and protecting the wiring.
+It carries the current away if something goes wrong and stops the appliance casing becoming live
+It’s also at 0V
What does it mean if an atom gains or loses an electron?
- If an atom gains an electron, it becomes a negative ion
- If an atom loses an electron, it becomes a positive ion.
What does the power transferred by an appliance depend on?
+This depends on the potential difference [p.d] across it, and the current flowing through it
What do non-linear components have?
+A curved I-V graph [eg. a filament lamp or a diode]
Show how the I-V graph for resistors and wires look like:
What is the formula for charge?
What does the a.c do?
How are alternating currents produced?
+In a.c supplies, the movement of the charges is constantly changing direction
+Alternating currents are produced by alternating voltages [the positive and negative ends of the p.d keep alternating]
What does the standard test circuit contain?
+Ammeter - this measures the current [in amps] flowing through the component. It can be put anywhere in the main circuit - but it must be placed in series with the component [never paralel]
+Voltmeter - this measures the potential difference across the component. It must be placed in paralel with the component under test
What happens in a parallel circuit?
+The total current supplied is split between the components on different loops
+Potential difference is the same accross each loop
+[If second resistor is added in parallel], the total resistance of the circuit is reduced as the current can follow multiple paths
+The total current going in to a junction equals the total current leaving [the same current flows through each component]
What are the advantages of the heating effect of an electric current?
+It can be useful when you want to heat something
+Toasters contain a coil of wire with a really high resistance
+When a current passes through the coil, its temperature increases so much that it glows and gives off infrared radiation.
+The radiation transfers energy to the bread and cooks it
+Filament bulbs and electric heaters work in a similar way
What is a fuse?
a safety device consisting of a strip of wire that melts and breaks an electric circuit if the current gets too high [exceeds a safe level]
What does the live wire do?
Where is the live wire situated?
+This carries the voltage [potential difference, p.d].
+It alternates between a high +ve and -ve voltage of about 230V
+It is situated on the right
Complete the table:
Name these circuit symbols:
What are appliances often given?
+A power rating - they’re labelled with the maximum safe power they can be operated at [usually maximum operating power]
What is an atom?
+A positively charged nucleus surrounded by negatively charged electrons.
+They are neutral
How does resistance vary within a diode?
+Current will only flow through a diode in one direction
+The diode has a very high resistance in the opposite direction
What is a thermistor?
+A thermistor is a temperature dependant resistor
+In hot conditions, the resistance drops
+In cool conditions, the resistance go up
Does any appliance transfer energy completely usefully?
+No appliance transfers all energy completely usefully
+The higher the current, the more energy is transferred to the thermal energy stores of the components [and then the surroundings]
For a parallel circuit, as the P.D increases…
So does the current through each bulb [non-linear relationship]
+The P.D across each bulb is the same as the P.D of the power supply
How does resistance vary within a filament lamp?
+An increasing current increases the temperature of the filament, which makes the resistance increase
+So their I-V graphs are curved
What is the power of an appliance?
+The power of an appliance is the energy is transfers per second
+So the more energy it transfers in a given time, the higher its power
In series circuits, how do you find the total resistance?
+The sum of the resistances
What happens when you add resistors in series?
+This increases total resistance
+This is because by adding a resistor in series, the two resistors have to share their total potential difference
+The P.D across each resistor is lower, so the current through each resistor is lower
+In a series circuit, the current is the same everywhere, so total current is reduced when a resistor is added [meaning total resistance increases]
+The bigger a components resistance, the bigger its share of the total potential difference
What is a.c?
Alternating current
What is resistance?
+Anything that slows the flow of electric charge [current] down
+Unit ohms [Ω]
What do the negative electrons do?
+They whizz around in fixed orbits called energy levels or shells.
What is the p.d between the live wire and the earth wire?
+This is 230V for a mains-connected appliance
What does all appliances with metal cases usually have?
+They are usually earthed - to reduce the danger of electric shock
What can any connection between a live or neutral wire be?
+Highly dangerous
+If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
What is the diagram of the standard test circuit?
What can current surges lead to?
+Leads to the circuits and wiring in appliances melting or causing a fire, and faulty appliances can cause deadly electric shocks
How is energy transferred in handheld fans?
+Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor.
What is the p.d between the live wire and the neutral wire?
+This equals the supply p.d [230V for the mains]
What normally happens the greater the resistance of a component?
+The smaller the current that flows [for a given potential difference across the component]
How can you find resistance on any point on any I-V graph?
+You can find resistance for any point on any I-V graph by…
+Reading the P.D and current at that point and putting them into the equation:
+V=IR
+So R = V/I
What does a diagram of a parallel circuit look like?
How should fuses be rated?
+Fuses should be rated as near as possible but just higher than the normal operating current.
What happens when an electrical current does work against electrical resistance?
+Electrical energy is dissipated as thermal energy in the surroundings
For a series circuit, as the potential difference increases
The current through the resistor increases [non-linear relationship for a filament bulb]
What are electrical appliances designed to do?
+Transfer energy to components in the circuit when a current flows
What does a voltmeter measure
Potential difference [volts]
When will the current only flow through an electrical circuit?
+If there is potential difference across that component, and if the circuit is complete [closed].
Unit: ampere A
What happens in direct current?
What is d.c created by?
+The movement of the charges is only in one direction
+D.c is created by a direct voltage [a p.d that is only positive or negative, not both]
For linear components, what happens if the line goes through [0,0]
+The resistance of the component equals the inverse of the gradient of the line [or 1/gradient]
+The steeper the graph, the lower the resistance
Are the colours for each of the three core cables the same?
+Yes, the colours are always the same for every appliance
+This is so that it’s easy to tell the different wires apart
Why does the fuse rating [needed for cables] usually increase with cable thickness?
+The larger the current, the thicker the cable you need to carry it [to stop the cable getting too hot and melting]
In parallel, the total current through the circuit…
Is the sum of the current through the two branches [which is larger than the total current in series with one filament bulb]
+This is because the overall resistance of a parallel circuit is lower, so a lower value of R causes a higher value of I
How do kettles transfer energy?
+Kettles transfer energy from the mains a.c supply to the thermal energy store of the heating element inside the kettle.
What does a diagram of a series circuit look like?
What is the equation for the energy transferred to an electrical component?
+The larger the current or P.D through a component, the more energy transferred to it
What does earthing and fuses prevent?
Electrical overloads
What is d.c?
Direct current
What does a circuit breaker do?
+Instead of melting a fuse, a large current instead may ‘trip’ [turn of] a circuit breaker
+Circuit breakers turn off quicker than the time taken for a fuse to melt
+They can also be reset, which is easier than replacing a fuse
What do microwaves have a range of?
+They have a range of power ratings
+Eg. A 500W microwave will take longer to cook food than one with a power rating of 750W
+This is because the 500W transfers less energy per second to the thermal energy store of the food, so it takes longer to cook
What happens when you add a resistor in parallel?
+This reduces the total resistance [their total resistance is less than the smallest of the resistors]
+By adding another loop, the current has more than one direction to go in
+This increases the total current that can flow around the circuit
+An increase in current means a decrease in the total resistance of the circuit
What are surges?
Sudden increases in current
What happens in a series circuit?
+The current is the same through each component
+The total potential difference of the power supply is shared between the components
+The total resistance of the circuit is the sum of individual resistors
[As you add resistors, the total resistance of the circuit increases]
What is a circuit breaker?
+A switch designed to protect an electrical circuit from damage caused by excess current or a short circuit
+They are safer than fuses
+They can be used in the place of household fuses
What are two core cables?
Cables that only carry live and neutral wires
What do plug sockets have?
+Switches which are connected in the live wire of the circuit.
+This is so the circuit can be broken - the appliance becomes isolated and the risk of an electric shock is reduced
What is the UK mains supply?
The UK mains supply [the electricity in your home] is an a.c supply at at around 230 V
What colour is the live wire?
Brown
What is double insulation?
+When an appliance has plastic casing and no metal parts showing
What normally happens the higher the potential difference across a given component?
The higher the current will be
What does resistance increase with?
Temperature [usually]
What can occur because of changes in a circuit? [or a fault]
+Surges [sudden increases] can occur because of changes in a circuit [eg. an appliance suddenly switching off] or because of a fault in an electrical appliance
What happens when there is an electric current in a resistor?
+There is an energy transfer which heats the resistor
What does the graph for a thermistor look like?
What is the formula for potential difference?
What is earthing?
+When the case is attatched to an earth wire
+An earthed conductor can never become live
What is a series circuit?
+A circuit where electrical components are connected one after the other in a single loop.
+An electron will pass through every component on its way round the circuit
+If a bulb is broken, the current will not be able to flow round the circuit
+If one of the bulbs go out, they all go out
Label the structure of the atom
What are some of the applications of thermistors?
+Thermistors make useful temperature detectors,
+Eg. car engine temperature sensors and electronic thermostats
What is the equation for power if you don’t know the potential difference?
What can you create I-V graphs for?
+Diodes, Thermistors and LDR’s
+However, resistance [and so potential difference] of these components can depend on other factors besides current. [like light intensity and temperature]
What could still happen even if a plug socket or light switch is turned off? [ie. the switch is open]
+There is still a danger of electric shock
+This is because even though a current isn’t flowing, there is still a potential difference in the live wire
+Making contact with the live wire, would mean your body will provide a link between the supply and the earth, so a current would flow through you.
What is the frequency of the a.c mains supply
50 cycles per second or 50 Hz [hertz]
What voltage is your body at?
+Your body [just like the earth] is at 0V.
What supplies d.c [direct current]?
Cells and batterries supply direct current
What is the disadvantage of a circuit breaker?
They are more expensive than fuses
What do atoms form when they join together?
+Molecules
+Eg, molecules of oxygen gas are made up of two oxygen atoms bonded together. Small molecules like this have a typical size of 10-10 m - the same sort of scale as the size of an atom.
What does heating usually do?
+This usually increases the resistance of the components
+This reduces its efficiency - less energy is transferred to useful energy stores [because more of it is being transferred to the thermal energy store of the component]
What other types of fuses are there?
+Household fuses, [eg, they blow when a light bulb fuses]
+These work in the same way, but protect the wiring in the house, not just an appliance
What does the current flowing through a component depend on?
+The potential difference across it and the resistance of the compoent.
How is energy transfered through a circuit?
+Energy is transferred when an electrical charge goes through a change in potential difference [as work is done against resistance]
What are the applications of an LDR?
+There are many applications including
+Automatic night lights, outdoor lighting and burglar detectors
What happens to the electrical transfer of energy?
- Some of this energy is transferred usefully but some of it is dissipated to the thermal energy stores of the componenent and the surroundings.
- So when a current flows through a resistor, the ressistor heats up
How does the earth wire and fuses work?
- If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, a large current flows through the live wire, through the case and out down the earth wire.
- This surge in current melts the fuse when the amount of current is greater than the fuse rating. Fuses are connected to the live wire, so that breaking the fuse breaks the circuit and cuts off the live supply.
- +This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current.
What is the p.d between the neutral wire and the earth wire?
+There is no p.d - they’re both at 0V
What does the graph for an LDR look like?
What can help customers choose between models of microwaves?
+The lower the power rating, the less electricity an appliance uses in a given time so the cheaper it is to run.
+But a higher power doesn’t neccesarily mean that it is more efficient. An appliance may be more powerful but might not transfer more energy usefully - meaning it might still only transfer the same amount of energy [or even less] to useful stores
Show how the graph for a diode looks like:
What does the p.d and current tell you?
+The p.d tells you how much energy each unit of charge transfers
+The current tells you how much charge passes per unit time
+So both will affect the rate energy is transferred to an appliance, and at the rate it transferess energy to other stores
What is the formula for energy transferred?
What does an ammeter measure?
Current [amps]
What do linear components have?
An I-V graph that is a straight line [eg. a fixed resistor]
What is the eqation for electrical power?
What happens if you touch a live wire?
+Body is at 0V
+This means if one touches a live wire, a large potential difference is produced across the body and a current flows through you.
+This causes an electric shock which could injure you or even kill you
What happens to the resistance of a thermistor as the temperature increases?
+The resistance of a thermistor decreases as the temperature increases
What happens if the temperature gets too high?
+This can cause components in the circuit to melt - which means the circuit will stop working, or not work properly
What is potential difference?
+The energy transferred per coulomb of charge that passes between two points in an electrical unit.
What is the standard test circuit?
How can this relationship be shown?
+This is an experiement to investigate the relationship between current [I], potential difference [V] and resistance for a range of components [such as a filament bulb or fixed resistor].
+This relationship can easily be shown with an IV graph
What colour is the neutral wire?
Blue
What does the total energy transferred by an appliance depend on?
+How long the appliance is on for and its power
What is current directly proportional to [if the temperature stays the same]
Potential difference [if the temperature stays the same]
What is potential difference also known as?
Voltage
What is current?
- The rate of flow of electric charge around the circuit.
- Current will only flow thrrough an electrical component if there is potential difference across that component, and if the circuit is complete [closed].
- Unit: ampere, A
What is an LDR?
+An LDR [Light Dependant Resistor] is a resistor that is dependant on the intensity of light.
+In bright light, the resistance falls
+In darkness, the resistance is the highest
What does the neutral wire do?
+Where is the earth wire situated?
+This completes the circuit - electricity [normally] flows in through the live wire and out through the neutral wire
+The neutral wire is always at 0V
+It is situated on the left
How does resistance vary within LDR’s?
+As the light level gets brighter, the current through the LDR increases as the resistance decreases.
+Condict experiment in a dim room. Keep P.D constant and slowly adjust the light level near to the LDR [eg. by using a lamp connected to the dimmer switch]
+You should find…
What doesn’t double insulation need?
+An earth wire, just a live and neutral
