topic 10 - electricity and circuits Flashcards
current
the flow of electric charge around the circuit/
the rate of flow of charge
what is current measured in and how
measured in Amperes (A)
measured using an ammeter connected in series
what is current in metals and how does it flow
current in metals is the FREE FLOW OF ELECTRONS
atoms in metals are bonded into a lattice of positive ions surrounded by a sea of delocalised electrons - these free electrons are free to move through the whole metal
potential difference/voltage
the energy transferred per unit of charge
the driving force that pushes the change around
what is voltage measured in and how
measured in volts (V)
measured using a voltmeter connected in parallel
resistance
anything that slows down the flow of charge
what is resistance measured in
Ohms (Ω)
charge, current, time relationship
charge = current x time
energy transferred, charge, p.d relationship
energy transferred = charge x potential difference
p.d, current, resistance relationship
potential difference = current x resistance
why does a resistor heat up as current flows through it (general)
when charge flows through a component, it must do work against resistance
this causes an electrical transfer of energy
some of this is transferred usefully but some is dissipated to the thermal energy stored of the component and the surroundings
why if a resistor is too hot can current not flow through it (atomic level)
electrons collide with ions in the lattice as they flow through, giving the ions energy and causing them to vibrate more
the more the ions vibrate, the harder it is for electrons to travel through so for a given p.d current decreases as the resistor heats up
what do low resistance wires do
reduce energy dissipated to thermal stores as the current flows between components
how to investigate the relationship between current potential difference and resistance for a range of components - describe circuit
a circuit connecting an ammeter, component and variable resistor in series and a voltmeter in parallel to the component
how to investigate the relationship between current potential difference and resistance for a range of components - process (5)
1) set resistance of variable resistor
2) measure current and potential difference across the component
3) repeat at lots of different resistances
4) swap over the wires to reverse the direction of the current, so the ammeter will give negative readings and repeat
5) plot results on I-V graphs
what do I-V graphs show
show how current changes as potential difference is changed
linear components
components which show a straight line I-V graph
non-linear components
components which have a curved line I-V graph
the steeper the I-V graph….
the lower the resistance
what is the relationship between variables for a fixed resistor
current is directly proportional to p.d
what is the I-V graph like for a filament lamp
increasing current, increases the temperature of the filament, increasing resistance so the graph is curved
what is the I-V graph like for a diode
current only flows through a diode in one direction, the diode has very high resistance in the opposite direction
light dependent resistor (what lowers resistance)
the brighter the light, the lower the resistance
thermistor (what lowers resistance)
the warmer it is, the lower the resistance
how to investigate how the resistance of an LDR changes with brightness (5)
1) conduct experiment in a dim room
2) measure p.d across and current through the LDR
3) change the light level near the LDR
4) measure p.d and current again for a range of light levels
5) calculate resistance for each measurement using (resistance = voltage/ current)
how to investigate how the resistance of a thermistor changes with temperature (4)
1) measure the p.d across and current through the thermistor
2) change temperature of thermistor by heating it
3) measure the p.d and current again for a range of temperatures
4) calculate resistance for each measurement using (resistance = voltage/current)
series circuit
different components are connected end to end in a line between the positive and negative of a power supply (excluding voltmeters which are always connected in parallel)
what happens if one component breaks or is removed in a series circuit
the whole circuit breaks and stops working
rules in a series circuit (4)
- current is the same everywhere
- voltage from cells adds up to give total p.d
- total p.d is shared between components according to its resistance (the greater the resistance, the greater the p.d)
- resistance is added up to give total resistance
parallel circuit
each component is connected separately to the negative and positive of the power supply (apart from ammeters which are always connected in series)
what happens if one component breaks or is removed in a parallel circuit
the other components are hardly affected
rules in a parallel circuit (5)
- current is shared between branches
- at junctions where current splits or rejoins the total current going into the junction equals the current leaving it
- if two identical components are connected in parallel, the same current will flow through each component
- p.d is the same across all components
- if you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors
why is the total resistance in a series circuit the sum of all the components’ resistances
by adding a resistor in series, the two resistors have to share the p.d
the p.d across each resistor is lower, so the current through each resistor is lower
the current is the same everywhere so the total current in the circuit is reduced when a resistor is added
this increases the total resistance of the circuit
how can you investigate the effect of increasing resistance on current IN A SERIES CIRCUIT (6)
- need 4 identical resistors
- build a series circuit with an ammeter, a resistor and battery (make a note of its p.d)
- measure the current using the ammeter and calculate the resistance of the whole circuit
- add another resistor in series, measure current and calculate resistance
- repeat until all resistors have been used
- plot a graph of number of resistors against total resistance
why if you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors?
in parallel, there is the same p.d
by adding another loops, the current has more than one direction to flow it, increasing the total current that can be flowing in the circuit
using voltage = current x resistance, an increase in current means a decrease in the total resistance of the circuit
how can you investigate the effect of increasing resistance on current IN A PARALLEL CIRCUIT (6)
- need 4 identical resistors
- build a series circuit with an ammeter, a resistor and battery (make a note of its p.d)
- calculate resistance of the whole circuit
- add another resistor in parallel and measure total current through the circuit, using it to calculate total resistance
- repeat until all resistors have been added
- plot a graph of number of resistors against total resistance
energy is transferred in an electrical circuit when…
an electrical charge goes through a change in potential difference
energy (J) =
current (A) x potential difference (V) x time (s)
what is energy supplied for
supplied to raise the charge through a potential
the charge gives up the energy at any potential drop at components elsewhere in the circuit
energy transfer in circuits (+note on efficiency)
energy is transferred from cells to components in a circuit when current flows
no appliance is completely efficient ; the higher the current, the more energy is transferred to the thermal energy stored of the components and then to the surroundings (heating usually increases with the resistance of components)
fuses
protect circuits by melting and breaking the circuit if the current gets too high
how can heating in circuits be beneficial in terms of heating
in toasters, there is a high resistance coil that once current is flowing through it, heats up so much that it glows and gives off infrared radiation, cooking the bread
(this also works in filament lamps and electric heaters)
power of appliance
the energy it transfers per second
what is power measured in
Watts (W)
power rating
the maximum safe power for operation (MAXIMUM OPERATING POWER)
relationship , power, energy, time
power = energy / time
relationship ; power, current, p.d
power = current x potential difference
relationship ; power, current, resistance
power = current^2 x resistance
what is a.c used as
the mains supply
what is UK mains supply p.d and frequency
230V
50Hz
what is d.c used as
to transfer energy from cells and batteries
wires
have a copper core and an insulatory coloured plastic coating
what are 3 wires
neutral wire blue
live wire brown
earth wire green and yellow
neutral wire blue
completes the circuit
live wire brown
carries the voltage
earth wire yellow and green
for safety and protecting the wiring
carries the current away if something goes wrong
stops the appliance casing from becoming live
p.d between live and neutral
230V
p.d between live and earth for mains connected appliance
230V
p.d between earth and neutral
0V
how does electricity flow through a plug
flows in through the live wire
flows out through the neutral wire
why can you get an electric shock
the body rests at 0V
if you touch a live wire, a large potential difference is produced across your body and current flows through you, causing an electric shock that could injure or kill you
surge
sudden increase in current
what can cause surges
changes in a circuit
fault in the electrical appliance
what can a surge result in
circuits and wiring melting
causing a fire
what can faulty appliances lead to
deadly electric shocks
what are used to prevent electric shocks or surges from harming others
an earth wire and a fuse
if the live wire in a toaster touches the metal casing what happens
the toaster has become faulty
this causes a current surge as current flows in though the live wire, through the casing and out down the earth wire
the surge causes the fuse to melt
this breaks the circuit and cuts off the live supply
the whole appliance is now isolated and it is impossible to get an electric shock from the case
circuit breaker
even safer than fuses
instead of melting, a large current will ‘trip’ a circuit breaker
they turn off quicker than how long it takes for a fuse to melt
can easily be reset but are more expensive
role of insulation of electrical appliances
appliances with metal casing should be ‘earthed; to reduce the danger of electric shock
a plastic casing can’t conduct so can’t become live
double-insulated appliance
an appliance with a plastic casing and no metal parts showing
what wires does a double-insulated appliance have
the two core cables (neutral blue and brown live)
it doesn’t need an earthing wire
