SP10: electricity and circuits Flashcards
switch
used to turn a circuit on (closed) and off (open)
lamp
an electrical current heats the filament in a bulb so that it gives out light
fixed resistor
-limits the flow of electrical current
-has a resistance that does not change
variable resistor
adjusting this resistor changes its resistance
thermistor
-resistance of a thermistor depends on its temperature
-low temperatures = high resistance
-as temp increases, resistance decreases
(thermostat)
light dependant resistor (LDR)
-the resistance of a LDR depends on light intensity
-at low light levels, the LDR has a high resistance
-as the light intensity increases, the resistance decrease
(automatic lights)
semiconductor diode
-allows current to flow in one direction only
- used to convert an alternating current into a direct current
electric current
a flow of electric charges
alternating current (AC)
an electric current that regularly changes its direction and size
direct current (DC)
electric current that moves in one direction only
charge
-Q
-a property of a body which experiences a force in an electric field
-measured in coulombs (C)
what is current?
-I
-the rate of flow of charge
-measured in amperes
units for charge equation (charge = current x time)
current (I) in amperes (A)
time (t) in seconds (s)
charge (Q) in coulombs (C)
one ampere is…
the current that flows when one coulomb of charge passes a point in a circuit in one second
how is current measured?
-using an ammeter
-the ammeter must be placed in series with that component
what is current measured in?
amperes (A)
potential difference/voltage
-symbol is V
-a measure of the energy given to the charge carriers in a circuit
-measured in volts
how to measure potential difference?
-using a voltmeter
-voltmeter must be placed in parallel with the component which’s potential difference is being measured
energy transferred equation units (charge x potential difference)
energy transferred (E) in joules
charge (Q) in coulombs (C)
potential difference (V) in volts
what is charge measured in?
coulombs (C)
one volt is the potential difference when…
one coulomb of charge transfers one joule of energy
potential difference equation (voltage = current × resistance
voltage (V) in volts (V)
current (I) in amperes (A)
resistance (R) in ohms (Ω)
what is resistance?
slows the flow of current
what is resistance measured in?
ohms (Ω)
increasing resistance, reduces…
current passing through
conductors & insulators vs resistance
-conductors have low resistance
-insulators have high resistance
practical: investigate current-voltage graphs (method)
- connect the circuit as shown in the first diagram
- adjust the variable resistor so that the potential difference is zero at the start
- record the reading on the voltmeter and ammeter
- use the variable resistor to increase the potential difference
- record the new readings on the voltmeter and ammeter
- repeat steps three to four, each time increasing the potential difference slightly
- reverse the power supply connections and repeat steps two to six
- repeat the experiment but replace the fixed resistor with a bulb
IV graph for a resistor
-straight line through origin
-the potential difference is directly proportional to the current
-a resistor is an ohmic conductor
ohm’s law
the current flowing through a resistor is directly proportional to the voltage across the resistor (when temperature remains constant)
IV graph for a filament lamp
-s-shaped curve which passes through the origin
-the current does not increase as fast as the potential difference
-not directly proportional, not an ohmic conductor
-the more energy is, the more resistance there is in the bulb
-as the voltage increases, so does the temperature of the filament inside the bulb
-increased vibrations of the ions in the filament make it harder for the electrons to get past
IV graph for semiconductor diode
-horizontal line along the x-axis until it reaches around 0.6 V, where it then becomes a straight line with a steep gradient
-only get a current when potential difference is around 0.6-0.7, after this current rises sharply as resistance has lowered
what is electricity?
the flow of electrons
what is conventional current?
the idea that current goes from the positive terminal to negative terminal
what direction does current go in?
negative to positive
what are delocalised electrons?
freely moving electrons
what happens as electrons meet the positive end of a circuit?
they have less energy
series circuit
-one route
-if one bulb fails, all fail
-current is same throughout
-voltage is split equally between components
why is it that if one bulb fails all fail in a series circuit?
-an electron will pass through every component on its way round the circuit
-if one of the bulbs is broken then current will not be able to flow round the circuit
resistance in series
-if resistors are connected in series, the current must flow through all of them, so the resistance is increased
-to find the total resistance, the resistances are added together
parallel circuit
-several branching routes
-bulbs can be individually switched off
-current is split
-voltage stays same
resistance in parallel
-if resistors are connected in parallel so that the current doesn’t flow through both, then the overall resistance is reduced
practical: investigating resistance in series and parallel circuits (method)
- set up the circuit (series with the voltmeter parallel) turn the power supply on and close the switch
- record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I
- change the resistor and repeat step two to find the resistance of a second resistor
- arrange the two resistors in series and close the switch
- record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I
- arrange the two resistors in parallel and close the switch
- record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel
steps of energy moving through a circuit
- cells contain energy
- energy is transferred to a charge
- charge is transferred to components
- energy is transferred from charge as it moves to lamp
- lamp transfers energy to surroundings by heat & light
heating up of wires
-as electrons flow through wires, they collide with the ions in the wire which causes the ions to vibrate more
-this increased vibration of the ions increases the temperature of the wire
-energy has been transferred from the chemical energy store of the battery into the internal energy store of the wire
what is power?
the rate at which energy is transferred
power equation units (power = current x potential difference)
power (P ) in watts
current (I) in amperes
potential difference (V) in volts
power dissipated by current in a resistor = current² x resistance (units)
power (P) in watts
current² (I) in amperes (A)
resistance (R) in ohms
what is power measured in?
watts
one watt is equal to…
one joule per second
how current affects heating of wires
-a high current will have a much higher heating effect on the transmission wires than a low current
-transmitting energy at a high voltage with a low current will keep the wires cooler and waste less energy
-reducing the resistance of the wires will also reduce unwanted energy transfer, but reducing the current will have a much more significant effect
how AC & DC are shown on voltage-time graphs
DC: straight horizontal line at a constant voltage which doesn’t go under 0V
AC: a curve alternating between positive and negative voltages, the positive and negative values indicate the direction of current flow
what current does the UK supply?
AC at 50Hz and 230v
what does a plug do?
-connects a device to the mains electricity supply
purpose of each part of a plug (outside)
- outer insulation: all three wires in the cable are bundled together and there is extra plastic insulation wrapped round them all for safety
- cable grip: holds the cable tightly in place so that wires do not become loose
live wire
-brown
-wire connects to the alternating potential difference pushing the current in the circuit
neutral wire
-blue
-this wire is connected to a voltage close to zero, to ensure the live voltage always has a difference in potential to make the push for the current
earth wire
-green and yellow
-provides a path for current to flow from the case of the device to the ground if there is a fault
-zero voltage connection)
how does the earth wire work?
-the earth wire is connected to the case and is attached to a metal plate underground
-as the wire is made of copper, the earth wire provides a low resistance path to the ground
-if a fault happens, the live current passing through the case will follow this path to the ground instead of passing through a person
(however, this would generate a very large current, leading to intense heating which could start a fire, so a fuse or a circuit breaker is also included in the circuit)
fuse
a glass or ceramic canister containing a thin wire that melts & breaks the circuit if the current gets too high
how does the fuse work?
-provides a built-in fail-safe to the electrical circuit for a device
-contains a thin wire that will melt if the current gets too high
-a large current will flow through the live wire and low-resistance earth wire
-this high current will cause the fuse to melt
-once the fuse has melted, the circuit is broken & no more current flows through the device
-the case of the device is no longer live and there is no more risk of electrocution
where must a fuse or circuit breaker be connected?
in the live wire side of a domestic circuit
circuit breaker
-serve the same function as a fuse but can be reset without the need for replacement if it trips
power in an appliance equation (power = energy transferred/time)
power (P) in watts
energy (E) in joules
time (t) in seconds
power in an appliance equation (power = current x voltage) units
power (P) in watts (W)
current (I) in amperes
voltage (V) in volts (V)
energy transferred in an electrical appliance equation units (energy transferred = current x
voltage x time)
energy transferred = current x
voltage x time
(E = IVt)
how does current flow through a metal?
-with a flow of negatively charged electrons
-the electrons are delocalised & move from one atom to another
a component with big resistance will need
a higher voltage to move electrons through it
