# physics topic 2

what is electrical current?
the flow of electric charge round a circuit, which will only flow if there is a potential difference
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what is potential difference?
the driving force that pushes the charge round
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what is resistance?
anything in the circuit which slows the flow down
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what is the relationship between resistance and current?
the greater the resistance across a component, the smaller the current that flows through it
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what is charge?
the size of the current is the rate of flow of charge, so more change passes around the circuit when a bigger current flows
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what is the relationship between current and pd in an ohmic conductor?
the resistance of ohmic conductors doesn't change with current, so at a constant temperature the current in an ohmic conductor is directly proportional to the pd across it
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what is the relationship between current and pd in a filament lamp?
when an electrical charge flows through a filament lamp, some energy is transferred to its thermal energy store, which is designed to heat up, and resistance increases with temperature, so as the current increases so does resistance
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what is the relationship between current and pd in a diode?
the resistance depends on the direction of the current, and will easily let current flow in one direction but have a very high resistance if reversed
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what does the IV characteristic for an ohmic conductor look like?
the current is directly proportional to the pd, so you get a straight line
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what does the IV characteristic for a filament lamp look like?
as current increases, the temperature of the filament increases, so resistance increases, meaning less current can flow per unit pd, giving the graph a curve
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what does the IV characteristic for a diode look like?
current will only flow in one direction and there is a very high resistance in the other, so the graph is flat until it reaches the other side of the origin and then goes up
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what is an LDR?
a resistor that depends on the intensity of light
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what happens in bright and dark light?
in bright light resistance falls, in dark light resistance increases
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what are the uses of LDRs?
automatic night lights, outdoor lighting, burglar detectors
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what is a thermistor?
a resistor that depends on the temperature
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what happens in hot and cold conditions?
in hot conditions resistance decreases, in cold conditions resistance increases
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what are the uses of thermistors?
temperature detectors, car engine sensors, electronic thermostats
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what are series circuits?
circuits where all components are connected in a line, end to end, so if you remove one component, the circuit breaks and they all stop
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how is pd in series circuits?
the total pd of the supply is shared between the various components, so the individual potential differences always add up to the source pd
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how is current in series circuits?
the same current flows through all components
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how is resistance in series circuits?
the resistance of each component adds up as, by adding another resistor, they have to share the source pd, meaning that the pd through each one is lower, so the current through each one, and the total current, is lower
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what are parallel circuits?
circuits in which each component is separately connected, so if you remove or disconnect a component, the others won't be affected
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how is pd in parallel circuits?
the pd across all components is the same
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how is current in series circuits?
current is shared between branches, so the total current is equal to the sum of all the currents through separate components
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how is resistance in series circuits?
adding a resistor in parallel reduces the total resistance
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why is this?
in parallel, both resistors have the same pd across them as the source, meaning that the pushing force making the current flow is the same as the source pd, so by adding another loop, the current has more directions it can go in, so current increases
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what is alternating current?
current which is produced by alternating potential difference in which the positive and negative ends keep changing
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what is the voltage and frequency of the UK mains supply?
230v, 50hz
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what is direct current?
current which is always flowing in the same direction, created by a direct potential difference
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what is the neutral wire?
blue, 0v, completes the circuit and carries away current
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what is the live wire?
brown, 230v, provides the alternating potential difference from the mains supply
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what is the earth wire?
yellow/green, 0v, protects the wiring and for safety as it stops the appliance from becoming live
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why does touching the live wire give you an electric shock?
your body is at 0v so if you touch the live wire a large pd is produces across your body and a current flows through you, which causes a large, potentially fatal, electric shock
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why are connections between live and earth dangerous?
if the link creates a low resistance path to earth, a huge current will flow which can result in a fire
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how is energy transferred in a kettle?
kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
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how is energy transferred in a handheld fan?
energy is transferred electrically from the battery to the kinetic energy store of the fan's motor
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what is power?
the amount of energy that an appliance transfers per second
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what is an appliance's power rating?
the maximum amount of energy transferred between stores per second, telling you the maximum safe power that they can operate at
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why is a low power rating better for customers?
the lower the power rating, the less electricity an appliance uses in a given time so the cheaper it is to run
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what is the national grid?
a giant system of cables and transformers that covers the UK and connects power stations to consumers
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how does demand for electricity change throughout the day?
demand increases when people get up in the morning, come home from school/work, when it starts getting cold and dark outside, during popular TV events
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why do power stations run below their capacity?
so there's spare capacity to cope with a high demand, even if there's an unexpected shut-down of another station
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what is kept on standby incase of need?
lots of smaller power stations
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what is needed to transmit the huge amount of power needed?
high pd or high current
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what's the problem with using high current?
lots of energy is lost as the wires heat up and energy is transferred to the thermal energy store of the surroundings
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why does the national grid use a high pd?
it's much cheaper to boost the pd up really high and keep the current quite low, as increasing pd decreases current, decreasing the energy lost by heating, making it more efficient
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what do transformers do?
they step the pd up at one end for efficient transmission, and step it down again to safe levels at the other end
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what happens when insulating materials are rubbed together?
negatively charged electrons will be scraped off one and dumped on the other, leaving the materials oppositely charged
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what happens when a polythene rod rubs a duster?
electrons move from the duster to the rod
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what happens when an acetate rod rubs a duster?
electrons move from the rod to the duster
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how can you observe electrostatic charges?
suspend a rod with a known charge from a piece of string, then place an object next to it and if it's repelled it's similarly charged, if it's attracted it's oppositely charged
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what causes a spark?
as electric charge on an object builds, the pd between it and the earth increases, and if the pd gets big enough electrons can jump across the gap between the charged object and the earth
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what is an electric field?
a region created around any electrically charged object, within which a force would be exerted on other charged objects
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how do you draw an electric field?
electric field lines go from positive to negative, they're always drawn at a right angle to the surface, the closer together they are the stronger the field is
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what happens when an object is placed in the electric field of another?
it feels a force, which causes attraction or repulsion, caused by the electric fields of each charged object interacting
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how can electric fields explain sparks?
a high pd causes a strong electric field between the charged object and the earthed object, causing air to be ionised which makes it conductive, so a current flows through it
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## Other cards in this set

### Card 2

#### Front

what is potential difference?

#### Back

the driving force that pushes the charge round

### Card 3

#### Front

what is resistance?

### Card 4

#### Front

what is the relationship between resistance and current?

what is charge?