Current Flashcards
(20 cards)
What is electric current?
Unit?
a rate of flow of electrical charge, which will only flow around a complete/closed circuit IF there is a potential difference.
Ampere (A)
What is potential difference?
The driving force that pushes the charge from one place to another.
What is resistance?
Anything in a circuit that reduces the flow of electric current.
What is charge? (And give equation)
The property of a body which experiences a force in an electric field.
One coulomb of charge = 6.25 x10^18 electrons
Q=It,
where Q is charge, I is current, t is time in seconds
Why is a thin wire used for the ‘investigating factors affecting resistance ( length )’ practical?
Its lower cross-sectional area
means there is less space for electtrons to flow through them,
making a higher resistance from the beginning,
so small changes in length will have a more signnificant impact.
Explain the change in resistance for a filmaent lamp as current increases.
As current increases, the electrical charge transfers some energy to the thermal energy store of the filament, which is designed to heat up.
Resistance increases with temperature, so:
as the current increases, the filament lamp heatts up more and resistance increases.
When current changes, what happens to the resistance of OHMIC conductors
Stays the same
Why does increasing temperature increase resistance?
Increased atomic vibrations
more frequent collisions bettween electrons and atoms,
hindering flow of current
Name two components of which resistance DOes change when current changes.
Diode and Filament Lamp
Explain the change in resistance for a diode when the current increases.
The resistance depends on direction of current.
Will let currennt flow in one direction but have a VERY high resistance if it is reversed.
Explain how a LDR works
+ examples
It changes its resistance based on the amount of light its exposed to:
- in bright light, resistance falls
- in darkness, resistance is highest
e.g
automatic night lights, outdoor lighting and burglar detectors.
Explain how a thermistor works.
+ examples
Resistance varies with temmperature (9 typically, as temp decreases, so resistance increases):
- hot condition, resistance drops
- cool conditions, resistance increases
This varies from typical resistors as thermistors use semi-conductive materials, so increasing heat increases, there are more elctrons released/ more holes so the elctrons can flow without resitance.
What is a sensing circuit?
A circuit that contains a component, like LDR or thermistor, whose resistance changes in reesponse to an environmental factor
What are sensing circuits used for?
To turn on or increase the power to components depending on the conditions they are in
Explain the sensing circuit of a thermistor
Fixed resistor + fan have same pd across them bc they r connected in parallel.
the ppd of the power supply is shared out between the thermistor and the lood made up of the fixed resistor and the fan according to the resisance - bigger resistance of a compound, the more pd iit takes
so when its hot, resistance of thermistor decreases, and it takes a smaller pf. from the poweer pupply, so the pd across the fan rises making the fan go faster.
Explain the sensing circuit of a LDR
iwhen the ldr and bulb are connected in parallel, the pd across both are high when its dark (because resistance is higher at dark). The grreater the Pd , the more energy it gets, so a bulb connected across an LDR would get brighter as the room gets darker.
How do u increase voltage
add more cells to battery/cells
What happens whenyou add resistors in parallel?
reduces the total resistance.
because:
- Both have same pd in parallel,
- the pushing force is making the current flow is the same as the soure pd for each resister added
- by addnig another loop, the current has more pathways to go through, meaning more current can flow around the circuit
- this mean total resistance is lower
How do you inestigate adding resistors in SERIES?
- find 4 identical resistors
- build a circuit (-cell-ammeter-resistor(s)-)
- make a note of the pd of the battery
- measure the current using the ammeter, and use R =V/I to find resistaance.
-repeat with more resisters each time, to calculate overall resistance - plot a grahph
How do u investigate adding resistors in series?
- find 4 identical resistors, set up (cell-ammeter-resistor-)
- measure total current and calc resistance using r = V/i, taking note of the battery’s pd.
- now add another resistor in parallel to the other resistor
- measure total current, and use the pd of battery to calculate resistance of circuit
-plot a graph after repeating with more resistors,
