10. Current, PD and Resistance Flashcards
what is the electric current?
Rate of flow of charge
what is the equation to calculate current from time and charge?
give units
current = charge(c)/time(s)
I=q/T
what is the definition of potential difference?
(voltage)
energy gained or lost by each electron (energy transferred per unit charge)
how do you calculate potential difference from energy transferred and charge?
potential difference = energy transferred (J) /charge (C)
v= e/q
what does an ammeter do?
measure current
what does a voltmeter do and where should it be in a circuit?
- measures voltage
- should be paralell to component
what is resistance?
what determines/reduces flow of current
what happens to the current when the potential difference increases?
increases (electrons gain more energy, move faster)
what happens to the current when the resistance of a circuit increases?
decreases
what is the equation v=ir?
potential difference = current x resistance
how do you convert from mA to A?
divide by 1000
there are 1000 milliamps in one amp
what is the unit for resistance?
ohm
what affects the resistance of a wire?
just list 4 things
- material
- length
- thickness
- temperature
why does length affect resistance?
- length of wire is increased, electrons have to travel further
- chance of collisions increases
- causes resistance to increase
why does thickness affect resistance?
- increasing thickness of wire increases area that electrons can flow through
- decreases chance of collisions w metal ions
- causes resistance to decrease
why does temperature affect resistance?
- ions vibrate faster
- harder for electrons to flow
- temp increases -> resistance increases
what is ohms law?
- current is directly proportional to poterntial difference if temp is constant
- thus component that obeys ohms law has constant resistance (the gradient)
why does the resistance of a wire increase when the current increases?
- current increases -> more FREQUENT collisions of electrons w ions
- rate of energy transer to ions increases
- ions vibrate faster
- resistance of wire increases
what can a diode or LED be used to determine?
whether a current is flowing
what are the similarities between resistors and bulbs?
- reduce the current in a circuit
- have some resistance
- energy is transferred
what are the similarities between resistors and bulbs?
- reduce the current in a circuit
- have some resistance
- energy is transferred
what are the differences between resistors and bulbs?
- resistor has fixed resistance
- bulb has a changing resistance
- bulb emits light
what are the similarities between bulbs and light emitting diodes?
- emit light
- resistance isn’t constant
- energy transferred
what are the differences between bulbs and light emitting diodes?
- current can only flow in one direction
- bulb -> current can flow in either direction
what is the use of a thermistor?
sense diff temps and switch off an oven when it gets too hot
why does the resistance of a thermistor drop with temperature?
as it gets hotter -> more electrons get excited + become available for conduction
resistance drops
what are LDRS used for
sense light levels and switch on streetlamps when it gets dark
why does the resistance decrease with the increase of light intensity?
as it gets brighter, more electrons get excited and become available for conduction -> R drops
