P4 - electric circuits Flashcards
how do you charge insulators
rub them with another insulator like a dry cloth
why does rubbing an insulator charge it
electrons are transferred from one of the materials to the other
what happens when two like charges are put near eachother
they repel
what happens when you put two unlike charges near to eachother
they attract
what is a non contact force
the force between two charged objects
what happens when a field is too strong and why
sparking happens because some electrons are pulled out of air molecules by the force of the field
what happens when you rub a polythene rod with a dry cloth (in terms of electrons)
electrons transfer from cloth to rod
rod becomes negatively charged
what happens when you rub a perspex rod with a dry cloth (in terms of electron transfer)
electrons transfer from rod to cloth
rod becomes positively charged
what is a cell used for
pushing electrons around a complete circuit
what does a switch do
enables the current in a circuit to be switched on or off
what does a bulb (indicator) do
emit light as a signal when a current passes through it
what does a diode do
allow current through in one direction only
what does a light emitting diode (LED) do
emit light when a current passes through it
what does an ammeter do
measure electric current
what does a fixed resistor do
limit current in a circuit
what does a variable resistor do
allow current to be varied
what does a fuse do
melt and ‘break’ the circuit if the current through it is higher than a certain amount
what does a heater do
transfer energy from an electric current to heat surroundings
what does a voltmeter do
measure potential difference (voltage)
what is the size of an electric current
the rate of flow of electric charge
flow of charge per second
what is the current at any point in a single closed loop
the same as the current at any other point
what is the equation for charge flow
charge flow (Q) = current (I) x time taken (t)
what is the current at any point in a series circuit
the same as any other point
what is the equation for potential difference across a component
potential difference across a component = energy transferred / charge
what is the equation for resistance
resistance = potential difference / current
what is ohms law
the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor
what does reversing the potential difference across a resistor do
reverse the current through it
in a filament lamp, is current directly proportional to the PD?
no
what happens in a filament lamp as the temperature increases
the resistance increases
why does the resistance increase as temperature increases in a filament lamp
the atoms in the metal filament vibrate more as temperature increases
so they resist passage of electrons through filament more
what happens to the resistance if you reverse the potential difference in a filament lamp
it’s the same
in a diode, is current directly proportional to PD?
no, the resistance changes as the current changes
what happens when you change a diodes direction
reverse direction resistance is a lot higher than forward resistance
what is a thermistor
a temperature dependent resistor
what happens to a thermistors resistance if it’s temperature increases
it decreases
what happens to an LDRs resistance if light intensity increases
it decreases
what is the current in each component in a series circuit
the same as every component
in a series circuit, what happens to the potential difference
it is shared between the components
what is the total PD of cells in series the sum of
the potential difference of each cell
what is the equation for total resistance in series
total resistance = R1 + R2
what happens when you add more resistors in series
the total resistance increases
current through resistors is reduced
PD is unchanged
what is the rule about current for components in parallel
total current through whole circuit is sum of currents through separate branches
what is the rule about PD for components in parallel
PD across each component is the same
in a parallel circuit, what happens to the current if the resistance increases
the current decreases
what is the equation for current through a component
current = PD/component resistance
what is the rule for resistors in parallel
total resistance of two (or more) components in parallel is less than resistance of the resistor with the least resistance
why does adding for resistors in parallel decrease the total resistance
the total current through the resistors in increased
PD across them is unchanged
