Electrical circuits Flashcards
what is kirchoffs second law
in any circuit the sum of the electromagnetic forces is equal to the sum of the pds around a closed loop
total energy transferred to the charges in a circuit is always equal to the total energy transferred from the charges as they move around the circuit
what is a closed loop
a single possible path for the current
what is a series curcuit
only has one path for the current
a single loop from one terminal to another
what is the current in a series circuit
the same is every position
what is the emf in a series circuit
emf is shared between the components
the sum of pds across the components is always equal to the emf
how do you share the emf between components in a series circuit
components with same resistance emf is shared equally
components have different resistance components take different proportions of the emf
what do you do when there are two sources of emf
add up the emf from both the sources before sharing
what happens if the sources of emf are connected with opposing polarities
emf is taken away and not added together
what is a parallel circuit
provides more than one possible path for the charges
what is the current like in a parallel circuit
how much charge flows down each path depends on the resistance of the path
current into each junction equals the current out of each junction - kirchoffs first law
how do you determine how much current goes down each path
greater the resistance the lower the current
once branch has half the resistance will have twice the current flowing through it, two thirds of total resistance
what is the emf and pd like in a parallel circuit
each branch of parallel circuit forms its own loop
around each loop the emf equals the pd - total pd across each branch equal to the emf of the power supply
once branch contains several components then the sum of the pds across these components must equal the emf
what happens when a resistor is connected in series
more resistors added the greater the resistance becomes
total resistance of a number of resistors connected in series is equal to the sum of the individual resistance
R=R1+R2+…
how do you derive the relationship of resistors in series R=R1+R2+…
consider kirchoffs two laws
kirchoffs second law - total pd is equal to the sum of the pds across each resistor V=V1+V2+…
because V=IR can be rewritten as IR=IR1+IR2+…
kirchoffs first law current through each resistor is the same so I is a constant so R=R1+R2+…
what are resistors like in parallel
total resistance drops - additional resistor provides another path current increasing the cross sectional area and lowering the resistance
how do you derive the expression for resistance in parallel
kirchoffs first law - total current is equal to the sum of the current in each resistor I=I1+I2+…
kirchoffs second law - pds across each resistor is constant and must be equal to V, divide first equation by V
I/V=I1/V+I2/V+…
V=IR so I/V=1/R
1/R=1/R1+1/R2+…
what are four key electrical relationships
I=Q/t
V=W/Q
P=IV
V=IR
what is internal resistance
the resistance of passing a current through the power source
what is the terminal pd
the pd across a power source when a current flows
what is lost volts
the potential difference across the internal resistor of a source of emf
emf-terminal pd
what is emf
maximum pd across a power source
what is the relationship between the emf, terminal pd and lost volts
electromagnetic force = terminal pd + lost volts
how does changing the current affect the lost volts and terminal pd
increasing current means more charges travel through the cell each second
more work done by the charges
increases the lost volts
lowers the terminal pd
what is the equation linking lost volts, current and internal resistance
lost volts = I x r
r is internal resistance in ohms
if r remains fixed the current in the power supply is directly proportional to the lost volts
