Studied by 6 peopleKirchhoff's First Law
the sum of currents into a junction equals the sum of currents out of a junction
Kirchhoff's Second Law
In any circuit, the sum of the electromotive forces is equal to the sum of pds around a closed loop
What law of conservation is used for KL1?
Law of conservation of energy, which is applied to electric circuits, so law of conservation of charge
sum of emf =
sum of pds around a closed loop
Current in series circuit
Is the same everywhere
Voltage in series circuit
split according to components resistance
Voltage in a parallel circuit
voltage is the same across each closed loop
in a parallel circuit the sum of the voltages in each closed circuit loop is equal to
the total emf of the power supply
If the emfs are connected in opposite ways to each other...
you subtract them
Parallel circuits: the greater the resistance of the branch
the lower the current that goes into it, because V=IR and V stays constant for the branches
If there are several components in a loop, the sum of the pds across these components is equal to
the emf supplied by the battery
Equation for resistors in series
R=R1 + R2 + R3
Explain the equation for resistors in series
According to KL2 V=V1+V2
so IR= IR1 + IR2
I is a constant according to KL1 so R=R1+R2
Equation for resistors in parallel
1/R = 1/R1 + 1/R2
Four key equations
Q=It, E=VQ (E=VIt or W=VIt), P=VI, V=IR
terminal pd
pd measured at the terminals
What is the difference between the emf and the terminal pd called?
lost volts
electromotive force=
terminal pd + lost volts, I(R+ r)
define internal resistance
The resistance of a source of emf due to its construction, which causes a loss in energy/voltage as the charge passes through the source
lost volts =
I x r, r=internal resistance
Connecting cells in series...
increases emf but also increases internal resistance
Connecting cells in parallel..
produce same emf as just one cell but provides greater current due to the reduced internal resistance
What does a battery of negligible internal resistance mean?
emf=terminal pd
Describe an experiment to determine internal resistance
connect an ammeter and variable resistor/potentiometer and switch in series and a voltmeter in parallel around the variable resistor
with switch open, record the reading V on the voltmeter
set variable resistor to max value, close switch and record V and I
make sure to open the switch between readings to avoid running the battery down/heating the circuit
vary the resistance of the variable resistor for 8 readings and take an average for each resistance
Plot V= -rI + E (V against I)
the negative gradient is the internal resistance
Why is there a cell and a resistor labelled r in a circuit?
The resistor isn't really there, it represents cell's internal resistance
Why do car batteries and many rechargeable batteries have small internal resistances?
allows them to be recharged using higher currents without overheating or wasting a lot of energy
Why do high voltage power supplies in classrooms have a very high internal resistance?
prevents power supply from delivering a fatal electric current
emf
energy transferred by the power supply per unit charge
What is the relationship between work done, emf and charge?
W=eQ
Potential dividers
circuits that produce an output voltage as a fraction of its input voltage
Purposes of potential dividers
Split PD of power source between 2 or more components
Potentiometer symbol
box with arrow pointing to it
When using a potential divider, how to calculate what reading will show on a voltmeter? (potential divider equation)
look at what resistor the voltmeter is over, R1 and other resistor will be R2
V out = R1/R1+R2 x V in
Loading a potential divider
connecting a component in parallel to the resistor that has a voltmeter over it in a potential divider circuit
How to produce a varying V out in a potential divider?
replace fixed resistor with a variable resistor
What is a potentiometer?
a variable resistor with 3 terminals and a sliding contact
If I have a potentiometer and I slide the dial towards the left side, what happens to the output voltage on the left?
increases because resistance decreases
State 4 advantages of using a potentiometer over a potential divider
can be made very compact, uses fewer components, can be easily turned into a rotary dial, allows full range of output potential difference from 0v to V in
Describe a graph of V against I that describes how the terminal pd of a supply changes with the amount of current drawn
y intercept = emf supplied negative gradient, not very steep, linear relationship, as voltage decreases, current decreases
How will VI graph change if you double emf and have the same internal resistance?
double y-intercept, same gradient: changes y intercept in same way
How will VI graph change if you have the same emf and have half the internal resistance?
same intercept, half the gradient: changes the gradient in the same way
What does a circuit with a potential divider that can be used as a temperature sensor so that output voltage increases with increasing temperature look like?
Thermistor in circuit, but since temp and resistance are inversely proportional, put voltmeter over the other resistor
When you draw a circuit that includes internal resistance, denoted r, how should you draw it?
Draw resistor next to battery with little r above it and draw a dotted box around battery and resistor to show they are the same thing
Note
Flashcard