the sum of currents into a junction equals the sum of currents out of a junction
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Kirchhoff's Second Law
In any circuit, the sum of the electromotive forces is equal to the sum of pds around a closed loop
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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
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sum of emf \=
sum of pds around a closed loop
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Current in series circuit
Is the same everywhere
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Voltage in series circuit
split according to components resistance
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Voltage in a parallel circuit
voltage is the same across each closed loop
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in a parallel circuit the sum of the voltages in each closed circuit loop is equal to
the total emf of the power supply
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If the emfs are connected in opposite ways to each other...
you subtract them
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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
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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
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Equation for resistors in series
R\=R1 + R2 + R3
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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
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Equation for resistors in parallel
1/R \= 1/R1 + 1/R2
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Four key equations
Q\=It, E\=VQ (E\=VIt or W\=VIt), P\=VI, V\=IR
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terminal pd
pd measured at the terminals
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What is the difference between the emf and the terminal pd called?
lost volts
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electromotive force\=
terminal pd + lost volts, I(R+ r)
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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
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lost volts \=
I x r, r\=internal resistance
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Connecting cells in series...
increases emf but also increases internal resistance
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Connecting cells in parallel..
produce same emf as just one cell but provides greater current due to the reduced internal resistance
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What does a battery of negligible internal resistance mean?
emf\=terminal pd
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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
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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
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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
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Why do high voltage power supplies in classrooms have a very high internal resistance?
prevents power supply from delivering a fatal electric current
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emf
energy transferred by the power supply per unit charge
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What is the relationship between work done, emf and charge?
W\=eQ
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Potential dividers
circuits that produce an output voltage as a fraction of its input voltage
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Purposes of potential dividers
Split PD of power source between 2 or more components
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Potentiometer symbol
box with arrow pointing to it
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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
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Loading a potential divider
connecting a component in parallel to the resistor that has a voltmeter over it in a potential divider circuit
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How to produce a varying V out in a potential divider?
replace fixed resistor with a variable resistor
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What is a potentiometer?
a variable resistor with 3 terminals and a sliding contact
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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
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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
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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
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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
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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
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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
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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