Chapter 10- Electrical Circuits Flashcards
Kirchhoff’s 2nd Law
In any circuit, the sum of the electromotive forces is equal to the sum of the potential differences in a closed loop
Closed loop
A series circuit or one branch of a parallel circuit
Series circuit rules
The current is the same everywhere
The potential difference is split between the components directly proportional to the resistance
Parallel circuit rules
Current is split between the loops inversely proportional to the resistance of each
Potential difference of each branch is equal to the emf of the power supply
If one loop has a resistor on it and another one doesn’t:
All the current flows through the loop without a resistor
Resistance in series
The total resistance is equal to the sum of the resistances of each individual resistor
Resistance in parallel
The inverse of the total resistance is equal to the sum of the inverses of the resistances of each path
Terminal p.d.
The potential difference where some of the energy is “lost” as heat in the power supply - the p.d. that gets used in the rest of the circuit
Lost volts
The difference between the emf and the terminal p.d.
Internal resistance
The resistance of the power supply due to the chemicals in the cell it can be effectively drawn as a resistor in the cell
Represented by r
Using e.m.f = terminal p.d. + lost volts
ε = V + Ir ε = IR + Ir ε = I(R+r)
Relationship between e.m.f. and p.d. at low currents
ε ≈ V
Cells in series
The total e.m.f. is the sum of each
Higher internal resistance so limited current
Cells in parallel
The e.m.f. is that of one cell
Lower internal resistance so higher current
Internal resistance as a graph
V = -rI + ε
In the form y = mx + c
What is a potential divider circuit
It splits the potential difference to give any required value up to the maximum provided by the power supply
Potential divider circuit makeup
It has two resistors, R1 and R2. R2 has the p.d. required by the component and the component is placed in parallel around that resistor,
Potential divider circuit ratios
V1/V2 = R1/R2
Potential divider circuit equation
( R2 )
Vout = ( ————) x Vin
( R1 + R2 )
Loading a potential divider circuit
Putting a component with resistance parallel to R2 will decrease the resistance for that part of the circuit, lowering Vout
Effects of loading with different resistances
Loading with a large resistance has little effect on Vout but loading with a small resistance massively decreases it
Using a variable resistor as R2 in a potential divider circuit
Allows you to vary the output p.d.
Increasing the resistance increases the output p.d.
LDR in a potential divider circuit
As R2, increased light intensity decreases resistance and decreases output p.d.
As R1, increased light intensity decreases resistance and decreases p.d. around R1, increasing R2
Thermistor in a potential divider circuit
As R2, increased temperature decreases resistance and decreases output p.d.
As R1, temperature decreases resistance and decreases p.d. around R1, increasing R2
Potentiometer
Made of a sliding contact which you can move between 3 terminals. Moving it towards A increases V out until it equals V in and moving towards B decreases it until it equals 0.
Potentiometer advantages
Compact
Output p.d. ranges from 0 to Vin
Made of less components
What causes a light to light up?
The current
