Electricity Flashcards
Principles in a series circuit
- There is only one path for current to follow
- The current is the same at every point
- The total voltage is split across components
What is Kirchhoff’s first law?
The sum of all current entering a junction is the same as the sum of all current exiting a junction (parallel circuit)
- Statement of conservation of charge
What is Kirchhoff’s second law?
The sum of the EMF’s in a closed loop is equal to the sum of p.ds
- Statement of conservation of energy
(closed loop = series or each loop of a parallel)
Principles of a parallel circuit
- Current splits at junctions/branches
- The total p.d is the same across the whole circuit
How do you find the total resistance in series in parallel circuits?
Series - R(T) = R(1) + R(2)…
Parallel - R(T) = [1/R(1) + 1/R(2)] ⁻¹
What is internal resistance?
The resistance within a power source (like a battery) that opposes the flow of current, causing some energy to be dissipated within the source itself
- The lost volts per unit current
- Charge carriers will lose energy due to collisions which will dissipate as heat
How to calculate the total EMF
EMF = Terminal p.d + Vlost lost
ε = V + Ir
ε = IR + Ir
ε = I(R+r)
PAG - determine internal resistance
- Create a circuit using an ammeter, variable resistor and voltmeter in parallel to the cell/battery
- Vary the resistance of the variable resistor to change the resistance in the circuit
- Measure the current in the circuit with the ammeter
- Measure the terminal p.d using the voltmeter
- Take multiple readings and average to improve the accuracy of this experiment
- Plot a graph of V against I
What is the y-intercept and gradient equal to in a V against I graph?
y-intercept = EMF
Gradient = resistance
Rearrange and put the EMF equation into y=mx+c
What is a potential divider?
A device used in a series circuit that splits the p.d across components
What is the equation to find the output voltage and potential dividers?
Vout = Vin x (R2/R2+R1)
- R2 is always the component that is targeted
A graph showing the effect of resistance on an LDR
Resistance against light intensity
- curve starting from the top of the y-axis and plateauing before reaching the bottom of the y-axis as light intensity is increasing
- As light intensity increases, resistance decreases, and current increases
- Number of charge carriers increase with light intensity
A graph showing the effect of resistance on a thermistor
Resistance against temperature
- curve starting from the top of the y-axis and plateauing before reaching the bottom of the y-axis as temp is increasing
- As temp increases, resistance decreases and current increases
- Number of charge carriers increase with temperature
The relationship between voltage and resistance of two resistors
V1 / V2 = R1 / R2 —>
or V1 x R2 = V2 x R1 —>
Rearrange for the value your finding
E.g. R1 = (V1/V2) + R2
When having a potential divider between a resistor (before) and LDR (after) in a series circuit, what is the relationship between output p.d and light intensity?
Output P.D decreases as light intensity increases
When having a potential divider between an LDR (before) and a resistor (after) in a series circuit, what is the relationship between output p.d and light intensity?
Output P.D decreases as light intensity decreases
Uses of potential dividers in circuits
Street lights or phone screens
What is a potentiometer?
A device that varies the P.D across the circuit and allows the Vout to vary from 0-Vin
What is a sensor?
Allows you to have a resistance that changes
Uses of sensors
- Temperature sensor in a car
- Rotation sensors in a car
- Acceleration sensors in airbags
Potential dividers - What happens to the Vout when R(1) is the same size as R(2)?
Vout decreases / is less than Vin
Potential dividers - What happens to the Vout when R(1) is much smaller than R(2)?
Vout decreases by a lot / is much less than the Vin
Potential dividers - What happens to the Vout when R(1) is much larger than R(2)?
Vout stays the same / is equal to Vin making it the most efficient
Conclusion - R1 must be of a very high resistance to conserve voltage in a circuit
Define internal resistance
The resistance of a cell or battery to the flow of charge within it
EMF = terminal p.d + lost volts
³ = V + Ir
Why are thermistors and LDRs used in potential divider circuits?
To vary the output if p.d
