Electricity (2): Direct Current Circuits Flashcards
How many laws does Kirchoff have?
5
What is Kirchoffs first law (current)?
At any junction current must be conserved/ total current leaving the junction is equal to total current entering the junction
What is Kirchoffs second Law (current)?
The current entering a component is the same as the current leaving the component (components do not use up current). Therefore, components in series have the same current flowing through them
What is Kirchoffs third Law (pd)?
For two or more components in series the total pd across all of the components is equal to the sum of the potential differences across each of the components
What is Kirchoffs fourth Law (pd)?
The potential difference across components in parallel is the same
What is Kirchoffs fifth Law (pd)?
For any complete loop of a circuit the sum of the emfs round the loop is equal to the sum of the potential drops around the loop
If the charge carriers lose energy this is known as a…?
potential drop
If the charge carriers gain energy this is known as a…?
potential rise
How do you approach a Kirchoffs law question?
- Draw in the junctions and label the current around each part of the series as values or I1, I2 etc.
- Choose two loops and draw a curved arrow showing the direction you are going round them
- Label your start position
- Get equations for the loops writing values as their values or in terms using the V=IR expression
- Equate your two equations and then solve for the value you are trying to find
NB: If you are going from +ve to -ve through a cell or battery you lose energy AND if you go backwards through a component (backwards relative to the direction of current) you gain energy
What is the equation for power supplied to a component and therefore its derivatives?
P = IV = I^2R = V^2/R
What is the equation for the rate of heat transfer to the surroundings?
P = I^2R
When a component is used and it has heat losses what do these heat losses depend on?
The power supplied to the component and the rate of heat energy transfer to the surroundings
Does the energy transferred per second to a component depend on the direction of current?
no
What is the equation of the energy transferred to a component in time t?
E = Pt = I^2Rt
What is the definition of a potential divider?
A circuit consisting of two or more resistors in series connected across a voltage source used to produce a required fraction of the source potential difference
What are the three uses of potential dividers?
- To supply a pd that is fixed at any value between zero and the source pd
- To supply a variable pd
- To supply a pd that varies with a physical condition such as temperature, light or pressure
What is the complicated potential divider equation and how do you derive it?
V1 = (R1/R1+R2)E
- Where E is the emf of the circuit
- R1 and R2 are the resistors in series
- V1 is the voltage across R1
V = IR
I = V/R
I = E/R
resistance (total) = sum of resistances in series = R1 + R2 + R3 …
I = E/ R1 + R2
V1 = IR1 (REMEMBER I IN SERIES IS THE SAME!!!)
substitute out I with I = V1/R1
V1 = (R1/R1+R2)E
NB: applies to any number of resistors no matter which one
What can we tell from the complicated potential divider equation?
Voltage is split in proportion to the ratio of R1 and R2 tot eh total resistance ( multiplied by circuit emf )
What is the simple potential divider equation?
V1/V2 = R1/R2
What can we tell from the simple potential divider equation?
By varying the value of R1 and R2 we can vary the pd across each side of the potential divider
What is a variable potential divider called?
A rheostat
What can a rheostat be used in?
- A simple audio volume control to change the loudness of the sound from the loudspeaker
- To vary the brightness of a light bulb between zero and and normal/maximum (can’t do this with a variable resistor as even when the resistor was at maximum resistance a current would still flow through the blub)
What is the definition of a sensor circuit?
A circuit that produces an output voltage which changes as a result of a change of a physical variable such as temperature or light intensity
Describe the set up of a light sensor
In series:
- Cell/battery
- LDR
- Second resistor
In parallel to LDR:
- LED
Describe how a light sensor works?
- For an LED to turn on the voltage has to be above the threshold voltage
- When it is dark the resistance of the LDR is high
- Therefore, the voltage across the LDR is high and so the threshold voltage of the LED is exceeded so in turns on in the dark
same holds for the opposite way around
Describe the set up of a heat sensor?
In series:
- Cell/battery
- Thermistor
- Second resistor
In parallel to thermistor:
- Voltmeter/ any component you want to use
Describe how a heat sensor works?
- When a thermistor is hot its resistance decreases therefore, the voltage across it will decrease altering the voltage across the component
- If the thermistor is not in parallel to the component then increasing the temperature will decrease the resistance of the thermistor and therefore increase the current in the circuit
- This means the resistance of the other resistor will increase and so the voltage across it increases
What is internal resistance of a source of electricity?
The electrical energy dissipated inside a cell/battery/electricity source due to electrons colliding with atoms inside the battery opposing the the flow of charge through the source
Is any electricity source ideal?
no
What is the definition of electromotive force, E(epsilon)?
The electrical energy per unit charge produced by the source/ electrical energy transferred per unit charge that passes through the source
What is the definition of the terminal pd?
The electrical energy per unit charge delivered by a source when it is in a circuit
Why is terminal pd always less than emf?
As there is always a voltage lost due to the internal resistance of the source
What is the definition of the internal resistance of a source?
The loss of potential difference per unit current in the source when a current passes through the source
What is the equation for the the emf of a non ideal cell relating current and resistance?
E (epsilon) = IR + Ir
I (total) = E (epsilon) / (R + r)
The lost pd inside the cell is called?
Lost volts
The lost pd inside the cell is…?
Emf - terminal pd
What is the equation for the power supplied by a non ideal cell?
IE (epsilon) = I^2R + I^2r
the power supplied by the cell is equal to the power delivered to R plus the power wasted in teh cell due to internal resistance
What is the equation for the terminal voltage?
V (term) = E (epsilon) - Ir
Describe how to determine the internal resistance of a non ideal cell?
Independent Variable: changing the values of current through the circuit by varying the variable resistor setting
Dependant Variable: measuring the voltage
Equipment:
- Voltmeter
- Ammeter
- Variable Resistor
- Wires
- Cell/battery
Set up:
In series:
- Cell
- Ammeter
- Variable Resistor
In parallel to the cell:
- Voltmeter
Method:
- The voltmeter is recording the terminal pd for different values of current as you adjust the variable resistor
- Plot a graph of terminal pd against current
- As current is increased the terminal pd will decrease proportionally as lost pd increases
- The terminal pd is equal to the cell emf at 0 current as lost pd is 0V
- Using equation V (terminal) = epsilon - Ir you can see that when V (terminal) is plotted against I your y-intercept is emf and the gradient is equal to -r (-ve internal resistance)
