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