Chapter 10 Flashcards
What is kirchoffs first law?
Sum of the current entering a junction = total current leaving
What is the difference between emf and pd?
Energy transferred per unit charge is emf
Energy transferred from a unit of charge is pd
E=VQ
What is Kirchhoff’s 2nd law?
In any circuit the sum of emf = the sum of pd’s around a close loop
A closed loop is a possible path for current
What happens in a series circuit?
There is only 1 path for current so current is the same everywhere
There is only 1 loop so the emf is shared between components.
Emf is shared equally if all components have the same resistance. Otherwise, the one with higher resistance a greater proportion of emf
What happens in a parallel circuit?
There is more than 1 path for current to flow so current is split at junctions depending on resistance. If the path has higher resistance, less current flows through it.
Current in = current out
Each branch is a different loop so they don’t affect each other
Total PD across each branch = emf because of 2nd law. V is shared between components on that path according to resistance
What is total resistance in a series circuit and how do I prove?
The sum of all the resistances
2nd law says that pd = sum of pd across all components so vtot=v1+v2…
And ohms law says that v=ir
So IRtot=IR1+IR2…
1st law says that current going through each resistor is the same so the current basically cancels
What is total resistance in a parallel circuit?
More paths so more resistors means that there are more paths for current
In a parallel circuit Itot = I1+I2…
So v/r tot = v/r + v/r ….
And 2nd law means that that the 1/rtot = 1/r1 + 1/r2 …
Define internal resistance
Resistance of a source of emf due to electrons having to do work when they move which causes the loss of energy
What causes internal resistance?
When a current flows, work must be done by the charges as they move through a power source
Work is done for chemical reactions in a chemical cell
Also electrons collide with atoms inside the cell which causes resistance so electrons have to do work to get through.
So some energy is lost as a current flows through a power source(lost volts)
Define emf
Work done on each colour of charge by the cell
What is the formula of terminal PD
Terminal pd = emf - loss volts
Terminal pd < emf
What happens to internal resistance as current increases
As current increases, more charge is travelling per unit time so more work is done by charge(also more collisions)
As current increases, lost volts increases so terminal PD decreases
Because V=IR
Current is directly proportional to lost volts
What equation links EMF, current and resistances
EMF= I(Load resistance + internal resistance)
What happens to emf in a series circuit?
Total emf is the sum of the individual emf’s. But the more cells in series increases the internal resistance which limits the current
What happens to emf in a parallel circuit?
In parallel circuit, emf of 1 branch = emf of another branch because current is equally split between at junctions. Charge only gains emf from ONE cell so overall emf doesn’t increase
E1=E2=Etot
Internal resistance decreases so current is larger
How do you measure internal resistance?
Vary the current by changing the resistance and measuring current
Plot a graph of V again I
Remember that Emf= V + lost volts
So Emf= V + ir(internal)
Y= mx+c V=-ri + Emf
So the y intercept is the emf and the gradient x-1 is the internal resistance
How do graphs vary with different internal resistances and emfs?
If the gradient is less steep, the internal resistance is less. If the y int is higher, emf is higher
What are potential dividers used for?
To divide the PD to give any pd up to the maximum supplied by source
How do potential dividers work?
They have resistors in series with different resistances. PD is divided in ratio of the resistances. The one with higher resistance receives more PD
If you put a bulb across a resistor in parallel, the total resistance would be lower than R so Vout must be less than you calculate
What is the formula for potential dividers?
V1/V2 = R1/R2
Vout = (R2/Rall) x Vin
What is loading?
When you connect a component or circuit in parallel to R2 which lowers the overall resistance which lowers Vout
If Rload is high, Vout changes a but
Is Rload is low, Vout changes a lot
What do you do if you want to vary Vout?
Put a variable resistor instead of a resistor so increasing the resistance increases Vout
What happens if you replace the resistor with a thermistor in a potential divider circuit?
An NTC thermistor has a high resistance at low temperatures so as temperature increases, resistance decreases so Vout decreases
What happens if you replace the resistor with a LDR in a potential divider circuit?
Vout varies with light intensity. There is higher resistance in the dark so as light intensity increases, resistance decreases so Vout decreases
You can flip this function if you replace the OTHER resistor with a LDR so that as light intensity increases, Vout increases
What is a potentiometer?
Is a variable resistor with 3 terminals and a slider. Sliding the slider varies Vout between the 2 terminals
Basically as you increase the distance between the 2 terminals, resistance increases so Vout increases
See notes
