4.3 Electrical Circuits Flashcards
What is Kirchhoff’s second law?
For any path (loop)of a circuit, the sum of all of the potential differences must equal to total emf of the circuit
How to calculate total resistance in a series circuit?
Add the individual resistances
How to find total resistance in parallel circuit?
Inverse sum of the inverses of the individual resistances
True or false: adding a resistor to a circuit in parallel will always reduce the total resistance
True
Total resistance drops the more paths there are for current to take
How can you measure current in a circuit?
An ammeter connected in series with the component
How do you measure potential difference of a component?
Using a voltmeter, connected in parallel over the component being measured
True or false: in parallel circuits, the total voltage in each loop is the same
True
V(total)=V1=V2=V3
How do you calculate the total voltage in series circuit circuits?
V(total)=V1+V2+V3
How does the current vary between each component of a series circuit?
The current through all of the components is the same so the current does not vary
Is the current in each component of a parallel circuit the same?
No, each branch of a parallel circuit has different currents.
Because the voltage across each branch is the same- the higher the resistance of a branch, the lower the current flow through that branch will be
If you connect two cells in series, with positive to positive, what would be the total emf?
E(total)=E(1)+E(2)
In a series circuit, if two cells are connected negative to negative, would their emf add up or cancel out?
They will cancel out. The total emf would be E(total)=E(1)-E(2)
True or false: the emf of a source and its terminal voltage will always be the same
False
Not all of the energy given to the charges in the source (emf) makes it out of the cell
Define internal resistance
Internal resistance is the resistance within a power source due to the materials and components which make it up
What are ‘lost volts’?
‘Lost volts’ refer to the difference in voltage between that supplied by the source and the amount available to the circuit
They are ‘lost’ due to the internal resistance of the source
Give an equation which relates emf to load and internal resistance
E=V+v
E=IR+Ir
E=I(R+r)
E=emf I=current R=load resistance r=internal resistance
Describe an experiment to determine the internal resistance of a source
Set up a circuit with a source, ammeter and variable resistor in series, and a voltmeter in parallel
Vary the resistance and measure the voltage and current (take several readings)
Plot a V-I graph
V=-rl+E corresponds to y=mx+c
The internal resistance=-m(-gradient)
What is the purpose of a potential divider?
To provide variable potential difference, or to provide a constant specific potential difference
How does the voltage across a component in a potential divider correspond to its resistance?
The proportion of the total voltage which is dropped across the component is equal to the proprtion of the circuit resistance which it contributes
V/Vt=R/Rt
How can a potential divider circuit be used as a sensing circuit? (eg. to switch on a fan when it gets warm)
Put a resistor that varies with the desired conditions (eg. a thermistor) in the potential divider. Insert the responding load (eg. fan) over either the variable or fixed resistor depending on how the circuit should work
eg. in this case as temperature increases, the resistance of the thermistor decreases so the voltage across it will decrease and the voltage across the other resistor will increase. Therefore, put the fan in parallel with the fixed resistor and as temperature increases it will turn on
Where should a bulb go to turn this circuit into a light-sensing circuit which switches the light on when it’s dark?
It should be added in parallel with the LDR
What equation can be used to calculate the voltage out of a potential divider in terms of the input voltage and resistances in the circuit?
Vout=Vin x (Rout/Rtotal)
