Module 4 EMF and Internal Resistance Flashcards
What is internal resistance?
the resistance within power supplies (or wires within a circuit)
What is the effect of internal resistance?
Reduces PD available to circuit
What causes internal resistance?
work being done by the charges as the move through the power source or wires
What is terminal PD?
PD available to the external circuit
When does internal resistance not apply?
when there is no current being drawn
What is lost volts?
PD lost in the power source due to internal resistance
What is the symbol for internal resistance?
r (lower case r)
How can lost volts be calculated?
I x r (current x internal resistance)
What happens with increasing internal resistance?
terminal p.d drops
How to draw a battery with internal resistance?
What is ε?
EMF of the battery
What is V?
terminal PD in a circuit
What is the EMF equation?
ε = V + Ir
ε is EMF
V is terminal PD
I is current
r is internal resistance
Factorise ε=V+Ir
V = IR where R is the resistance of the circuit
ε = IR + Ir (current is same through circuit and power source)
ε=I(R+r)
What is the effect of connecting identical cells in series
increases available emf (add EMFs)
also increases internal resistance (add in series),
therefore limiting the current that can be produced (less current)
What is the effect of connecting identical cells in parallel
Produce the same emf as one cell
much smaller internal resistance (adding resistors in parallel decreases resistance)
Provides a greater current
Describe (or draw) the circuit used to find the internal resistance of a battery
A battery (with internal resistance) in series with a variable resistor and an ammeter, with a voltmeter in parallel with the battery
Describe the measurements which would be taken to find the internal resistance of a battery
Using a voltmeter, measure the pd across the battery whilst changing current using a variable resistor
Use the ammeter in series to measure the current
Use this information to plot a graph of V on the y-axis and I on the x
gradient = -r
Explain, by using an internal resistance equation, how using a graph of V terminal PD on the Y-axis and I on the x, we can find the internal resistance of a battery and it’s EMF
rearrange ε=V+Ir to match y=mx+c
V=-rI + ε
We can see that the graph will give us a gradient of -r (r is positive of this) and the y intercept is the EMF of the source
What is the effect on terminal PD and lost volts of increasing the current?
As the current through the cell/battery/power source increases the terminal pd decreases
Meaning lost volts increases with current
When current is zero the terminal pd=EMF of cell. Lost volts = 0
When is the power output of a variable resistor highest for a circuit with internal resistance?
when R=r
R resistance of whole circuit
r internal resistance of power supply
