13.3 electromotive force and internal resistance Flashcards
what is the internal resistance of a source of electricity due to
opposition to the flow of change through the source
this causes electrical energy produced by the source to be dissipated inside the source when charge flows through it
what is the electromotive force (e) of a source
the electrical energy per unit charge produced by the source
electromotive force equation
e = E/Q
the pd across the terminals of a source is the
electrical energy per unit charge delivered by the source when it is in a circuit
why is the terminal pd less than the emf whenever current passes through the source
due to the internal resistance of the source
defintion for the internal resiatcne of a source
the loss of pd per unit current in the source when current passes through the source
how may internal resistance of a source be shown in a circuit diagram
with a resistor laybelled internal resistance in series with the source
how would you find the current of an emf with internal resistance r and an external resistor with resistance R
I = e/ r+R
how do you work out a cells emf
e = IR+Ir
the terminal pd + the wasted pd
what is the lost pd inside the cell (the pd across the internal resistance of the cell) equal to
the difference between the cells emf and the pd across its terminals
the lost pd is the energy per coulomb charge dissipated/ wasted inside the cell due to its internal resistance
power supplied by a cell =
ie = I^2R +I^2r
the power delivered to R + the power wasted in the cell due to its internal resistance
the power delivered to R =
I^2 R = e^2/ (R+r)^2. R
what is a load
a component that consumes electrical energy to perform work e.g to produce light
when is the maximum power delivered to a load
when the load resistance is equal to the internal resistance of the source
the load is then said to be matched to the source
how can the potential difference across the terminals of a cell when it is in a circuit be measured
connecting a high resistance voltmeter directly across the terminals of the cell
what can a current be adjusted by
a variable resistor
what does a fixed resistor such as a lamp do
limits the maximum current that can pass through the cell
what is an ammeter used ro measure
the cell current
what does a graph of terminal pd against current for a given cell show
as the terminal pd decreases the current increase
this is because lost potential difference increases as current increase
what is terminal pd
voltage measured across the terminals of a components in a circuit
sum of pd
when is terminal pd equal to the cells emf
when there is zero current
this is because the lost pd is zero with a zero current
why is the graph terminal pd against current for a given cell have a straight line with a negative gradient
by rearranging e = IR + Ir to become
IR = e - Ir
because IR represents terminal pd
V = e - Ir
comparison of V = e - Ir to y =mx+c
V is on the y axis against I on the x axis
giving a straight line with the gradient -r and a Y intercept of e
what does the change over the y axis re[present in this graph
the lost potential difference
what does the gradient of this graph represent
lost voltage / change in current = internal resistance (r)
when can internal resistance and the emf of the cell be calculated
if the terminal pd is known for 2 different values of current
current 1 =
terminal pd 1 = e - I1r
curent 2 =
terminal pd 2 = e -I2r
subtract the 2 equations to get
r = v1 - v2 / I2 - I1
therefore r can be calculated and then subbed back into the equation to find e
