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Electricity
Electromotive Force and Internal Resistance
RP: Investigating EMF and Internal Resistance
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Created by
Harry Parker
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Cards (19)
What is the overall aim of the experiment?
Investigate the relationship between
e.m.f
and
internal resistance
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What is the independent variable in the experiment?
Resistance
,
R (Ω)
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What are the dependent variables in the experiment?
Voltage, V
(V) &
current, I
(
A
)
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What are the control variables in the experiment?
E.m.f
of the cell and
internal resistance
of the cell
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What is the resolution of the voltmeter used?
1 mV
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What is the resolution of the ammeter used?
0.1 mA
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What is the method for measuring voltage and current in the experiment?
Connect the
cell
and
resistor
in series.
Record the
voltmeter
reading with the
switch
open.
Set the
variable resistor
to maximum, close the switch, and record V and I.
Vary the resistance and record V and I for at least 8-10
readings
.
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What is the equation relating e.m.f and internal resistance?
E = I (R + r)
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How can the equation for e.m.f be simplified?
E =
IR
+
Ir
= V +
Ir
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What is the rearranged equation for voltage, V?
V =
–rI
+
E
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How does the equation for voltage compare to the equation of a straight line?
y = V, x = I,
gradient
= –r, y-intercept = E
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What should you do to analyze the results of the experiment?
Plot a graph of V against I.
Draw a line of best fit.
Measure the gradient and compare with
manufacturer's value
.
Identify the y-intercept as
e.m.f
and gradient as negative
internal resistance
.
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What are systematic errors in the experiment?
Only close the switch briefly to avoid resistance changes
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What are random errors in the experiment?
Use fairly new
cells
and wait for readings to
stabilize
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What safety considerations should be taken during the experiment?
Switch off
power supply
if
burning smell
occurs.
Keep liquids away from electrical equipment.
Monitor electrical components for overheating.
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What is the first step in determining e.m.f and internal resistance from a graph?
Plot the data on a graph of
V
against
I
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How do you calculate the gradient from the graph?
Gradient =
Δ
y
Δ
x
\frac{\Delta y}{\Delta x}
Δ
x
Δ
y
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How is the internal resistance related to the gradient of the graph?
Gradient = –r (
Ω
)
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How is the e.m.f related to the y-intercept of the graph?
Y-intercept
=
E (V)
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