SACE Stage 1 Physics B Experiment
DC Motor Efficiency
Electric Motors consume roughly half of all electricity generated in 1st world economies. This means that their
efficiency is of crucial importance to minimise waste.
Aim: To accurately measure the efficiency of an electric D.C. motor.
Method: Before the advent of efficient transformers, voltage/current
conversion could only be achieved through the use of
motor/generator sets. They are still used in certain situations where
transformers are not appropriate such as high power AC – DC
conversion.
Ai
2-12 V D.C.
level
Ao
Vi
M
G
Vo
5Ω
Build the circuit shown. When you are ready to begin ask to have your circuit checked
/2
Why is it important to place the voltmeter across the motor and generator as against across power supply and
the resistor?
/2
Explain what efficiency is and how it is calculated from the voltage and current readings.
/2
Explain why a column showing theoretical efficiency is necessary and how it is calculated.
/2
Measure all quantities as accurately as possible for six different input voltages.
Results:
Vi (volts)
Ii (amps)
Pi (watts)
Vo (volts)
Io (amps)
Po (watts)
Efficiency
Theoretical
Efficiency
/2
Graph the results of your experiment showing input power vs voltage, output power vs voltage and theoretical
efficiency vs voltage on the grid below. On the left vertical axis plot power. On the right vertical axis plot
theoretical efficiency.
/2
Conclusions & Questions:
What was the maximum theoretical efficiency achieved by the motor and at which voltage did this occur?/1
Why wasn’t the motor 100% efficient? Identify three causes of energy loss.
1.
2.
3.
/3
Identify two unavoidable inaccuracies of the experimental setup.
1.
2.
/2
Where and how was the useful output energy dissipated?
/2
Knowledge and
Understanding
Application
Analysis and Evaluation
Investigation
Stage 1 Physics
PERFORMANCE STANDARDS FOR ASSESSMENT
A
B
C
D
E

Designs and implements a
logical, coherent, and detailed plan for a
physics investigation.

Critically and logically selects
and consistently and appropriately
acknowledges information about physics
and issues in physics from a range of
sources.

Designs and implements a
well-considered and clear plan for a
physics investigation.

Logically selects and
appropriately acknowledges
information about physics and issues in
physics from different sources.

Designs and implements a
considered and generally clear plan for
a physics investigation.

Selects with some focus, and
mostly appropriately acknowledges,
information about physics and issues in
physics.

Prepares the outline of a
physics investigation.

Identifies a simple
procedure for a physics investigation.

Selects and may partly
acknowledge one or more sources of
information about physics or an issue in
physics.

Identifies a source of
information about physics or an issue
in physics.

Manipulates apparatus and
technological tools carefully and highly
effectively using well-organised safe and
ethical investigation procedures.

Obtains, records, and displays
findings of investigations using
appropriate conventions and formats
accurately and highly effectively.

Manipulates apparatus and
technological tools carefully and mostly
effectively using organised safe and
ethical investigation procedures.

Obtains, records, and
displays findings of investigations using
appropriate conventions and formats
mostly accurately and effectively.

Uses clear and wellorganised analytical skills to examine
connections between data, concepts,
and issues in physics.

Evaluates procedures and
suggests some appropriate
improvements.

Uses apparatus and
technological tools with inconsistent care
and effectiveness and attempts to follow
safe and ethical investigation procedures.

Obtains, records, and displays
findings of investigations using
conventions and formats inconsistently,
with occasional accuracy and
effectiveness.

Describes basic connections
between some data, concepts, and issues
in physics.

Attempts to use apparatus
and technological tools with limited
effectiveness or attention to safe or
ethical investigation procedures.

Attempts to record and
display some descriptive information
about an investigation, with limited
accuracy or effectiveness.

Uses perceptive and thorough
analytical skills to examine connections
between data, concepts, and issues in
physics.

Logically evaluates procedures
and suggests a range of appropriate
improvements.

Systematically analyses and
evaluates data and other evidence to
formulate logical and highly relevant
conclusions.

Applies physics concepts and
evidence from investigations to suggest
solutions to complex problems in new and
familiar contexts.

Uses appropriate physics
terms, conventions, formulae, and
equations highly effectively.

Uses mostly logical
approaches to analyse and evaluate
data and other evidence to formulate
consistent and relevant conclusions.

Applies physics concepts and
evidence from investigations to suggest
solutions to problems in new and
familiar contexts.

Uses appropriate physics
terms, conventions, formulae, and
equations effectively.

Manipulates apparatus and
technological tools generally carefully
and effectively using safe and ethical
investigation procedures.

Obtains, records, and
displays findings of investigations using
generally appropriate conventions and
formats with some errors but generally
accurately and effectively.

Uses generally organised
analytical skills to examine connections
between data, concepts, and issues in
physics.

Evaluates some procedures
in physics and suggests some
improvements that are generally
appropriate.

Analyses and evaluates data
and other evidence to formulate simple
and generally relevant conclusions.

Demonstrates initiative in
applying constructive and focused
approaches to individual and collaborative
work skills.

Consistently demonstrates a
deep and broad knowledge and
understanding of a range of physics
concepts.

Uses knowledge of physics
perceptively and logically to understand
and explain contemporary applications.

Applies mostly constructive
and focused approaches to individual
and collaborative work skills.

Applies a variety of forms to
communicate knowledge and
understanding of physics coherently and
highly effectively.

Applies a variety of forms to
communicate knowledge and
understanding of physics coherently
and effectively.

Demonstrates some depth
and breadth of knowledge and
understanding of a range of physics
concepts.

Uses knowledge of physics
logically to understand and explain
contemporary applications.

Applies physics concepts and
evidence from investigations to suggest
some solutions to basic problems in
new or familiar contexts.

Uses generally appropriate
physics terms, conventions, formulae,
and equations with some general
effectiveness.

Applies generally
constructive approaches to individual
and collaborative work skills.

Demonstrates knowledge
and understanding of a general range of
physics concepts.

Uses knowledge of physics
with some logic to understand and
explain one or more contemporary
applications.

Applies a variety of forms to
communicate knowledge and
understanding of physics with some
general effectiveness.

For some procedures,
identifies improvements that may be
made.

Acknowledges that
connections exist between data,
concepts, and/or social issues in
physics.

Acknowledges the need
for improvements in one or more
procedures.

Attempts to extract meaning
from data and other observations and to
formulate a simple conclusion that may
be relevant.

Applies some evidence to
describe some basic problems and
identify one or more simple solutions, in
familiar contexts.

Attempts to use some physics
terms, conventions, formulae, and
equations that may be appropriate.

Attempts to organise
some limited data or observations.

Attempts individual work
inconsistently, and contributes
superficially to aspects of collaborative
work.

Demonstrates some basic
knowledge and partial understanding of
physics concepts.

Shows emerging skills in
individual and collaborative work.

Identifies and explains some
physics information that is relevant to one
or more contemporary applications.

Shows an emerging
understanding that some physics
information is relevant to
contemporary applications.

Attempts to communicate
information about physics.

Communicates basic
information to others using one or more
forms.

Identifies a basic problem
and attempts to identify a solution in
a familiar context.

Uses some physics terms
or formulae.

Demonstrates some
limited recognition and awareness of
physics concepts.