PRELIMINARY STAGE 2018
West African Senior High School
Konongo Odumase SHS
Kadjebi Asato SHS
ROUND 5 - RIDDLE
RIDDLE
I have a coil suspended in a magnetic field.
A small current deflects it by an amount depending on the current.
Used with shunt/series resistors, I become an ammeter or voltmeter.
Who am I?
ANSWER: Moving-coil galvanometer
EXPLANATION:
A shunt resistor in parallel converts it to an ammeter.
A series (multiplier) resistor converts it to a voltmeter.
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PRACTICE QUESTIONS
1. RIDDLE:
I measure electric current.
I am connected in series.
An ideal one of me has zero resistance.
A galvanometer with a small shunt becomes me.
My scale is marked in A or mA.
I must never be connected straight across a cell.
Who am I?
ANSWER: AMMETER
EXPLANATION:
A low resistance keeps the circuit current unchanged.
2. RIDDLE:
I measure potential difference.
I am connected in parallel.
An ideal one of me has infinite resistance.
A galvanometer with a large series resistor becomes me.
My scale is marked in V.
I draw almost no current.
Who am I?
ANSWER: VOLTMETER
EXPLANATION:
A high resistance draws almost no current from the circuit.
3. RIDDLE:
I am a small resistance.
I am connected in parallel with a galvanometer.
Most of the current bypasses the coil through me.
I turn a galvanometer into an ammeter.
My value is $\dfrac{I_gG}{I - I_g}$.
My name means to push aside.
Who am I?
ANSWER: SHUNT
EXPLANATION:
The shunt carries the excess current.
4. RIDDLE:
I am a large resistance.
I am connected in series with a galvanometer.
I turn a galvanometer into a voltmeter.
My value is $\dfrac{V}{I_g} - G$.
I limit the current through the coil.
Another name for me is a series resistor.
Who am I?
ANSWER: MULTIPLIER
EXPLANATION:
It extends the voltage range of the meter.
5. RIDDLE:
I measure resistance directly.
I contain my own cell.
My scale reads backwards, with zero on the right.
I must be used with the circuit switched off.
My scale is marked in Ω.
I am set to zero by joining my leads.
Who am I?
ANSWER: OHMMETER
EXPLANATION:
It passes a known current and reads the resulting p.d. as a resistance.
6. RIDDLE:
I compare emfs without drawing current at balance.
I use a long uniform wire.
My balance length is proportional to the p.d. being measured.
I behave like an ideal voltmeter at balance.
I can find the internal resistance of a cell.
A jockey slides along my wire.
Who am I?
ANSWER: POTENTIOMETER
EXPLANATION:
At balance no current flows from the cell being tested.
7. RIDDLE:
I display how a voltage changes with time.
An electron beam draws my trace on a screen.
My time base sweeps the beam across.
I can measure the frequency of a signal.
My short name is CRO.
My Y-gain sets the volts per division.
Who am I?
ANSWER: OSCILLOSCOPE
EXPLANATION:
The deflection of the beam is proportional to the input voltage.
8. RIDDLE:
I can measure current, voltage and resistance.
A rotary switch selects my range.
Electricians carry me.
I may have a digital display.
I combine several meters in one case.
My short name is AVO meter.
Who am I?
ANSWER: MULTIMETER
EXPLANATION:
One instrument covers amperes, volts and ohms.
9. RIDDLE:
I am a variable resistor.
A sliding contact changes my resistance.
I control the current in a circuit.
I can dim a lamp.
My symbol is a resistor with an arrow.
I often have three terminals.
Who am I?
ANSWER: RHEOSTAT
EXPLANATION:
Moving the slider changes the length of wire in the circuit.