Download Voltmeter, Ammeter and Resistance

Grade 9 Science – Unit 2
Voltmeter, Ammeter and Resistance
A VOLTMETER measures the voltage drop between two points of an
electric circuit. To make the measurement, the voltmeter must be
connected in PARALLEL.
An AMMETER measures the amount of current IN an
electrical circuit. Since the measurement is “in” the circuit,
the ammeter is connected in SERIES.
V = Voltmeter
A = Ammeter
Laboratory
We can measure electric current and the voltage drop of a circuit
using an ammeter and a voltmeter.
Question – How can you accurately measure voltage drop and electric
current in series and parallel circuits?
Hypothesis ---- What is your idea?
Materials
? Three light bulbs
? Three light bulb holders
? One six-volt battery
? One voltmeter
? One ammeter
? One switch
? 12 connecting wires with alligator clips
Setup
? Draw this table in your lab report for your observations.
Number of
Lights
1
2
3
1
2
3
Series
or
Parallel Circuit
Voltage
across the
battery
(V)
Voltage
across the
light
(V)
Current
(A)
?
Draw the following five circuits
1. A circuit that contains a switch, one light, an ammeter and a voltmeter.
2. A circuit that contains a switch, two lights in parallel, an ammeter and a
voltmeter.
3. A circuit that contains a switch, three lights in parallel, an ammeter and a
voltmeter.
4. A circuit that contains a switch, two lights in series, an ammeter and a
voltmeter.
5. A circuit that contains a switch, three lights in series, an ammeter and a
voltmeter.
?
?
?
?
?
?
Show your drawings to the teacher
Build Circuit 1
Show your circuit to your teacher
Close the switch so the circuit is complete
Record the readings on the voltmeter and the ammeter
Connect the voltmeter ACROSS THE BATTERY, close the switch and record the
voltmeter reading.
Repeat these steps for Circuit 2 and Circuit 3
Remove one bulb from its socket and record your observations
Repeat this step by removing another bulb from its socket
?
?
?
?
?
?
?
?
?
?
?
?
Build Circuit 4
Show your circuit to your teacher
Close the switch so the circuit is complete
Record the readings on the voltmeter and the ammeter
Connect the voltmeter ACROSS THE BATTERY, close the switch and record the
voltmeter reading.
Repeat these steps for Circuit 5
Remove one bulb from its socket and record your observations
Replace the bulb
Remove a different bulb from its socket and record your observations
Questions
1. How does the voltage drop across the power source compare with the
voltage drop across the light bulbs?
2. What happens to the brightness of the light from the bulbs as a new bulb
is added in each circuit?
3. What happened when you unscrewed one of the light bulbs?
4. How many paths for current flow does each circuit have?
5. Is electric current shared in the series circuit? Explain your answer using
your observations.
Resistance
When electricity meets resistance, the electrical energy is changed into one of FOUR TYPES OF
ENERGY:
1. Heat energy
2. Light energy
3. Sound energy
4. Mechanical energy (motion)
Conductors
A conductor is a substance that carries electrical energy without much resistance. In other
words, electrons move EASILY through a conductor and lose little energy.
? Copper is a good conductor
There is a relationship between CURRENT and VOLTAGE DROP. It is called OHM’s LAW.
OHM’s LAW
The voltage drop between two points on a conductor is proportional
to
the electric current flowing through the conductor.
Voltage Drop = Current X Resistance (V + I * R)
This simple law allows us to calculate the resistance of the load when designing electrical circuits.
Sample problems
1. What is the VOLTAGE DROP across the filament of a 100 watt light bulb if the resistance
of the filament is 150 ? and a current of 0.8 A is flowing through it?
I = 80 A
R = 150 ?
V=?
Formula V = I * R
V = 80 X 150
V = 120 V
The voltage drop is 120 V
Resistors slow electrical current.
2.
You have a circuits. Circuit 1 has a power source (V = 3 V) and a resistance of 5 ? .
What current is required?
RECALL V = I * R
R=5?
V=3V
I=?
I = V/ R
I = 3 / 5 = 0.6 A