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Voltage
Investigation 3
Measuring Voltage
Part 1
Volts
 What makes electricity move through a
circuit?
 Electricity is a flow of electrons. The flow of
electrons is called a current. Electrons move
when a force is applied to them. The force
moving the electrons is called voltage.
When you hear someone mention the
voltage of an electric source, they are
talking about the amount of push available
to move current through a circuit.
Volts
 Voltage in a circuit is sometimes
compared to pressure in a water system.
The greater the pressure, the more work
you get out of the system.
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Voltmeter
 Turn the rotary
switch to the 20 V
setting.
 At this setting, the
voltmeter can
measure voltage
between 0 and 20
V.
Voltmeter
 Set up a circuit as shown and close
the switch.
 Voltage is ALWAYS measured with
current flowing through the circuit.
 Touch the 2 probes to the 2 sides of a
component to measure voltage drop.
The red probe goes on the positive
side of the component.
 Reminder: Do not move the switch on
the meter to measure resistance while
you are working with electrified
circuits!
Voltage drop
 The difference in the amount of voltage
on the 2 sides of a component is what
voltage measures. The voltmeter
measures the amount of voltage “used”
by a component.
 The amount of voltage used by a
component is called a voltage drop.
 We will always refer to a voltmeter
reading as a volt drop.
Voltmeter Inquiry – 8 minutes
Voltage drop
across the poles of your battery
across a lamp connected directly to a battery
across a lamp connected directly to a battery
a lamp is in series with a 75 Ω resistor
a lamp is in series with a 330 Ω resistor
across one lamp
across two lamps in series
across two lamps in parallel
across three lamps
across three lamps in parallel
How to Record Voltage
 Draw a schematic of your circuit.
 Measure the voltage drop across
a component.
 Write the voltage drop in a circle
near the component symbol on
the schematic.
 Draw 2 lines from the sides of the
circle to the schematic showing
where you placed the 2 probes
OR write the voltage on a short
line next to the component.
Recording Voltage Drops
 Lab book – p.11
Measuring Voltage
Drops in 4 Circuits.
 Build the circuits.
 Measure the
voltage drops.
 Use your lab books
to record voltage
drop.
Share Discoveries
Summary
 Batteries have the highest voltage drop.
 The lamp drops more voltage when it is
bright.
 The switch doesn’t drop any voltage
when it is closed, but it drops a lot of
voltage when it is opened.
Notes
 Voltage is what moves current through a circuit.
 Voltage is like force – a push or a pull. When the
electricity is pushed through some components,
like resistors and lamps, the amount of available
push is reduced. We can measure how much the
available push is reduced with a voltmeter.
 The difference in the amount of voltage on the 2
sides of a component is what he voltmeter
measures. We call that reduction in voltage a volt
drop.
Clean up
 Turn in lab book – Measuring voltage
Drops in Four Circuits.
 Secure battery leads
 Return components to the bag and
inventory.
 Return all materials to the materials
station.
Resistance/Voltage
Relationship
Part 2
Quick Write
On your Study Guide Sheet Part 2
How do you explain the voltage
drop across a lamp in a circuit?
Share
 Share with the class.
Lab book p. 7 – Revisit Resistor
Investigation
 The greater the resistance in series
with a lamp, the dimmer the lamp
glows.
 Repeat the series of investigations
with the resistors (75, 150, 330 and
1000 Ω), but this time measure the
voltage drop across the lamp and
resistor in each circuit. ADD A
COLUMN
 Use lab book – Resistor/Voltage
Investigation (p. 12) to record your
data.
 Complete the task in 15 minutes.
How can you make a resistor
with 0 Ω of resistance?
 Use the copper wire
Observations
 The larger the resistor, the greater the
voltage drop.
 The sum of the voltage drops in a circuit
equals the voltage at the source.
Lab book p. 12

Is there a pattern in the amount of voltage
drop across the resistor?

The greater the resistance, the greater the
voltage drop across the resistor.

Is there a pattern in the amount of voltage
drop across the lamp?

The greater the resistance, the less the
voltage drop across the lamp.

How do these voltage drop patterns relate to
the size of the resistor in the circuit with the
lamp?

As resistance increases, the voltage across
the resistor increases, and the voltage across
the lamp decreases.

Add up the lamp and resistor voltages for
each resistor. Is there a pattern?

The sum of the voltage drops in the circuit
equals the available voltage across the
terminals of the battery.
Clean up
 Secure battery leads
 Return components to the bag and
inventory.
 Return all materials to the materials
station.
Resistors “Sharing” Voltage
Part 3
Review
 The greater the resistance, the greater
the voltage drop across the resistor.
 The greater the resistance, the less the
voltage drop across the lamp.
 As resistance increases, the voltage
across the resistor increases, and the
voltage across the lamp decreases.
 The sum of the voltage drops in the
circuit equals the available voltage
across the terminals of the battery.
Task
 Today’s circuits will only contain resistors
and wires as components.
 Your task is to find out what share of the
voltage drop each resistor is responsible
for.
Procedure
 Set up a series circuit with 3 1000 Ω
resistors only.
 Measure and record the voltage drop
across each resistor.
 Record and organize your results.
 You have 8 minutes to complete the
task.
Discuss
 What is the total resistance in the 3
resistor circuit?
 3000 Ω
 This means that 3000 Ω equal 100 % of
the resistance in the circuit.
 What percentage of the resistance is
contributed to one of the resistors? Ω
1000 Ω X 100 = 33.3%
3000 Ω
Discuss
 What is the total voltage available in the
circuit?
 About 8 volts.
 Then how much voltage is dropped by
resistor 1?
 About 2.7 volts
 What is the percentage of voltage
dropped by resistor 1?
2.7 V X 100 = 33.3%
8V
Lab book – Percentage of
Resistance and Voltage
 Calculate the
percentages
using your data.
Outcomes - Notes
 The sum of the percentages of resistance
imposed by the individual resistors equals
100%.
 The sum of the percentages of voltage
drop across the resistors equals 100%.
 The percentage of resistance imposed
by a resistor is equal to the percentage
of the voltage it drops.
Assignment - Terms
Terminals (positive
and negative
Insulator

Open circuit
Schematics

Closed circuit
Resistors

Current
Ohms

Direct current
Voltage

Series circuit
Multimeter

Parallel circuit
Potentiometer

Conductor
Voltage drop

Short circuit
Assignment
 Resource Book – Read The Three Great
Truths of Electricity.
Break Point
Lab book p. 14
Investigate with 6 Resistors

Put all 6 resistors into one series
circuit (75 Ω, 150 Ω, 330 Ω, and
three 1000 Ω.)

Record the total resistance.

Calculate the percentage of
resistance contributed by each
resistor.

Measure and record the voltage
drops across each resistor.

Calculate the percentage of
voltage drop contributed by each
resistor.

Complete the data chart.
What is the relationship
between the percentage of
resistance imposed by a
component and the
percentage of voltage drop
associated with that
component?
The percentage of resistance imposed by a
component is equal to the percentage of the
voltage dropped by that component.
Great Truths of Circuitry
#1 The sum of the component voltage in a series circuit is equal
to the voltage of the source.
#2 The greater the resistance of a resistor, the greater the
voltage it drops. Larger resistors cause large voltage drops; small
resistors cause small voltage drops.
#3 In a series circuit, the percentage of resistance contributed by
a component is equal to the percentage of voltage dropped by
that component.
Create a poster for one of the
Great Truths of Circuitry
Clean up
 Secure battery leads
 Return components to the bag and
inventory.
 Return all materials to the materials
station.
Series Circuit Exercises
Part 4
Response Sheet –
Lab book p. 15 - Voltage

Read the text and
respond to the
ideas presented.

Be sure to use
complete
sentences.

Turn in your
response sheet
when finished.
Lab book - How Do Resistors
Divide Voltage? p. 17
Lab book - How Do Resistors
Divide Voltage?
 Work on this sheet with your team.
How Do Resistors Divide
Voltage?
How Do Resistors Divide
Voltage?
Voltage Response Sheet
Review
 The first Great Truth is about voltage, not
resistance; it states that the sum of the
voltage drops equals the voltage of the
source.
 The second great truth says that the
larger the resistance a component
imposes in a series circuit, the greater the
voltage drop.
Terms Crossword
Midsummative Exam 3