Ammeter and VoltmeterObjectives:3.10 Explain the effect of incorporating a measuring instrument into an electric circuit.

What is the resistance of a circuit with a current intensity of 5A & a potential difference of 10V?R = V

IR = 10

5R = 2 Ω

Adding an Ammeter into a Circuit

In order to measure the current traveling through a circuit, an ammeter must be inserted into the circuit.

An ammeter is always connected inside the circuit in series.

A

In each diagram, is the ammeter hooked up correctly to these circuits?

Connecting An Ammeter

A

A

A

A

A A

5

3 10

In a series circuit an ammeter can be placed anywhere along the wire and it will have the same current intensity reading.

Adding An Ammeter In A Series Circuit

SERIES PARALLEL

Current (I) SAME DIFFERENT

Potential Difference (V)

DIFFERENT SAME

Resistance (R) Req = R1 + R2 + R3 + …

1 2 3

1 1 1 1= + +

Req R R R

A1

A4

A2

A3

A1 = 3 amps

A2 = 3 amps

A3 = 3 amps

A4 = 3 amps

IT =3 amps

Adding an Ammeter into a Parallel Circuit

Ammeters placed in a parallel circuit can have different

readings.

SERIES PARALLEL

Current (I) SAME DIFFERENT

Potential Difference (V)

DIFFERENT SAME

Resistance (R)

Req = R1 + R2 + R3 + …

1 2 3

1 1 1 1= + +

Req R R R

Recall in parallel, current is different.

IT = 8 A

AI1 = 6A

I2 = 2 AA A

A

IT = 8 A

IT = 12A

A4

I1 = 4A

I2 = 8 A

A1 A3

A2

What is the reading on the ammeters?

A1 = 12A A2 = 4A A3 = 8A A4 = 12A

Req

A4

R1

R2

A1

A3

A2

Which ammeter correctly measures the current flowing through R1?

Ammeter #2 measures the current flowing through R1

Which ammeter correctly measures the current flowing through R2?

Ammeter #3 measures the current flowing through R2

Which ammeter correctly measures the total current?

Ammeter #1 & 4 measures the total current

IT = 14A

I1 = 9A

I2 = 5 A

A1

What is the reading on the ammeter (A1)?

Recall IT = I1 + I2

So 14A = 9A + I2

I2 = 5A

A1 must read 5 amps

A3

If the reading on ammeter 2 (A2) is 10 amps & the reading on ammeter 3 (A3) is 4 amps, what is the

reading on ammeter 1 (A1)?

Recall IT = I1 + I2

So IT = 10A + 4A

IT = 14A

A1 must read 14 amps

A1

A2

I1 = 10 A

I2 = 4 AIT = 14 A

Adding a Voltmeter to a Circuit

In order to measure the voltage or potential difference traveling through a circuit, a voltmeter must be inserted into the circuit.

A voltmeter is always connected outside the circuit in parallel.

V

In each diagram, is the voltmeter hooked up correctly to these circuits?

Connecting A Voltmeter

VV

V

V

V

V

In a series circuit a voltmeter does NOT read the same throughout the entire circuit.

Voltmeter In A Series Circuit

SERIES PARALLEL

Current (I) SAME DIFFERENT

Potential Difference (V)

DIFFERENT SAME

Resistance (R) Req = R1 + R2 + R3 + …

1 2 3

1 1 1 1= + +

Req R R R

V1

Notice….Voltmeter 1 = Voltmeter 2 + Voltmeter 3

+ Voltmeter 4

VT =11 VV3

V4

V =6 V

V =2 V

V2V =3 V

For this series circuit, what is the reading on the voltmeter given the following information?

Note…. VT = V1 + V2 +V3

30 = V1 + 15 + 10

V1 = 5 V

VT =30 VV2 = 15

V3 =10 V

V

V1 =?The reading on the

voltmeter is 5 V

For this series circuit, what is the reading on the voltmeter given the following information?

Note…. VT = V1 + V2 +V3

VT = 10 + 5 + 3

V1 = 18 V

VT = ?V2 = 5 V

V3 =3 V

V

V1 =10 VThe reading on the

voltmeter is 18 V

Adding a Voltmeter into a Parallel Circuit

Voltmeters placed in a parallel circuit will have the same

readings.

SERIES PARALLEL

Current (I) SAME DIFFERENT

Potential Difference (V)

DIFFERENT SAME

Resistance (R)

Req = R1 + R2 + R3 + …

1 2 3

1 1 1 1= + +

Req R R R

Recall in parallel, potential difference is the same.

VT = 8V

V1 = 8V

V2 = 8VV

V

V

VT = 12V

V1

V2

What is the reading on the voltmeters?

V1 = 12V

V2 = 12V

V3 = 12V V3

Problem Solving Using Ammeters and Voltmeters

It is important to be able to use all of

the information discussed in this section and apply it to

solving circuits.

Let’s try some problems….

IT =

I2 =

I3 =

I1 =VT =

V2 =

V1 =

V3 =

R1 = 12 Ω

R2 = 6 Ω

R3 = 2 Ω

Req = 20 Ω

2 A

2 A

2 A12 V

SERIES PARALLEL

Current (I) SAME DIFFERENT

Potential Difference (V)

DIFFERENT SAME

Resistance (R) Req = R1 + R2 + R3 + …

1 2 3

1 1 1 1= + +

Req R R R

1. What is the voltage going through V2 if the reading on the ammeter is 2 amps?

A

2 A

2. What is the voltage going through VT if the reading on the ammeter is 2 amps?

R V

1 I

6 V

1 2

V=12V

R V

1 I

20 V

1 2

V=40V

If the voltmeter reads 20V & the ammeter reads 1A, solve for the equivalent resistance, Req?

V

R1 = 5Ω

A

VT = 20V

I2 = 1A

V1 = 20V

V2 = 20V

I

V

1

R

2

22

I

V

1

R

1

20

1

R2

R2 = ?

21

11

Re

1

RRq

5

1

20

1

Re

1

q

1

25.0

Re

1

q

Req = 4Ω

20Ω

KEY POINTS

Voltmeter – always placed outside of circuit attached in parallel

Ammeter – is always placed inside circuit attached in series

Activities

Worksheet # 14

References

Student Study Guide – Physical Science 416/436 - MEQ

Science Quest – Grenier, Daigle, Rheaume – 1998, Cheneliere

Google Images Animation Factory

Alternative Educational Experiences through e-Learning