12.4

Ohm’s Law / Energy and Power / Electric Meters

Within a battery, a chemical reaction occurs that transfers electrons

from one terminal to another terminal.

This potential difference across the terminals is called the voltage.

Voltage produces a flow of charge, or current, within a conductor.

The flow is restrained by the resistance it encounters.

The rate at which energy is transferred by electric current is power

Voltage

Ohm’s Law

The relationship among voltage, current, and resistance is called

Ohm’s Law.

Ohm’s Law states:

Current in a circuit is directly proportional to the voltage across

the circuit, and is inversely proportional to the resistance of the circuit.

Voltage

(Volts)

Current

(Amps)

Resistance

(Ohms Ω)

Ohm’s Law Formula

Example #1

An iron supplies 6 amps of current and has 20 Ω of resistance.

What is the voltage?

120 V

Ohm’s Law

Example #1

The current in a wire is 24 amperes when connected to a 1.5

volt battery. Find the resistance of the wire.

0.0625 Ω

Ohm’s Law

Example #2

In a simple electric circuit, a 24 Ω resistor is connected across a

6 volt battery. What is the current in the circuit?

0.25 A

Ohm’s Law

Electric Circuits

• In an electric circuit, an energy source and an energy

consuming device are connected by conducting wires through

which electric charges move.

Electric Circuits

• Electric Circuits are typically represented using diagrams know as schematics.

• Schematics are simplified, standard representation in which common circuit elements are represented with specific symbols, and wires connecting the elements in the circuits are represented by lines.

Electric Circuits

Electric Circuits

V = 12 V R = 6 Ω

I = 2 A

V = 12 V R = 3 Ω

I = 4 A

Ohm’s Law with Schematics

In order for current to flow through a circuit, you must have a

source of Voltage.

Typical sources of potential difference are batteries (which are

just two or more cells connected together), and power

supplies (electron pumps).

In drawing a cell or battery on a circuit schematic, remember

that the longer side of the symbol is the positive terminal.

Electrical Schematics

Voltage

Electric circuits must form a complete conducting path

(closed loop) in order for current to flow.

In the example circuit shown below, the circuit is incomplete

because the switch is open, therefore no current will flow

and the lamp will not light.

Electrical Schematics

In the circuit below, the switch is closed, creating a closed

loop path. Current will flow and the lamp will light up.

Electrical Schematics

Just like mechanical power is the rate at which mechanical

energy is expended, electrical power is the rate at which

electrical energy is expended.

Formula(s):

VIP RIP 2

R

VP

2

SI Unit of Power: Watts (W)

Energy and Power

Example #1

A 110 volt toaster over draws a current of 6 amps on its highest

setting as it converts electrical energy into thermal energy. What

is the toaster’s power rating?

P = V ∙ I

P = 110 v ∙ 6 A

P = 660 W

Energy and Power

Example #2

A potential drop of 50 volts is measured across a 250 Ω resistor.

What is the power in the resistor?

Energy and Power

P = V2 / R

P = (50 V)2 / 250 Ω

P = 10 W

Example #3

Complete the table

20 W

0.5 W

15 V

6 V

20 Ω

132.25 Ω

Energy and Power

Series and Parallel Connections

Electrical Meters

The basic idea of a “series” connection is that components are

connected end-to-end in a line to form a single path for electrons to flow.

Series Connections

The basic idea of a “parallel” connection, on the other hand, is

that all components are connected across each other’s leads.

There are multiple paths for electrons to flow.

Parallel Connections

Meters

Electrical Meters

Voltmeters are tools used to measure the voltage between

two points in a circuit.

The voltmeter is connected in parallel with the element to

be measured, meaning an alternate current path around the

element to be measured and through the voltmeter is

created.

Electrical Meters - Voltmeters

In the diagram below, a voltmeter is connected to correctly

measure the potential difference (voltage) across the lamp.

Electrical Meters - Voltmeters

Ammeters are tools used to measure the current in a circuit.

The ammeter is connected in series with the circuit, so that

the current to be measured flows directly through the

ammeter.

Electrical Meters - Ammeters

In the diagram below, a ammeter is connected to correctly

measure the current flowing through the circuit.

Electrical Meters - Ammeters

Electrical Meters

Electrical Meters - Voltage

Electrical Meters - Series

Electrical Meters

Electrical Meters

Example #1

In the electric circuit diagram, possible locations of an ammeter

and voltmeter are indicated by circles 1, 2, 3, and 4. Where should

the ammeter be located and where should a voltmeter be located

to correctly measure the total current and voltage?

Ammeter: 1

Voltmeter: 4

Electrical Meters

Example #2

Which circuit diagrams below correctly shows the connection of

ammeter A and voltmeter V to measure the current through and

voltage across resistor R?

Electrical Meters

Example #3

A student uses a voltmeter to measure the voltage across a

resistor. To obtain a correct reading, the student must connect

the voltmeter:

a. In series with the resistor

b. In parallel with the resistor

c. Before connecting the other circuit components

d. After connecting the other circuit components

Electrical Meters