13.4 ohm’s law / energy and power / electric meters

13.4 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 The current in a wire is 24 amperes when connected to a 1.5 volt battery. Find the resistance of the wire.

R = 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?

I = 0.25 A

Ohm’s Law

Example #3 A high-beam filament of an automobile headlight carries a current of 4.5 amps. The voltage difference between its terminals is 12 V. Calculate the resistance of the filament.

R = 2.67 Ω

Ohm’s Law

Example #4 Calculate the current when a 12-V battery is connected across a 4 Ω resistor.

I = 3 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

Circuit Symbols

Voltage

V=12V R=6Ω

I=2A

V=12V R=3Ω

I=4A

Ohm’s Law with Schematics

Electric Circuits

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.

Electric Circuits

§  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.

Electric Circuits

§  In the circuit below, the switch is closed, creating a closed loop path. Current will flow and the lamp will light up.

Energy and Power

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=

RVP2

=

SI Unit of Power: Watts (W)

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?

Energy and Power

P = 660 W

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 = 10 W

Example #3 How much power is produced from a 4A current flowing through a resistance of 6Ω

Energy and Power

96 W

Example #4 What is the resistance of a 60 watt light bulb operated at 120 volts?

Energy and Power

R = 240 Ω

Example #5 Complete the table

20

0.5

15

6

20

132.25

Series and Parallel

Electric Meters - Voltmeters

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:


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:


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.

§  Voltmeters have very high resistance so as to minimize the current flow through the voltmeter and the voltmeter's impact on the circuit.


In the diagram below, a voltmeter is connected to correctly measure the potential difference (voltage) across the lamp.


§  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.

§  Ammeters have very low resistance to minimize the potential drop (voltage) through the ammeter and the ammeter's impact on the circuit, so inserting an ammeter into a circuit in parallel can result in extremely high currents and may destroy the ammeter.

Electric Meters - Ammeters

In the diagram below, a ammeter is connected to correctly measure the current flowing through the circuit.

Electric Meters - Ammeters

Electric Meters

Electric Meters - Voltage

Electric Meters - Series

Electric 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?

Electric Meters

Ammeter: 1 Voltmeter: 4

Example #2 Which circuit diagrams below correctly shows the connection of ammeter A and voltmeter V to measure the current through and potential difference across resistor R?

Electric Meters

Example #3 A student uses a voltmeter to measure the potential difference (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

Electric Meters

13.4 ohm’s law / energy and power / electric meters

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