Chapter 13: Electric Circuits

Genius is one percent inspiration, ninety-nine percent perspiration.

Thomas Alva Edison (1847 – 1931)

2

13.1 Electric Circuits

• Explain how electrical energy is supplied to devices in a circuit.

• Use electrical symbols to draw simple circuit diagrams.

• Distinguish between open and closed circuits.

3

Electricity

• When we think of “electricity” we usually think of electric current in wires, motors, light bulbs, and other devices.

• Electric current flows and does work similar to water flowing and doing work.

4

How to develop a current – A water Analogy

• Water flows from high pressure to a lower pressure.

• Electric current flows from a high voltage to a lower voltage.

• A “pump” or “battery” keeps the current flowing!

• A “resistance” in the pipe will restrict the flow of water.

• An electrical resistance will do the same in an electrical circuit.

5

Electric Circuits

• An electric circuit is a complete path through which electricity travels.

• Wires carry electricity in electric circuits in a way similar to the way pipes carry water.

6

Components of Electrical Circuits

• Circuits are made up of wires and electrical devices such as batteries, light bulbs, motors, and switches.

• A resistor is an electrical device that uses the energy carried by electric current. Many electrical devices have a “resistance” such as light bulbs, heating elements, speakers, and motors.

7

A Schematic (or Circuit Diagram)

8

Open and Closed Circuits

• You need a continuous path for electric current to flow.

• All electric circuits must have a source of energy.

• In the examples to the right it is the battery that provides the energy.

• An unintended break can cause the current to stop.

9

13.2 Current and Voltage

• List the units used to measure current and voltage.

• Describe how to measure current and voltage in a circuit.

• Explain the function of a battery in a circuit.

10

Current

• Electric current is measured in Amperes (A), or amps, for short.

• An Ammeter is used to measure electrical current.

11

Voltage

• Voltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy.

• Voltage is measured in Volts (V).

• A Voltmeter is used to measure voltage.

12

How to develop a current – A water Analogy

• Water flows from high pressure to a lower pressure.

• Electric current flows from a high voltage to a lower voltage.

• A “pump” or “battery” keeps the current flowing!

• A “resistance” in the pipe will restrict the flow of water.

• An electrical resistance will do the same in an electrical circuit.

13

A Couple Diagrams from the Textbook

14

Batteries Use Stored Chemical Energy

15

A battery uses chemical energy to create a voltage difference between its two terminals.

Multimeters

• A Multimeter can be used to measure both current and voltage.

• Hook up a multimeter in series to measure current, and in parallel to measure voltage!

16

Multimeters

In parallel to measure voltage In series to measure current

17

Fuses & Circuit Breakers

• Fuses and circuit breakers are two different ways of protecting against suddenly large overloads of electrical current.

• Large power overloads are dangerous, potentially destroying electrical equipment or causing a fire.

• During normal flow of electricity, the fuse permits the current to pass freely. But during an unsafe overload, the small piece of metal melts, stopping the flow of electricity. When a fuse blows out, it must be thrown away and replaced with a new fuse.

• Circuit breakers are switches that are tripped when the electrical current passes a safe limit. Once tripped, the switch is simply turned off. That stops the flow of electricity, which will remain off until the switch is reset.

18

13.3 Resistance and Ohm’s Law

• Explain the relationships between current, voltage, and resistance.

• Use Ohm’s law to calculate current, resistance, or voltage.

• Distinguish between conductors and insulators.

19

Resistance and Ohm’s Law

• A high resistance leads to a smaller “current” of water!

• A low resistance leads to a larger “current” of water!

• The same holds true for electrical currents.

20

Resistance and Ohm’s Law

• Electrical resistance is measured in Ohms (W).

• Ohm’s law states that the current, i, is directly proportional to the voltage, V, and inversely proportional to the resistance, R.

21

iRV

Resistance and Ohm’s Law

22

iRV

Copper wire has a very low resistance, the resistance in circuits comes from the light bulbs, motors, toasters, hair dryers, or other “loads” connected to the circuit.

Class Problems

1. Section 13.3: A typical light bulb has a resistance of 240 W. What is the current going through the light bulb when hooked up to the household voltage of 120 Volts?

2. Section 13.3: On average your skin can feel a current of about 0.0005 amps. If your skin has a resistance of 100,000 W, what voltage is needed to just feel a tingle?

3. Section 13.3: In lab, we will hook up a Christmas tree light to a 5 volt battery. If the current is 0.03 amps, what is the resistance of the light bulb?

23

Conductors and Insulators

• Current passes easily through some materials, such as copper, which are called conductors. A conductor can conduct, or carry, electric current. The electrical resistance of wires made from conductors is low. Most metals are good conductors.

• Other materials, such as rubber, glass, and wood, do not allow current to easily pass through them. These materials are called insulators, because they insulate against, or block, the flow of current.

24

Resistors

• Resistors are designed to regulate the current flow in a circuit.

• They are found in many common electronic devices such as computers, televisions, telephones, and stereos.

25

Resistor Chart

26

11 W 68 W

330 W 47,000 W

Potentiometers

• Simple resistors have a predetermined resistance over a wide range of currents.

• Variable resistors can have their resistance adjusted.

27

Class Problems

4. Section 13.3: Using the formula i = V/R, describe two ways of increasing a circuits current.

5. Section 13.3: The resistance of the human body when the skin is dry is about 1 x 105 ohms. Find the current through the body from a 120 V household outlet. A person will feel a tingle at about 2 mA (milliamps). If the skin becomes wet the resistance falls to about 1500 ohms, what would the current be now? Respiratory paralysis occurs between 20 and 100 milliamps! It is dangerous to work with electricity around water!

28