intro to electricity and circuits pg. 45. objectives define electric current. identify the electric...

Intro to Electricity andcircuitspg. 45

Objectives• Define electric current.

• Identify the electric symbols for basic circuit elements.

• Use a multimeter to test continuity.

• Identify and construct open, closed, and short circuits.

• electric current

• ampere (A)

• electric circuit

• open circuit

• closed circuit

• short circuit

Physics terms

• Define voltage source.

• Characterize materials as conductors or insulators based on their electrical properties.

• Demonstrate the use of multimeters, power supplies, and resistors.

• State and apply Ohm’s law to calculate current, voltage or resistance in an electric circuit involving a single resistor.

Objectives Physics terms• voltage

• volt (V)

• battery

• Voltmeter

• resistance

• resistor

• ohm (Ω)

• electrical conductor

• electrical insulator

Legend has it that Benjamin Franklin flew his metal key on his kite in a lightning storm & learned about what electricity was.

Important People in HistoryPrior to 1800, no one knew how to create a steady supply of electric current. Alessandro Volta was the first to succeed. • He put two different metals in

an acid or salt solution & produced an electric current between the terminals.

• He connected multiple voltaic piles together to create a battery of them.

Electricity is the flow of electric charges, typically through wires, conductors, and electrical devices.Sometimes we can see the effects of electricity in nature.

What is electricity & how is it used

Many household appliances and personal devices use electricity.The room you are in right now is probably full of devices that use electricity.

Key Terms in Electricity• There are 3 key terms that we’ll use

throughout our discussion of electricity:– Current– Voltage– Resistance

What is electric current?

Current is measured in amperes or amps (A).Its symbol is I

Electric current is the flow of charged particles, usually through wires and circuits.

Animated illustration, page 472

Electric circuits & conserving chargeAn electric circuit is a conducting path through which electric current can flow.

Charge can never be created or destroyed. All the charge that flows through one point in the circuit must flow through the next point also.

Electric current (I) must be the same at all points in a simple, single-loop circuit.

An open circuit does not have a complete path so no current flows.

A closed circuit has a complete path, allowing current to flow.

Open and closed circuits

Common circuit elements

What is a short circuit?

A battery is a device that converts stored chemical energy into electrical energy.

Batteries come in many different kinds – even lemons!

What is a battery?

chemical energy

electrical energy

Batteries come in many sizes andvoltages: 1.5, 6, 9, 12 V, for example.

Voltage is the electric potential difference between the two terminals of a battery.

Voltage is measured in volts (V), named after Volta, the inventor of the battery.

Reading a battery

Going further: What’s inside a battery

How do you dispose of used batteries?Batteries last for a long time, but what do you do when they are used up?

Alkaline batteries: These batteries used to contain mercury, and so would have to be disposed with hazardous waste. Today, most alkaline batteries have no mercury and can be disposed in the trash. Do not pile many batteries together, however, because they can connect with each other and become hot!

Rechargeable batteries: (nickel cadmium, lithium, car batteries, etc.) These must be recycled directly at a waste management center or through an auto dealer or home hardware store.

Current and voltage are the two most important physical quantities for understanding electric circuits.

Volts and amps

• Current measures the flow of charge through a wire.

• Voltage measures the change in electrical potential energy per unit of charge.

A useful analogy

Charges in a wire are analogous to water in a pipe:• Electric current is analogous to the flow of the water. • The battery is analogous to the water pump.• Voltage is analogous to the change in gravitational Ep.

Analogy: water and electricityWater flows because there is an energy difference, measured in height. Current flows because there is an energy difference, measured in volts. View this short video on page 475

of your e-Book.

Electricity is all around us, yet we are not electrocuted on a daily basis. Why not?

Electrical conductors

• Some materials are good electrical conductors, while others are good insulators.

• Air, plastic, glass, wood, and rubber are all good insulators.

A typical electrical wire has copper on the inside and plastic insulation on the outside.

Electrical wire

• Copper is a good conductor because it has many electrons that are free to move through the material.

• The plastic covering on the wire protects us from getting electrocuted.

Have you ever done any kind of wiring where you had to strip a wire? Why do you have to strip it in the first place?A small amount of plastic insulation must be removed from the ends of wires to use them in circuits.

If your wires are not “pre-stripped,” then you will need to use a wire stripper to remove a small amount of the insulation.

Going further

Ordinary tap water may conduct electricity which is why you should never use electrical devices in or near a bathtub or shower!

But also think about it, what is the cord to all your electronics made of?

Water is conductive too!

How much current?This lamp conducts electricity.

When the lamp is connected to a 10 V source, 2 amps of current flows through it.

But why? What determines how much current will flow?The lamp itself helps determine the current flow. Is the lamp filament a good conductor, or does it resist the flow of current?

ResistanceSome materials are better conductors than others. How do we quantify this difference?

Electrical resistance (R) is a measure of how hard it is for electricity to flow through a material.

• Resistance is measured in ohms (Ω).[Ω is the Greek letter “Omega.”]

• An ohm is a volt per amp

Resistance: an analogy

Electric circuit elements that have resistance are called resistors.

• Resistors control the flow of current in a circuit.

• Light bulbs have resistance, too.

Resistors

A resistor has three bands on one end, giving its value using two digits and a multiplication factor.

Reading resistors

How do we measure current, voltage & resistance?Electric current ammeterVoltage voltmeterResistance ohmmeter

More commonly we use a device that can measure all of them, a multimeter.

Label what each stands for & their units.

EquationsOhm’s law relates current to voltage and resistance in electrical circuits.Ohm’s “law” is not really a law: it’s a mathematical relationship closely obeyed by most conducting materials.

Three ways to use Ohm’s law

Determine the current through a resistor.

Determine the voltage drop across a resistor.

Determine the resistance of an unknown resistor.

Write the equation (rearranged if necessary) that you would use to do each of the following.

Applying Ohm’s law What is the current through a 10 Ω resistor when a voltage of 5 V is applied across it?

5 ohms

Practice1. What is the current through a 10 Ω resistor when it is connected

to a 5 V battery?

2. To reduce the current, should resistance be increased or decreased?

3. If the resistance increases by a factor of two, how does the current change?

4. What if resistance is halved?

5. What is the voltage drop across a 50 Ω resistor when a current of 0.10 A flows through it?

I = 0.5 amps

increased

Current is half as much

Current doubles

5.0 volts

Homework1.Which statement below is a correct definition of

electric current? Electric current is . . .

A. the energy of moving electrical charges.

B. the flow of electric charges through a wire or circuit.

C. the number of electrons in a circuit element.

D. the attraction between charged particles in a wire.

Homework1.Which statement below is a correct definition of

electric current? Electric current is . . .

A. the energy of moving electrical charges.

B. the flow of electric charges through a wire or circuit.

C. the number of electrons in a circuit element.

D. the attraction between charged particles in a wire.

Homework2.Label each of these electrical symbols with the name

of the electrical component it represents: battery; resistor; lamp; switch; or wire.

resistorlamp

batteryswitch

wire

Homework3. Identify the following circuits:

Short circuit Open circuit Closed circuit

Homework4. Yuri connected his power supply (pictured below) to his circuit using the red plug. The lamp did not light. What did he do wrong?

When using a power supply (voltage source) you must connect both the positive (red plug) and the negative (black plug).

Homework5. Which statement below provides a correct practical definition of a voltage source? A voltage source . . .

A. creates an electric potential difference between two points in a circuit, such as the two ends of a battery.

B. provides the electrical potential energy needed for a circuit to operate.

C. is analogous to the pump in a system of circulating water.

D. all of the above.

Homework5. Which statement below provides a correct practical definition of a voltage source? A voltage source . . .

A. creates an electric potential difference between two points in a circuit, such as the two ends of a battery.

B. provides the electrical potential energy needed for a circuit to operate.

C. is analogous to the pump in a system of circulating water.

D. all of the above.

Homework6. What does the “1.5 V” label on the battery mean?

A.The voltage of the positive terminal is 1.5 V.

B.The voltage of the negative terminal is 0 V.

C.The voltage between the positive and negative terminals is 1.5 V.

D.All of the above

6. What does the “1.5 V” label on the battery mean?

A. The voltage of the positive terminal is 1.5 V.

B. The voltage of the negative terminal is 0 V.

C.The voltage between the positive and negative terminals is 1.5 V.

Voltage is a potential difference that is always measured between two points in a circuit.

D.All of the above

Homework

Homework7. Which materials listed below are conductors?

a. aluminum

b. rubber

c. copper

d. gold

e. diamond

Homework7. Which materials listed below are conductors?

a. aluminum

b. rubber

c. copper

d. gold

e. diamond

Homework8. Which equation below is incorrect?

A. B. C. D.

9. What is the voltage drop across a 50 Ω resistor when a current of 0.1 A flows through it?

10. What is the current through a 10 Ω resistor when it is connected directly to a 15 V battery?

Homework8. Which equation below is incorrect?

A. B. C. D.

9. What is the voltage drop across a 50 Ω resistor when a current of 0.1 A flows through it?

10. What is the current through a 10 Ω resistor when it is connected directly to a 15 V battery?