Topic 3.3 - Electrical Circuits

Topic 3.3 - Electrical Circuits

Key Points

• Chemical energy separates electrical charges in cells.

• Charges can flow through conductors, but not insulators.

• Moving electrical charges form an electric current.

• A load resists the flow of current.

• Conductors must form a closed loop to allow current to flow.

Concept 1: Chemical energy separates electrical charges in cells.

Electrochemical cell

• Transforms chemical energy into electrical energy

• Example: An AA “battery” is an electrochemical cell (even though it is commonly known as a “battery”)

How an Electrochemical Cell Works

• Chemical reactions of two different metals or metal compounds occur on the surface of electrodes (zinc and carbon)

• Electrodes are in a solution called an electrolyte (aqueous solution of ammonium chloride)

How an Electrochemical Cell Works

• The chemical reactions cause one electrode to become positively charged, and the other to become negatively charged

• The electrodes are in contact with terminals in the cell (Figure 3.13: negative terminal and positive terminal)

How an Electrochemical Cell Works

• When the terminals are connected to an electrical pathway, charges flow through

Battery

• Battery: A connection of two or more electrochemical cells

• Example: Several electrochemical cells can be packaged together to make a battery

• Electrochemical cells and batteries are sources (anything that supplies electrical energy)

Electrochemical Cells

• An electrochemical cell transforms chemical energy into electrical energy

• How does an electrochemical cell become “charged”? (How does a cell transform chemical energy into electrical energy?)

Chemical to Electrical: How it works

• The worker (chemical energy) carries negative charges (electrons) up a ladder and places them at the negative terminal of the cell

• The worker leaves positive charges on the bottom at the positive terminal

Chemical to Electrical: How it works

• The first electron is easy to carry up, since one only pair of charges is being separated

• Only a small amount of electrical energy is stored in the cell

Chemical to Electrical: How it works

• After a few charges have been separated, the attraction between the positive charges at the positive terminal and the negative charge of the electron being carried increases

Chemical to Electrical: How it works

• The negative charges of the electrons at the negative terminal are repelling the negative charge of the electron being carried

• Therefore, it takes more energy to carry each additional electron

Chemical to Electrical: How it works

• The worker (chemical energy) has done a lot of work to separate the charges

• This energy is stored in the electrical potential energy of the separated charged

Chemical to Electrical: How it works

• Eventually, repulsion of the electron by the negative charges at the negative terminal and the attraction by the positive charges gets so strong that the worker cannot carry any more electrons

Chemical to Electrical: How it works

• No more chemical energy will be transformed into electrical potential energy

• The battery is now charged

Electrical Potential Difference

• A unit of charge gains electrical potential energy when it passes through a source (such as a battery)

Electrical Potential Difference

• Electrical potential difference: A quantity that provides a measure of the electrical potential energy a unit of charge gains when passing through a source

  •Often called voltage

  •Symbol: V

  •Units: volts (V)

Electrical Potential Difference 

• Why is electrical potential difference called a difference?

• It measures the difference in electrical potential energy per unit of charge between the positive terminal and the negative terminal in an electrochemical cell

Electrical Potential Difference 

• Electrical potential difference is often called the voltage

• 1.5V cell: It took 1.5 units of energy to separate the last unit of charge (e.g., carry the last unit of charge “up the ladder”)

Voltmeter

• Measures electric potential difference

Concept 2: Charges can flow through conductors, but not insulators.

Conductors and Insulators

• When two different solid materials are rubbed together, electrons can be transferred from one material to the other

• •Electrons will either stay on the surface of the new material or travel through it

Conductors and Insulators

• Insulator: A material charges cannot travel through

• Conductor: A material charges can travel through

Conductivity

• Conductivity: An indication of how easily charges travel through a material

• Electrons can move through almost all metals (conductors); some metals more easily than others

• The higher the conductivity of a material, the more easily electrons can move through

Concept 3: Moving electrical charges form an electric current.

Current

• Chemical energy from a source (cell or battery) causes charges to move through a conductor (wires), carrying energy to an electrical device load (cellphone)

Current

• The moving charges are called an electric current

• Symbol for current: I

• Current is measured in amperes: A

Current

• Ammeters measure electric current

Concept 4: A load resists the flow of current.

Electrical Loads

• Load: A device that converts electrical energy into another form of energy

• As electrons pass through a load, they lose energy as electrical energy is converted to another type of energy

Electrical Loads

• Light bulb: A load that transforms electrical energy into light energy

• Radio: A load that transforms electrical energy into sound energy

Electrical Loads

• A load resists (hinders) the flow of current

• Electrons in the current collide with atoms that make up the load, or with each other

• Collisions interfere with the flow of current

Resistance

• Resistance: Describes the amount that current is hindered by a load

  •Symbol: R

  •Units: Ω (Ohm)

• Example of Resistance: Filament in a Light Bulb

Resistance

• •Charges move from a large wire (electrical cord) into a very thin wire (filament)

• Since the charges have less room in the filament (the filament resists the movement of charges), they collide with atoms so hard that the filament gets very hot

• The heat makes the filament glow (“light up”)

Resistance

• Resistance can be measured with an Ohmmeter

Open Ended

Use the terms source, current, and load to describe how you think a flashlight works.

Concept 5: Conductors must form a closed loop to allow current to flow.

Electrical Circuits

• Electrical circuit: A source, a load, and wires in a closed loop that allow current to flow

• Source (electrochemical cell)

• Load (lightbulb)

• Wires

Short Circuits

• Short circuit: A circuit with a resistance that is too low, making the current so high that it is dangerous

Short Circuits

• Example: If there wasn’t a load (light bulb) to resist the flow of current, the current would be so large that the conductor would get very hot and start a fire

Modelling the Flow of Current 

• Negative terminal repels the negative charges already in the conductor

• Positive terminal attracts the negative charges already in the conductor

• Electrons move along the conducting wires; electrons from the electrochemical cell move into the conductor

Modelling the Flow of Current 

• As the electrons pass through the load, they transfer some of their energy to the load

• The electrons then leave the load and return to the electrochemical cell

Modelling the Flow of Current 

• Electrons enter the electrochemical cell; combine with positive ions to become neutral

• Over time: fewer electrons at negative terminal; fewer positive ions at positive terminal

• The worker (chemical energy) can carry more electrons up the ladder, keeping the number of separated charges equal

Controlling the Flow of Current

• In a typical circuit, a switch controls current in a circuit

Controlling the Flow of Current

• A. The switch is open. The circuit is open so the current cannot flow.

• B. The switch is closed. The circuit is closed so the current can flow and the light is on.

Comparison: Water Circuit and Electrical Circuit

• Water circuit: A pump lifts the water to a higher level against the pull of gravity

• Electrical circuit: The cell or battery is similar to the pump