How Do Charges Flow Through the Components of a Circuit? Topic 3.3

Review 1. What will happen if a positively charged object gets close to another positively charged object? 2. What will happen if a positively charged object gets close to a negatively charged object? 3. Using + and -, draw a positively charged object, a negatively charged object and a neutral object Question for today: How do you think a battery works?

Review 1. What will happen if a positively charged object gets close to another positively charged object? They will repel! 2. What will happen if a positively charged object gets close to a negatively charged object? They will attract! 3. Using + and -, draw a positively charged object, a negatively charged object and a neutral object Question for today: How do you think a battery works?

Topic 3.3: Key Concepts 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.

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Concept 1: Chemical Energy Separates Electrical Charges In Cells Transforms energy into energy Example: an AA “battery” is an electrochemical cell, even though it is commonly known as a “battery”) Electrochemical cell: chemical electrical

Electrodes In a cell, chemical reactions of two different or compounds occur on the surface of an electrode is a conductor through which an object, substance, or region Electrodes in solutions are called metals metal electrodes electricity enters or leaves electrolytes

How Electrochemical Cells Work The reactions cause one electrode to become charged and the other becomes charged The electrodes are in contact with the in the cell When terminals are connected to an electrical pathway, charges flow through it positively negatively terminals

Two Types of Electrochemical Cells Both types transform chemical energy into electrical energy - contains a as an electrolyte Dry cell moist paste Figure 3.13: A dry cell (electrolyte is a paste)

Two Types of Electrochemical Cells - the electrodes sit in a Wet cell liquid solution Figure 3.13: A wet cell

Battery two or more cells sources Battery – a connection of E.g. you make a battery when you put AA cells together in an electrical device Often, several electrochemical cells can be packaged together to make a battery Electrochemical cells and batteries are (anything that supplies electrical energy) two or more cells Electrical outlets are also sources Figure 3.14: The battery shown here is made up of six individual electrochemical cells. sources

Understanding How an Electrochemical Cells Works Chemical energy separates the positive and negative charges Energy is the Work is done on an object when a acts on it and moves it some Electrons now have energy to do other work Electrical energy now stored in the cell is a form of (it has the potential to do work) ability to do work force distance Because opposite charges attract each other, it takes energy to separate the positive and negative charges. From the last topic, friction can provide this energy Chemical energy from chemical reactions does the work of separating the charges. Energy is the ability to do work. Work is done on an object when a force acts on it and moves it over a certain distance. Because work went into separating the charges, the electrons now have energy to do other work, e.g. running a fan It has the potential to do work because of the separation or position of the charges https://www.connectschool.ca/mod/mheresource/view.php?id=757763 potential energy

Understanding How an Electrochemical Cells Works The worker (chemical energy) carries (electrons) up a ladder and places them at the The worker leaves on the bottom of the Only a small amount of electrical energy is stored in the cell negative charges negative terminal positive charges The first electron is easy to carry up, since only one pair of charges is being separated and the attraction is not very strong positive terminal Figure 3.15: The worker represents chemical energy

Understanding How an Electrochemical Cells Works After a few charges have been separated, the attraction between the charges at the terminal and the charge of the electron being carried The negative charges of the electrons at the negative terminal are the negative charge of the electron being carried Therefore, it takes more energy to carry each additional electron The worker (chemical energy) has done a lot of work to separate the charges This energy is stored in the of the separated charges positive positive negative increases repelling All of the positive charges of the positively charges ions are attracting to the negative charge of the electron that the worker is carrying. At the same time the negative charges of the electrons at the negative terminal are repelling the negative charge of the electron So it takes more energy each time to carry the electron up the ladder The worker (chemical energy) has to done a lot of work to separate the charges. This energy is now stored in the electrical potential energy of the separated charges Figure 3.15: The worker represents chemical energy electrical potential energy

Understanding How an Electrochemical Cells 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 No more chemical energy will be transformed into electrical potential energy The battery is now Complete handout Figure 3.15: The worker represents chemical energy charged

Classifying Cells Cells are either or Another classification depends on whether they can be Primary cell – can be use , then discarded Secondary cell – can be recharged To recharge a secondary cell, an electric current is passed in the direction through the cell from a source The charging current reverses the chemical reactions in the cell, restoring the cell to full capacity dry wet recharged only once many times opposite

Electrical Potential Difference A unit of charge is called a When a unit of charge passes through a , it The quantity used to measure this is called Often call this Symbol: V Units: volts (V) coulomb source gains electrical potential energy electrical A coulomb gains electrical potential energy when it passes through a source (such as a battery) Because of the symbol and units, electrical potential difference is often called voltage For this reason voltage is used on cells and batteries potential difference voltage Figure 3.16: A typical AA or AAA cell provides an electrical potential difference (voltage) of 1.5V.

Electrical Potential 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 The 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 (i.e. carry the last unit of charge “up the ladder” Why is electrical potential difference called a difference? Think of it this way: to charge a cell, a chemical reaction does work to separate electrons and positive ion. It takes more energy to carry each electron up the ladder. This is because the forces of attraction and repulsion acting on it get stronger and stronger Separating the final charges requires the most energy of all The electrical potential difference of the cell represents the amount of energy it took to carry the last unit of charge up the ladder The electrical potential difference of a cell is determined by the nature of the chemical reaction that happens in the cell Cells and batteries are rated according to their electrical potential difference between one terminal and the other Other sources, like electrical outlets, are rated in a similar manner If two cells are linked together, their voltages add up E.g. 2 1.5V cells placed together in a radio = their voltage or electrical potential difference is 3V Figure 3.16: A typical AA or AAA cell provides an electrical potential difference (voltage) of 1.5V.

Discussion Questions Use an analogy other than a worker and a ladder to explain how chemical energy is transformed into electrical energy in a cell. Why is the electrical potential difference of a source referred to as a difference? https://www.connectschool.ca/mod/mheresource/view.php?id=757763 Do video before discussion questions