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Grade 9 Academic Science – Unit 4 Physics (Electricity) Electric Current Section 13.3 Pages 556-557 Electric Charge The unit of electric charge is the coulomb. Electric Current (I) A measure of the RATE of electron flow past a given point in a circuit Measured in amperes (A) Ordinary matter is made up of atoms that have positively charged nuclei and negatively charged electrons surrounding them. As noted, the rate of flow of electric charge is called electric current and it is measured in amperes. NOTE: In introducing one of the fundamental properties of matter, it is important to realize that simplified sketches and constructs are used to introduce concepts. There is much more to the story. NO significance should be attached to the circles representing the proton and electron. The circles do not imply size or shape. Electrically, the most important opening idea, is that electrons and protons have a property called "charge" which is the same size but opposite in polarity (i.e., proton is + and electron is -). The proton has 1836X the mass of the electron but exactly the same size charge. The idea of charge suggests the proton and electron will strongly attract each other. As well, like charges (two protons or two electrons) strongly repel each other. "Like charges repel and unlike charges attract". For any electrical device to work, there must be a FLOW of electrons. For an analogy of current flow to water moving over a waterfall, read Page 556 Current is measured in units called 'amperes'. The number of amperes in a wire relates to how many electrons pass a cross section of the wire per second. With a higher the current (higher amperes), the number of electrons that pass a cross section of the wire per second is increased or greater (see illustration). NOTE: The unit of measurement of current is AMPERES (symbol A). It is named after André-Marie Ampère the founder of a device to measure current. The symbol of current is I Measuring Current An Ammeter is device used to measure electric current. As shown, an ammeter is connected in SERIES with the load. This setup ensures that all the electrons that flow through the load also flow through the ammeter. Current involves the flow of a VERY HUGE NUMBER of electrons. If an ammeter indicates a current of 1 A, it means 6.2 X 1018 electrons are passing through the ammeter each second. Current flows from a higher potential (…more negative) to a low potential (…few electrons). NOTE: The direction of the current will change if the negative and positive poles of the source exchange. Safety with Electric Currents Large currents can damage equipment and cause fires A fuse or circuit breaker in your home ensures the proper amount of current reaches your electrical equipment. The fuse or breaker is connected in series with the electrical circuits of your house. If too much current is demanded, the current also flows through the fuse or breaker. The fuse breaks or the breaker trips. It is like a switch….the current is turned off. Electrical Shock and the Human Body Your brain coordinates movement and reactions via electrical signals sent through your nervous system. Small electrical shocks can be dangerous to your health A current of 0.001 A can give a tingling sensation A current of 0.05 to 0.15 A causes muscle contraction or convulsions. This level is called the “LET GO” threshold. Beyond this value, you CANNOT let go of the object giving the shock. A current of 1.0 to 4.3 A causes your heart to stop beating. NOTE: a wall outlet delivers a current of 15 A. HOMEWORK Page 557, Questions 1-4 Grade 9 Academic Science – Unit 4 – Physics – Electricity Notes on Current Electric Charge The unit of electric charge is the coulomb. Ordinary matter is made up of atoms that have positively charged nuclei and negatively charged electrons surrounding them. The rate of flow of electric charge is called electric current. Current is measured in amperes. NOTE: In introducing one of the fundamental properties of matter, it is important to realize that simplified sketches and constructs are used to introduce concepts. There is much more to the story. NO significance should be attached to the circles representing the proton and electron. The circles do not imply size or shape. Electrically, the most important opening idea, is that electrons and protons have a property called "charge" which is the same size but opposite in polarity (i.e., proton is + and electron is -). The proton has 1836X the mass of the electron but exactly the same size charge. The idea of charge suggests the proton and electron will strongly attract each other. As well, like charges (two protons or two electrons) strongly repel each other. "Like charges repel and unlike charges attract". Although it is electrons that are the mobile charge carriers and responsible for electric current in conductors, it is convention to take the direction of electric current as if the positive charges which are moving. In common applications such as determining the direction of force on a current carrying wire, treating current as positive charge motion or negative charge motion gives identical results. The conventional current direction is the direction from high voltage to low voltage -- high energy to low energy. In this way, the current flow is similar to the flow of water from high pressure to low (see diagram). Current – Note Summary Electric Charge The unit of electric charge is the coulomb. Ordinary matter is made up of atoms that have positively charged nuclei and negatively charged electrons surrounding them. NO significance should be attached to the circles representing the proton and electron. The circles do not imply size or shape. Electrons and protons have a property called "charge" which is the same size but opposite in polarity (i.e., proton is + and electron is -). The proton has 1836X the mass of the electron BUT exactly the same size charge. The idea of charge suggests the proton and electron will strongly attract each other. Like charges (two protons or two electrons) strongly repel each other. "Like charges repel and unlike charges attract". The rate of flow of electric charge is called electric current. Current is measured in amperes. The conventional current direction is the direction from high voltage to low voltage -- high energy to low energy.