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Electric Current – SPH 3U1 Consider the structure of the atom: The centre is called the ___________________ It contains the _______________(+) and ___________________ (0) The outer rings are called ________________ which contain the _________________ ( - ) Only the electrons are able to move from atom to atom so their motion is the foundation of ELECTRIC CURRENT. Electric Current: the amount of charge that passes a given point in a conductor per second Symbol: Unit: Formula: We don’t count the number of electrons passing a point because that value would simply be too BIG. We group electrons into units of charge called COULOMBS where: 1 C = 6.24x1018 electrons Example: A light bulb with a current of 0.80A is left radiating for 45 minutes. How much electric charge passes through the filament of the light bulb? How many electrons passed through the bulb? An ammeter is used to measure electric current. To use an ammeter, it must be connected as part of the electric circuit loop so that the electrons go through the meter. Electric Potential Difference – SPH 3U1 Because electric charges exert forces of attraction or repulsion on each other, we can consider how much force would be needed to move an electric charge from one point to another while feeling an electric force from another charged particle. Electric Potential Difference: the work done to move a charged object through an electric field (essentially it is the amount of work per unit charge). Electric potential energy is converted into another useful form of energy. Symbol: Unit: Formula: Example: The current flowing through a flashlight bulb is 0.50A and the potential difference across the filament is 2.4V. How much work is done in moving the charge in 2.0s? Example: It takes a current of 10A for 300s to boil a kettle of water requiring 3.6x10 5J of heat energy, what is the potential difference across the kettle’s heating element? A voltmeter is used to measure the electric potential difference between two points in an electric circuit. The voltmeter must be connected across these two points and is therefore NOT part of the original electric circuit. Electrical Resistance – SPH 3U1 As electrons are travelling through a circuit, they will lose energy as their motion is resisted by the atoms that they are travelling across. This is called electrical resistance. Symbol: Unit: Formula: Example: Find the resistance of an electric light bulb if there is a current of 0.8A when the potential difference across the bulb is 120V? Example: What is the potential difference across a toaster of resistance 13.7Ω when the current through it is 8.75A? Electric Circuits and Symbols Component Symbol Component Symbol Cell resistor Voltmeter fuse Connecting wires crossing wires Ammeter light Switch ground Battery capacitor The Four Parts of a Simple Circuit: Diagram: 1. 2. 3. 4. Direction of Electron Flow Since the wires are made of conductors, the electrons in the metal wires can ___________ from atom to atom. The negatively charged electrons are going to be ___________________ by the negative terminal of the energy source and ____________________to the positive terminal of the energy source. Current flow is actually defined as being ___________________ to electron flow! Series vs. Parallel Circuits Series Circuits: electrons must pass through ___________ __________________ in the circuit before returning to the ______________ Example: Parallel Circuits: the electrons reach a point where they can follow _______________ or another so they do not have to pass through _________ ________________ in a circuit before returning to the source Example: Combining Resistors In Series: R1 R2 R3 𝑹𝒕𝒐𝒕𝒂𝒍= In Parallel: R1 R2 R3 𝟏 𝑹𝒕𝒐𝒕𝒂𝒍 = Kirchhoff’s Laws – SPH 3U1 Conservation of Energy: as electrons move through a _______________, they gain ________________ in sources and lose energy through ____________ but the total amount of energy gained in one ___________ through a circuit is equal to the total amount of energy ____________ in that loop. Kirchhoff’s Voltage Law: Around any complete ________________ in a circuit, the sum of the electric potential increases is equal to the sum of the electric potential _______________________. Mathematically: Example: Kirchhoff’s Current Law: At any __________________ in a circuit, the total amount of electric current ___________ that junction is equal to the total amount of electric current ____________ that junction. Mathematically: Example: Ohm’s Law: The potential resistance across a conductor is directly proportional to the current flowing through it. Mathematically: Circuit Analysis – SPH 3U1 In order to completely analyze a circuit, you must find all voltage, current and resistance values. According to notation, I1 is the current flowing through resistor 1 and V1 is the potential difference across resistor 1. Example: Completely analyze the following circuit R1=3Ω R2=10 Ω Vo = 10V R3=10 Ω R4 = 2 Ω Step 1: simplify the circuit to find the total resistance Step 2: Use Kirchhoff’s voltage law to find the current going through the source Step 3: Determine where else that current flows and start finding all V’s, R’s and I’s