Electric Current symbol: I Current = flow of charges unit: Amperes (A) (The rate at which charge flows by a given cross section) 1C 1A = 1s I= Electric Current symbol: I Current = flow of charges unit: Amperes (A) (The rate at which charge flows by a given cross section) 1C 1A = 1s I= To have an electric current, you need two things: • A closed circuit / path for the charges • A power supply maintain the potential difference Electric Current symbol: I Current = flow of charges unit: Amperes (A) (The rate at which charge flows by a given cross section) 1C 1A = 1s I= To have an electric current, you need two things: • A closed circuit / path for the charges • A power supply maintain the potential difference Remember: Current will flow from high potential to low potential, but charges need to be pushed (by an energy source) back from low potential to high potential. Closed vs. Open Circuits No, the switch is open, so the path is not complete Yes, charge will flow, BUT the light will not light No, the circuit needs to go from + to - Maybe! If one battery has higher V than the other Direction of Current Current is defined as the direction positive charges would flow • From high potential to low potential • From + side of battery to – side of battery Fun Fact Benjamin Franklin defined current in this manner long before we knew much about charges. Now, we know that positive charges stay put and negative charges flow. So, electrons actually flow opposite current. Calculating Current Current is the rate of charge flow per unit time I= So, if you have 6 C of charges passing through a section of wire every 2 seconds, then your current is: I = 6C / 2 s = 3 A An important note about current: Although potential decreases across a circuit, current is the same everywhere in a circuit! Circuit Analogy – CFU Circuit Analogy - CFU E F B C A Electrical resistance (symbol R) • Electrons flow due to potential difference. If the potential difference is removed, then • As electrons move, they bump into other atoms, this slows them down and impedes their motion. atoms (actually positive ions) free electron . Resistance (R) is a measure of the degree to which an object impedes the flow of current. Resistance is measured in Ohms () path OHM’S LAW - Current, Voltage and Resistance Current = the potential difference applied across a circuit divided by the total resistance of the circuit. • R - resistance • I – current V I= R V – potential difference across R Examples • If a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw? • I = V / R = 3 V / 9 = 1/3 Amps • If a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the resistance of the light bulb? • R = V / I = 120 V / 2 A = 60 Check for Understanding Which of the following would cause the current in a circuit to decrease the most? 1) 2) 3) 4) Increased voltage and increased resistance Increased voltage and decreased resistance Decreased voltage and decreased resistance Decreased voltage and increased resistance Check for Understanding If the resistance of a circuit were tripled, then the current through the circuit would be ____. 1. one-third as much 2. three times as much 3. unchanged 4. ... nonsense! There would be no way to make such a prediction. Just for Fun … Effects of electric current on the BODY- electric shock Current (A) Effect 0.001 can be felt 0.005 painful 0.010 involuntary muscle contractions (spasms) 0.015 0.070 loss of muscle control if through the heart, serious disruption; probably fatal if current lasts for more than 1 second human body resistance varies: 100 ohms if soaked with salt water; moist skin - 1000 ohms; normal dry skin – 100 000 ohms, extra dry skin – 500 000 ohms. What would be the current in your body if you touch the terminals of a 12-V battery with dry hands? I = V/R = 12 V/100 000 = 0.000 12 A quite harmless But if your hands are moist and you touch 12 V battery, how much current would you draw? I = V/R = 12 V/1000 = 0.012 A a dangerous amount of current.