2 Electric Current Current = flow of charges unit: Amperes (A) symbol: I (The rate at which charge flows bya given cross section)I = 𝑄 𝑡
3 Electric Current Current = flow of charges unit: Amperes (A) symbol: I (The rate at which charge flows bya given cross section)I = 𝑄 𝑡To have an electric current, you need two things:A closed circuit / path for the chargesA power supply maintain the potential difference
4 Electric Current Current = flow of charges unit: Amperes (A) symbol: I (The rate at which charge flows bya given cross section)I = 𝑄 𝑡To have an electric current, you need two things:A closed circuit / path for the chargesA power supply maintain the potential differenceRemember: 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.
5 Closed vs. Open Circuits No, the circuit needs to go from + to -No, the switch is open, so the path is not completeMaybe! If one battery has higher V than the otherYes, charge will flow, BUT the light will not light
6 Direction of CurrentCurrent is defined as the direction positive charges would flowFrom high potential to low potentialFrom + side of battery to – side of batteryFun FactBenjamin 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.
7 An important note about current: Calculating CurrentCurrent is the rate of charge flow per unit timeI = 𝑄 𝑡So, 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 AAn important note about current:Although potential decreases across a circuit, current is the same everywhere in a circuit!
10 Electrical resistance (symbol R) Electrons flow due to potential difference. If the potential difference is removed, thenAs electrons move, they bump into other atoms, this slows them down and impedes their motion..pathatoms(actually positive ions)free electronResistance (R) is a measure of the degree to which an object impedes the flow of current.Resistance is measured in Ohms ()
11 OHM’S LAW - Current, Voltage and Resistance Current = the potential difference applied across a circuit divided by the total resistance of the circuit.R - resistanceI – current V – potential difference across R
12 ExamplesIf 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 AmpsIf 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
13 Check for Understanding Which of the following would cause the current in a circuit to decrease the most?Increased voltage and increased resistanceIncreased voltage and decreased resistanceDecreased voltage and decreased resistanceDecreased voltage and increased resistance
14 Check for Understanding If the resistance of a circuit were tripled, then the current through the circuit would be ____.one-third as muchthree times as much3. unchangednonsense! There would be no way to make such a prediction.
15 Effects of electric current on the BODY- electric shock Just for Fun …Effects of electric current on the BODY- electric shockCurrent (A)Effect0.001can be felt0.005painful0.010involuntary muscle contractions (spasms)0.015loss of muscle control0.070if through the heart, serious disruption; probably fatal if current lasts for more than 1 second
16 human body resistance varies: 100 ohms if soaked with salt water;moist skin ohms;normal dry skin – ohms,extra dry skin – 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/ W = A quite harmlessBut if your hands are moist and you touch 12 V battery, how much current would you draw?I = V/R = 12 V/1000 W = Aa dangerous amount of current.