Current, Volts and Resistance (Ohm’s Law)

Coulomb Recall that one Coulomb has 6.25 X 1018 electrons. If the current coming out of the outlet on the wall has 600 C, how many electrons are coming out of the outlet? 1C = 6.25 X 1018 electrons 600 C #e- Cross multiply and you get 3.75 X 1021 electrons

Symbol: I Units: Amperes, A Current Intensity Current Intensity - the amount of electrons that flow past a given point in a circuit every second (i.e through a wire) Example: 0.7 amperes is shown by I = 0.7 A An ammeter is the instrument used to measure current intensity A

I = q/t Current Intensity The current intensity in a circuit can be determined using the formula: I = q/t I is the current intensity in amps (A) q is the charge in coulombs (C) t is time in seconds (s) NOTE: 1 Amp (A) = 1 Coulombs (C)/1 second (s) A=C/s

Example #1 What is the current flowing through a car headlight if there are 900 C of charge used in 1 minute? I = q/t I = 900 /60 I = 15A

Example #2 How much charge does it take to operate an MP3 player for 15 minutes if the current is 2A? I = q/t 2 = q/900 q = 1800C

Potential Difference (Voltage) Symbol: V Units: Volts, V Potential Difference (Voltage) Potential Difference is the amount of energy transferred between two points in an electrical circuit. It is the voltage that is measured at a load. Example: Energy that is provided by a battery or power supply. It could be a 12 volts battery (also know as 12V) A voltmeter is used to measure potential difference V

Potential Difference (Voltage) The potential difference can be determined using the formula: V=E/q V is potential difference in volts (V) E is the energy in joules (J) q is the electric charge in coulombs (C) Note: 1 Volts = 1 Joule / 1 Coulombs V=J/C

Example #3 In a house, how much energy is provided by 120V service providing 200C of charge? V=E/q 120 = E/200 E = 24 000J

Resistance Resistance is how difficult it is for current to flow Symbol: R Units: Ohms, Ω Resistance Resistance is how difficult it is for current to flow Resistance analogy: imagine a river flowing. If you put a large boulder in the river, the current on the other side of the boulder will slow down. If you put a small boulder in the river, the current on the other side of the boulder will still be quick. Now replace river flow with flow of electrons, and a boulder with a resistor. A resistor is used to slow current down and convert electrical energy into heat energy (e.g. light bulb, stove element). Resistor– high resistance & low current flow Conductor (wire) - low resistance & high current flow

Ohm’s Law Ohm’s Law states – that for a given resistance, the potential difference in an electrical circuit is directly proportional to the current intensity.

Formula to Solve Circuits Resistance, current intensity and potential difference are all related R = V/I R is the resistance in Ohm’s (Ω) V is the potential difference (Volts) I is the current intensity (Ampere) Note: 1 Ohms= 1 Volts / 1 Amper 1 (Ω) = 1 V / 1 A

Example #4 An stove element 30 Amperes going through it, and carries 120 volts of electricity, how much resistance is in this circuit? R = V/I R = 120 V/30 A R = 4 V/A = 4 Ω

Example #5 A circuit has a potential energy difference of 240 V and offers a resistance of 6Ω, what is the intensity of the current flow? R=V/I 6 = 240 /I I = 240 /6 I = 40 A

Example #6 A circuit has a resistance of 10Ω & a current intensity of 5A. What is the potential difference? R=V/I 10 = V/ 5 10 X 5 = 50V

Causes electrons to flow Summary Table Resistance Current Potential Difference Definition Slows current down The flow of electrons Causes electrons to flow Symbol R I V Units Ohms (Ω) Amps (A) Volts (V) Formula (if applicable) R=V/I Ω=V/I I=q/t A=C/s V=E/q V=J/C How measured Circuit Formula Ammeter or formula Voltmeter or formula

Assignment Page 319 – Questions 1 – 11 Handout