Topics Current Electromotive Force Potential Difference Resistance
Current Symbol for current: I Units for current: Ampere (A) Definition: Rate of flow of electric charge Formula: I = Q/t Q is charge (in Coulombs, C) t is time (in seconds, s) Current is a vector, it has both magnitude and direction
Direction of Current If positive charges are moving, the direction of current is the same direction as the positive charges If negative charges (e.g. electrons) are moving, the direction of the current is the opposite direction as the negative charges Note: 99% of the time, it is electrons which are moving
Worked Example 1 2.0 C of positive charge moved from left to right in 1.0 s. (a) what is the current? (b) what is the direction of the current?
Worked Example 2 5.0 C of electrons moved downwards in 4.0 s. (a) what is the current? (b) what is the direction of the current?
Worked Example 3 A current of 2.0 A flowed for 0.3 s. How much charge did that current carry?
Worked Example 4 Simple Circuit Diagram: direction of current direction of electrons
Did You Know? (not in syllabus) How much current does it take to kill a person? Ans: 0.1 A, 0.0001 A if through the heart How much current in a lightning bolt? Ans: 40 000 A (on average) Did you know that majority of people survive a lightning strike?? (10-30% mortality rate)
Voltage In Primary School, you used the word “voltage” in electricity. DO NOT EVER USE THIS WORD FOR O LEVELS Actually this is not a wrong term, but O levels prefer you to differentiate between e.m.f. and p.d.
Electromotive Force (e.m.f) The electromotive force (e.m.f.) is a measure of a source of electrical energy (usually a battery) Symbol: ɛ (epsilon) Units: volts (V) Definition: the work done by the source in driving a unit charge around a complete circuit
Electromotive Force (e.m.f) (not in syllabus) A battery with e.m.f. 1 volt will supply 1 joule of energy to 1 coulomb of charge around a complete circuit In equation form: ɛ = W/Q
E.M.F. in series Recall from primary school when batteries are arranged in series, the e.m.f. add up
Potential Difference (p.d.) Symbol: V Unit: volts (V) Definition: Work Done to drive a unit charge through the component
E.M.F. vs P.D. e.m.f. is a quantity describing sources of electrical energy (i.e. they supply electrical energy) batteries, electrical generators p.d. is a quantity describing sinks of electrical energy (i.e. they use up electrical energy) resistors, bulbs, etc.
E.M.F. vs P.D. How do I use a voltmeter ? When I attach a voltmeter across a resistor, what am I measuring? When I attach a voltmeter across a battery, what am I measuring?
Resistance Symbol: R Units: Ohm ( Ω) Definition: the ratio of the potential difference across the component to the current flowing through it Equation: R = V/I
Resistance Simple Circuit Diagram: Resistance = (Voltmeter Reading )/(Ammeter Reading) R = V/I V A
Resistors in Series If there are two or more resistors in series, the total resistance is given by: R total = R 1 + R 2 + R 3 + …..
Worked Example 5 What is the total resistance of this arrangement of resistors? R total = 1+2+3 = 6.00 Ω 1 Ω 2 Ω 3 Ω
Resistors in Parallel When there are two or more resistors in parallel, the total resistance is given by: 1/R total = 1/R 1 + 1/R 2 + 1/R 3 + …..
Useful Hint! Most questions only ask for you to calculate two resistors in parallel It may be useful to memorize this equation: R total = R 1 R 2 /(R 1 + R 2 ) Note: this equation may only be used for 2 parallel resistors. If 3 or more resistors, use back original formula
Worked Example 6 What is the total resistance of this arrangement of resistors? Method 1: 1/R total = ½ + ¼ = ¾ R total = 4/3 = 1.33 Ω (3 sf) Method 2: R total = R 1 R 2 /(R 1 + R 2 ) = (2)(4)/(2+4) = 8/6 = 1.33 Ω 2 Ω 4 Ω
Important Concept When a resistor is added in series, the total resistance always increases When a resistor is added in parallel, the total resistance always decreases
Problem Solving Strategy For more complex arrangement of resistors, break it down into parts and determine subtotals of resistance before finally combining to find total resistance
Worked Example 7 What is the total resistance of this arrangement of resistors? Step 1: find the subtotal of the parallel resistors first Step 2: add this subtotal to the other resistor in series Ans: 3.71 Ω (3sf) 2 Ω 3 Ω 4 Ω
Worked Example 8 What is the total resistance of this arrangement of resistors? Ans: 2.77 Ω 4 Ω 3 Ω 1 Ω 2 Ω 3 Ω 2 Ω
Summary I = Q/t Conventional Currvent vs Electron Flow Electromotive Force Potential Difference Resistance R = V/I Resistors in Series Resistors in Parallel
Quiz will only be done after Part 2 is completed No Quiz!