 Textbook Chp 17. Topics  Current  Electromotive Force  Potential Difference  Resistance.

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Textbook Chp 17

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!

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