Textbook Chp 17. Topics Current Electromotive Force Potential Difference Resistance.
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Transcript of 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?
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)
Half-Time: Electricity Men? Mohan (India) http://
www.youtube.com/watch?v=PRf9MqqkMDM
Biba Struja (Serbia) http://www.youtube.com/watch?v=PpIXNZjAvpA
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
Resistors in Series
If there are two or more resistors in series, the total resistance is given by:
Rtotal = R1 + R2 + R3 + …..
Worked Example 5
What is the total resistance of this arrangement of resistors?
Rtotal = 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/Rtotal = 1/R1 + 1/R2 + 1/R3 + …..
Useful Hint!
Most questions only ask for you to calculate two resistors in parallel
It may be useful to memorize this equation:
Rtotal = R1R2/(R1 + R2) 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/Rtotal = ½ + ¼ = ¾
Rtotal = 4/3 = 1.33 Ω (3 sf)
Method 2: Rtotal = R1R2/(R1 + R2) = (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 Ω3 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 Ω2 Ω
Summary
I = Q/t Conventional Currvent vs Electron
Flow Electromotive Force Potential Difference Resistance
R = V/I Resistors in Series Resistors in Parallel