Download Current of electricity (Part 1)

Textbook Chp 17
CURRENT OF ELECTRICITY
(PART 1)
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)
Half-Time: Electricity Men?
 Mohan (India)
 http://www.youtube.com/watch?v=PRf9Mqq
kMDM
 Biba Struja (Serbia)
http://www.youtube.com/watch?v=PpIXNZjA
vpA
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:
A
V
 Resistance = (Voltmeter Reading )/(Ammeter
Reading)
 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?
1Ω
2Ω
 Rtotal = 1+2+3 = 6.00 Ω
3Ω
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?
2Ω
4Ω
 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
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?
2Ω
3Ω
4Ω
 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)
Worked Example 8
 What is the total resistance of this
arrangement of resistors?
4Ω
2Ω
 Ans: 2.77 Ω
3Ω
1Ω
2Ω
3Ω
Summary
 I = Q/t
 Conventional Currvent vs Electron Flow
 Electromotive Force
 Potential Difference
 Resistance
 R = V/I
 Resistors in Series
 Resistors in Parallel
No Quiz!
Quiz will only be done after Part 2 is completed