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Critical Inquiry 2.1 Engage in effective critical inquiry through a process of problem definition, evidence gathering, and evaluation. 2.1.1.

Definition – identify the problem and associated constraints; set the parameters of the inquiry.

2.1.2. Evidence – identify and gather sufficient relevant information of appropriate quality.

2.1.3. Evaluation – assess defined problem in the light of available evidence.

2.1.4. Discernment – distinguish fact from opinion.

Natural Sciences 1
Electricity
Ramsay Khadeir, PhD
James Stone, PhD
Reminder
To do well in this course, every week you must complete any:
1.
2.
3.
Lecture activity
Problem set
Lab coursework
CHECK Due Dates in Blackboard
2
Learning
Objectives
•
Describe key electrical concepts
•
Understand conventions for drawing circuits
•
Differentiate series and parallel circuits
KFSC – Bachelor of Security Sciences
3
Background
Circuits
Just like highways, metro systems,
blood vessels, and plumbingCircuits are closed paths that
allow things like electricity to flow
4
Electrical Circuits
Electric charge
• an amount of electrical energy
Electric current
• a flow of electrical charge, often a flow of electrons
• conventional current is in the opposite direction to a flow of electrons
Current flow in a circuit
• a sustained current needs a complete circuit
• also requires a stimulus to cause the charge to flow
5
Electrical charge and current
• Two types of current: DC and AC. With direct current (DC), the flow of electrons is
consistently in one direction around the circuit.
• With alternating current (AC), the direction of electron flow continually reverses.
• Electrons are negatively charged particles. They transfer energy as electricity.
• Charge is a property of a body which experiences a force in an electric field.
Charge is measured in coulombs (C).
charge = current × time
Q=Ixt
charge (Q) is measured in coulombs (C)
current (I) is measured in amps (A)
time (t) is measured in seconds (s)
6
Concept Checks 1,2
1.
What unit is current measured in?
2.
What do AC and DC stand for?
Active
Current and
Direct
Current
Alternating
Current and
Diverging
Current
Alternating
Current and
Direct
Current
Activating
Current and
Direct
Current
7
Drawing Circuits
8
Resistors, Capacitors and Inductors
Resistors provide resistance they oppose the flow of
electricity measured in Ohms ()
Capacitors provide capacitance they store energy in an
electric field measured in Farads (F)
Inductors provide inductance they store energy in a
magnetic field measured in Henry (H)
9
Ohms Law
The current flowing in a conductor is directly proportional to the
applied voltage V and inversely proportional to its resistance R
V = IR
I = V/R
R = V/I
10
Concept Check 3,4
3. How much charge has moved if a current of 13A flows for 10 s?
Q=Ixt
Q = 13 x 10
Q = 130C
4. How much current flows when 10C passes down a wire in 2 s?
Q=Ixt
I=Q/t
I = 10 / 2
I = 5A
11
Potential and resistance
The current through a component depends on both
the resistance and the potential difference across the
component.
Potential difference is called voltage, measured in (V)
•
•
•
potential difference (V) is measured in volts (V)
energy (E) is measured in joules (J)
charge (Q) is measured in coulombs (C)
12
Concept Checks 5,6
5. Which of these is true in a parallel circuit?
•
Voltage is constant across each component
•
Resistance is constant for each component
•
Current is constant for each component
6. Electrical potential difference also means which of these?
•
Current, Voltage, Charge, Amperes
13
Resistance
When a charge moves through a potential difference, electrical work is done and
energy transferred. The potential difference can be calculated using the equation:
potential difference = current × resistance
potential difference (V) is measured in volts (V)
energy (E) is measured in joules (J)
charge (Q) is measured in coulombs (C)
1V is the potential difference when 1 coulomb of charge transfers 1J of energy
14
Concept Check 7,8
7. What’s the potential difference if 2 C of charge moves 4 J of energy ?
V=E/Q
V=4/2
V=2
8. What’s the resistance if 12V moves a current of 2A through it?
V=IxR
12 = 2 x R
R=6
15
Kirchkoff’s Current Law
At any instant the algebraic sum of the currents flowing into any junction
in a circuit is zero…
For example
I1 – I2 – I3 = 0
I2 = I1 – I3
= 10 – 3
=7A
16
Kirchkoff’s Voltage Law
The sum of the voltages around any loop in a circuit is zero
E – V1 – V2 = 0
V1 = E – V2
= 12 – 7
= 5V
17
Power and Resistors
Power dissipation, P, of a resistor is the product of the voltage across
and the current passing through it:
P = IV
P = I2R
P = V2/R
CC9: What power is transmitted by a
5 A current through a 10 Ω resistor?
18
Resistors in Series and Parallel
• Series
R = R1 + R2 + R3
• Parallel
1
1
1
1
R
R1 R2
R3
19
How much potential
is lost across resistors?
General case
V V2 (V1 V2 )
R2
R1 R2
20
Example
V V2 (V1 V2 )
10
R2
R1 R2
R2
R1 R2
300
200 300
6V
10
21
Concept Check 10
V V2 (V1 V2 )
3 12
R2
R1 R2
500
1000 500
34
7V
22
ADVICE:
KEEP IT SIMPLE
xkcd.com/730/
23
Background:
Circuits
Ammeters, Voltmeters, and a couple equations are almost all
you need to fully analyze a circuit!
P = IV
V = IR
But how do parts of a circuit interact with each other?
24
...

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