Electricity
ALL OF ELECTRICITY
Section Objectives
• To understand the basics of a circuit
• To define current
The Circuit ‘Game’
• Draw the symbol
How Many Correct? /12
Electric Current
• What is it?
Electric Current
• Flow of charge
•
Electric Current
•
• Unit: Coulomb per second, C/s = ampere, A
Checkpoint: Show high current and low current in two drawings (3 marks)
Answer: Show high current and low current in two drawings (3 marks)
Current
• High vs. Low
AC/DC
• You know all about it, right?
AC vs DC
• DC - Direct Current: flows in only one direction
• AC – Alternating Current: Continually changes
directions
• UK – Changes 50 times a second (50Hz)
Batteries
• What do they do? (can be simplified)
Batteries (Simplified)
• Use a chemical reaction..
• To create a difference in potential between its terminals
Chemistry of a Battery
• They will explain it better than me
Ultimately….
• Chemical energy -> Electrical potential energy
Battery (Simplified)
• Potential Difference:
• Allows flow of charge
Battery (Simplified)
• No Potential Difference:
• No flow of charge (switch)
Gravitational Potential
• Bodies move from high potential to low
High to Low
Electrical Potential (Convention)
Electrical Potential (Convention)
• How do positive charges move?
Potential Difference
• Ultimately, a consequence of energy conservation
Potential Difference – Conservation of E
• The kinetic energy has to equal the change in potential energy, so KE=qV
Current
• So what about current? What conservation law ultimately applies?
Current – Conservation of ‘Q’
• Net current in a volume is inherently dependent on the net charge
• Charge cannot be created or destroyed
Current
• We model it as a flow of positive charge
Current – IGNORE
• I’m not going to confuse you about election flow
Current – IGNORE
• I’m not going to confuse you about election flow
Electric Circuit
• What is it?
Electric Circuit
• When charge flows through a closed path and returns to its starting point
• (Conservation of “q”)
Checkpoint Time
1. How many coulombs of charge are in one ampere-hour?
2. A flashlight bulb carries a current of 0.18 A for 78 s. How much charge flows through the
bulb in this time? How many electrons?
3. Highly sensitive ammeters can measure currents as small as 10.0 fA. How many electrons
per second flow through a wire with a 10.0-fA current (f – ‘femto’ – 10-15)
Answer - 1
1. How many coulombs of charge are in one ampere-hour?
3,600C
Answer - 2
2. A flashlight bulb carries a current of 0.18 A for 78 s. How much charge flows through the
bulb in this time? How many electrons?
8.78 x 1019 electrons
Answer - 3
3. Highly sensitive ammeters can measure currents as small as 10.0 fA. How many electrons
per second flow through a wire with a 10.0-fA current (f – ‘femto’ – 10-15)
8.78 x 1019 electrons
Simple Circuits
• What do you see here?
• Series
•All in a row
•One path for electricity
•If one light goes out, the circuit is broken
• Parallel
•Many paths for electricity
•If one light goes out, the others stay on
Section Completed
• To understand the basics of a circuit
• To define current
Section Objectives
• To understand resistance
• To understand Ohm’s Law
• To understand potential difference (more)
Inside a Wire
• What’s really happening when a current flows?
Inside a Wire
• Simply put – Electrons movement, atom stuff, collisions
A Question
• What stops the box/desk from sliding?
Current
• The electrons don’t have a clear path
Resistance
• There is resistance.
A Question
• So how to we ‘overcome’ the resistance?
We apply a potential difference
Checkpoint:
1) Draw a ‘low’ potential difference between two plates.
2) Draw a ‘high’ potential difference between two plates.
Answer and discussion
1) Draw a ‘low’ potential difference between two plates.
2) Draw a ‘high’ potential difference between two plates.
Simple Explanations = Good Explanations
A Question
• How do you ‘help’ the current flow in a circuit?
Answer
• More voltage (potential difference)
Potential Difference Voltage – The Push!
•Electrons move due to “voltage difference”
• Higher p.d. = stronger “push”, which causes an increase in current
• Unit – Volts (V)
Work Done – Real Life
• Work is done when a force causes displacement of an object
• Work done = Energy Transferred
Work Done – In Electricity
• Do you know?
Work Done – In Electricity
•The voltage between two points is the work done per coulomb travelling
between the two points
• Voltage = ; or
Work Done – In Electricity
• Voltage = ; or
• W = Work done (J)
• E = Energy transferred (J)
• V = Voltage (V)
• Q = Charge (C)
Power – In Real Life
• What is it again?
• Hint: Watts
Power
• Power =
• So what about circuits?
Power – In Electricity
• Power = Current x Voltage
• P = IV
• P - Power (W)
• I – Current (A)
• V – Voltage (V)
Space on Next Pages!
Ohm’s Law
• V = IR
• V – Voltage (V)
• I – Current (A)
• R – Resistance (Ω)
What Type of Circuit is Better?
• Why?
Series Parallel
Current is the SAME through all Current ‘’SPLITS UP’’ through
series components parallel branches
Voltages SPLITS and adds to the Voltage is THE SAME across all
total circuit voltage parallel branches
Resistance: Adding resistance Adding resistance reduces total R
increases total R
It’s that time
• What time is it?
That time
• Time for a highly patronising simulation
Checkpoint – A potential difference of 24V is applied to a 150Ω resistor. How
much current flows through the resistor?
Answer – A potential difference of 24V is applied to a 150Ω resistor. How much
current flows through the resistor?
Resistor – Symbols
Resistance
Ideal Wire - 0Ω
Resistor
(added in to indicate
resistance of circuit wires)
Resistance – Ohm’s Law
•
• V – Voltage (V)
• I – Current (A)
• R – Resistance (Ω)
Resistance – Ohm’s Law
•
• Special case, when I α V for a constant temperature
• Why?
Resistance – Temperature
• Resistance increases with temperature
• Heating a metal conductor causes atoms to vibrate more; makes it more
difficult for the electrons to flow
Resistance – Temperature (I sketch)
Section Objectives
• To understand resistance MORE
• To understand resistivity
• To apply our formulas/understanding to circuits
Let’s Think - Inherent Property
• The mass of this cube ultimately depends on _________
Let’s Think - Inherent Property
• The material it’s made from…..it’s density
Density
• An inherent property of the material
Let’s Think - Inherent Property
• The resistance of this wire ultimately depends on _________
Let’s Think - Inherent Property
• The material it’s made from….it’s resistivity
Checkpoint
• Which material makes the best wire? Why?
Answer
• Silver. Lowest resistivity. (Highest conductivity)
But
•We use copper because it’s cheaper, and still kinda good…
Resistivity of a wire
•Higher resistivity = higher resistance
Conductivity of a wire
•Lower resistivity = higher conductivity
Resistivity - Defined
• “A measure of the resisting power of a specified material to the flow of an
electric current.”
• An inherent property of a material
Bad analogy?
Let’s Think
• Water flows easier when your straw is…
Let’s Think
• Short and fat straw = less resistance
(what an idiot)
Resistance
• Therefore depends on which three quantities?
Resistance
• Length ‘L’
• Cross-sectional Area ‘A’
• Resistivity of the material
• Can you write a formula for resistance?
Resistance “R”
•R=
It’s That Time Again!
• Highly patronizing simulation time!
• https://phet.colorado.edu/sims/html/resistance-in-a-wire/latest/resistance-in-a-wire_en.html
Resistivity “ “
•
• Has a huge range!
• What makes a great circuit wire?
• What doesn’t? Why?
Resistivity “ “
•
•https://phet.colorado.edu/sims/html/resistance-in-a-
wire/latest/resistance-in-a-wire_en.html
• Look at the simulation. What happens to “R”
with a change in “ ?
Resistance “R” and Resistivity “ ”
•
• Resistance “R” is directly proportional to resistivity
•Which means….
Ohm’s Law
•I=
• Less current!
Simulation Time – Again!
•https://phet.colorado.edu/sims/html/ohms-law/latest/ohms-law_en.html
Section Objective
• To understand resistance in series/parallel circuits
• To calculate effectively
Ohm’s Law
• Will help you here!
• V = IR
Section Objective
• To understand resistance in series/parallel circuits
• To calculate effectively
Resistors In Series
• Rtotal = R1 + R2 + R3 ….
Resistors In Parallel
• = + + ….
Rtotal 𝟏 𝟐 𝟑
Power Through A Resistor
• What is it?
Power Through A Resistor
• Hint. Combine:
• V = IR
• P = IV
Power Through A Resistor
•Power = I x IR = I2R Or P = (V/R)V = V2 /R
• P = I2R
𝟐
•P=
Current
Electricity 2
NEXT STUFF ON ELECTRICITY
Section Objective
• To understand potential dividers
4 x 1.5V Batteries in Series
•PhET Time!
In Circuit
• As we move around the circuit, the voltage should drop
from 6V to 0V
In Circuit
• Assuming the wires have negligible resistance, and the resistors
are identical, the potentials will be as shown
Potential Dividers
IMPORTANT
•We use this when the voltage across components is different!
IMPORTANT
• Most questions we did already assume that the bulbs are the same
Remember: Potential Divider Equation
• We may need this when the two bulbs/resistors are different!
Section Objective
• To understand EMF and Internal Resistance
Reminder
•Voltage =
•V =
Terminal Potential Difference
