A-LEVEL Physics 3.5.1: Electricity PMT Flashcards
What is ‘Electric Current’?
Current is the Rate of Flow of Charge.
Current = …
Charge / Time
I = …
ΔQ / Δt
What does ‘Δ’ Mean?
Change in. Eg Δt = Change in Time.
What is ‘Potential Difference’?
Potential Difference in the Energy Transferred per Unit Charge, between 2 Points in a Circuit.
Potential Difference = …
Energy Transferred / Charge
What is ‘Resistance’?
Resistance is the Measure of how Difficult it is for Charge Carriers to Pass Through a Component.
Resistance = …
Potential Difference(Voltage) / Current
R = …
V / I
What Can you say about the Potential Difference (Voltage) between 2 Points in a Circuit?
The Potential Difference (Voltage) between 2 Points in a Circuit is the Work Done per Unit Charge Passing between the 2 Points.
P.D. = …
W / Q
=
ΔE / Q
What is ‘Ohm’s Law’?
Ohm’s Law States that the Potential Difference (Voltage) Across an Ohmic Conductor is Directly Proportional to the Current Flowing Through it, Provided all Physical Conditions (eg Temperature) Remain Constant.
V=IR
What is an ‘Ohmic Conductor’?
An Ohmic Conductor is a Conductor that Obeys Ohm’s Law.
What will the Current-Voltage Graph of an Ohmic Conductor look like? Why?
The Current-Voltage Graph of an Ohmic Conductor Will be a Straight Line Through the Origin. Because Ohmic Conductors Obey Ohm’s Law, meaning that their Current is Directly Proportional to their Potential Difference (Voltage). (This is Provided Physical Condition Remain Constant.)
What is a ‘Semiconductor Diode’?
A Semiconductor Diode has a Forwards Bias & Reverse Bias:
-The Forward Bias of a Diode is the Direction in which it Will Allow Current to Flow Easily Past the ‘Threshold Voltage’ (which is the Smallest Voltage Needed to let Current Flow)
-In the Direction of the Reverse Bias, the Resistance of the Diode is Extremely High, Meaning that Only a Very Small Current Can Flow.
-
What is a ‘Filament Lamp’?
A Filament Lamp is a Component that Contains a Length of Metal Wire, which Heats Up as the Current through it Increases. Therefore, the Resistance of this Component Increases as Current Increases. (Higher Temp = Higher Resistance; more difficult for Charge-Carrying Electrons to get Through). At Low Currents, the Metal Wire Will not Heat Up significantly. Therefore, for very Low Currents, Ohm’s Law is Obeyed. However, as the Current Through the Filament Lamp Increases (in either Direction), the Current-Voltage Graph begins to Curve, Due to the Increasing Resistance.
When is Ohm’s Law Obeyed in a Filament Lamp? Why?
In a Filament Lamp, Ohm’s Law is Obeyed only at Low Currents. This is because at Low Currents, the Metal Wire in the Filament Lamp Will not Heat up much. Hence, there Will be Less Resistance in the Filament Lamp at Lower Currents, & the Current Will be Directly Proportional to the P.D. (Voltage).
Explain what the Current-Voltage Graph of a Filament Lamp Looks like. Why is this?
The Current-Voltage Graph of a Filament Lamp is Curved Through the Origin. This is Because if the Current through the Metal Wire in a Filament Lamp gets too High, the Metal Wire Will Heat up, Leading to an Increase in Resistance. When the Current is Low, this does not Happen, & the Current through the Metal Wire is Directly Proportional to the P.D. (Voltage) (it obeys Ohm’s Law).
Unless a Question States Otherwise, Ammeters Can be Assumed to… Why is this?
Have 0 Resistance. This is so that they Will Not Affect the Measurement of Current in a Circuit at All.
Unless a Question States Otherwise, Voltmeters Can be Assumed to… Why is this?
Have Infinite Resistance. This is so that No Current Can Flow Through them, Meaning that their Measurement of Voltage (Potential Difference) Across a Component is Exact.
What is ‘Resistivity’?
Resistivity (p-rho) is a Measure of how Easily a Material Conducts Electricity.
R = p L/A
p is Resistivity
It is an Intrinsic Property of the Material.
Unit: Ohmic Meters
What Will ‘Resistivity’ give?
Resistivity Will give the Value of Resistance Through a Material of Length 1m & Cross-Sectional Area of 1m^2.
Resistivity is Also Dependent on Environmental Factors, such as…
Temperature.
As the Temperature of a Metal Conductor Increases, …
Its Resistance Will also Increases.
In a Metal Conductor, the Temperature & the Resistance are…
Directly Proportional.
As the Temperature of a Metal Conductor Increases, its Resistance Will also Increase. Why is this?
This is Because the Atoms of the Metal Gain Kinetic Energy, & Move More, which Causes the Charge Carriers (the Electrons) to Collide with the Atoms More Frequently, Causing them to Slow Down. Therefore, Current Decreases, & so Resistance Increases (as R = V/I).
What does a Temperature-Resistance Graph look like?
It Will be a Downwards Curve.
In Thermistors, as the Temperature Increases, …
Resistance Decreases.
In Thermistors, Temperature & Resistance are…
Inversely Proportional.
In Thermistors, as the Temperature Increases, the Resistance Decreases. Why is this?
This is Because Increasing the Temperature of a Thermistor Causes Electrons to be Emitted from Atoms, therefore the Number of Charge Carriers (Electrons) Increases, and so Current Increases, Causing Resistance to Decrease.
What is One Application of a Thermistor in a Circuit?
One Application of a Thermistor in Circuits is a Temperature Sensor, which Can Trigger an Event to Occur Once the Temperature Drops or Reaches a Certain Value.
e.g. it Could be Used to Turn On the Heating Once the Room Temperature Drops Below a Certain Value.
What is a ‘Superconductor’?
A Superconductor is a Material which, Below a Certain Temperature, known as the ‘Critical Temperature’, has Zero Resistivity.
What is ‘Critical Temperature’?
When the Temperature of a Superconductor Passes Below the Critical Temperature, the Superconductor will have Zero Resistivity.
The Critical Temperature of a Superconductor Depends on the Material it is Made Out of. Most known Superconductors have an Extremely Low Critical Temperature which lies Close to 0 K.
With a Resistivity of Zero, ___ also Drops to Zero.
Resistance.
With a Resistivity of Zero, the Resistance also Drops to Zero. Therefore, the Applications of Superconductors Include: (2)
-Power Cables: Which would Reduce Energy Loss through Heating to Zero during Transmission.
-Strong Magnetic Fields: Which would Not Require a Constant Power Source. These Could be Used in Maglev Trains, where there Would be No Friction Between the Train & the Rail. Also Useful in Certain Medical Applications.
What is the Rule for Adding the Resistances of Resistors in Series?
In Series:
Rt = R1 +R2 + R3 …
(Where Rt is Total Resistance, & Rn is the Resistance of Resistor n)
What is the Rule for Adding the Resistances of Resistors in Parallel?
In Parallel:
1/Rt = 1/R1 + 1/R2 + 1/R3 …
(Where Rt is Total Resistance, & Rn is the Resistance of Resistor n)
What is ‘Power’?
Power is the Rate of Energy Transfer.
Power is Energy Transferred over Time Taken.
Power = …
Energy Transferred / Time Taken
Power = …
Potential Difference * Current
Potential Difference = …
Current * Resistance
As Power is the Energy Transferred over Time Taken, the Product of Power & Time is…
The Energy Transferred.
Therefore:
E = V I t
Energy Transferred = Potential Difference * Current * Time Taken
In a Series Circuit, the Current…
Is the Same Everywhere in the Circuit.
In a Series Circuit, the ___ is the Same Everywhere in the Circuit.
Current.
In a Series Circuit, the Current is the ___ Everywhere in the Circuit.
Same.
In a Series Circuit, The Battery P.D. is Shared Across all Elements in the Circuit, therefore…
The Total Sum of the Voltages Across All Elements is Equal to the Supply P.D..
What Can you Say about the Potential Difference in a Series Circuit?
In a Series Circuit, the Battery P.D. is Shared Across all Elements in the Circuit. Therefore, the Total Sum of the Voltages Across all Elements in the Circuit is Equal to the Battery Supply of P.D..
In a Parallel Circuit, The Sum of Current in Each Parallel Set of Branches is Equal to…
The Total Current.
What Can you Say about the Current in a Series Circuit?
In a Series Circuit, the Current is the Same Everywhere in the Circuit.
What Can you Say about the Current in a Parallel Circuit?
In a Parallel Circuit, the Sum of the Current of Each Parallel Branch is Equal to the Total Current in the Circuit.
What Can you Say about the Potential Difference in a Parallel Circuit?
In a Parallel Circuit, the Potential Difference Across Each Branch is the Same.
In a Parallel Circuit, the ___ ___ Across Each Branch is the Same.
Potential Difference (Voltage).
In a Parallel Circuit, the Sum of the ___ of Each Parallel Branch is Equal to the Total ___ in the Circuit.
Current, Current.
When Joining together Battery Cells, you Can Use Either a ___ or ___ Configuration.
Series, Parallel.
In a Series Circuit, the ___ is the Same at any Point in the Circuit.
Current.
When Cells are Joined in Series, …
The Total Voltage Across the Cells is Equal to the Sum of the Individual Voltages of the Cells:
VT = V1 + V2 + V3 …
When Cells are Joined in ___, the Total Voltage Across the Cells is Equal to the Sum of the Individual Voltages Across the Cells.
Series.
When Identical Cells are Joined in Parallel, the Total Voltage is Equal to the…
Voltage of One Cell.
This is Because the Current is Split Equally Between Branches. Therefore, the Overall Potential Difference is the Same as if the Total Current was Flowing Through a Single Cell:
VT = V1 = V2 = V3 = …
When ___ Cells are Joined in ___, the Total Voltage is Equal to the… Why is this?
Identical, Parallel.
Voltage of One Cell.
This is Because the Current is Split Equally Between Branches. Therefore, the Overall Potential Difference is the Same as if the Total Current was Flowing Through a Single Cell:
VT = V1 = V2 = V3 = …
In DC Circuits, Charge & Energy are Always ___.
Conserved.
In DC Circuits, Charge & Energy are Always ___. Kirchoff’s 2 Laws Describe how this is Achieved: (2)
Conserved.
-Kirchoff’s First Law: The Total Current Flowing Into a Junction is Equal to the Current Flowing Out of that Junction. This Shows that No Charge is Lost at any Point in the Circuit.
-Kirchoff’s Second Law: The Sum of all the Voltages in a Series Circuit is Equal to the Battery Voltage. This Shows that No Energy is Lost at any Point in a Circuit.
What is ‘Kirchoff’s First Law’?
The Total Current Flowing Into a Junction is Equal to the Current Flowing Out of that Junction. This Shows that No Charge is Lost at any Point in the Circuit.
What is ‘Kirchoff’s Second Law’?
The Sum of all the Voltages in a Series Circuit is Equal to the Battery Voltage. This Shows that No Energy is Lost at any Point in a Circuit.
What is a ‘Potential Divider’?
A Potential Divider is a Circuit with Several Resistors in Series, Connected Across a Voltage Source, Used to Produce a Required Fraction of the Source Potential Difference, which Remains Constant.
You can Also Make a Potential Divider Supply a ___ Potential Difference by Using a ___ ___ as One of the Resistors in Series, therefore by Varying the Resistance Across it, you Can Vary the Potential Difference Output.
Variable, Variable Resistors.
In a Light Dependent Resistor (LDR), as Light Intensity Increases, …
Resistance Decreases.
In an LDR, Light Intensity & Resistance are…
Inversely Proportional
Batteries have an ___ ___, which is Caused by…
Internal Resistance.
Electrons Colliding with Atoms Inside the Battery. Therefore, some Energy is Lost Before Electrons even Leave the Battery.
What is ‘Electromotive Force’?
Electromotive Force is the Energy Transferred by a Cell per Coulomb of Charge that Passes through it:
E.F. = E / Q
Electromotive Force = …
Energy Transferred / Charge
In a Circuit, the Sum of the Internal Resistance (r) & the Load Resistance (R) = …
The Total Resistance (RT)
RT = r + R
The P.D. Across Resistor R is Known as the ___ ___, Whereas the P.D. Across the Resistor r is Known as ___ ___. This is Because this Value is Equal to the…
Terminal P.D. (V), Lost Volts (v).
Energy Wasted by the Cell Per Coulomb of Charge.
How Can you Measure the emf of a Battery?
The emf of a Battery Can be Measured by Measuring the Voltage Across a Cell Using a Voltmeter, When there is No Current running through the Cell, which means it is an Open Circuit.
What is the Difference between Resistance & Resistivity?
Resistance is the Obstruction to the Flow of Current given by a Conductor. Resistivity is the Obstruction offered to the Flow of Current by UNIT AREA OF the Conductor.
e.g: Resistance is the Property of the Object (say a Copper Wire), whereas Resistivity is the Property of the Substance (Copper).
emf is the Sum of…
Terminal P.D. + Lost Volts
e = V + v
emf is the Product of…
Why is this?
Total Resistance * Current
e = IR + Ir
can also be written as: e = I(R + r)
This is because V=IR
