5. Electricity Flashcards
Electric Current
Rate of flow of charge
Potential Difference
Work done per unit charge
Current Equation
I = ΔQ / Δt
I is current in A
ΔQ is change in charge in C
Δt is change in time in s
PD Equation
V = W / Q
V is potential difference in V
W is work done in J
Q is charge in C
Resistance Defined as ____
R = V / I
Conservation of ____ in DC Circuits
Charge and energy
Potential Divider
A combination of resistors in series connected across a voltage source to produce a required pd
Examples of Potential Dividers Include the Use of (3)
- Variable resistors
- Thermistors
- Light dependent resistors (LDRs)
Internal Resistance
The resistance inside a power supply (from electrons colliding with atoms in the power supply)
EMF Equations (2)
ε = E / Q, ε = I (R + r)
ε is emf in V
E is electrical energy in J
Q is charge in C
I is current in A
R is load / external resistance in Ω
r is internal resistance in Ω
Terminal PD
The potential difference across the load / external resistance
EMF
Electromotive force is the amount of chemical energy converted to electrical energy per unit charge by the power supply
Resistivity
The resistance of a 1 m length of a material with 1 m² cross-sectional area
Resistivity Equation
ρ = R A / L
ρ is resistivity in Ω m
R is resistance in Ω
A is cross-sectional area in m²
L is length in m
Qualitative Effect of Temperature on Resistance of Metal Conductors
Increasing temperature increases resistance of a metal conductor as the nuclei in the material vibrate more so are more likely to collide with electrons, restricting their flow
Qualitative Effect of Temperature on Resistance of Thermistors
Increasing temperature decreases the resistance of a thermistor as heating the material gives electrons enough energy to escape from atoms so more charge carriers, that can flow, are present, decreasing resistance
Applications of Thermistors (2)
- Temperature sensors
- Resistance-temperature graphs
Superconductivity
A property of certain materials, which have zero resistivity at and below a critical temperature, which depends on the material
Applications of Superconductors (2)
- Production of strong magnetic fields
- Reduction of energy loss in transmission of electric power
Current-Voltage Characteristics for an Ohmic Conductor
https://revise.im/content/02-physics/01-unit-1/03-current-electricity/IVohmic.jpg
Current is directly proportional to voltage so the characteristic graph is a straight line
Current-Voltage Characteristics for a Semiconductor Diode
https://th.bing.com/th/id/OIP.Mgrw_KjjxjT9kDlb2uC3fgHaGA?pid=ImgDet&rs=1
Current will only flow in the forward bias after the threshold voltage (usually ~0.6 V) has been exceeded, but very little current can flow in the reverse bias
Current-Voltage Characteristics for a Filament Lamp
https://images.twinkl.co.uk/tr/image/upload/illustation/Filament-Lamp-Iv-graph-Electricity-Science-KS4-bw-RGB.png
Current starts to increase with voltage but then it plateaus because a higher current heats up the thin filament increasing the resistance, which reduces current
Ohm’s Law
A special case where I ∝ V under constant physical conditions
Resistors in Series
R_T = R₁ + R₂ + R₃ + …
Resistors in Parallel
1 / R_T = 1 / R₁ + 1 / R₂ + 1 / R₃ + …
Energy Equation
E = I V t
E is energy in J
I is current in A
V is potential difference in V
t is time in s
Power Equation
P = I V = I² R = V² / R
P is power in W
I is current in A
V is potential difference in V
R is resistance in R
Relationship between Currents, Voltages &
Resistances in Series Circuits (3)
- There will be the same current at all points of the circuit
- The terminal pd is split between components, so: V = V₁ + V₂ + V₃ + …
- The terminal pd splits proportionally to the resistance as V = I R
Relationship between Currents, Voltages &
Resistances in Parallel Circuits (3)
- The current is split at each junction, so: I = I₁ + I₂ + I₃ + …
- There is the same pd across each branch of the circuit, which is equal to the terminal pd
- The current is split inversely proportionally to the resistance as I = V / R
Conservation of Charge
Kirchoff’s 1st law: the total current entering a junction = the total current leaving it
Conservation of Energy
Kirchoff’s 2nd law: the total emf around a series circuit = the sum of the pds across each components
Required Practical 5
Determination of resistivity of a wire using a micrometer, ammeter and voltmeter
Required Practical 5 Method (5)
https://docs.google.com/document/d/1_5bXHrnEENfssSP0dCiQRqP–_FMR0H8x5I93OxipJM/edit?usp=sharing
1. Setup the apparatus as shown in the diagram
2. Measure the diameter of the wire using a micrometre
3. Attach a known length of wire to the circuit
4. Close the switch and read the current and pd and then open the switch
5. Repeat steps 3 and 4, varying the length of the wire and trying to keep the current constant using the variable resistor
Required Practical 6
Investigation of the emf and internal resistance of electric cells and batteries by measuring the variation of the terminal pd of the cell with current in it
Required Practical 6 Method (3)
https://docs.google.com/document/d/1WBHQoWRPvcF_r6iEzcOAXu8s2-Tc5mSROuqsQjpgmU4/edit?usp=sharing
1. Set up the circuit as shown in the diagram
2. Record the potential difference across the variable resistor and the current
3. Repeat step 2, adjusting the variable resistor to change the current through the circuit
