5. Electricity

Question 1

Electric Current

Answer 1

Rate of flow of charge

Question 2

Potential Difference

Answer 2

Work done per unit charge

Question 3

Current Equation

Answer 3

I = ΔQ / Δt
I is current in A
ΔQ is change in charge in C
Δt is change in time in s

Question 4

PD Equation

Answer 4

V = W / Q
V is potential difference in V
W is work done in J
Q is charge in C

Question 5

Resistance Defined as ____

Answer 5

R = V / I

Question 6

Conservation of ____ in DC Circuits

Answer 6

Charge and energy

Question 7

Potential Divider

Answer 7

A combination of resistors in series connected across a voltage source to produce a required pd

Question 8

Examples of Potential Dividers Include the Use of (3)

Answer 8

• Variable resistors
• Thermistors
• Light dependent resistors (LDRs)

Question 9

Internal Resistance

Answer 9

The resistance inside a power supply (from electrons colliding with atoms in the power supply)

Question 10

EMF Equations (2)

Answer 10

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

Question 11

Terminal PD

Answer 11

The potential difference across the load / external resistance

Question 12

EMF

Answer 12

Electromotive force is the amount of chemical energy converted to electrical energy per unit charge by the power supply

Question 13

Resistivity

Answer 13

The resistance of a 1 m length of a material with 1 m² cross-sectional area

Question 14

Resistivity Equation

Answer 14

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

Question 15

Qualitative Effect of Temperature on Resistance of Metal Conductors

Answer 15

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

Question 16

Qualitative Effect of Temperature on Resistance of Thermistors

Answer 16

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

Question 17

Applications of Thermistors (2)

Answer 17

• Temperature sensors
• Resistance-temperature graphs

Question 18

Superconductivity

Answer 18

A property of certain materials, which have zero resistivity at and below a critical temperature, which depends on the material

Question 19

Applications of Superconductors (2)

Answer 19

• Production of strong magnetic fields
• Reduction of energy loss in transmission of electric power

Question 20

Current-Voltage Characteristics for an Ohmic Conductor

Answer 20

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

Question 21

Current-Voltage Characteristics for a Semiconductor Diode

Answer 21

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

Question 22

Current-Voltage Characteristics for a Filament Lamp

Answer 22

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

Question 23

Ohm’s Law

Answer 23

A special case where I ∝ V under constant physical conditions

Question 24

Resistors in Series

Answer 24

R_T = R₁ + R₂ + R₃ + …

Question 25

Resistors in Parallel

Answer 25

1 / R_T = 1 / R₁ + 1 / R₂ + 1 / R₃ + ...

Question 26

Energy Equation

Answer 26

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

Question 27

Power Equation

Answer 27

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

Question 28

Relationship between Currents, Voltages & Resistances in Series Circuits (3)

Answer 28

- 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

Question 29

Relationship between Currents, Voltages & Resistances in Parallel Circuits (3)

Answer 29

- 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

Question 30

Conservation of Charge

Answer 30

Kirchoff's 1st law: the total current entering a junction = the total current leaving it

Question 31

Conservation of Energy

Answer 31

Kirchoff's 2nd law: the total emf around a series circuit = the sum of the pds across each components

Question 32

Required Practical 5

Answer 32

Determination of resistivity of a wire using a micrometer, ammeter and voltmeter

Question 33

Required Practical 5 Method (5)

Answer 33

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

Question 34

Required Practical 6

Answer 34

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

Question 35

Required Practical 6 Method (3)

Answer 35

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