Formulas for CT4

Question 1

Charge formulas x2

• Note on charge

Answer 1

Q = It
Q = ne

Q is electrical charge in coulombs (C)
I is current in amperes (A)
t it time in seconds (s) 
n is the number of electrons
e is charge on one electron (1.6x10^-19)

• Charge is a quantised property

Question 2

Current formula

Answer 2

I = Anqv

I is current in amperes (A)
A is area in m²
n in the number density of charge carriers in the material
q is electrical charge in coulombs (C)
v is the drift speed of the electrons in m⁻¹

Question 3

Potential difference definition

Answer 3

Energy transferred per unit charge

Question 4

Voltage formulas x2

Answer 4

V = W/Q
V = IR

Question 5

Power formulas x3

Answer 5

P = I²R 
P = VI
P = V²/R

Question 6

I/V Graph of metallic conductor at constant temp, semiconductor diode and filament lamp

Answer 6

Metallic conductor at const temp is positive linear

Non-ohmic resistor is curved accelerating towards v or decelerating away from I

Linear at zero from negative till just over 0 and then positive linear between I/V

Negative half parabola till zero then sloping into a positive half parabola in positive

Question 7

Ohm’s Law

Answer 7

For a metallic conductor at constant temperature the current in the conductor is proportional to the potential difference across it

Question 8

Resistivity formula

Answer 8

R = ρL / A

Question 9

Resistance definition

Answer 9

Resistance is a measure of the opposition to current flow in an electrical circuit.

Question 10

Resistance in terms of a filament lamp

Answer 10

Resistance in a filament lamp increases as the current increases as does the temperature

Question 11

Resistance in terms of an LDR

Answer 11

Resistance in an LDR decreases as the light intensity increases

Question 12

Resistance in terms of a thermistor

Answer 12

Resistance of a thermistor decreases as the temperature increases

Question 13

Emf definition

Answer 13

Energy transferred per unit charge in driving charge around a complete circuit

Question 14

Difference between emf and pd

Answer 14

Emf is the driving electric force from a power supply, the pd results from current passing through resistance within a circuit

Question 15

Effect of internal resistance on emf

Answer 15

Decreases voltage supplied by power source to circuit or reduces pd across circuit

Question 16

Factors of current

Answer 16

pd carried by each coulomb of charge and nature of compound (resistance)

Question 17

Factors of resistance

Answer 17

Longer length greater resistance

Greater cross sectional area lesser resistance

Question 18

Emf formula

Answer 18

ε = I(R + r)
Iᵣ = ε - Vᵣ

Question 19

Power resistance graph

Answer 19

Negative parabola where in the middle drawn a line down is r

Question 20

Voltage current graph with emf

Answer 20

Emf is y-int and is a negative linear curve with the gradient equal to negative r

Question 21

Current voltage graph with emf

Answer 21

Emf is y-int and is a negative linear curve with the gradient equal to 1 / negative r

Question 22

Total resistance in series

Answer 22

ε = I(R₁ + R₂ + R₃)

Question 23

Total resistance in parallel

Answer 23

ε = I(R₁⁻¹ + R₂⁻¹ + R₃⁻¹)⁻¹

Question 24

Potential divider purpose

Answer 24

A circuit with a potential divider can allocate different voltages across different components

Question 25

Kirchhoff’s first law

Answer 25

Charge conservation, ΣIᵢₙ = ∑Iₒᵤₜ

Question 26

Kirchhoff’s second law

Answer 26

Voltage law, ∑V = 0
where in the direction of current power suppliers are added and resistance or power dissipators are subtracted

or against the direction of current power suppliers are subtracted and resistance or power dissipators are added

through a cell the current flows out from the larger terminal of the cell