Current and Charge
Definitions:
Electric Current ( I ): The rate of flow of charge.
The Coulomb (C ): The unit of electric charge. One Coulomb is
defined as the amount of charge that passes a point in a circuit in 1
second when the current is 1 Ampere (1 C=1 As).
Key Equation:
ΔQ
I=
Δt
I = Current (Amperes, A)
Q = Charge (Coulombs, C)
t = Time (Seconds, s)
Quantisation of Charge:
Charge is quantised, meaning it comes in discrete packets.
The smallest possible non-zero charge is the elementary charge (e
).
C (This is given in your data sheet).
−19
e=1.60 ×10
The net charge on an object is always a multiple of e :
Q=±ne (Where n is the number of electrons added or removed).
2. Direction of Flow
You must distinguish between the two ways of describing flow in a circuit:
1. Conventional Current:
Flows from Positive (+) to Negative (-).
This is used for all circuit analysis, arrow diagrams, and rules
(like Fleming's Left Hand Rule).
2. Electron Flow:
Electrons are negatively charged, so they are repelled by the
negative terminal and attracted to the positive.
Actual electron flow is from Negative (-) to Positive (+).
Charge Carriers:
, Metals: The charge carriers are delocalised electrons.
Electrolytes (Liquids): The charge carriers are ions (both positive
and negative).
3. Kirchhoff’s First Law
Definition:
The sum of currents entering a junction is equal to the sum of currents
leaving the junction.
Physics Principle:
This is a statement of the Conservation of Charge. Charge cannot be
created or destroyed; therefore, whatever flows into a junction must flow
out.
Equation:
∑ I in =∑ I out
4. Mean Drift Velocity ( I = Anev )
This is the most common calculation topic for this section.
The Concept:
Inside a wire, electrons move randomly at very high speeds
(approx 105 ms−1). When a potential difference is applied, they drift slowly
towards the positive terminal. This slow, overall movement is the Mean
Drift Velocity (approx 10−4 ms−1 ).
The Equation:
I = Anev
Breakdown of Variables:
I : Current (Amperes, A)
A : Cross-sectional Area (m2).
2 π d2
Watch out: Wires are usually cylinders. Area = π r or .
4
Unit Trap: If diameter is given in mm, convert to meters first (
−3
1 mm=1 ×10 m).
n : Number density of charge carriers (m−3).
, This represents the number of free electrons per cubic metre
of material.
e : Elementary charge (1.60 ×10−19 C).
v : Mean drift velocity (m s−1).
5. Classification of Materials
Materials are classified as conductors, semiconductors, or insulators based
on their Number Density (n ).
1. Conductors (Metals):
Very high number density (n ≈ 1028 m−3).
Because n is so high, even a small drift velocity ( v) produces a
large current ( I ).
2. Semiconductors (Silicon, Germanium):
Intermediate number density (n ≈ 1019 m−3).
Unique property: As temperature increases, n
increases (more electrons break free), so resistance
decreases.
Drift velocity needs to be much higher than in metals to
maintain the same current.
3. Insulators (Rubber, Plastic):
Very low number density (n ≈ 0).
Almost no free electrons to carry charge
Energy, Power, and Resistance
1. Electromotive Force (e.m.f.) vs Potential Difference (p.d.)
Both are measured in Volts (V), which is defined as Joules per Coulomb
(1 V=1 JC−1). However, the energy transfer happens in opposite directions.
Potential Difference (V ):
Definition: The energy transferred from electrical energy to other
forms (heat, light, motion) per unit charge.
Example: Across a bulb or resistor.
Definitions:
Electric Current ( I ): The rate of flow of charge.
The Coulomb (C ): The unit of electric charge. One Coulomb is
defined as the amount of charge that passes a point in a circuit in 1
second when the current is 1 Ampere (1 C=1 As).
Key Equation:
ΔQ
I=
Δt
I = Current (Amperes, A)
Q = Charge (Coulombs, C)
t = Time (Seconds, s)
Quantisation of Charge:
Charge is quantised, meaning it comes in discrete packets.
The smallest possible non-zero charge is the elementary charge (e
).
C (This is given in your data sheet).
−19
e=1.60 ×10
The net charge on an object is always a multiple of e :
Q=±ne (Where n is the number of electrons added or removed).
2. Direction of Flow
You must distinguish between the two ways of describing flow in a circuit:
1. Conventional Current:
Flows from Positive (+) to Negative (-).
This is used for all circuit analysis, arrow diagrams, and rules
(like Fleming's Left Hand Rule).
2. Electron Flow:
Electrons are negatively charged, so they are repelled by the
negative terminal and attracted to the positive.
Actual electron flow is from Negative (-) to Positive (+).
Charge Carriers:
, Metals: The charge carriers are delocalised electrons.
Electrolytes (Liquids): The charge carriers are ions (both positive
and negative).
3. Kirchhoff’s First Law
Definition:
The sum of currents entering a junction is equal to the sum of currents
leaving the junction.
Physics Principle:
This is a statement of the Conservation of Charge. Charge cannot be
created or destroyed; therefore, whatever flows into a junction must flow
out.
Equation:
∑ I in =∑ I out
4. Mean Drift Velocity ( I = Anev )
This is the most common calculation topic for this section.
The Concept:
Inside a wire, electrons move randomly at very high speeds
(approx 105 ms−1). When a potential difference is applied, they drift slowly
towards the positive terminal. This slow, overall movement is the Mean
Drift Velocity (approx 10−4 ms−1 ).
The Equation:
I = Anev
Breakdown of Variables:
I : Current (Amperes, A)
A : Cross-sectional Area (m2).
2 π d2
Watch out: Wires are usually cylinders. Area = π r or .
4
Unit Trap: If diameter is given in mm, convert to meters first (
−3
1 mm=1 ×10 m).
n : Number density of charge carriers (m−3).
, This represents the number of free electrons per cubic metre
of material.
e : Elementary charge (1.60 ×10−19 C).
v : Mean drift velocity (m s−1).
5. Classification of Materials
Materials are classified as conductors, semiconductors, or insulators based
on their Number Density (n ).
1. Conductors (Metals):
Very high number density (n ≈ 1028 m−3).
Because n is so high, even a small drift velocity ( v) produces a
large current ( I ).
2. Semiconductors (Silicon, Germanium):
Intermediate number density (n ≈ 1019 m−3).
Unique property: As temperature increases, n
increases (more electrons break free), so resistance
decreases.
Drift velocity needs to be much higher than in metals to
maintain the same current.
3. Insulators (Rubber, Plastic):
Very low number density (n ≈ 0).
Almost no free electrons to carry charge
Energy, Power, and Resistance
1. Electromotive Force (e.m.f.) vs Potential Difference (p.d.)
Both are measured in Volts (V), which is defined as Joules per Coulomb
(1 V=1 JC−1). However, the energy transfer happens in opposite directions.
Potential Difference (V ):
Definition: The energy transferred from electrical energy to other
forms (heat, light, motion) per unit charge.
Example: Across a bulb or resistor.
