2. Current of Electricity

Question 1

Electric Current (I)

Definition

Answer 1

Electric current is the rate of flow of electric charge (Q).

Unit: ampere (A)
I = Q / t
current = electric charge / time

Question 2

Conventional Current (I)

Definition

Answer 2

Conventional current is the flow of positive charges from the positive to the negative terminal (from high to low potential).

Question 3

Electron Flow (e^-)

Answer 3

Electron flow is the flow of electrons from the negative to the positive terminal (from low to high potential).

Question 4

Electromotive Force (ϵ)

Definiton

Answer 4

Electromotive force (e.m.f.) of an electrical source is the work done by the source in driving a unit charge around a complete circuit.

Unit: volt (V)
ϵ = W / Q
e.m.f. = work done / electric charge

Chemical potential energy of cell is transferred to kinetic energy of the electric charge as it flows through the circuit (electrical energy).

Question 5

Potential Difference (V)

Definition

Answer 5

Potential difference (p.d.) across a component in an electric circuit is the work done to drive a unit charge through the component.

Unit: volt (V)
V = W / Q
p.d. = work done / electric charge

Kinetic energy of the electric charge as it flows through the circuit (electrical energy) is transferred to other forms of energy by the component.

Question 6

Resistance (R)

Definition

Answer 6

Resistance of a component is the ratio of the potential difference (V) across it to the current (I) flowing through it.

Unit: ohm (Ω)
R = V / I
resistance = p.d. / current

Question 7

Resistance of a Wire

4 Factors which affect it

Answer 7

1. Length of wire (longer wire, more collisions between electrons and metallic ions of wire, higher resistance)
2. Cross-sectional area (larger cross-section, more elctrons can flow through it per unit time, lower resistance)
3. Type of material (affects resistivity)
4. Temperature (higher temperature, metallic ions vibrate more vigorously, more collisions between electrons and metallic ions of wire, higher resistance)

R (resistance) = ρℓ / A
resistance = resistivity x length / cross-sectional area

Question 8

Resistivity (ρ)

Definition

Answer 8

Resistivity is a fixed property of a material that measures its electrical resistance or how strongly it resists electric current at a certain temperature.

Metals with lower resistivities:

• Copper: good conductor of electricity

Metals with higher resistivities :

• Nichrome: Generate a lot of heat when current passes through it (used in heating coils of electric kettles)
• Tungsten: Converts electrical energy to light and thermal energy (used as a filament in light bulbs)

Question 9

Ohm’s Law

Definition

Answer 9

Ohm’s law states that the current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions such as temperature remain constant.

I ∝ V
V = IR
voltage = current x resistance

Question 10

Ohmic Conductors vs Non-Ohmic Conductors

Answer 10

Ohmic conductors obey Ohm’s law:

• Current passing through them has a directly proportional relationship with potential difference across them.
• Constant resistance at different currents/potential differences

Non-ohmic conductors do not obey Ohm’s law:

• Current passing through them does not have a directly proportional relationship with potential difference across them
• Resistance changes at different currents/potential differences

Question 11

Semiconductor Diode

Answer 11

Semicondcutor diode:

• Only allows electric current to flow in 1 direction
• Conducts current (low resistance) when it is forward-biased
• No current flows (high resistance) when it it reverse-biased
• Used to convert from alternating current (a.c.) to direct current (d.c.)