Section Nine: Potential Difference/Voltage - Power Dissipation

Diode

*the curve points to 'v' in both graphs

Filament Lamp

*the curve points to 'v' in both graphs

Conventional direction of current

-> but electrons, in reality, actually flows from negative to positive terminal -> the actual direction of electron flow is opposite to that of the conventional current

Note about resistance

1. every equipment in a circuit is a resistor 2. there are standard resistors -> specifically made with specific values

Factors that Affect Resistance

1. the cross-sectional area (A) 2. length of the conductor (L) 3. the type of material its made up of R ∝ L/A

Formula for Electric Current

I = ∆Q/∆t Unit: Ampere (A) - 1 amp = 1 Coulomb/1 second - another unit -> C/s

Formula for Resistance

R = V/I -> UNIT: Ohms (Ω) or V/A 1 Ohm = 1 V/A

Another Formula for Resistance

R = ρL/A - ρ (Rho) is a constant called resistivity (of the conductor) -> this means that current flows faster through a short, fat conductor

The 3 Power Dissipation Formulas

Recall, P.d = ΔE/q and Current = q/t therefore, P.d x current, ΔE/q x q/t = ΔE/t but V = IR and also, I = V/R

Formula for Ohm's Law

V = IR unit -> V

Formula for Potential Difference

V = W/q (work/charge) ... (1) V = ΔE/q ... (2) UNIT -> V, J/C -> 1V = 1 J/C -> it is a scalar quantity -> its source is a battery or mains supply -> measured using a voltmeter

Why conductors heat up during current flow

as the conduction electrons move, they collide with the metal atoms or fixed lattice (positive) ions - this leads to a transfer of their KE to the metal atoms/ions, resulting in an increase in the KE of the metal atoms - this increases the temperature of the metal atoms and eventually heats up the conductor

Representation of direction of current

conventionally, the flow of current is from the positive to the negative terminal

Alternating Current (ac)

current that constantly changes direction -> source is the mains -> measured using an ammeter or galvanometer

Direct Current (dc)

current that flows in only one direction -> source is the battery

Metallic/Conduction Electrons

delocalized electrons

Non-Ohmic Conductors

devices that do not obey Ohm's law -> ex. filament lamp (light bulb), diode etc.

Ohmic Conductors

devices that obey Ohm's law -> ex. metals at a constant temperature

Power Dissipation

energy dissipated by a circuit per unit time -> unit: J/s or Watt (W)

Ohm's Law

states that the current flowing through a conductor is ∝ to the P.d across it, PROVIDED THE TEMPERATURE STAYS CONSTANT V ∝ I {if temp. is constant}

Electric Current (I)

the rate of flow of electric charges - current = #charge that flows/time taken

Resistance (R)

the ratio of potential difference and current R∝V R 1/I - this means that a conductor with a very high resistance needs a large P.d to get current to flow across it - this results in high temperature

Drift Velocity

the velocity of the electrons due to current

Potential Difference (P.d) or Voltage (V)

work done PER UNIT CHARGE to move the charge from one place to another - also defined as the change in energy per unit charge involved when a charge moves from one point to another