Lecture 6 - Electric Potential and Currents

Electric Potential Energy

When a charge, q, moves from point a to point b in an electric field, the change in its PE is equal to the work down on it by the electric force

The larger the charge the larger the…

electric potential energy

Electric Potential V Equation

V(a) = PE(a)/q

Unit = Volts

relationship between V and E

V = W/q → W = -qV

E = F/q AND W = Fd

W = qEd

therefore: E =-V/d

Battery in car is rated as 12V, what does this mean

12V terminal is at higher potential and the other is at 0 potential, potential difference between the terminal is 12V

when you switch on your headlights, what does the battery do?

the battery does 12V of work in pushing each coulomb of charge through the filament of the headlight

electric cell

• a cell transforms chemical energy into electrical energy

• chemical reactions within the cell create potential difference between the terminals

• this potential difference can be maintained even if a current is kept flowing, until one or the other terminal is completely dissolved

• many cells connected = battery

Electric Current

the amount of charge flowing per unit time at any point in the circuit

battery in circuit charges

long side = positive

short side = negative

electrons flow from the negative to positive terminal

Ohm’s Law

V = IR

Why are resistors used in circuits

used to limit the amount of current flowing through the circuit

• symbol on circuit is a jagged line

resistivity

resistivity of any material is directly proportional to its length L and inversely proportional to its cross-sectional area A

NOTE: p = the resistivity of the material in ohm metres

R = p * L/A

If a wire is stretched to twice its original length, what will be its new resistance

new resistivity = 4x the previous resistance

Temperature and resistivity

Electric Power

Electric energy can be transformed into other forms of energy such as mechanical, heat and light energy

POWER is the energy transformed by a device by unit time

P = IV = I²R = V²/R

Electron Volt

1 eV is the energy an electron gains or loses when acceleration through a potential difference of 1 Volt

1 eV = 1.60 × 10^(-19) J

Capacitor

a device used to store electric charge, consisting of two conductors separated by an insulator

unit = farad (F)

Capacitance equation

C =ε₀ A/d

• A = area

• d = distance

• ε₀ = permittivity of free space

Dielectric material in capacitor

C = k(ε₀ A/d)

when inserted between the plates of an empty capacitor, it increases the capacitance by a factor k

EXAMPLES:

• vacuum = 1

• teflon = 2.1

• water = 80.4

charge and capacitance equation

Q = CV

Storage of electric energy

in charging up capacitor the battery does work in transferring the charge from one plate to the other This energy is stored in the capacitor as electric potential energy

• W = 0.5 Q/V

• W = 0.5 CV²

• Q = CV

• W = 0.5 x Q²/C

use of capacitors

• electronic circuits

• RAM chaos

• electronic flash camera

• mobile x-ray unit

• defibrillators