Physics Electric Fields flashcards

Electric field

the field in which an electric charge attracts or repels

• Can be both radial and uniform

Electric charge

certain elementary particles, such as electrons and protons, that cause them to interact with reach other

Coulomb

unit of electric charge 

• One coulomb is equivalent to the charge transported by 1 ampere of current in a second 

• Protons charge - 1.6 x 10^-19 C

• Electrons have an electric charge of -1.6 x 10^-19 

Static electricity

build up of electric charge on the surface of an object due to the removal or addition of electrons, commonly caused by friction

Electric field

a region where a charged particle will experience a force

• All charged objects surrounded by one

• Direction of the lines represents the direction of the field, where it shows the direction a positive charge would move in the field

• The concentration of field lines represent the strength of the field, the more concentrated the lines, the stronger the field

Coulomb's law / Force between two point charges equation

F= 1/4πε₀ x Q₁Q₂/r²

• Magnitude of F is proportional to the product of q1 and q2 

• Inversely proportional to the square of the distance between them 

• 1/(4πε₀) - coulombs constant

  • ε₀  - permittivity of free space - 8.85 x 10^-12 C²N^-1m^-2 or Fm^-1

Radial electric field

Pointing in the direction which a positive test charge would go

Uniform electric field

• Pointing in the direction which a positive test charge would go

• Only in between two charged plates

• electric field between two parallel plates, 

• Charge will experience same force wherever it is placed between the plates 

• Depends on the voltage between the plates and the distance between them

• Typically measured in NC^-1, but can also be in Vm^-1

• NC^-1 typically for point charge and Vm^-1 for parallel plates

Test charge

A small enough charge that does not impact the shape of a field

Fields around a pair of charges

Field on the side without the other charge behaves as a normal electric field

Electric field strength

The electrostatic force experienced per unit of positive test charge placed at a specific point in an electric field

Electric field strength equations

E = F / Q

• All fields

• E - Electric field strength

• F - Force

• Q - Charge

E = 1/(4πε₀) x Q/r²

• Radial fields only

• E - Electric field strength

• 1/(4πε₀) - coulombs constant

  • ε₀  - permittivity of free space - 8.85 x 10^-12 Fm^-1

• Q - Charge

• r - distance between charges

Uniform electric field strength equation

E = V/d

• E - Electric field strength

• V - P.D. / voltage

• d - distance

Electric Potential

• If a positive test charge is moved from infinity to a position in an electric field, the electric potential energy of the charge will change.

• Work is done on the charge

• In an electric field the E_p is at infinity when it is around a point charge. 

• The strength of the field would be zero at infinity.

Work done from Electric potential equation

∆W = Q∆V 

• W - work done to move a charge

• Q - Charge 

• V - Electric potential

Electric potential equation for a point charge (radial field)

V =  Q / (4πε_or)

Potential gradient

the rate of change of electric potential per unit change of distance in a certain direction within that field

• Also known as the the electric field strength at a point

• The gradient in a V-r graph 

Potential gradient equation

E = ∆V/∆r 

• E - Electric field strength

• ∆V - change in potential

• ∆r - change in distance

Capacitor

a pair of plates with an insulator between them that store charge

• Only works in a dc circuit (as charge needs to build up)

• AC will remove and add charge to each plate resulting in an overall neutral charge

• The electric field around capacitors is uniform

Capacitance

the amount of charged stored per unit potential difference between the plates

Capacitance equation

Q=CV

• Q - Charge, measured in Coulombs

• C - Capacitance, measured in Farads

  • Coulomb per volt = farad

• V - Potential difference, measured in Volts

Charged stored factors

• Distance between plates - Greater distance, hence less charge stored

• Voltage - Greater voltage, hence more charge stored 

• Capacitance - Greater Capacitance, hence more charge stored 

• Size of plates - Greater area of plates, hence more charge stored 

Work done from capacitance

• The area under a Q-V graph

• W = ½ QV

  • W is not proportional to V due to Q changing when V is changed so Q cannot be made constant while V varies with W

• W = ½ CV²

  • W is proportional to V²

• W = Q² / 2C

  • W is proportional to Q²

Capacitor charging

• Increasing the potential difference between the two plates of the capacitor

• This stores charge within the capacitor

Capacitor discharging

• Decreasing the potential difference between the two plates of the capacitor by using the potential difference to power a component

• This decreases/uses charge within the capacitor

Capacitor charging graphs

• The gradient of a charge time graph is the current 

• The area under a current time graph is the charge

Capacitor discharging graphs

• The gradient of a charge time graph is the current 

• The area under a current time graph is the charge

Factors for time take for capacitor discharge

• Time constant

  • Resistance of the resistor in the circuit

  • capacitance of capacitor

Time constant (𝛕)

the time taken for the charge on the capacitor to discharge to 1/e (~ 37%) of its initial value or the time taken to charge to 63%

• 1 time constant drops by 37% of its previous / initial value

• The charge drops by the same proportion each time

Time constant equation

𝛕 = RC

• 𝛕 - time constant [s]

• R - resistance [Ω]

• C - capacitance [F]

Rate of removal of charge

Proportional to the amount of charge remaining 

Half life of a discharging capacitor

the time taken for the charge, current or voltage of a discharging capacitor to reach half of its value

Half life of capacitor equation

t_1/2  = 0.69𝛕 = 0.69RC