Chapter 3 Flashcards
A flow of charged particles produces an [ ]
Electric current
Each electron has the same [ ] measured in [ ]
Charge (Q)
Coulombs (C)
What equation links I, Q and t?
I = ΔQ/Δt
Current is measured in [ ]
Amperes (A)
The [ ] is the same at all points along the stream of [ ]
current
charged particles
Describe current when a wire forks
The current splits
I = I₁+I₂
What is Kirchhoff’s first law?
That the current at a junction must add up
When will charges move?
When attracted by charges of the opposite sign or when repelled by charges of the same sign
What happens inside a battery cell?
Chemical reactions produce an electrical potential energy difference between the 2 terminals resulting in a +ve charged terminal and a -ve charged terminal
If the poles of the cell are joined by a [ ], charges will flow
conducting path
Describe which charges flow in which directions
+ve charges flow from the +ve pole to the -ve pole, and -ve charges move the other way
Which charges are more likely in circuits?
As metals contain free electrons, which are -ve charged, the movement of -ve charges is more usual in a circuit
When charges move between 2 points in a circuit, their [ ] changes by amount ΔE
electrical potential energy
What is the equation of voltage (Using E,Q and W)?
V = ΔE / Q = W/Q
The P.d between ends of a wire will [ ] the free elctrons
Accelerate
What obstructs the free electrons moving down a wire?
By their interactions with the +ve ion cores of the metal atoms, so they don’t gain kinetic energy
What does the p.d lost by electrons obstructing do? What’s this called?
The p.d lost does work on the wire, heating it
This ‘wasted’ energy is called dissipation and the process is called ‘Joule Heating’
Combine the equations V=W/Q and I=Q/t
V = W/Q -> W = VQ I = Q/t -> Q=It W = V(It) - > W = VIt
What is the equation and units of power? (Using work done)
P = W/t = VIt/t = IV
Power (Js⁻¹ or W)
What is power?
It is the rate at which energy is transferred
What is the equation for resistance (voltage and current)?
R = V/I V = IR
What is the equation for power (w/ resistance)?
P = IV = (IR)I = I²R
When there is a constant current in a conductor, the forward push of the moving charges from the p.d, between the ends of the conductor is balanced by the …
…obstructing effect of the atoms in the conductor interacting with the charges.
What is the unit of resistance?
VA⁻¹ = Ω
What is resistance?
Is a measure of how difficult it is to get the current through the component
For most metals at a constant temp. the resistance is [ ]
Constant
What is Ohm’s law?
I is proportional to V
A conductor that obeys Ohm’s law is…
Ohmic
What is the equation for conductance (With Voltage)?Units?
G = I/V
AV⁻¹ or S (Siemen)
What is the equation of conductance (w/ Resistance)?
G = 1/R
Equation for conductance is a parallel circuit (including steps)
G = I/V G = I₁+I₂ / V G = I₁/V + I₂/V G = G₁ + G₂
What is the equation for resistance in parallel?
1/R = 1/R₁ + 1/R₂ + …
Equation for voltage in series?
V = V₁ + V₂ + …
Equation for resistance in series?
R = R₁ + R₂ + …
Equation for conductance in series?
1/G = 1/G₁ + 1/G₂ + …
From a p.d - current graph, how can you tell if the component is ohmic?
If the line is straight (proportional) then the component is ohmic, otherwise non-ohmic
What are extensive properties?
Properties that depend on the dimensions or material of the thing (eg, wire)
What are intensive or bulk properties?
Properties dependant of dimensions.
Are resistance and conductance extensive or bulk properties?
Extensive
Give examples of bulk properties
Density, Young’s modulus, Refractive index
What is σ?
σ = Electrical conductivity (Sm⁻¹)
What is ρ?
ρ = electrical resistivity (Ωm)
Conductance = (with sigma)
G = σA / L
Resistance = (w/ rho)
R = ρL / A
What are the 3 types of material?
Conductor, semiconductor and insulator
How can you measure the resistivity/conductivity of an insulator?
You need a tiny value of L, a large value of A, a large p.d, and a very, very sensitive ammeter.
How can you measure the resistivity/conductivity of an metal?
You need a large L and a very small A, and you must be aware of the complications cause by I²R heating of the wire and systematic errors such as p.d drops in places other than across the conductor in question
