Electricity Flashcards
Define CHARGE
Measured in COULOMBS
The measure of the amount of ELECTRICAL CHARGE an OBJECT has
What is an ELECTRICAL CURRENT?
The rate of flow of charge
(OR:
The flow of free electrons around an electrical circuit)
What is CONVENTION CURRENT?
When the current flows from positive to negative
What is ELECTRON FLOW?
Electrons flow from positive to negative
I = ΔQ/Δt
I = Current [A]
Q = Charge [C]
t = Time [S]
How can the total charge in a current-time graph be found?
Area under graph
Total charge =
Number of electrons * Charge per electron
Define DRIFT VELOCITY
The speed at which ‘free’ electrons flow from negative to positive, transferring energy.
What are the factors that AFFECT CURRENT?
Drift velocity
Number of free electrons per m³
Cross-sectional area
Length
Derive the TRANSPORT EQUATION
A = Cross-sectional area of wire
L = Lenth of wire
Diagram of wire, with c-s area, length, and electrons
Volume of wire = AL
Number of free electrons / m³ = n
Drift velocity per free electron = v
Charge per electron = e
Number of free electrons in volume = nAL
Total charge flowing in volume = nALe
I = Q/t = (nALe)/t
L/t = v (as s = D/t)
I = nAve
Define potential difference
Measured in volts
The amount of electrical energy transferred into other forms of energy per unit of charge between two points
V = (The equation with work done)
W/Q
Define resistance
The opposition of the flow of a current
R = V/I
R = Resistance
V = Potential difference
A = Current
What is Ohm’s law?
V ∝ I when the temperature is constant
Derive V = RI
V ∝ I
V = kI, where k is a constant
V = RI
Why does resistance occur?
Free electrons collide with the ions of the conductor, losing energy
What affects resistance?
Length of wire
Temperature
Cross-sectional area
Type of material OR number of electrons
Define power
The rate of energy transfer
P = W/t
P = Power
W = Work done OR energy
t = Time
Derive P = IV (I = Qt, V = W/Q)
P = Wt
I = Qt
t = Q/I
P = WI/Q
V = W/Q
W = VQ
P = VQI/Q
P = IV
Derive P = I²R (P = IV, V = IR)
P = IV
V = IR
P = IIR
P = I²R
Derive P = V²/R (P = IV, V = IR)
P = IV
V = IR
I = V/R
P = V/R * V
P = V²/R
What is the purpose of resistivity?
To compare the resistance of different materials
Define resistivity
Measured in Ωm
The resistance of a material that is 1m long, and has a cross-sectional area of 1m², at a specific temperature
R = (ρL) / A
R = Resistance
ρ = Resistivity
L = Length
A = Cross-sectional area
When ρ and A in the equation R = (ρL)/A are constant, what is the relationship between R and L?
R ∝ L
When ρ and L in the equation R = (ρL)/A are constant, what is the relationship between R and A?
R ∝ 1/A
What happens when metals are cooled to near 0 Kelvin?
They become superconductive
They have no resistance
What is a critical temperature?
The temperature at which resistance disappears
What does superconducting mean?
The range of temperatures at which a material has no resistance
What is used to cool materials to their critical temperatures?
Liquid nitrogen
What is a high temperature superconductor?
A material who’s transition temperature is above the boiling point of nitrogen
Explain P.D. in a superconductive circuit
Needed to start the current flowing, does not need to be maintained
What is the purpose of using superconductive wires?
No resistance means no heat loss
Large currents with 0V P.D.
What are some uses of superconductivity?
Nuclear fusion
Large Hadron Colliders
MRI scanners
More efficient and smaller generators
What are the negatives of superconductive wires?
Hard to maintain temperature
Brittle and non-ductile material
Hard to manufacture
What is the rule for current in a series circuit?
I_1 = I_2 = I_3
What is the rule for P.D. in a series circuit?
V = V_1 + V_2
What is the rule for resistance in a series circuit?
R_T = R_1 + R_2
What is the rule for current in a parallel circuit?
I_1 = I_2 + I_3
What is the rule for P.D. in a parallel circuit?
V_T = V_1 = V_2
What is the rule for resistance in a parallel circuit?
1/R_T = 1/R_1 + 1/R_2
Define electromotive force (Units)
The amount of chemical energy transferred into electrical energy per coulomb as it passes through the battery, measured in V or JC^-1
Derive the EMF equation
E = V + v
[V = IR]
E = IR + Ir
E = I(R+r)
or
E = V + v
E = V + Ir
V = E - Ir
V = External P.D.
v = ‘Volts lost’ within cell
E = EMF
