2.2 Resistance Flashcards
What is voltage?
In general, Voltage is the potential difference between two points in an electric field.
In a circuit, it is the pressure that pushes the charged electrons and enabling them to do work such as illuminating a light
unit measured in: energy transferred per unit charge (work done / time)
5V means that the 5J of energy are transferred for each coulomb of charge in the circuit.
How to calculate Voltage?
Voltage = work done ÷ charge
V (joules per coulomb) = W(joules) ÷ Q(coulombs)
W = work done
What is work done?
measure of energy transfer that occurs when an object is moved over a distance by an external force which is applied in the direction of the displacement (distance moved in a straight line)
Calculate the energy transferred when a charge of 28C flows through p.d (potential difference) of 12V
W = V x Q
= 12 x 28
= 336 J
What is resistance?
a measure of the opposition to current flow in an electrical circuit.
What is resistance measured in?
ohms (Ω)
How to calculate resistance?
resistance = voltage ÷ current
R = V ÷ I
How to work out resistance in a graph?
construct triangle (size of over half the graph) and work out gradient gradient = change in V ÷ change in I
What does gradient mean?
Is used to measure the steepness of an object.
if the gradient is 0.75, it means y =0.75x, so every 1 across, every 0.75 up.
if the gradient is 1.278, it means y =1.278x, so every 1 across, every 1.278 up.
if the gradient is 90, it means y =90x, so every 1 across, every 90 up.
What is the ohm’s law?
Voltage is proportional to current if the external factors such as temperature and pressure remain constant.
Does lamp filament obeys ohm’s law?
No, because when the lamp filament gives out light, it also releases heat. Therefore voltage is not proportional to current as the resistance increases as temperature increases.
Fixed resistance has __ gradient
constant
Non-ohmic devices has __ gradient
non-linear (not arranged in a straight line)
What is power?
Power (watt) = energy transferred per second (Js⁻¹)
= energy transferred (J) ÷ time (s)
P = W / S
How to calculate power from using other variables?
Power = Voltage x current P = V x I
How to calculate power using resistance?
Since R = V ÷ I
so V = I x R
P = V x I
so V can be replaced as I x R
Hence P = RI x I
= I² x R
How to calculate power using voltage?
Since I = V ÷ R and P = V x I I can be replaced with V ÷ R So P = V x (V ÷ R) Hence P = V² ÷ R
What is resistivity?
Resistivity is a property of material, it describes its resistance only when its dimensions are considered.
e.g gold has a constant resistivity but different shapes of gold will have different resistances.
What is resistivity measured in?
ρ (pronounced as ‘roe’) is measured in Ωm (ohm metre)
How to calculate resistance by using resistivity?
R = ρL ÷ A
R = resistance (Ω) ρ = resistivity (Ωm) L = length of the material (m) A = cross sectional area (m²)
What is the relationship between resistance and the length of the material?
R ∝ L
means that the bigger the length, the bigger the resistance and vice versa
What is the relationship between resistance and cross sectional area?
R ∝ 1/A
means that the bigger the current, the smaller the resistance and vice versa
