2.2 Resistance

Question 1

What is voltage?

Answer 1

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.

Question 2

How to calculate Voltage?

Answer 2

Voltage = work done ÷ charge
V (joules per coulomb) = W(joules) ÷ Q(coulombs)
W = work done

Question 3

What is work done?

Answer 3

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)

Question 4

Calculate the energy transferred when a charge of 28C flows through p.d (potential difference) of 12V

Answer 4

W = V x Q
= 12 x 28
= 336 J

Question 5

What is resistance?

Answer 5

a measure of the opposition to current flow in an electrical circuit.

Question 6

What is resistance measured in?

Answer 6

ohms (Ω)

Question 7

How to calculate resistance?

Answer 7

resistance = voltage ÷ current

R = V ÷ I

Question 8

How to work out resistance in a graph?

Answer 8

construct triangle (size of over half the graph)  and work out gradient
gradient = change in V ÷ change in I

Question 9

What does gradient mean?

Answer 9

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.

Question 10

What is the ohm’s law?

Answer 10

Voltage is proportional to current if the external factors such as temperature and pressure remain constant.

Question 11

Does lamp filament obeys ohm’s law?

Answer 11

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.

Question 12

Fixed resistance has __ gradient

Answer 12

constant

Question 13

Non-ohmic devices has __ gradient

Answer 13

non-linear (not arranged in a straight line)

Question 14

What is power?

Answer 14

Power (watt) = energy transferred per second (Js⁻¹)
= energy transferred (J) ÷ time (s)
P = W / S

Question 15

How to calculate power from using other variables?

Answer 15

Power = Voltage x current
P = V x I

Question 16

How to calculate power using resistance?

Answer 16

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

Question 17

How to calculate power using voltage?

Answer 17

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

Question 18

What is resistivity?

Answer 18

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.

Question 19

What is resistivity measured in?

Answer 19

ρ (pronounced as ‘roe’) is measured in Ωm (ohm metre)

Question 20

How to calculate resistance by using resistivity?

Answer 20

R = ρL ÷ A

R = resistance (Ω)
ρ = resistivity (Ωm)
L = length of the material (m)
A = cross sectional area (m²)

Question 21

What is the relationship between resistance and the length of the material?

Answer 21

R ∝ L

means that the bigger the length, the bigger the resistance and vice versa

Question 22

What is the relationship between resistance and cross sectional area?

Answer 22

R ∝ 1/A

means that the bigger the current, the smaller the resistance and vice versa