Energy, Power and Resistance

Question 1

Potential Difference

Answer 1

• Is used to measure work done by charge carriers, which lose energy as they pass through the components in a circuit
• Energy transferred from electrical energy to other forms

Question 2

What is Voltage?

Answer 2

• Voltage is the push to allow current to flow around
• Voltage is the work done per unit charge:
  V = W/Q

Question 3

Definition of 1 Volt

Answer 3

• Potential difference of 1 volt is 1 joule of energy transferred per coulomb

Question 4

Electromotive force (e.m.f)

Answer 4

• Is used to measured the work done to charge carrier, which lose energy as they pass through a cell of power supply
• Energy transferred from chemical energy to electrical energy

Question 5

What is EMF?

Answer 5

• The total voltage produced by a cell due to some voltage used by internal resistance in the cell
• EMF = I(R+r)
  R = Circuit resistance
  r = Internal resistance

Question 6

Resistance

Answer 6

• Each components in a circuit resists the flow of charge
• R = V/I

Question 7

Definition of Resistance

Answer 7

• Resistance of 1 ohm is defined as 1 volt per unit ampere

Question 8

How to determine resistance of a component?

Answer 8

• A circuit can be set up with a variable power supply, ammeter in series and voltmeter parallel with component
• By varying the power supply, we can vary the potential difference across the component
• Record the pd and current to calculate the resistance, 1 over the gradient of the I-V graph

Question 9

Ohm’s Law

Answer 9

• For a metallic conductor kept a constant temperature, the current in the wire is directly proportional to the potential difference
• I-V graph will have a constant linear gradient

Question 10

Non Ohmic components

Answer 10

• Some components don’t obey Ohms law due to when the current across the component increases
• The metal ions are heated gaining kinetic energy and vibrate more in their fixed points in the metallic lattice
• This increases the frequency of collision with electrons, so more work is done on charge carriers, increasing the resistance

Question 11

Resistivity

Answer 11

• Is a physical property of a material
• It is the same for any shape of a given material at a set temperature and it acts as a constant to link the resistance of a material with its area and length
• 𝜌 = 𝑅A/L

Question 12

How does resistivity vary with temperature?

Answer 12

• For metals, when temperature increases, the fixed ions vibrate with greater frequency and amplitude, this increases number of collision of electrons, therefore increases resistance
• For semiconductors, when number density of charge carrier increases with increasing temperature, so the resistance of material decreases
• As resistivity is constant linking the shape of the material with the resistance, if resistance increases or decreases, the value of resistivity will be affected in the same way

Question 13

How to determine resistivity?

Answer 13

• ## Record the cross sectional area of the wire by taking multiple reading with Vernier calliper at different points along the wire and take an average

Question 14

Electrical power

Answer 14

• The rate of energy transfer
• P = W/t