C9 Energy, Power and Resistance

Question 1

Define Potential Difference and explain with a filament lamp as an example

Answer 1

• A measure of the transfer of energy by charge carriers.
• The p.d. across a filament lamp is a measure of the electrical energy being transferred into heat and light as charge carriers move through the lamp.

Question 2

P.d. in terms of joules

Answer 2

• 1 volt is the p.d. across a component when 1J of energy is transferred per unit charge.
• 1V = 1 JC -1
• A p.d. of 1000V means that 1000J of energy is transferred per coulomb of charge.

Question 3

Formula relating Charge, Voltage, Energy

Answer 3

V = W / Q
Voltage = Work Done / Charge

Question 4

Describe Potential Difference in terms of work done

Answer 4

Potential difference is when work is done by the charge carriers.

Question 5

Electromotive Force (e.m.f) idea in terms of work done, and equation

Answer 5

This is when work is done on the charge carriers, as each charge carrier gains energy as they pass through a source of energy e.g. cell.

ε = W / Q

Question 6

Electromotive Force in terms of energy transfer from sources definiton

Answer 6

The energy transferred from a chemical energy source (or another form) to electrical energy per unit charge.

Question 7

Calculating work done with charge, emf and voltage

Answer 7

W = V x Q, work done = voltage x charge
W = ε x Q, work done = e.m.f x charge

Question 8

How do electron guns work?

Answer 8

• A small metal filament is heated by an electric current.
• The electrons will gain enough kinetic energy to escape from the metal’s surface (thermionic emission).
• The heated filament is placed in a vacuum, and a high p.d is applied between the filament and an anode, the filament acts as a cathode.
• The emitted electrons accelerate toward the anode.
• The anode as a small hole in it, so the electrons in line with this hole pass through it creating a beam of electrons with a specific KE.

Question 9

How to calculate work done on electron from electron gun

Answer 9

work done on a single electron = eV
V = accelerating p.d

eV = 1/2mv^2

this assumes the electrons have negligible kinetic energy at the cathode

Question 10

Formula for resistance

Answer 10

R = V / I

Question 11

Define the ohm

Answer 11

• The resistance of a component when a p.d. of 1V is produced per ampere of current.

Question 12

Define Ohm’s law, and what is a device that obeys Ohm’s law called?

Answer 12

• For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d across its ends, V = IR.
• If the voltage across a wire doubles, so does the current.
• A component that obeys ohm’s law is an ohmic conductor, e.g. fixed resistor.

Question 13

How and why does temperature affect resistance?

Answer 13

• As temperature increases, the ions inside a wire have more internal energy, therefore vibrate with greater amplitude about their mean positions.
• The frequency of collisions between charge carriers and ions increases, so the charge carriers have do more work to move.

Question 14

Describe Filament Lamps

Answer 14

• They are non-ohmic conductors.
• Their resistance is not constant, and increases as the p.d. increases.
• They behave in the same way regardless of polarity.

Question 15

Describe the properties of a diode

Answer 15

• Diodes are non-ohmic conductors.
• They do not have a constant resistance.
• Behaviour depends on polarity of circuit.

Question 16

What is Diode Threshold Frequency

Answer 16

• The minimum voltage required for the diode to conduct significant current.

Question 17

What factors affect the resistance of a wire?

Answer 17

• Material
• Length
• Cross sectional area

Question 18

Explain the relationship between resistance and length

Answer 18

• Resistance is directly proportional to length.
• Electrons have to travel a longer distance through a wire. This means they encounter more collisions with ions in the wire, which impedes their flow, increasing resistance.

Question 19

State and explain the relationship between cross sectional area and resistance

Answer 19

• They are inversely proportional, if cross sectional area increases, resistance decreases.
• There is more space for electrons to flow through. This reduces the likelihood of collisions between electrons and the ions in the wire, allowing the current to flow better.

Question 20

Formula to calculate resistance given resistivity

Answer 20

R = pL / A
where p = resistivity of a material

Question 21

Formula to calculate resistivity given resistance

Answer 21

p = RA / L

Question 22

What does a negative temperature coefficient mean for a semiconductor

Answer 22

• As temperature increases, their resistance drops.
• The number density of the charge carriers increases as temp increases.

Question 23

Thermistor & its relationship between resistance and temperature

Answer 23

• Thermistors are made from semiconductors with a negative temperature coefficient.
• Noh-ohmic conductor
• As temperature increases, resistance drops.

Question 24

Light-dependant resistor

Answer 24

• Made from a semiconductor where density of charge carriers depends on intensity of incident light.
• When light is more intense, resistance decreases.

Question 25

Define Power, and how to convert it into joules

Answer 25

• The rate of energy transfer by an electrical component.
• 1W = 1J per second

Question 26

Formulas for power

Answer 26

P = IV
P = I^2 * R
P = V^2 / R

Question 27

Deriving P = IV

Answer 27

P = W / t
V = W/Q
rearrange: W = QV
P = VQ / t
however, Q/t = I
P = IV

Question 28

Kilowatt-hour (kWh)

Answer 28

The energy transferred by a device with a power of 1kW for 1 hour.