Chapter 9 Energy, Power and Resistance

Question 1

Potential Differnece

Answer 1

Measure of transfer of energy by charge carriers

Question 2

Electromotive Force

Answer 2

Energy transferred from chemical energy to electrical energy per unit charge

Question 3

Electron Gun:

Answer 3

• Used to ionise particles by adding or removing electrons from atoms

Question 4

Thermionic Emission

Answer 4

The emission of electrons from the surface of a metal

Question 5

How does an electron gun work?

Answer 5

• Need a source of electrons
• Small metal filament heated by electric current
• Electrons gain kinetic energy
• Escape from the surface of the metal
• If heated filament is placed in a vacuum and high pd is applied between filament and anode
  THEN:
• Filament acts as a cathode and freed electrons accelerate towards the anode
• If anode has a hole:
• Electrons can pass through it
• Beam of electrons with a specific Ke

Question 6

Electron Energy Equations:

Answer 6

Ke = eV
=>
eV = 1/2 m v^2

Question 7

Mass of an electron:

Answer 7

9.11 * 10^-31

Question 8

Resistance

Answer 8

Measure of opposition to flow in electrical circuit

Question 9

Resistance Equation:

Answer 9

R = V / I

Question 10

Ohm’s Law:

Answer 10

For a metallic conductor kept at a constant temp:

- Current in wire is directly proportional to the p.d. across its ends

Question 11

Relationship between temp and resistance:

Answer 11

AS WIRE GETS HOTTER, ITS RESISTANCE INCREASES

Why:

1. When temp increases, POSITIVE IONS inside the wire have more INTERNAL ENERGY and vibrate with greater amplitude about means positions
2. Frequency of collisions increase
3. Carriers transfer more energy

Question 12

Why is filament lamp non-Ohmic?

Answer 12

• Increase in resistance is caused by wire getting so hot it glows
• Current increasing so rate of flow of charge increases
• More collisions between metal ions and electrons
• Collisions lead to transfer of energy from electrons to ions
• Causes ions to vibrate more
• Increasing temp

Question 13

Diodes:

Answer 13

• Non-Ohmic Conductors

- Resistance of a diode is not constant

Question 14

Factors which affect resistance:

Answer 14

• Material of the wire
• Temperature
• Length of the wire
• Cross-sectional area of the wire

Question 15

Formula for resistivity:

Answer 15

R = pL / A

Question 16

NTC:

Answer 16

• Negative Temperature Coefficient

- Resistance drops as temperature increases

Question 17

Explain NTC semiconductors:

Answer 17

In some semiconductors, as temp increases, no. density of charge carriers increases

Question 18

Thermistor

Answer 18

Electrical component made from semiconductor with NTC

Question 19

I-V characteristic of Thermistor:

Answer 19

• Non- Ohmic

- Similar to Filament Lamp

Question 20

LDR and light:

Answer 20

• As light intensity increases, resistance of LDR falls

Question 21

Why does resistance of LDR change with light intensity?

Answer 21

When light shines onto LDR, no. density of charge carriers increases dramatically leading to a rapid decrease in resistance of component

Question 22

Electrical Power:

Answer 22

Rate of energy transfer

Question 23

Power Equation:

Answer 23

P = V I

Question 24

What is 1 kWh is Joules?

Answer 24

3.6 MJ