Unit 2 - Electricity

Question 1

Current

Answer 1

• Rate of flow of chared particles
• I= Q / T
• I = nqvA

Question 2

Potential difference

Answer 2

• Energy provided per unit charge by charge carriers to components
• pD = W / Q

Question 3

EMF

Answer 3

• Energy provided per unity charge by source to charge carriers
• ε = W / Q

Question 4

Series circuit

Answer 4

• Current is equal
• Voltage split
• Rₜ = R₁ + R₂ + … + Rₙ

Question 5

Parallel circuit

Answer 5

• Current is split
• Voltage equal
• Rₜ⁻¹ = R₁⁻¹ + R₂⁻¹ + … + Rₙ⁻¹

Question 6

Ohm’s law

Answer 6

• Current is proportional to potential difference in a conductor as long as temperature remains constant
• V = IR

Question 7

Semiconductors

Answer 7

• Better conductivity than insulator, worse than conductor
• The more energy given, the better they conduct

1. Energy provided (light, temperature, voltage) increases
2. Charge carriers gain energy
3. Charge carriers become delocalised
4. Number of charge carriers increases
5. Current increases
6. Resistance decreases as inversely proportional to Current

Question 8

Filament bulb

Answer 8

• Non-omhic
• Heats up to produce light

1. Temperature increases
2. Amplitude of oscilations of lattice ions increases
3. Collision frequency of charge carriers with lattice ions increases
4. Current decreases as temperature increases
5. As V = IR, resistance increases as temperature does

Question 9

Diode

Answer 9

• Semiconductor
• Allows current to flow in one direction
• No current for negative voltage
• Small potential difference until a threshold voltage is reached
• Current increases as potential difference increases

Question 10

Thermistor

Answer 10

• The higher the temperature, the lower the resistance (negative coefficient)
• Semiconductor

Question 11

Light dependent resistor (LDR)

Answer 11

• The higher the light intensity, the lower the resistance
• Semiconductor

Question 12

Resistivity

Answer 12

• Measure of resistance of a material due to electron density density, electron and lattice structure
• Resistance = (Resistivity · Wire length) / Cross-sectional area
• R = (pl) / A
• Temperature dependent
• Conductor: p ∝ T - more vibrations
• Semiconductor: p ∝ T⁻¹ - free electrons

Question 13

Electric field lines

Answer 13

• Flow direction of positive charge
• Density represents strength

Question 14

Kirchhoffs first law

Answer 14

• For any junction in an electrical circuit, the sum of the currents flowing in is equal to the sum of currents flowing out

Question 15

Kirhoff’s second law

Answer 15

-The sum of potential differences in any circuit loop is zero (pD = ε)

Question 16

Potential divider

Answer 16

• Circuit that uses changeable resistances to provide an output voltage that’s a fraction of the EMF
• Vout = (R2 · Vin) / (R1 + R2): assuming infinite resistance and negligible current

Question 17

Potentiometer

Answer 17

• Fine control of voltage
• Divides resistances on ratio of lengths
• Greater range and more compact than potential divider

Question 18

Sensing circuits

Answer 18

• Potential divider circuit with LDR or thermistor

Question 19

Power

Answer 19

• Rate of energy transfer
• P = IV

Question 20

Internal resistance

Answer 20

• Cells are not 100% efficient
• ε = V + Ir
• Find by plotting V = ε - rI

Question 21

Conduction band

Answer 21

• Electrons with enough energy to become delocalised

Question 22

Valence band

Answer 22

• Electrons without the energy required to become delocalised

Question 23

Band

Answer 23

• Represents an energy level needed
• Insulators have largest band gaps
• Semiconductors small band gaps
• Conductors have no band gaps