Electricity

Question 1

Area under IT graph

Answer 1

Charge

Question 2

Drift velocity equation:

Answer 2

I = nAve

Current = (no. if charge carriers per m3) x (cross sectional area) x (average drift speed of carriers) x (1.6 x10-19)

Question 3

Kirchhoff’s current distribution rule?

Answer 3

At any junction in a circuit, the total current leaving the junction equals the total current entering the junction

Question 4

How can conductors become charged?

Answer 4

If they are insulated from their surroundings (suspended by nylon threats/in a vacuum)

Question 5

Resistance of an ammeter:

Answer 5

Negligible resistance/ very low in comparison to current so it doesn’t alter the current in the circuit, only used as a measuring tool.

Question 6

Kirchhoff’s potential difference rule?

Answer 6

The sum of the potential differences in a circuit is equal to zero (principle of conservation of energy)

Question 7

Charge, electrons relationship?

Answer 7

Charge: electrons x (1.6x10-19)

Question 8

Potential difference definition?

Answer 8

The pd between two points in an electric circuit is numerically equal to the work done by an electric field per unit charge passing between the points.

Question 9

1 ev in Joules?

Answer 9

1eV = 1.6x10-19 J

The work of one electron volt

Question 10

Define a volt

Answer 10

= The potential difference between two points when the electrical energy converted (to other forms of energy) is one joule per coulomb of charge

Question 11

Resistance of voltmeter?

Answer 11

Very high so the current doesn’t pass through the voltmeter

Ideal: infinite

Question 12

IV for fixed resistor:

Answer 12

Straight line through origin

Question 13

IV filament bulb and why?

Answer 13

Initial region straight line, curve in negative and positive sections of graph - increasing vibrations of lattice ions scatter electrons and transfers energy into thermal, increase of temperature- resistance increases (non-ohmic conductor)

Question 14

What is resistance caused by?

Answer 14

Caused by collisions between the free charges and the lattice of ions which make up the conductor. (High current - more collisions - resistor gets hotter

Question 15

What is ohms law?

Answer 15

Current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant

Question 16

Investigating resistance important:

Answer 16

• Magnitude of current kept small, so temperature doesn’t heat up and therefore would not be constant throughout experiment
• switch only on while taking readings otherwise temperature would build up
• high resistance voltmeter

Question 17

Is there a delay in bulbs turning on? Do long wires take longer than shorter?

Answer 17

No - all the wires and components are full of electrons and as soon as a power supply is connected, a current starts flowing.

Question 18

What’s the charge of an electron?

Answer 18

-1.6x10^-19

Question 19

How many electrons in one coulomb?

Answer 19

6.25 x10^-18

Question 20

What is terminal pd

Answer 20

The potential difference across the source terminals

Question 21

What is the terminal pd/lost volts equation?

Answer 21

E = V + v = Ir + IR = I(r + R)

terminal pd = E - Ir = V

Question 22

Lost volts

Answer 22

The energy expended in compelling the charge carriers to drift inside the battery itself when a battery drives a current through an external circuit

Ir = lost volts

Question 23

How to measure emf?

Answer 23

Connect high resistance voltmeter directly across its terminals

(High R because less current will flow, therefore Ir (lost volts) will be smaller. E - Ir = v is nearly zero. So the voltage measured is almost the Emf

Question 24

What is a potential divider?

Answer 24

Consists of two or more resistors connected in series across a pd source

(Used to supply required values of pd to a circuit from a fixed emf source - pd required is obtained by tapping a sliding contact along the wire)

Question 25

Resistance relationship with length / area

Answer 25

• Proportional to length
• Inversely proportional to cross sectional area

R = k L/A

k is the content of proportionality - resistivity

Question 26

How do LDRs work?

Answer 26

• Semi conductor material
  Light landing on the material can boost electrons from valence energy band up to the conduction band, increasing number of conduction electrons ➡️ conducts with lower resistance

Question 27

How do thermistors work?

Answer 27

Thermal energy landing on the material can boost electrons from valence energy band up to the conduction band, increasing number of conduction electrons ➡️ conducts with lower resistance

Question 28

Why are insulators?

Answer 28

There is an energy gap between the valence and conduction band that is so big that there are almost zero electrons available for conduction