BMAT Section 2: Physics - Electricity

Question 1

How can insulators be charged?

Answer 1

• Via friction (rubbing against each other)
• As electrons are transferred from one object to the other
• Forming +ve charge on one object and -ve charge on the other

Question 2

What is charging caused by?

Answer 2

The gain or loss of electrons

Question 3

Direct current

Answer 3

The flow of electric charge in only one direction

Question 4

Alternating current

Answer 4

The flow of electric charge that periodically reverses direction

Question 5

Difference between conductors and insulators

Answer 5

Insulators:

• Do not conduct electricity
• As their electrons cannot flow throughout the material, they are fixed

Conductors:

• Can conduct electricity
• As their electrons can flow, and are not fixed (they are delocalised)

Question 6

3 examples of conductors:

Answer 6

• Metals
• Aqueous solutions of salts
• Graphite
• The human body

Question 7

3 examples of insulators:

Answer 7

• Plastics
• Paper
• Styrofoam
• Rubber
• Glass

Question 8

Symbol, unit and definition of current

Answer 8

• I
• Amperes (A)
• The rate of flow of electric charge

Question 9

Symbol, unit and definition of charge

Answer 9

• Q
• Coulombs (C)
• A physical property of a material, the fundamental quantity of electricity

Question 10

Equation for charge

Answer 10

• Charge = current x time

- Q = I x t

Question 11

Ammeter

Answer 11

• Measures the current (in amps) flowing through a wire

- Must always be placed in series with whatever is being investigated

Question 12

Voltmeter

Answer 12

• Measures the potential difference (in volts) across the test wire
• Must always be placed in parallel with whatever is being investigated

Question 13

Equation for voltage

Answer 13

• Voltage = Current x resistance

- V = I x R

Question 14

Explain the voltage-current graphs for filament lamps

Answer 14

• The resistance is not constant so it changes with the current through the component
• The resistance of a filament lamp increases as the temperature of the filament increases and
  the current through a diode flows in one direction only, so it has a higher resistance in the
  reverse direction

Question 15

Negative temperature coefficient thermistors (NTC thermistors)

Answer 15

• As temperature increases, resistance decreases

- E.g. thermostats - the heating automatically turns on/off at a certain temperature

Question 16

Light dependent resistors (LDR)

Answer 16

• As light intensity increases, resistance decreases

- E.g. automatic night lights - a light automatically turns on as it gets dark)

Question 17

Ideal diode

Answer 17

• A semiconductor which only allows current flow in one direction
• When positive voltage is applied across the anode to the cathode, the diode conducts forward current instantly

Question 18

Current, potential difference and resistance in a series circuit

Answer 18

• Same current everywhere
• Potential difference is shared between various components
• Total resistance is the resistance of each component combined

Question 19

Current, potential difference and resistance in a parallel circuit

Answer 19

• Current is shared between each branch
• Potential difference is the same in all components
• Thee total resistance for two resistors in parallel is less than the resistance of any individual resistor

Question 20

Equation for voltage

Answer 20

• Voltage = Energy / Charge

- V = E / Q

Question 21

Equation for power

Answer 21

• Power = current x voltage

- P = IV or P = I^2R

Question 22

Equation for energy transfer

Answer 22

• Energy transfer = power x time

- E = V x I x T

Question 23

Symbol and units for power

Answer 23

• P

- Watts (W)

Question 24

Symbol and units for energy transfer

Answer 24

• E

- Joules (J)