electricity

Question 1

charging by friction

Answer 1

• friction causes displacement of electrons
• When 2 insulators move relative to each other, friction can result in electrons being transferred from one to the other which can result in both objects becoming electrically charged.
• One of the objects (the one that lost electrons) has +ve charge, and the other (the one that gained electrons) has -ve charge, depending on the nature of the materials
• e.g. plastic being rubbed by a duster; hair being combed; an aeroplane moving through the air.

Question 2

induction

Answer 2

• when a neutral object becomes charged by being placed near a charged object
• the neutral object is oppositely charged to the already charged object

Question 3

repulsion vs attraction

Answer 3

• like charges repel each other
• unlike charges attract each other
• the larger the charges, the larger the force
• the larger the distance of separation, the smaller the force

Question 4

conductors and charge

Answer 4

• it is possible for 2 conductors to charge each other by friction. However, the fact that electrons in them are free to move means that any charge transfer between them will instantly return, given that the objects must be in contact for friction to occur between them.
• they will only retain that charge if they are insulated from their surroundings. If they are not insulated, then any charge that builds up will leak away

Question 5

photocopier

Answer 5

• The scanning process results in charge being placed on the paper at the locations where the image is to be printed
• The paper is then exposed to toner powder, which ‘sticks’ to the paper at those locations as a result of electrostatic induction
• The paper is then heated so that the toner powder melts and then re-solidifies on the paper

Question 6

aircraft refuelling

Answer 6

• When aircraft are refuelled, large volumes of fuel flow through the pipe very quickly creating large amounts of friction
• This results in the fuel and the pipe becoming charged
• Sparking presents a significant risk of explosion of the fuel in the fuel tank, so, the tank and pipe are always earthed beforehand.
• This earthing prevents the build-up of charge and so eliminates the risk of explosion.

Question 7

direct current (DC)

Answer 7

• a current that is always in the same direction

- cells/batteries are sources of dc

Question 8

alternating current (AC)

Answer 8

• a current that repeatedly changes direction, usually very rapidly
• Generators in power stations produce ac
• its waveform must include a graph line that is sometimes above the axis and sometimes below it

Question 9

diode

Answer 9

• A diode only allows current in one direction (the direction of the arrow on the symbol)
• The output from a power supply from mains electricity can be converted from ac to dc using diodes as a ‘rectifier’
• for current to flow the diode arrowhead must point from the +ve terminal of the battery to the -ve terminal

Question 10

conductors

Answer 10

• material that allows flow of charge
• e.g: all metals (because of the presence of delocalised electrons which can flow through the metal), particularly copper, gold and silver, carbon (in the form of graphite), ionic solutions

Question 11

insulators

Answer 11

• material in which electric current does not flow freely

- e.g: most non-metals, particularly plastics, rubber, dry wood, air, vacuum

Question 12

water

Answer 12

Water, unless extremely pure, is a conductor, so wet or damp materials are not good insulators

Question 13

cables

Answer 13

An electric cable usually uses an insulator on the outside and a conductor on the inside

Question 14

directions of flow

Answer 14

• current flows from the +ve end of a conductor to the -ve end
• electrons flow from the -ve end of a conductor to the +ve end

Question 15

electrolytes

Answer 15

• a substance that conducts electricity when dissolved in water
• this is because it contains charged ions
• If two electrodes are placed in the liquid and a voltage is applied across them, the cations move towards the -ve cathode, and the anions move towards the +ve anode

Question 16

voltmeters

Answer 16

• connected in parallel with a component
• has very high resistance (infinite), otherwise it would ‘short circuit’ the component across which it was connected (because there would be a significant amount of current in the voltmeter instead of in the component)

Question 17

ammeter

Answer 17

• connected in series with a component
• has very low resistance (zero), otherwise it would tend to reduce the amount of current that it was being used to measure

Question 18

Ohm’s Law

Answer 18

-a current flowing through a conductor is directly proportional to the voltage, given the temperature of the conductor remains constant

Question 19

constant resistance

Answer 19

• A fixed resistor at constant temperature has a constant resistance
• This means that a graph of 𝑉 against 𝐼 will be a straight line passing through the origin
• The resistance is given by: Δ𝑉 / Δ𝐼 (or 1/grad)

Question 20

varied resistance

Answer 20

• in lamps, the filament heats up as more current passes through it
• the heat generated increases the resistance of the filament (resistance is no longer constant)
• This means that a graph of 𝑉 against 𝐼 will be a curve (steep gradient then levels off)
• The resistance is given by dividing one value of 𝑉 by the corresponding value of 𝐼 (or 1/grad)

Question 21

thermistor

Answer 21

• A thermistor is a resistor with a resistance that depends on its temperature.
• as its temperature increases, its resistance decreases (TURD)

Question 22

light dependant resistor (LDR)

Answer 22

• An LDR is a resistor with a resistance that depends on the intensity (brightness) of light that falls on it
• As the light intensity increases, the resistance of the LDR decreases (LURD)

Question 23

battery symbol

Answer 23

-long line is +ve end and short line is -ve end

Question 24

series circuits

Answer 24

• the current in each component is the same

- total voltage across the components = sum of the voltages across each individual component

Question 25

parallel circuits

Answer 25

- the voltage across each component is the same | - total current moving into the branch = total current moving out of it

Question 26

current definition

Answer 26

the flow of charge passing a point in the circuit per unit time

Question 27

voltage defintion

Answer 27

the energy required to move a unit of charge between two points

Question 28

resistance in series

Answer 28

-The combined resistance 𝑅𝑇 resistors connected in series is the sum of the individual resistances: 𝑅𝑇=𝑅1+𝑅2+…... -𝑅𝑇 > 𝑅1 /𝑅2

Question 29

resistance in parallel

Answer 29

- for 2 resistors in parallel: RT=(R1xR2)/(R1+R2) - for 3 or more resistors in parallel: 1/RT=1/R1 + 1/R2 +.... - RT < R1 / R2

Question 30

fixed resistor

Answer 30

the resistance will not change no matter what the voltage across it or the current flowing through it

Question 31

voltage formula

Answer 31

voltage = energy / charge (V=E/Q)

Question 32

power formulae

Answer 32

𝑃 = 𝑉𝐼 = 𝐼squared𝑅 = 𝑉squared/𝑅 E=Pt E=IVt

Question 33

voltage-resistance relationship in series circuits

Answer 33

R1/R2 = V1/V2

Question 34

short circuits

Answer 34

-when the charge bypasses a component by taking the path of least resistance

Question 35

household electricity

Answer 35

- voltage = 230V - frequency = 50Hz - ac

Question 36

resistance in wires

Answer 36

- doubling the length of the wire doubles the resistance | - halving the diameter/radius of the wire quadruples the resistance

Question 37

power energy formula

Answer 37

E=Pt OR P=E/t