P45 Electricity

Question 1

state the equation that links charge, current, and time

Answer 1

charge = current x time
Q = I x t
(C) (A) (s)

Question 2

define current

Answer 2

the rate of flow of charge
measured in Amps

Question 3

define potential difference

Answer 3

-> the force pushing charge around a circuit
-> energy used to move charge around a circuit (work done)
-> energy transferred per unit charge to move charge between two circuit points
* the voltage/potential diff across two circuit points
* also known as voltage
* measured in volts using a voltmeter

Question 4

state the equation linking potential diff, energy transferred, and charge

Answer 4

energy transferred = potential diff x charge
E = V x Q
(J) (V) (C)

Question 5

describe the relationship between resistance and current

Answer 5

when resistance increases, current decreases
-> providing the potential diff is fixed

Question 6

describe current, voltage and resistance in a series circuit

Answer 6

• current through each component is the same - all ammeters will have the same reading
• voltage across circuit is sum of voltages in all components
• total resistance of the circuit is the sum of the resistances of all components
  -> resistance increases as no. components increases

Question 7

describe current in a parallel circuit

Answer 7

• total current is the sum of the current through each branch
• current splits between branches in proportion to resistance
  -> more current flows through the path of least resistance

Question 8

describe voltage in a parallel circuit

Answer 8

• voltage along each branch is the same as the supply voltage
  -> if pd. of battery is 5V, each branch will share 5V between its components

Question 9

describe resistance in a parallel circuit

Answer 9

• resistance of the entire circuit is less than the branch with the lowest resistance
  -> if one branch is 5Ω and one is 10Ω, the resistance of the circuit will be lower than 5
  -> resistance decreases as more resistors are added

Question 10

state the equation linking resistance, current, and potential diff

Answer 10

potential diff = current x resistance
V = I R
(V) (A) (Ω)

Question 11

state Ohm’s law

Answer 11

• in ohmic conductors, voltage is directly proportional to current
  -> provided all conditions (eg. temp) are fixed
• shown on a graph by a straight line of pos gradient through the origin

Question 12

describe a fixed resistor

Answer 12

• ohmic conductor as it follows Ohm’s law - the voltage is directly proportional to the current (provided all conditions eg. temp remain constant)
  *maintains constant resistance regardless of potential diff
• particles in a resistor give a constant obstruction to current
• current increases as potential diff increases - direct proportion

Question 13

describe a fixed resistor graph

Answer 13

straight line of pos gradient through the origin

Question 14

describe a filament lamp

Answer 14

• resistance increases as potential diff increases
• filament wire heats up so particles move faster and more collisions happen between electrons in moving current+atoms - increased resistance
• current increases less for higher potential differences - gradient becomes shallower

Question 15

describe a filament lamp graph

Answer 15

initially straight line through origin then gradient becomes shallower (s shape curve)

Question 16

describe a diode

Answer 16

• high resistance to negative potential diff so doesn’t allow current to flow
• resistance above 0.7V is constant and current can flow
• particles in a diode act as valves+ only allow charge to travel in one direction
• current is 0 until pd=0.7 then current is directly proportional to pd

Question 17

describe a diode graph

Answer 17

flat line at 0 and negative pd. values, then gradient becomes very steep at 0.7, current is directly proportional to pd once pd. is higher than 0.7V

Question 18

describe the general method for RP: I-V characteristics in fixed resistor/ filament lamp/ diode

Answer 18

(resistor + lamp / diode)
* set up apparatus as shown
* use variable resistor to ensure voltmeter reads (3.5V / 0.7V)
* measure current with ammeter
* repeat steps for voltages of (3 to 0.5V / 0.6 to 0.1V )
* switch terminals of the battery and repeat steps with negative pd. values

Question 19

describe the circuit+apparatus used for RP: I-V characteristics in fixed resistor/ filament lamp/ diode

Answer 19

• FIXED RESISTOR: fixed resistor+ battery/ ammeter/ variable resistor connected in series, with voltmeter connected at points before+after the fixed resistor
• FILAMENT LAMP: lamp+ battery/ ammeter/ variable resistor connected in series, with voltmeter connected at points before+after the lamp
• DIODE: diode+ battery/ ammeter/ variable resistor +FIXED RESISTOR connected in series, with voltmeter connected at points before+after the diode

Question 20

describe a thermistor +graph +uses

Answer 20

• resistance decreases as temp increases
• thermal energy helps particles line up/form regular arrangement so charge can move through more easily
• current increases as temp increases
• shown on graph as decreasing curve
• used in fire alarms, air-conditioning, (fridge) thermostats

Question 21

describe a light dependent resistor +graph +uses

Answer 21

• resistance decreases as light intensity increases
• light energy helps particles line up/form regular arrangement so charge can move through more easily
• current increases as light intensity increases
• shown on graph as decreasing curve
• used in night lights, outdoor lighting eg. street lamps

Question 22

describe the method for RP: how the length of wire affects resistance

Answer 22

• set up circuit: place a cell, ammeter+test wire in series then connect a voltmeter to places in the circuit before+after the test wire
• place jockey on test wire at length of 100cm
• measure the potential diff using voltmeter and current using ammeter
• calculate resistance = potential diff / current
• repeat steps for lengths 90cm-30cm in intervals of 10cm
  -> wire resistance is directly proportional to wire length

Question 23

describe sources of error, prevention and improvements for RP: how length of wire affects resistance

Answer 23

• wire gets hot which changes the temp of the wire:
  start taking results at the longest length as wire would heat up less
  connect resistance wire to the circuit by the jockey for the shortest possible time to minimise current flow
  pause between readings to allow wire to cool down
  alternative method - have resistance wire submerged in water to help regulate temp
• results precision - improved by repeating experiment+ calculating mean results

Question 24

describe the conclusion for RP: how length of wire affects resistance

Answer 24

• as the length of wire increases the resistance increases
• shown on graph as straight line passing through the origin
• the current flowing through the wire contains charge that carries electrons, which collide with atoms in the wire - the longer the wire, the more collisions between electrons+ atoms so increased resistance

Question 25

describe how the national grid works

Answer 25

• voltage from a generator in a power station is transported to a step-up transformer
• the transformer increases voltage (from around 25,000V-700,000V) so the current decreases - less current means less energy disspiated through heating of cables to thermal energy store of surroundings
• transmission cables carry high voltage electrical power - above ground to keep people safe from electric shocks
• a step-down transformer reduce voltage to 230V for safe use in consumer homes/industry

Question 26

function of a fuse in a circuit

Answer 26

• thin wire that melts if too large a current flows through it - causes circuit to break
• this protects electrical circuits

Question 27

function of a resistor and variable resistor in a circuit

Answer 27

resistor: controls amount of current in a circuit
variable: adjusted to control the amount of current

Question 28

function of a diode and a LED in a circuit

Answer 28

diode: allows current to flow in one direction - prevents damage to other components
LED: emits light when current flows correct way through it

Question 29

describe direct current

Answer 29

• power supply provides a direct pd.
• so current (electrons) flow in one direction from + to - of battery

Question 30

describe alternating current

Answer 30

• pd. constantly changes direction
• so current (electrons) constantly change direction

Question 31

describe the mains electricity

Answer 31

• alternating current/ supply
• UK domestic supply has frequency 50Hz and 230V
  (current alternates 50 times per second)

Question 32

describe the live wire

Answer 32

• bRown insulation - right side
• carries alternating pd. from plug socket to appliance
• can be dangerous even when circuit is switched off
• connected to a fuse which melts/blows to disconnect the circuit if too large a current passes through it

Question 33

describe the neutral wire

Answer 33

• bLue insulation - left side
• completes the circuit
• very close to earth pd. = 0V

Question 34

describe the earth wire

Answer 34

• green and yellow striped insulation - top
• safety wire - brings current to the earth if circuit develops a fault

Question 35

describe the purpose of double insulated appliances

Answer 35

• outer case made of insulating plastic
• eg. hair dryers, mobile phone chargers, hand whisks
• do not need an earth wire
• two layers of insulating material between the electrical parts+ outside parts that can be touched - insulating plastic case and the insulated cable carrying the live+neutral wires
• no direct contact with internal electrical parts
• shown by symbol containing an inner+outer square

Question 36

name the three equations for power

Answer 36

work done (energy transferred) = power x time
W = P x t
(J) (W) (s)
electrical power = current x pd.
P = I x V
(W) (A) (V)
power dissipated = current² x resistance
P = I² x R
(W) (A) (Ω)