physics - electricity & circuits (10.1 - 10.42)

10.1 atom structure - position, mass, charge of protons, neutrons, electrons

10.2 + & - terminals symbol

10.2 cell symbol & definition

provides circuit with source of potential difference

10.2 battery symbol & definition

provides circuit with source of potential difference

two/more cells

10.2 open switch symbol & definition

circuit off

10.2 closed switch symbol & definition

circuit on

10.2 voltmeter symbol & definition

measures voltage

connected in parallel

10.2 ammeter symbol & definition

measures current

connected in series

10.2 resistor symbol & definition

limits flow of electrical current

can’t change resistance

10.2 variable resistor symbol & definition

limits flow of electrical current

can change resistance

10.2 lamp symbol

10.2 motor symbol & definition

converts electrical energy to mechanical energy

10.2 diode symbol & definition

only lets current flow in one direction

10.2 thermistor symbol & definition

resistance depends on temperature

10.2 LDR (light-dependent resistor) symbol & definition

resistance depends on light-intensity

10.2 LED (light-emitting diode) symbol & definition

only lets current flow in one direction

emits light as result of current flow

10.3 series circuits

one route current can take around circuit

diagram: lamps cannot be switched on & off individually; one lamp fails = all lamps switch off

10.3 parallel circuits

junctions that let current take diff. routes around circuit

diagram: lamps can be switched on & off individually; one lamp fails = not all lamps fail

10.4 voltmeter - connected in, measures

connected in parallel with component

measures potential difference (voltage) across component - in volts

10.5 potential difference (voltage) equal to

energy transferred per unit charge

1 V = 1 J/c

10.6 energy transferred equation

energy transferred (J) = charge moved (C) x potential difference (V)

E = QV

10.7 ammeter - connected in, measures

connected in series with component

measures current in component - in amps

10.8 electric current definition

rate of flow of charge

10.8 current in metals

flow of electrons

10.9 charge equation

charge (C) = current (A) x time (s)

Q = It

10.10 what will there be when closed circuit includes source of potential difference?

current in circuit

10.11 what happens to current at junction in circuit?

current is conserved (current entering junction = current leaving junction)

10.12 changing resistance in circuit effect on current

resistance increased = current decreased

V = IR

e.g. 10 = I x 2 → I = 5, resistance increased from 2 → 5, 10 = I x 5 → I = 2 - current decreased from 5 → 2

10.12 how to change resistance in circuit?

use variable resistor

10.13 potential difference equation

potential difference (V) = current (A) x resistance (Ω)

10.14 two resistors in series effect on net resistance & why

total resistance increases

pathway harder for current to flow through

10.14 two resistors in parallel effect on net resistance & why

total resistance decreases

more paths for current to flow through

10.15 calculate current in series circuit

V = IR → I = V/R

I_T = I₁ = I₂ = I₃

10.15 calculate potential difference in series circuit

V = IR

V_total = V₁ + V₂ + V₃

10.15 calculate resistance in series circuit

V = IR → R = V/I

R_total = R₁ + R₂ + R₃

10.16 design & construction of series circuits for testing & measuring

variable resistor changes current in circuit

measurements of current & p.d. recorded

resistance of fixed resistor calculated - checks if resistor has correct value/measures unknown resistance

10.17 core practical: construct electrical circuits to investigate relationship between potential difference, current & resistance for resistor & filament lamp

1. set up circuit in diagram, use power pack that can provide diff. p.d.s

2. set power pack to lowest voltage & switch on

3. record ammeter & voltmeter readings

4. switch power pack off

5. repeat steps 2-4 up to max. of 6V

6. replace resistor in circuit with 2 filament lamps & repeat steps 2-5

10.17 core practical: construct electrical circuits to test series & parallel circuits using resistors & filament lamps

1. set up 1st circuit in diagram

2. set power pack to lowest voltage & switch on

3. record ammeter & voltmeter readings

4. repeat steps 2-3 up to max. of 6V

5. set up 2nd circuit in diagram

6. repeat steps 2-4 for several diff. voltage settings

10.18 filament lamps - current, potential difference, resistance

current makes filament heat up

greater p.d. = more current flows = filament gets hotter

filament gets hotter = resistance increases

p.d. changes → current doesn’t change by same % - not in direct proportion

10.18 diodes - current, potential difference, resistance

p.d. in one direction = low resistance

p.d. in opposite direction = high resistance

current only flows in one direction

10.18 fixed resistors - current, potential difference, resistance

p.d. changes → current changes by same % - in direct proportion

resistance stays same

10.19 light intensity effect on resistance of LDR

light increases = resistance decreases

light decreases = resistance increases

10.20 temp. effect on resistance of thermistor

temp. increases = resistance decreases

temp. decreases = resistance increases

10.21 filament lamps, diodes, thermistors, LDRs - design & use of circuits to explore variation of resistance

explores how resistance of lamp changes as p.d. across lamp is changed

current through lamp measured by ammeter & recorded for diff. values of p.d. measured on voltmeter

(same for diodes, thermistors & LDRs just with diff. symbol)

10.22 what happens when there is electric current in resistor?

there is an energy transfer - heats resistor

10.23 what happens when electrical current does work against electrical resistance?

current passes through resistor - energy transferred because electrical work done against resistance

electrical energy dissipated as thermal energy in surroundings

10.24 why is energy transferred when there is electrical current in resistor?

electrons flow through lattice of vibrating ions

electrons collide with ions - transfer energy to ions

electrons make more collisions with ions = harder for electrons to pass through = higher electrical resistance

10.25 reducing unwanted energy transfer - low resistance wires

reduce resistance in circuits:

• use wires made from metals with low resistance

• use thicker wires - have lower resistance

• cool metals - lattice ions not vibrating as much

lower resistance = less energy transferred by heating = less energy dissipated

10.26 advantages of heating effect of electric current

energy transferred from circuit by heating

surroundings gain thermal energy

useful in electric heater/kettle - purpose is to heat surroundings

10.26 disadvantages of heating effect of electric current

useful energy transferred from circuit by heating

surroundings gain thermal energy

not useful in computers/plugs/wires - purpose is not to heat surroundings

10.27 energy transferred equation

energy transferred (J) = current (A) x time (s) x p.d. (V)

E = ItV

10.28 power definition

energy transferred per second

10.28 power units

watts (W)

10.29 power equation

power (W) = energy transferred (J)/time taken (s)

P = E/t

10.30 how is power transfer in any circuit device related to p.d. across it & current in it?

power transfer in component/appliance proportional to p.d. across it & current in it

energy transferred = current x time x p.d.

power = energy transferred/time

power = current x time x p.d./time = current x p.d.

10.31 electrical power equation - current & p.d.

electrical power (W) = current (A) x p.d. (V)

P = IV

10.31 electrical power equation - current & resistance

electrical power (W) current² (A²) x resistance (Ω)

P = I²R

10.32 how is energy transferred from batteries → energy of motors in fan?

some energy stored in battery —transferred by electricity→

motor —transferred→

store of kinetic energy in fan

some energy —transferred by heating→

wires, motor & surroundings

all energy dissipated by heating - increases store of thermal energy of surroundings

10.32 how is energy transferred from batteries → energy of heating devices in heating gloves?

gloves contain wire of high resistance

energy stored in battery —transferred by electricity→

high resistance wire —transferred by heating→

store of thermal energy in wire —transferred by heating→

gloves & hands of wearer & then surroundings

10.32 how is energy transferred from a.c. mains → energy of motors washing machine?

motor in washing machine —transfers energy→

kinetic energy in washing machine drum

10.33 direct voltage

cells & batteries have a positive & a negative terminal

direction of movement of charge (current) stays same

voltage stays same

10.33 alternating voltage

direction of movement of charge (current) changes

voltage changes - increases to peak voltage, decreases to 0

10.34 direct current (d.c.) definition

movement of charge in one direction only

10.34 what current do cells & batteries supply?

direct current (d.c.)

10.35 alternating current (a.c.) definition

in a.c. movement of charge changes direction

10.36 UK domestic supply - kind of current

alternating current (a.c.)

10.36 UK domestic supply - frequency

50Hz

10.36 UK domestic supply - voltage

about 230V

10.37 live mains input wire function

connects appliance to generators at power station

10.37 neutral mains input wire function

return path to power station

10.38 earth wire function

fault in appliance - metal parts at high voltage

touch wire = electric shock - current flows through you to ground

connects metal parts of appliance to large metal spike/ tubing in ground

current flows to ground through earth wire not you

for safety

10.38 fuses function

tube with thin wire inside

marked with current it can carry

for safety

fuse melts before wiring/parts of appliance can overheat

1. current passes through wire

2. wire gets hotter

3. current exceeds certain value

4. wire melts

5. breaks circuit - cuts off mains electricity supply

6. stops current

fuses disadvantages

must be replaced

take time to melt - don’t prevent electric shock

10.38 circuit breakers function

detect change in current

switch off current supply

for safety

circuit breakers advantages

once fault fixed, circuit breakers can be switched on again

can prevent electric shock

10.39 why should switches & fuses be connected in live wire of domestic circuit?

when off, no current goes through appliance

10.40 live wire p.d.

230V

10.40 neutral wire p.d.

0V

10.40 earth wire p.d.

0V

10.41 dangers of connection between live wire & earth

if fault makes live wire touch metal part, makes very low resistance circuit between 230V & 0V

makes very large current flow to earth - heats up wire, could cause fire

10.42 relationship between power ratings for domestic electrical appliances & changes in stored energy when in use

e.g. kettle with power rating 3kW:

• transfers 3000 joules of energy each second

• transfers from mains electricity supply → store of thermal energy in water