electric circuits

Question 1

what happens when you rub 2 objects together?

Answer 1

become electrically charged as electrons (negatively charged) are transferred to 1 object to another:

• objects receiving electrons negatively
• objects giving up electrons positively

electrical charge = static electricity

Question 2

what happens when 2 charged materials are brought together?

Answer 2

exert force on each other

• 2 materials with the same type of charge repel
• 2 materials with diff charges attract

eg:

• positively charged perspex rod another pc rod, rod will repel
• negatively charged ebonite rod near to positively charged Perspex rod, will attract

Question 3

what is an electric current?

Answer 3

flow of charge, measured in amps

Question 4

what happens in a electric circuit?

Answer 4

• components + wires full of charges that are free to move
• battery causes free charges to move
• charges aren’t used up but flow in a loop

Question 5

what happens in metal conductors and insulators?

Answer 5

metal conductors:
- lots of charges free to move

insulators:
- no charges free to move

metals contain free electrons in structure which move to create an electric current

Question 6

what is a direct current?

Answer 6

always flows in same direction

Question 7

what is an alternating current?

Answer 7

changes direction of flow back and forth continuously

Question 8

why is alternating used for mains supply instead of current?

Answer 8

• easier to generate
• distributed efficiently
• used in a transformer

Question 9

what is potential difference?

Answer 9

voltage:

• pd between 2 points in the circuit is the work done on a given amount of charge as it moves between these points
• measured in volts using voltmeter connected in parallel across a component

Question 10

e

Answer 10

greater pd, greater current

more batteries in a series, pd increases

Question 11

what happens if you add more batteries in parallel?

Answer 11

• total pd + current remain same

- each battery supplies less current

Question 12

what do components do?

Answer 12

resist flow of charge through them

examples:

• resistors
• lamps
• motors

Question 13

explain resistance?

Answer 13

greater resistance, smaller the current

Question 14

if you add another cell in a circuit, what does it provide?

Answer 14

• greater pd

- greater current

Question 15

what happens if you add resistors in series + parallel?

Answer 15

series:
- batt push charges through more resistors so resistance increases

parallel:
- more paths for charges to flow along so resistance reduces

Question 16

what happens when a current flows through a component?

Answer 16

causes component to heat up, heating effect large enough to make lamp filament glow

Question 17

what happens as the current flows?

Answer 17

• moving charges collide with vibrating ions in wire, giving energy
• inc in energy causes component to heat

Question 18

how can you calculate resistance?

Answer 18

resistance = voltage / current

Question 19

explain current-voltage graphs?

Answer 19

as components resistance stays constant, current through resistor is directly proportional to voltage across resistor

means graph showing current + voltage in component will be a straight line through 0

Question 20

what does the thermistor depend on?

Answer 20

temp

as temp increases:

• resistance decreases
• more current flows

Question 21

what does the LDR depend on?

Answer 21

light intensity

as light intensity increases:

• resistance increases
• more current flows

Question 22

what are the uses of some electric motors?

Answer 22

hard disk drive - rotate @ high speed
dvd player - rotate so info read
washing mach - rotate drum + agitate wash
tumble dryer - rotate drum + provide uniform heat
microwave - rotate food so evenly heated

Question 23

e

Answer 23

when electric charge flows through device, work is done by power supply

energy is transferred from power supply to device and surroundings

Question 24

what is power?

Answer 24

• measured in watts

- measure of rate of energy transfer to device and surroundings

Question 25

how can you calculate power?

Answer 25

power = voltage x current

Question 26

what are transformers?

Answer 26

used to change voltage of alternating current consisting of 2 coils of wire wrapped around a soft iron core

Question 27

when 2 coils of wire are close to each other, a changing magnetic field in one coil caused by changes in current can induce a voltage in the other how?

Answer 27

• alternating current flowing through pc creates alternating magnetic field
• changing field induces an alternating current in the secondary coil

Question 28

what is the equation of the transformer?

Answer 28

amount of which transformer changes voltage depends on number of turns on coils

changing current causes changing magnetic field in iron core which causes a changing pd acriss sc

equation:
v on primary/v on secondary = # of turns on primaru/# of turns on secondary

Question 29

what is an electric motor?

Answer 29

consists of coil of wire that rites in between opposite poles of a permanent magnet when a current flows through the coil

Question 30

what can a coil do?

Answer 30

• exert force on magnet or another current carrying wire
• exp a force if placed in a magnetic field whos lines of force are at right angles to the wire. force will be at right angle to both current direction and lines of force of magnetic field. if parallel to magnetic field lines, doesn’t experience force

Question 31

what happens when a current flows through the coil?

Answer 31

cut field lines in opposite directions on each side of coil creating pair of forces in opposite directions and causing coil to rotate

Question 32

what is a commutator?

Answer 32

rotary switch that turns with coil but brushes that touch it remain fixed

this has effect of making sure that as coil rotates, has current direction into the coil is switched

Question 33

when does the current always cut the field lines?

Answer 33

• nearest south pole of magnetic in same direction

- nearest north pole in opposite direction producing pair of opposing forces whose directions remain same

Question 34

what is mains electricity produced by?

Answer 34

generators that use principal of electromagnetic induction to generate electricity by rotating a magnetic inside a coil

Question 35

what can the size of voltage be increased by?

Answer 35

• inc speed of rotation of magnet
• inc strength of magnetic field by using electromagnet
• inc # of turns on coil
• placing iron core inside coil

Question 36

what happens as the magnetic rotates?

Answer 36

voltage induced in coil changes direction and size

current is alternating current as it reverses its direction of flow every half turn of the magnetic, the direction of voltage + current in same direction

Question 37

what is the electromagnetic induction?

Answer 37

when you move a magnetic into coil of wire, voltage is induced between ends of wire bc magnetic field is being cut

if ends of coil are connected to make a complete circuit, a current will be induced

Question 38

how can you induce a current in the opposite direction?

Answer 38

• moving magnet out of coil
• moving pole of magnet into coil

no movement of coil or magnet, no induced current

Question 39

what happens when 2 components are connected in series to a battery?

Answer 39

• current flowing through each component is same = A1 = A2 = A3
• pd across components adds up to pd across battery = V1 = V2 + V3
• pd is largest across components with the greatest resistance