Electric Currents

Question 1

what are conductors

Answer 1

materials which electrons are free to wander

• electrons only loosely anchored to the atoms
• all metals

Question 2

what are insulators

Answer 2

materials in which electrons are not free to wander

• electrons tightly bound and belong to particular atoms
• rubber, glass, air plastics (which is why wires are covered with plastic)

Question 3

What are semiconductors

Answer 3

neither good conductors nor good insulators

-can be used to control the flow of electrons

Question 4

true or false; air is an insulator

Answer 4

true

Question 5

During lightning millions of electrons move from a cloud to earth. How is this possible? describe 3 initial steps

Answer 5

1. ) negative charge builds up on the bottom side of a cloud (through induction)
2. ) electrons pulled away from the air molecules by the electric field
3. ) if the electric field is sufficiently large the bond is broken and the electron is removed from the atom
   - insulator (air) is converted to conductor; as electrons are free to wander

Question 6

What happens when the electric field between the ground and the cloud is strong enough?

Answer 6

• air molecules are ionized (electrons kicked off)
• a conductive plasma channel (gas of electrons and ionized molecules) form
• electrons can move from the cloud to the ground via the plasma channel
• cloud discharges (lightning)

Question 7

Define electric potential

Answer 7

a charged object has the potential to do work by virtue of its location in an electric field

Question 8

Define electric potential energy (U)

Answer 8

the energy a charged particle possesses by virtue of its location in an electric field

Question 9

What does the U depend on

Answer 9

charge

Question 10

true or false; the larger the charge the larges the potential to do work

Answer 10

true

Question 11

what does work equal?

Answer 11

the change in potential energy

Question 12

What does electric potential energy describes

Answer 12

how much a positive test charge wants to be at a given location

Question 13

true or false; electric potential is independent of charge, dependent on location

Answer 13

true

Question 14

what is the unit of electric potential

Answer 14

V

volts

Question 15

electric potential= electric potential energy/ charge

Answer 15

true

Question 16

For a positive test charge:
high potential: does not want to be there at all
low potential: wants to be there a lot
what about a negative test charge?

Answer 16

opposite
high potential: wants to be there
low potential: does not want to be there

Question 17

true or false; electric potential decreases when moving away from positive charge and towards negative charge

Answer 17

true

Question 18

direction of flow of electrons

Answer 18

low to high potential

Question 19

define electric current

Answer 19

flow of electric charge through a conductive medium

Question 20

what are the units of current

Answer 20

ampere (A)

Question 21

equation of calculating electric current, given the charge and time

Answer 21

I = Q/t

Question 22

direction of electron flow?

Answer 22

low to high potential

Question 23

direction of current flow?

Answer 23

high to low potential

Question 24

true or false; the electric field inside a current-carrying conductor is not zero

Answer 24

true

- electric field is only zero if there is no current (electric shielding)

Question 25

what is the direction of electric field?

Answer 25

same as the current

Question 26

how is potential difference maintained?

Answer 26

by doing work

Question 27

what must be maintained to keep the charge flowing?

Answer 27

potential difference

Question 28

true or false; eventually flow of charge will neutralize the potential difference

Answer 28

true

Question 29

What do batteries do?

Answer 29

maintain a constant voltage (potential difference) between the terminals - energy supplied by chemical reactions - current flows when terminals are connected

Question 30

what does the amount of current depend on?

Answer 30

voltage and resistance | - large voltage, large current

Question 31

what does resistance do?

Answer 31

opposes the passage of current

Question 32

what are the units of resistance?

Answer 32

ohms (Ω)

Question 33

What does ohms law say?

Answer 33

I = V/R

Question 34

What causes resistance

Answer 34

electric field accelerates electrons electrons collide with positive ions and other electrons - electrons have to be re-accelerated - hinders their movement

Question 35

Electrons power (P)

Answer 35

rate of which electric energy is converted to another form | - energy lost by electrons on collisions is converted to eg. light

Question 36

equation of power

Answer 36

P= I*V

Question 37

true or false; electron energy at the positive terminal is zero (potential energy is zero and KE is negligible) so all energy is converted

Answer 37

true

Question 38

What is an electric circuit

Answer 38

any path along which electrons can flow | - for continuous flow, there must be no gaps

Question 39

What is a series circuit

Answer 39

all components are connected along a single path

Question 40

What is a parallel circuit

Answer 40

components form branches, each of which is a separate path for electrons

Question 41

Describe a parallel circuit

Answer 41

- voltage remains the same in all components - current through each component obeys the Ohms law independently - total current equals the sum of currents through various components 1/Rtot = 1/R1 + 1/R2 + 1/R3....

Question 42

Describe a series circuit

Answer 42

- current flowing through each component is the same - voltage drops across each component - sum of voltages across each voltage equals the supply voltage Rtot = R1 + R2+ ....

Question 43

Define direct current

Answer 43

charge flows continuously in one direction

Question 44

define alternating current

Answer 44

direction of current alternates - cheaper for long distance transport (less power loss) - easier to make devices than for DC