Electric Currents Flashcards
what are conductors
materials which electrons are free to wander
- electrons only loosely anchored to the atoms
- all metals
what are insulators
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)
What are semiconductors
neither good conductors nor good insulators
-can be used to control the flow of electrons
true or false; air is an insulator
true
During lightning millions of electrons move from a cloud to earth. How is this possible? describe 3 initial steps
- ) negative charge builds up on the bottom side of a cloud (through induction)
- ) electrons pulled away from the air molecules by the electric field
- ) 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
What happens when the electric field between the ground and the cloud is strong enough?
- 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)
Define electric potential
a charged object has the potential to do work by virtue of its location in an electric field
Define electric potential energy (U)
the energy a charged particle possesses by virtue of its location in an electric field
What does the U depend on
charge
true or false; the larger the charge the larges the potential to do work
true
what does work equal?
the change in potential energy
What does electric potential energy describes
how much a positive test charge wants to be at a given location
true or false; electric potential is independent of charge, dependent on location
true
what is the unit of electric potential
V
volts
electric potential= electric potential energy/ charge
true
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?
opposite
high potential: wants to be there
low potential: does not want to be there
true or false; electric potential decreases when moving away from positive charge and towards negative charge
true
direction of flow of electrons
low to high potential
define electric current
flow of electric charge through a conductive medium
what are the units of current
ampere (A)
equation of calculating electric current, given the charge and time
I = Q/t
direction of electron flow?
low to high potential
direction of current flow?
high to low potential
true or false; the electric field inside a current-carrying conductor is not zero
true
- electric field is only zero if there is no current (electric shielding)
what is the direction of electric field?
same as the current
how is potential difference maintained?
by doing work
what must be maintained to keep the charge flowing?
potential difference
true or false; eventually flow of charge will neutralize the potential difference
true
What do batteries do?
maintain a constant voltage (potential difference) between the terminals
- energy supplied by chemical reactions
- current flows when terminals are connected
what does the amount of current depend on?
voltage and resistance
- large voltage, large current
what does resistance do?
opposes the passage of current
what are the units of resistance?
ohms (Ω)
What does ohms law say?
I = V/R
What causes resistance
electric field accelerates electrons
electrons collide with positive ions and other electrons
- electrons have to be re-accelerated
- hinders their movement
Electrons power (P)
rate of which electric energy is converted to another form
- energy lost by electrons on collisions is converted to eg. light
equation of power
P= I*V
true or false; electron energy at the positive terminal is zero (potential energy is zero and KE is negligible) so all energy is converted
true
What is an electric circuit
any path along which electrons can flow
- for continuous flow, there must be no gaps
What is a series circuit
all components are connected along a single path
What is a parallel circuit
components form branches, each of which is a separate path for electrons
Describe a parallel circuit
- 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….
Describe a series circuit
- 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+ ….
Define direct current
charge flows continuously in one direction
define alternating current
direction of current alternates
- cheaper for long distance transport (less power loss)
- easier to make devices than for DC
