Transcript current - Helios

Ohm’s Law
Physics 102
Professor Lee Carkner
Lecture 14
PAL #13 Capacitors

What is capacitance?
Q= CV
C = Q/V = 0.005/1000 =
Jury-rig a replacement out of metal foil and
Teflon coating (k = 2.1, thickness = 0.01 mm).
C = ke0A/d
A = Cd/ke0 = (5X10-6)(0.00001)/(2.1)(8.85X10-12)
A =
How can such a device be portable?

Circuit Theory

We have already discussed potential
difference

This charge motion is called the current
(symbol: I)
Energy can be extracted from the
current due to resistance (symbol: R)
Current

I = DQ/Dt
The units are amperes (amps) or
coulombs per second

The most common charge carrier is the
electron
Inside a Wire

The wire is full of electrons

However, the electrons do not move in a
straight line

Electrons do not move freely, directly or
rapidly
Drift Speed

We can find the drift speed in terms of the
properties of the wire:
Where I is the current, n is the electron
density, q is the charge on the electron and
A is the cross sectional area of the wire

Electron Motion
Current Conundrums
The drift speed is very small (~mm per
second), yet the effect of current is felt
instantaneously

Electrons move randomly, yet current flows
in only one direction

The direction of the current is opposite the
motion of the electrons

Resistivity

Why?
The materials resist the flow of current

Good conductors have low resistivity,
good insulators have high resitivities
Resistivity is a property of a particular type of
material rather than of a particular wire
Resistance

Short, wide wires have less resistance than
long, narrow wires
The resistance can be written as:
R = r (L/A)

The units of resistance are ohms (volts
per ampere)
Ohm’s Law
How much current do you get if you put a potential
difference V across a wire with resistance R?
High voltage, low resistance means large current

Commonly written as:
V = IR

However, the law only holds for certain types of materials
(called ohmic)
Simple Circuit
Using Ohm’s Law
Ohm’s law quantifies the way circuits work

Can write in different ways:

The voltage varies directly with resistance
when current is constant

High voltage and low resistance means high
current
Today’s PAL
A 1.5 volt battery produces 167 A of
current when connected to a 1 meter
long, 2 mm thick wire. What is the wire
made of?
Discuss the validity of the following
claim:
“The relationship R = V/I tells us that the
resistance of a wire is directly proportional
to the potential difference applied to it.”
Temperature and Resistance
Resistors convert energy from the current
into heat

Temperature also affects electronic properties

This increased random motion means collisions
are more frequent and it is harder for current to
flow
Resistance generally increases with
temperature
Superconductivity
If we set up a current in a wire and then take away
the battery the current fades to zero

If the resistance was zero the current would keep
flowing even without a battery

Such materials are called superconductors

Superconductors have R go to zero as T gets small
Energy in Electric Circuits

As the charges flow (as current) they
convert the potential energy to kinetic
energy

We should be able to relate the
potential difference, current and
resistance to the energy produced
Energy Dispersion Rate

The energy per electron is DPE = qDV so energy per
coulomb is DV

(Energy/Coulomb)(Coulomb/Second) =
(Energy/Second)
IDV = P
Power
Using Ohm’s law (DV = IR) we can
write:
In general we will know the values of R
(since it depends on the properties of
the resistor) and DV (since we should
know the voltage of our source or
battery)

Lightbulbs
A common circuit element is the lightbulb

Household lightbulbs are rated in watts

In the US, most power outlets produce 120
volts of potential difference

Those that do not use a transformer
Coronal Loop on the Sun
Joule Heating

This is how an electrical resistance heater
works
Joule heating is seen in the natural world:

Can produce energy in the Earth’s atmosphere

Next Time
Read: 19.1-19.4, 18.6, 19.7
Homework: Ch 18, P 35, (+one not in
book), Ch 19, P 5, 9