Forces encountered in PHYSICS I:
Gravitational
“Contact” forces
“Normal” force
Springs
Friction
Tension in strings
Pressure
Still to come later in Phys 2012: Magnetic forces
Not covered in 2012:
Inside the nucleus
“weak” force
“strong” nuclear force
What do you think of when you hear or see the word
ELECTRICITY?
We need additional, more precisely defined and
measurable concepts and quantities to describe
phenomena of “electricity”
Electric charge – new quantity beyond mass, length
and time so far
Electric force arising from charges (still in newtons)
Electric field
Electric potential
Electric potential energy
Electric CHARGE is an intrinsic property of the fundamental
particles that make up atoms.
Atoms = (protons + neutrons) + electrons
Mass (kg)
m =
Charge (C)
(coulomb)
q =
Spin Ang.
Momentum.
L=
Proton 1.673x10-27 +1.602x10-19
(+e)
+/- 1/2 (h/2)
Neutron 1.675x10-27 0 +/- ½ (h/2)
Electron
9.11x10-31 -1.602x10-19
(-e)
+/- 1/2 (h/2)
New unit to quantify charge: the coulomb (C)
1 coulomb is a lot of charge.
Proton charge is e = +1.6 x 10-19 C
So 1 C requires about 6.25 x 1018 protons
Electron and proton have exactly opposite
charges, so most matter is neutral – having an
equal number of protons and electrons.
Atom = electrically neutral
Ion = atom that has gained or lost electrons
Evidence of two kinds of charge on objects (positive/negative)
Frictional charging of rods – two kinds of behavior among three rods
A and B = attraction B and C = repulsion
Opposite signs attract Same signs repel
Convention from B. Franklin: Glass is + after rubbing with silk.
Effect of charged rod on a neutral object (polarization)
Insulators vs conductors
Charging by induction
Reading Question: According to Coulomb’s law, the electric
force between two charges, q1 and q2 is proportional to:
1 2 3 4
10%3%
11%
76%
1. q1 + q2
2. q1q2
3. q1 / q2
4. q1 - q2
Coulomb’s Law – electric force between point charges
Force is directed along the line connecting the charges so that
Same sign charges repel
Opposite sign charges attract
Magnitude given by
2
21
r
qqkF c
Simple example: Two bowling balls (m=7.3 kg) 1 meter apart are
charged, each to +1.0 C. What’s the electric force created? What
would be the (initial) acceleration of one of the bowling balls?
Two equal mass balls are charged, and hang on strings as shown. What can you say about the magnitudes of the charges Q1 and Q2 on the two balls?
1 2 3
78%
22%
0%
1. Q1 must equal Q2
2. Q1 cannot equal Q2
3. Can't decide/not enough
information.
©University of Colorado, Boulder (2008)
Channel Setting Instructions for ResponseCard RF
1. Press and release the "GO" or "CH" button.
2. While the light is flashing red and green, enter the 2 digit
channel code (i.e. channel 1 = 01, channel 21 = 21).
Channel is 41
3. After the second digit is entered, Press and release the
"GO" or "CH" button. The light should flash green to
confirm.
4. Press and release the "1/A" button. The light should
flash amber to confirm.
Multiple charges – Superposition of Forces
...32313 FFF
In which of the situations below is there a point to the left of
the charges where an electron will be in equilibrium?
1 2 3 4 5 6 7
7%
25%
11%
7%
19%
26%
6%
1. (a)
2. (b)
3. (c)
4. (d)
5. (a) and (c)
6. (a) and (b)
7. None
Two charges are on the x axis, equidistant from the origin. In which of
the situations below is the net force on the third charge on the y axis to
the purely in the +x direction?
1 2 3 4 5 6
2% 1%
10%
4%2%
82%
1. 1
2. 2
3. 3
4. 4
5. More than 1
6. none
Reading question: Initially, sphere A has a charge of -50e
and sphere B has +20e. They are both conductors and
identical in size and material. After the spheres touch and
are separated, sphere A has a charge of:
1 2 3 4
9%
2%
22%
67%
1. -35e
2. -15e
3. -30e
4. +20e
Charge is quantized
All observed charged objects are an integer (N) multiple
on the smallest unit (quantum) of charge e.
q = Ne
[ But protons, neutrons = 3 quarks with (1/3)e , (2/3)e ! ]
Charge is conserved
The total electric charge of an isolated system is constant.
Charges can be created or destroyed
in matching pairs (+ / - ) only.
Insulating spherical shell, total charge Q uniformly
spread over its surface
To a charge q placed OUTSIDE, the shell acts like a point charge Q
concentrated at the center
To a charge q placed INSIDE, the electrostatic force on q due to the
shell is 0 ---- everywhere inside!!
Static vs Current Electricity
Static -- fixed charges, (apart from some
transient moving of charge to set up the
situation
Current electricity – charges in motion,
perhaps briefly (lightning bolt) or in a steady
and controlled way (electric circuits).
Moving charges = electric current
(rate at which charge is transferred)
Electric current measured in ampere (A)
1 C/s = 1 A (or 1 C = 1 A·s)
dt
dqi
The figure shows four arrangements of electrons (e) and protons (p)
near a charge +Q. Which situation produces the largest magnitude
electric force on Q?
1 2 3 4 5 6
0% 0% 0%0%0%0%
1. (a)
2. (b)
3. (c)
4. (d)
5. Two tie
6. All 4 tie