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