Topic 5 - Electricity and Magnetism

Question 1

what is : charge

Answer 1

nobody knows

but e- has negative charge
p+ has positive charge
n0 has no charge

Question 2

charge laws

Answer 2

opposites attract
like charges repel

there must be some force then

Question 3

test charge

Answer 3

q - infinitely small positive charge

no effect on space time and no own electric field

this is how charge is figured out

Question 4

types of charge transfer (electrostatic)

Answer 4

friction
* physical stripping of e- from one surface to another
* Q1 = 0 Q2 = 0 –> Q1 = 6C Q2 = -6C

conduction
* two objects made contact and charge transferred as a result
* ground - transfer via conduction to a ‘sink’ of e- (earth)

induction
* NO CONTACT, but an alignment of charge in two areas of opposite charge
* the image is for induction

Question 5

conductor + insulator

Answer 5

conductor - allows the free flow of e-

insulator - doesn’t allow it
no ‘sea’ of e-

Question 6

Electromotive Force

Answer 6

ϵ - not a force - voltage (potential difference)

Question 7

single charge

Answer 7

monopole

Question 8

two charges

Answer 8

dipole

Question 9

capacitor

Answer 9

two equal, but opposite charged metal plates

create a uniform electric field in between the plates

Question 10

Potential Difference

Answer 10

from one equipotential to another
∆V = Voltage (in volts)
it is the work that must be done to move a charge
we normaly use eV not V
1eV = 1.6*10^-19J

Question 11

Drift Velocity

Answer 11

I=nAVq

if we know the number (n) of free charges (q) per unit volume in a conductor, known as the number density, and the cross sectional area (a) of the conductor, and the drift velocity (v) of the charges, we can get the current (I)

Question 12

what is : resistor

Answer 12

• filled with carbon
• thermistor (temperature dependent)
• light dependent resistor
• variable resistor /potentiometer (you can change)

when V and R are proportional, Ohmic behavior is present

when many resistors in a circuite, create a simple equivalent circuit (Req)

Question 13

resistor in series

Answer 13

One after another

I = constant
V = decreases
R(eq) = ∑R

Question 14

resistors in parallel

Answer 14

Side by side

V = constant
I = not constant
1/R(eq) = ∑1/R

Question 15

battery vs cell

Answer 15

a battery is a group of cells

Question 16

ideal voltmeter

Answer 16

measure across (in parallel)

R = infinity
I = O

Question 17

ideal amemeter

Answer 17

in series
R = 0

Question 18

Kirchoff’s Laws : Junction

Answer 18

Junction Rule : Total current in a junction is equal to the total current out
I(in) = I(out)
I1 = I2 + I3
Conservation of current

Question 19

Kirchoff’s Laws : Loop Rule

Answer 19

Loop Rule : The sum of voltages in a closed loop is equal to zero
V1 + V2 + V3 = 0

a) choose a direction in a CLOSED loop (CW or CCW)
b1) if the direction of the loop crosses a resistor AGAINST I -> +IR
b2) if the direction of the loop crosses a resistor WITH I -> -IR
c1) if the direction of the loop crosses cell (big line to small line) -> -ϵ
c2) if the direction of the loop crosses cell (small line to big line) -> +ϵ

conservation of energy

Question 20

Potential Divider Circuits

Answer 20

just a series circuit
Vout = Vin[R2/(R1+R2)]
something that uses voltage

Question 21

Potential Divider Circuits (equation)

Answer 21

Question 22

cells

Answer 22

cells store electrical energy in the form of chemical energy

loaded - being used
unloaded - not used

r = the cost of doing business with the battery / the internal resistance

V = unloaded
ϵ = loaded

ϵ = I(R + r)

the smaller the r, the more efficient the battery. r increases over time due to ageing

Question 23

what causes : magnetism

Answer 23

1) a moving charge (right hand rules)
2) materials with aligned domains (magnets)

domains
1) magnetic moment of moving e-
2) spin of e-
3) spin of nucleus

unlike electric dipole, magnetic dipole forms ‘rings’ not always the same direction facing

Question 24

mass spectrometer

Answer 24

Fc = FB