some electricity questions

Below are soem electricity questions.. I jus have problem with some parts of the question.. so i will post all the details.. of answers of other parts linked..

1) aluminium has density 8.9 X 10^3 kgm^-3 and a relative atomic mass of 64. A particular aluminium wire carrying a current of 2.0 A has a cross sectional area of 0.10mm^2
Calculate
a) the no. density of aluminium atoms... (ans = 8.3 X 10^28 m^-3)
the other parts of this question are ok

2)Ina cathode ray Oscilloscope there are 6.4X10^10 electrons per metre along the electron beam. The are travelling at 7.3X10^7 ms^-1. 70% of those that arrive at the screen flow off aloing a wire.What is the current in this wire? (ans = 0.52A)

3)a tape recorder is operated by a battery of a 5 X 1.5V cells in series, each of terminal resistance 0.2 ohms.
a)What is the combined EMF and the internak resistence of the battery..(this part is ok)
b)What is the p.d across the battery when the current of 150mA is being taken from it. (ans= 7.35V)

4)2 cells having EMF of 1.5V and an internl resistence of 1 ohm and are connected to resistance of 4 ohms. find the currect in this resistance if the cells are connected in parallel.

thanks so much in advance

Reply 1

katykt77

usman_s

3)a tape recorder is operated by a battery of a 5 X 1.5V cells in series, each of terminal resistance 0.2 ohms.
a)What is the combined EMF and the internak resistence of the battery..(this part is ok)
b)What is the p.d across the battery when the current of 150mA is being taken from it. (ans= 7.35V)

sorry i have only worked this question out

a) EMF=7.5V
Internal resistance = 1 Ohm

b) Total Resistance in circuit= Voltage/current
= 7.5 / 0.15
= 50 ohms

This value includes the internal resistance as well.
50-1 = 49 ohms

Then you need to work out pd
V = IR
= 0.15 x 49
= 7.35V

Reply 2

Jonathan

usman_s

2)Ina cathode ray Oscilloscope there are 6.4X10^10 electrons per metre along the electron beam. The are travelling at 7.3X10^7 ms^-1. 70% of those that arrive at the screen flow off aloing a wire.What is the current in this wire? (ans = 0.52A)

Current is charge over time, or coulombs per second.

How many coulombs are in a metre of beam? One electron has a charge (magnitude, forget plus or minus) of 1.6x10^-19 C. So a metre of electrons has charge, in coulombs:

6.4x10^10 x 1.6x10^-19 = 1.024 × 10^-8 C

How many coulombs flow in a second? The number of coulombs in a metre times the number of metres in a second:

7.3X10^7 x 1.024 × 10^-8

= 0.74752 C m^-1 m s^-1
or 0.74752 C s^-1
or 0.74752 A

but it's only 70% of that current:

0.74752 x 0.7 = 0.523264 A
= 0.52 A (3 s.f.)

Hope that helps.

Reply 3

=Keith=

thankyou So much... ur help is appreciated