1. Electricty concept review

2. What makes a lightbulb bright?
POWER!
Brightness is related to how much power is dissipated in the bulb.
P = V I
using Ohm’s law V = I R…
P = V2 = I2 R R

3. Which bulb is brightest? dimmest?
All bulbs have the same resistance A
6 V
6 V C B
C > (B=A)

4. Rank the currents, IA, IB, IC
from highest to lowest A
6 V
6 V C B
(IA = IB )< IC

5. Add more bulbs in series with A & B
What happens to the brightness of bulb A? A
6 V B
more bulbs, more R, less current, less bright (P = I2R at each resistor)

6. A 6V battery is attached to:
A light bulb (Rbulb= 3Ω) and motor (Rmotor= 9Ω) in series.
The wires have a combined resistance Rwire = 0.5Ω.
Calculate:
IT in circuit
V drop across the bulb and motor
Power consumed by bulb and motor
Number of electrons that leave the battery each second
Energy carried by each electron
Energy “dropped off” by an electron in bulb? Motor?

7. A 6V battery is attached to:
A light bulb (Rbulb= 3Ω) and motor (Rmotor= 9Ω) in series.
The wires have a combined resistance Rwire = 0.5Ω.
IT in circuit
IT = VT/RT = 6/ = .48 A
V drop across the bulb and motor
Vb = Ib Rb= .48 (3) = 1.44V Vm = Im Rm= .48 (9)= 4.38 V
Power consumed by bulb and motor
P = IV Pb= .48(1.44) = .69W Pm = .48(4.32)= 2.1 W

8. A 6V battery is attached to:
A light bulb (Rbulb= 3Ω) and motor (Rmotor= 9Ω) in series.
The wires have a combined resistance Rwire = 0.5Ω.
Number of electrons that leave the battery each second
I = [C/s], Charge on electron = [C/electron]
→ I/(Charge on e-) = [C/s ÷(C/electron)] = electrons/s
= .48 C/s ÷ 1.6 x C/e- = 3 x 1018 electrons/s
Energy carried by each electron
Voltage is a measure of EPE per unit charge
6V = J/C (1.6 x C/electron)
= x J/electron

9. Which bulb is brightest? dimmest?
All bulbs have the same resistance D E
6 V
6 V C
Is circuit current, IT, the same in both circuits?
What if we keep adding bulbs in parallel? What happens to brightness of A?
PC = PD = PE; IT(DE) > IT(C) ; add more in parallel and power stays the same in all (IC = ID = IE)

10. Apply to home circuits...
When you turn on lights in one room, what happens to the other lights in the house?
Parallel or series?
Have you ever noticed a change if you use the dryer, electric oven, or hairdryer?
These are high power devices. They need large current.
Each circuit in the house has a current limit
= fuses in fuse box

11. Home electrical power
For a home running 120 V, how much current is drawn by a 60 W incandescent bulb?
What is the current if we replace the incandescent bulb with an 8W LED bulb (which give the same light output).
How much current is drawn by a 1875 W hairdryer?
Why do houses have circuit breakers?

12. Switches (circuit before vs after)
Find the voltages and currents 3Ω 2Ω 3Ω
6 V
6 V 2Ω 4Ω

13. tomorrow...

14. Charge accelerator from electric field
Electron is released near plate A
Accelerates through a small hole in plate B
Find
Polarity of the battery (+/-)
Potential difference between A&B
Electric field between the plates
Force on the electron when it is between A&B
Speed of electron as it leaves plate B
If we reverse the polarity between plates A&B and release an alpha particle from near plate A, how fast will the alpha particle be moving when it gets to plate B?
(alpha particle has 2 neutrons + 2 protons)

15. Kinetic energy, J and eV Electron is released near plate A
Accelerates through a small hole in plate B
Find
Kinetic energy of the electron
in Joules and in eV
Define eV = electron volt

16. Another one? Electron acceleration
direction of field direction of e- motion
Battery
* e- will accelerate through
1.5V hole = particle accelerator
* What is EPE?
(in J? in eV?)
* What is velocity of
electron?
EPE e- = qV = 2.4x10-19 J = 1.5 eV; v ~ 7 x 105 m/s;

17. What path is followed for a positively charged sphere? (plates are
vertical)
B(FE to the left, and FG down)

18. Electric field, charged plates
What is the direction and strength of the electric field between the two charged plates?f
E = 2500 N/C to the left

19. Charged particles, Electric force
What is the:
Electric force on each charge?
Electric field at a point midway between the 2 charges?
Voltage at the point midway between the 2 charges?
7.88 x 1014 N; 1.8 x 1014 N/C; 1.8 x 1012 V

20. Ohmic circuit
For an ohmic resistor, if the voltage is doubled without changing the resistance, the current will
decrease by a factor of 4
decrease by a factor of 2
increase by a factor of 2
increase by a factor of 4 C

21. If a 60 W bulb is used in a 120 V circuit, what is the resistance of the bulb?
2 Ω
20 Ω
240 Ω
720 Ω
7200 Ω C

22. intensity of the other bulbs decreases
If 3 bulbs are in parallel and the central bulb burns out, what happens?
all bulbs go out
intensity of the other bulbs decreases
intensity of the other bulbs increases
intensity of the other bulbs remains the same
more current is drawn from the source D

23. Power
How much energy is dissipated (as heat) in 20 seconds, from a 100 Ω resistor at 0.5 A?
50 J
100 J
250 J
500 J
1000 J

24. Batteries in series 1
What is the overall EMF? (voltage difference between top and bottom)
What is the current in the circuit?
4 V
3 Ω
4 V
1 Ω
8V, 2A

25. Batteries in series 2
What is the overall EMF? (between top and bottom)
What is the current in the circuit?
12 V
3 Ω
4 V
1 Ω
8V, 2A

26. Voltage divider
What combination of resistors can be used to generate 6V from a 15 V supply?
15 V
6 V
any where Rtop = 1.5 x Rbottom (for example Rtop = 3 Ω, Rbottom = 2 Ω, or Rtop = 9 Ω, Rbottom = 6 Ω…)