Transcript Current Electricity Review

Current Electricity
Review - Answers
CP Physics
Ms. Morrison
1. Define the following vocabulary terms.

Potential difference: the difference in potential energy
through a circuit (voltage)

Current: the flow of electric charges

Voltage source: device that provides a potential
difference in a circuit (charge pump)

Resistance: opposition to the flow of electric charges
(current)
1. Define the following vocabulary terms.

Resistor: device designed to provide a specific
resistance

Direct current: current that flows in only one direction

Alternating current: current that flows back and forth in
two directions

Electric power: the rate at which electrical energy is
used
1. Define the following vocabulary terms.

Charge pump: source of electrical charges in a circuit,
provides voltage

Circuit: closed loop through which electric charges can
move

Fuse: safety device that melts and opens the circuit if
too much current goes through it

Circuit breaker: safety device that pops open the circuit
if too much current goes through it
2. What three conditions must be met for charges
to flow through a circuit?
1.
Must have a closed loop
2.
Must have a potential difference
3.
Must have a charge pump (voltage source)
3. Identify the variables and units for the following:
Variable
Symbol
Unit
Unit symbol
voltage
V
volt
V
current
I
amp
A
resistance
R
ohm
Ω
time
t
second
s
time
t
hour
h
power
P
watt
W
energy
E
joule
J
energy
E
kilowatt-hour
kWh
4. What are the circuit symbols for the following:
battery
load
fuse
lamp
resistor
switch
5. What do generators do? Give several
examples of generators.

Convert mechanical (kinetic) energy to electrical energy

Examples: windmills, hand-cranked, stationary bikes,
hydroelectric, nuclear, coal-burning
6. What do batteries do? What are the three
classifications of batteries?

Convert chemical energy to electrical energy

Classifications:



Primary – use up and throw away
Secondary – rechargeable
Fuel cells – chemicals continually pumped in as they are used
up (ex. Space shuttle uses H2 and O2 to generate electricity and
make H2O)
7. What do photocells do?
Convert light (solar) energy to electrical energy
8. What is direct current? What voltage source
provides direct current?

Current that flows in only one direction

Batteries
9. What is alternating current? What voltage
source provides alternating current?

Current that moves back and forth in two directions

Voltage sources: electricity from power companies; the
outlets in our homes, schools, and businesses;
generators
10. Explain how each of the following affects
resistance through a wire:

Conductivity: greater conductivity = less resistance

Thickness: thicker the wire = less resistance

Length: longer the wire = more resistance

Temperature: colder the temperature = less resistance
11. What is true about superconductors?
Superconductors operate at temperatures close to
absolute zero and have NO electrical resistance
(resistance = ZERO)
12. What are the proportions between current and
voltage and current and resistance according to
Ohm’s Law?

Current and voltage have a direct proportion – more
voltage means greater current flow

Current and resistance have an inverse proportion –
more resistance means less current flow
13. Why do some electrical plugs have a third
prong?
The third prong grounds the device. Home electrical
systems are grounded – there is actually wiring that goes
into the ground from the electrical system to help remove
excess charge in the system
14. What level of current is fatal to humans?
0.10 A or 100 mA
15. Why can birds sit on high voltage lines without
being harmed?

Their entire body is at the same electric potential so they
do not experience a potential difference.

A potential difference is necessary for current to flow
through – voltage causes current!

Therefore, as long as they only touch one line they are
fine.
16. Do you buy electrons from the power
company?
NO, you buy energy from the power company. The
electrons are in your devices and they flow because of
the energy received from the power company.
17. What are two safety devices used in circuits?
How do they work?

Fuse



Melts if current exceeds a certain value
Must be replaced
Circuit breaker


Breaker pops open if current exceeds a certain value
Can be reused – simply reset breaker by turning off then on
again
18. What is the standard voltage in a house?
All outlets in homes and businesses are 120 V
19. How much current flows in a 175 Ω resistor
when a voltage of 5.0 V is across it?
R = 175 Ω
V = 5.0 V
I=?
V = IR
5.0 = I (175)
I = 5.0/175
I = 0.029 A
20. What is the resistance of a cold incandescent
bulb filament if it draws a current of 0.43 A when
plugged into a 120 V circuit?
I = 0.43 A
V = 120 V
R=?
V = IR
120 = 0.43 R
R = 120/0.43
R = 279 Ω
21. What voltage is applied to a 18.5 Ω resistor if
the current is 7.3 A?
R = 18.5 Ω
I = 7.3 A
V=?
V = IR
V = (7.3)(18.5)
V = 135.05 V
22. What is the resistance of an electric frying pan
which draws 5.8 amps when connected to a 120 V
circuit?
I = 5.8 A
V = 120 V
R=?
V = IR
120 = 5.8 R
R = 120/5.8
R = 20.69 Ω
23. What is the power of the frying pan in #22?
I= 5.8 A
V = 120 V
P=?
P = VI
P = (120)(5.8)
P = 696 W
24. How much energy does the pan in #23 use in
21.0 minutes?
P = 696 W
t = 21.0 min x 60
= 1260 s
E=?
E = Pt
E = (696)(1260)
E = 876,960 J
25. What current flows through a 100.0 W light
bulb connected to a 20.0 V electrical source?
P = 100.0 W
V = 20.0 V
I=?
P = VI
100.0 = 20.0 I
I = 100.0/20.0
I = 5.00 A
26. A small heater connected across a 220 V line
dissipates 0.94 kW of power as heat. How much
energy does this heater use if it operates for 7.5
hours?
V = 220 V
P = 0.94 kW
t = 7.5 h
E=?
E = Pt
E = (0.94)(7.5)
E = 7.05 kWh
27. What does it cost to operate the heater in #26
for 7.5 hours if the rate charged by Georgia Power
is $0.08/kWh?
E = 7.05 kWh
rate = $0.08/kWh
Cost = ?
Cost = E x rate
Cost = (7.05)(0.08)
Cost = $0.56
28. What is the cost of operating an electric
device that is rated at 350 W for 5.0 h a day for 30
days if the cost of electrical energy is $0.06/kWh?
P = 350 W
= 0.350 kW
t = 5.0 h(30) = 150 h
rate = $0.06/kWh
Cost = ?
E = Pt
E = (0.350)(150)
E = 52.5 kWh
Cost = E x rate
Cost = (52.5)($0.06)
Cost = $3.15