Current Electricity Review - Answers

CP PhysicsMs. 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 = IR

V = 5.0 V 5.0 = I (175)

I = ? 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 = IR

V = 120 V 120 = 0.43 R

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 V = IR

I = 7.3 A V = (7.3)(18.5)

V = ?

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 = IR

V = 120 V 120 = 5.8 R

R = ? R = 120/5.8

R = 20.69 Ω

23. What is the power of the frying pan in #22?

I= 5.8 A P = VI

V = 120 V P = (120)(5.8)

P = ?

P = 696 W

24. How much energy does the pan in #23 use in 21.0 minutes?

P = 696 W E = Ptt = 21.0 min x 60 E = (696)(1260)

= 1260 sE = ?

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 P = VI

V = 20.0 V 100.0 = 20.0 I

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 E = PtP = 0.94 kW E = (0.94)(7.5)t = 7.5 hE = ?

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 Cost = E x rate

rate = $0.08/kWh Cost = (7.05)(0.08)

Cost = ?

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 E = Pt

= 0.350 kW E = (0.350)(150)

t = 5.0 h(30) = 150 h E = 52.5 kWh

rate = $0.06/kWh

Cost = ? Cost = E x rate

Cost = (52.5)($0.06)

Cost = $3.15