Power Problems

Resistance

By Betty Nagy

Multiple Choice

The function of a resisitor is to ____________in a circiut

• reduce the current

• resistance

• lower the current

• volts

Multiple Choice

Resistance is measured in ___. 

• watts

• volts

• amps

• ohms

Multiple Choice

A(n) __________ is used to measure resistance.

• voltmeter

• ohmmeter

• ammeter

• flowmeter

Multiple Choice

Substances with high resistivity such as plastics and ceramics.

• conductors

• semiconductors

• fixed resistors

• insulators

​Resistance (R)

•Opposition to the flow of charges.   “electrical friction” of the moving charges.

•Measured in Ohms (Ω)

•Resistance in a wire depends on   

   - material (low R for copper)

   - temperature (as T increases, R increases)

   - length (as length increases, R increases)

   - cross-sectional area ( as A increases, R decreases)

•The “electrical friction” encountered by the charges moving through a material.

•Depends on material, length, and cross-sectional area of conductor

•Measured in Ohms (Ω)​

Some text here about the topic of discussion

Resistivity (ρ)

•Property of material that resists the flow of charges (resistivity, ρ, in Ωm)

•The inverse property of conductivity

•Resistivity is temperature dependent…as temperature increases, then resistivity increases, and so resistance increases.

Multiple Choice

If the length of a wire increases, the resistance ____________

• decreases

• increases

• stays the same

• unknown

Multiple Choice

If the cross sectional area of a wire decreases, the resistance in the wire will ____________

• increases

• stays the same

• decreases

• unknown

Multiple Choice

Materials that have a high electrical resistance

• less length

• more thickness

• less cross sectional area

• less temperature

Multiple Choice

The length of a certain wire is doubled while its radius is kept constant. What is the new resistance of this wire?

• It is 4 times as large.

• It is 2 times as large.

• It is 0.5 times as large.

• It is 3 times as large.

•A relationship between voltage, current, and resistance in an electric circuit

•used to make calculations in all circuit problems

•V = potential difference (voltage) in volts

•I = electric current in amperes (amps , A)

•R = resistance in ohms ( Ω )

Multiple Choice

Ohm's law is ________________

• $$V=IR$$  

• $$I=VR$$  

• $$R=\frac{I}{V}$$  

• $$I=\frac{R}{V}$$  

Multiple Choice

The voltage is 5 Volts. The Resistance is 25 ohms. What is the current?

• 5A

• 20A

• 0.2A

• 125A

Multiple Choice

If the resistance remains the same in a circuit but the current increases, what happened to the voltage?

• increases

• remains the same

• decreases

• unknown

Multiple Choice

 

If the voltage remains the same in a circuit but the resistance increases, what happens to the current?

• increases

• remains the same

• decreases

• unknown

Multiple Choice

The current is 10 amps. The resistance is 5 ohms. What is the voltage?

• 2V

• 5V

• 50V

• 10V

–measures current in amps or mA

–must be placed in series with what is being measured

​Ammeter

–measures voltage in volts

–must be placed in parallel with what is being measured

Voltmeter

Multiple Choice

Which device measures the current in a circuit?

• ammeter

• voltmeter

• amplifier

• rheostat

Electric Energy

​

E=Pt​

​

​

​

•Electric energy can be measured in Joules (J) or Kilowatt hours ( kWh )

•for Joules use Power in watts and time in seconds

•for kWh use Power in kilowatts and time in hours​

Subject | Subject

Some text here about the topic of discussion

Electric Power (Watts)

​<----used for thermal energy

Multiple Choice

What quantities do you need to calculate energy?

• Current and Voltage 

• resistance and current

• power and time

• Power and voltage

Multiple Select

What quantities do you need to calculate power?

• current and voltage

• Resistance and current

• Power and Voltage

• Power and time

Multiple Choice

What are the units for energy?

• kWh

• Joules

• both

• neither

Multiple Choice

How many kilowatts are in 6500 W

• 6,500,000 kW

• 6.5 kW

• 0.65 kW

• 65kW

Multiple Choice

How many watts are in 2 kW

• 2 W

• 2000 W

• 200 W

• 0.2W

Multiple Choice

What is the electrical energy of a 750 W waffle iron that is used for 15 min

• 11,250 kWh

• 11.25 kWh

• 50 kWh

• 0.1875 kWh

Multiple Choice

What is the power rating of a space heater that uses 1.2 kWh of energy in 45 min?

• 0.026 kW

• 0.9 kW

• 54 kW

• 1.6 kW

Multiple Choice

 How much time did you play video games if the PS4 uses 110 W of power and you used a total of 16.5 kWh of energy?

• 6.63 hrs

• 50 hrs

• 0.15 hrs

• 150 hrs

Multiple Choice

A pair of 15-watt computer speakers are connected to a 12-volt power supply. what is the electric current running through the speakers? 

• 12.5A

• 0.8A

• 1.25A

• 180A