fizikzauglom

GRADE 8 PRACTICE

Physics Problems: Work and current power, Grade 8

Electrical energy, heating and safety.

18 problems across 4 sub-topics
Practise in the interactive mode

Here the conditions and analysis of the solution can be read without an account. In the simulator, you need to collect the same analyzes yourself: “Given,” formulas, order of steps and answer.

Problem statements

The answer options are there to check against, and the walkthrough opens under each problem.

18 problems
Position in this set: 1 from 18

Problem #55

Work, power and energy consumption

What concept does the description correspond to: “Energy transferred by an electric field to charges and converted by the device into other forms.»?

Answer options

  1. \text{Work done by electric current}
  2. \text{Electrical power}
  3. \text{Electrical energy metering}
  4. \text{Capillary action}
Show the walkthroughHide the walkthrough
Given
\text{Description}
\text{Energy transferred by an electric field to charges and converted by the device into other forms.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    At constant voltage and current, work is equal to their product times time.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Work done by electric current}

Answer\text{Concept} = \text{Work done by electric current}

Position in this set: 2 from 18

Problem #56

Work, power and energy consumption

What concept does the description correspond to: “The work done by electric current per unit of time, showing the rate of energy consumption.»?

Answer options

  1. \text{Electrical power}
  2. \text{Work done by electric current}
  3. \text{Electrical energy metering}
  4. \text{Capillary action}
Show the walkthroughHide the walkthrough
Given
\text{Description}
\text{The work done by electric current per unit of time, showing the rate of energy consumption.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    The power on the device plate helps to estimate the current, energy consumption and permissible network load.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Electrical power}

Answer\text{Concept} = \text{Electrical power}

Position in this set: 3 from 18

Problem #57

Electrical work

The lamp operates at a voltage of 220 V and a current of 0.5 A for 120 s. Find the power and work of the current.

Answer options

  1. 110\,\text{Вт};\ 13{,}2\,\text{кДж}
  2. 440\,\text{Вт};\ 52{,}8\,\text{кДж}
  3. 110\,\text{Вт};\ 220\,\text{Дж}
  4. 220\,\text{Вт};\ 26{,}4\,\text{кДж}
Show the walkthroughHide the walkthrough
Given
U
220\,\text{В}
I
0{,}5\,\text{А}
t
120\,\text{с}
Find

PA

  1. P=UIP=220\cdot0{,}5\,\text{Вт}P = 110\,\text{Вт}
  2. A=PtA=110\cdot120\,\text{Дж}13200\,\text{Дж}=13{,}2\,\text{кДж}

Answer110\,\text{Вт}13{,}2\,\text{кДж}

Position in this set: 4 from 18

Problem #58

Thermal effect of current and safety

What concept does the description correspond to: “The energy released by the current in the resistance during time t is equal to Q = I²Rt, if the current I and resistance R are constant.»?

Answer options

  1. \text{Joule-Lenz law}
  2. \text{Short circuit}
  3. \text{Electrical safety}
  4. \text{Capillary action}
Show the walkthroughHide the walkthrough
Given
\text{Description}
\text{Энергия, выделяемая током в сопротивлении за время t, равна }Q = I^{2}Rt\text{, если ток I и сопротивление R постоянны.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    At the same R and t, doubling the current quadruples the released energy. Temperature also depends on mass, heat capacity, and cooling; its growth does not have to be fourfold. Some of the energy can be transferred to surrounding bodies.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Joule-Lenz law}

Answer\text{Concept} = \text{Joule-Lenz law}

Position in this set: 5 from 18

Problem #59

Thermal effect of current and safety

What concept does the description correspond to: “An unintended very low-resistance path between circuit points with a voltage difference; it may cause excessive current.»?

Answer options

  1. \text{Short circuit}
  2. \text{Joule-Lenz law}
  3. \text{Electrical safety}
  4. \text{Capillary action}
Show the walkthroughHide the walkthrough
Given
\text{Description}
\text{An unintended very low-resistance path between circuit points with a voltage difference; it may cause excessive current.}
Find

\text{name concepts}

  1. \text{Let's compare approval with definition}

    The actual current is limited by the resistance of the source, wires and contacts. A properly selected and functioning fuse or circuit breaker will cut off excessive current, reducing the risk of wiring overheating. Such a device does not guarantee human protection upon any touch.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Short circuit}

Answer\text{Concept} = \text{Short circuit}

Position in this set: 6 from 18

Problem #60

Joule-Lenz law

A current of 2 A flows through a 5 Ohm resistor for 60 s. How much heat will be released?

Answer options

  1. 1200\,\text{Дж}
  2. 300\,\text{Дж}
  3. 600\,\text{Дж}
  4. 2400\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
R
5\,\text{Ом}
I
2\,\text{А}
t
60\,\text{с}
Find

Q

  1. Q=I^2RtQ=2^2\cdot5\cdot60\,\text{Дж}Q = 1200\,\text{Дж}

Answer1200\,\text{Дж}

Position in this set: 7 from 18

Problem #181

Work, power and energy consumption

Under tension 12 B device consumes current 2 A. Find power.

Answer options

  1. 24\,\text{Вт}
  2. 48\,\text{Вт}
  3. 12\,\text{Вт}
  4. 72\,\text{Вт}
Show the walkthroughHide the walkthrough
Given
U
12\,\text{В}
I
2\,\text{А}
Find

P

  1. P=UIP=12 \cdot 2P=24\,\text{Вт}

    For direct current, power is equal to the product of voltage and current.

Answer24\,\text{Вт}

Position in this set: 8 from 18

Problem #182

Work, power and energy consumption

Under tension 24 B device consumes current 3 A. Find power.

Answer options

  1. 144\,\text{Вт}
  2. 72\,\text{Вт}
  3. 36\,\text{Вт}
  4. 216\,\text{Вт}
Show the walkthroughHide the walkthrough
Given
U
24\,\text{В}
I
3\,\text{А}
Find

P

  1. P=UIP=24 \cdot 3P=72\,\text{Вт}

    For direct current, power is equal to the product of voltage and current.

Answer72\,\text{Вт}

Position in this set: 9 from 18

Problem #183

Work, power and energy consumption

device power 800 W works 3 h. How much energy he will consume B kWh?

Answer options

  1. 4{,}8\,\text{кВт·ч}
  2. 2{,}4\,\text{кВт·ч}
  3. 1{,}2\,\text{кВт·ч}
  4. 7{,}2\,\text{кВт·ч}
Show the walkthroughHide the walkthrough
Given
P
800\,\text{Вт}
t
3\,\text{ч}
Find

E

  1. p=\frac{P}{1000}p=\frac{800}{1000}p=0{,}8\,\text{кВт}

    Convert watts to kilowatts.

  2. E=ptE=0{,}8 \cdot 3E=2{,}4\,\text{кВт·ч}

    The time is already set in the clock.

Answer2{,}4\,\text{кВт·ч}

Position in this set: 10 from 18

Problem #184

Work, power and energy consumption

device power 1200 W works 2 h. How much energy he will consume B kWh?

Answer options

  1. 4{,}8\,\text{кВт·ч}
  2. 1{,}2\,\text{кВт·ч}
  3. 2{,}4\,\text{кВт·ч}
  4. 7{,}2\,\text{кВт·ч}
Show the walkthroughHide the walkthrough
Given
P
1200\,\text{Вт}
t
2\,\text{ч}
Find

E

  1. p=\frac{P}{1000}p=\frac{1200}{1000}p=1{,}2\,\text{кВт}

    Convert watts to kilowatts.

  2. E=ptE=1{,}2 \cdot 2E=2{,}4\,\text{кВт·ч}

    The time is already set in the clock.

Answer2{,}4\,\text{кВт·ч}

Position in this set: 11 from 18

Problem #185

Work, power and energy consumption

device included To the network 200 B, consumes current 5 A and works by 2 h B day B current 10 days. Tariff 6 rub./(kWh). Find cost energy.

Answer options

  1. 240\,\text{руб.}
  2. 60\,\text{руб.}
  3. 120\,\text{руб.}
  4. 360\,\text{руб.}
Show the walkthroughHide the walkthrough
Given
U
200\,\text{В}
I
5\,\text{А}
t
2\,\text{ч}
n
10\,\text{дней}
r
6\,\text{руб./(кВт·ч)}
Find

C

  1. P=\frac{UI}{1000}P=\frac{200 \cdot 5}{1000}P=1\,\text{кВт}

    We immediately express the power in kilowatts.

  2. E=PtnE=1 \cdot 2 \cdot 10E=20\,\text{кВт·ч}

    We take into account the number of days of work.

  3. C=ErC=20 \cdot 6C=120\,\text{руб.}

    We multiply the energy by the tariff from the condition.

Answer120\,\text{руб.}

Position in this set: 12 from 18

Problem #186

Work, power and energy consumption

device included To the network 240 B, consumes current 5 A and works by 3 h B day B current 10 days. Tariff 5 rub./(kWh). Find cost energy.

Answer options

  1. 360\,\text{руб.}
  2. 90\,\text{руб.}
  3. 540\,\text{руб.}
  4. 180\,\text{руб.}
Show the walkthroughHide the walkthrough
Given
U
240\,\text{В}
I
5\,\text{А}
t
3\,\text{ч}
n
10\,\text{дней}
r
5\,\text{руб./(кВт·ч)}
Find

C

  1. P=\frac{UI}{1000}P=\frac{240 \cdot 5}{1000}P=1{,}2\,\text{кВт}

    We immediately express the power in kilowatts.

  2. E=PtnE=1{,}2 \cdot 3 \cdot 10E=36\,\text{кВт·ч}

    We take into account the number of days of work.

  3. C=ErC=36 \cdot 5C=180\,\text{руб.}

    We multiply the energy by the tariff from the condition.

Answer180\,\text{руб.}

Position in this set: 13 from 18

Problem #187

Thermal effect of current and safety

Via resistor 5 Ohm flows current 2 A B current 20 s. Find quantity stood out warmth.

Answer options

  1. 800\,\text{Дж}
  2. 400\,\text{Дж}
  3. 200\,\text{Дж}
  4. 1200\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
R
5\,\text{Ом}
I
2\,\text{А}
t
20\,\text{с}
Find

Q

  1. Q=I^2RtQ={2}^{2} \cdot 5 \cdot 20Q=400\,\text{Дж}

    The heat dissipation depends on the square of the current.

Answer400\,\text{Дж}

Position in this set: 14 from 18

Problem #188

Thermal effect of current and safety

Via resistor 8 Ohm flows current 3 A B current 10 s. Find quantity stood out warmth.

Answer options

  1. 1440\,\text{Дж}
  2. 360\,\text{Дж}
  3. 720\,\text{Дж}
  4. 2160\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
R
8\,\text{Ом}
I
3\,\text{А}
t
10\,\text{с}
Find

Q

  1. Q=I^2RtQ={3}^{2} \cdot 8 \cdot 10Q=720\,\text{Дж}

    The heat dissipation depends on the square of the current.

Answer720\,\text{Дж}

Position in this set: 15 from 18

Problem #189

Thermal effect of current and safety

heater resistance 10 Ohm connected K voltage 20 B on 30 s. Find stood out warmth.

Answer options

  1. 2400\,\text{Дж}
  2. 600\,\text{Дж}
  3. 1200\,\text{Дж}
  4. 3600\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
R
10\,\text{Ом}
U
20\,\text{В}
t
30\,\text{с}
Find

Q

  1. I=\frac{U}{R}I=\frac{20}{10}I=2\,\text{А}

    First we find the current.

  2. Q=I^2RtQ={2}^{2} \cdot 10 \cdot 30Q=1200\,\text{Дж}

    We substitute the current into the Joule–Lenz law.

Answer1200\,\text{Дж}

Position in this set: 16 from 18

Problem #190

Thermal effect of current and safety

heater resistance 20 Ohm connected K voltage 40 B on 20 s. Find stood out warmth.

Answer options

  1. 3200\,\text{Дж}
  2. 800\,\text{Дж}
  3. 4800\,\text{Дж}
  4. 1600\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
R
20\,\text{Ом}
U
40\,\text{В}
t
20\,\text{с}
Find

Q

  1. I=\frac{U}{R}I=\frac{40}{20}I=2\,\text{А}

    First we find the current.

  2. Q=I^2RtQ={2}^{2} \cdot 20 \cdot 20Q=1600\,\text{Дж}

    We substitute the current into the Joule–Lenz law.

Answer1600\,\text{Дж}

Position in this set: 17 from 18

Problem #191

Thermal effect of current and safety

K networks 200 B at the same time connected devices capacities 600 and 400 W. Resistance wires extension cord 0{,}4 Ohm. Find warmth, stood out B wires for 60 s. For current use voltage networks, fall voltage neglect.

Answer options

  1. 1200\,\text{Дж}
  2. 300\,\text{Дж}
  3. 1800\,\text{Дж}
  4. 600\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
U
200\,\text{В}
P_1
600\,\text{Вт}
P_2
400\,\text{Вт}
R
0{,}4\,\text{Ом}
t
60\,\text{с}
Find

Q

  1. P=P_1+P_2P=600 + 400P=1000\,\text{Вт}

    The powers of simultaneously operating devices are added up.

  2. I=\frac{P}{U}I=\frac{1000}{200}I=5\,\text{А}

    Based on the total power, we find the current in the common wire.

  3. Q=I^2RtQ={5}^{2} \cdot 0{,}4 \cdot 60Q=600\,\text{Дж}

    The heating of the wires increases as the square of the total current.

Answer600\,\text{Дж}

Position in this set: 18 from 18

Problem #192

Thermal effect of current and safety

K networks 240 B at the same time connected devices capacities 720 and 480 W. Resistance wires extension cord 0{,}6 Ohm. Find warmth, stood out B wires for 120 s. For current use voltage networks, fall voltage neglect.

Answer options

  1. 1800\,\text{Дж}
  2. 3600\,\text{Дж}
  3. 900\,\text{Дж}
  4. 5400\,\text{Дж}
Show the walkthroughHide the walkthrough
Given
U
240\,\text{В}
P_1
720\,\text{Вт}
P_2
480\,\text{Вт}
R
0{,}6\,\text{Ом}
t
120\,\text{с}
Find

Q

  1. P=P_1+P_2P=720 + 480P=1200\,\text{Вт}

    The powers of simultaneously operating devices are added up.

  2. I=\frac{P}{U}I=\frac{1200}{240}I=5\,\text{А}

    Based on the total power, we find the current in the common wire.

  3. Q=I^2RtQ={5}^{2} \cdot 0{,}6 \cdot 120Q=1800\,\text{Дж}

    The heating of the wires increases as the square of the total current.

Answer1800\,\text{Дж}

The theory behind these problems

Work through the topic if a problem will not give in.

Work, power and energy consumptionElectrical work, appliance power and household energy metering.Thermal effect of current and safetyJoule-Lenz law, heaters, short circuits and protection.

Formulas by topic

Look up symbols and units of measurement in the reference.

Timet, τ · сHeatQ · ДжPowerP, N · ВтWorkA · Дж