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GRADE 8 PRACTICE

Physics Problems: Resistance and Ohm's Law, Grade 8

Conductor resistance, Ohm's law and rheostat.

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 #43

Resistance and Ohm's Law

What concept does the description correspond to: “One ohm is the resistance of the section in which a voltage of one volt creates a current of one ampere.»?

Answer options

  1. \text{Ом}
  2. \text{Ohm's law for a circuit section}
  3. \text{Electrical resistance}
  4. \text{Capillary action}
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Given
\text{Description}
\text{One ohm is the resistance of the section in which a voltage of one volt creates a current of one ampere.}
Find

\text{name concepts}

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

    The unit relates the readings of the ammeter and voltmeter through Ohm's law.

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

Answer\text{Concept} = \text{Ом}

Position in this set: 2 from 18

Problem #44

Resistance and Ohm's Law

What concept does the description correspond to: “For an ohmic section under constant physical conditions, current is proportional to voltage: I = U/R, where R is constant over the range considered.»?

Answer options

  1. \text{Ohm's law for a circuit section}
  2. \text{Ом}
  3. \text{Electrical resistance}
  4. \text{Capillary action}
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Given
\text{Description}
\text{For an ohmic section under constant physical conditions, current is proportional to voltage: I = U/R, where R is constant over the range considered.}
Find

\text{name concepts}

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

    The graph of I(U) of such a section is a straight line through the origin. Heating can change R. However, constant temperature alone is not enough: the diode and some other elements remain non-ohmic, so the applicability of the dependence with constant R is tested by experiment.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Ohm's law for a circuit section}

Answer\text{Concept} = \text{Ohm's law for a circuit section}

Position in this set: 3 from 18

Problem #45

Ohm's law

A 6 ohm resistor maintains a voltage of 12 V. Find the current.

Answer options

  1. 2\,\text{А}
  2. 0{,}5\,\text{А}
  3. 6\,\text{А}
  4. 72\,\text{А}
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Given
R
6\,\text{Ом}
U
12\,\text{В}
Find

I

  1. I=\frac{U}{R}I=\frac{12}{6}\,\text{А}I = 2\,\text{А}

Answer2\,\text{А}

Position in this set: 4 from 18

Problem #46

Conductor resistance and rheostat

What concept does the description correspond to: “The resistance of a homogeneous conductor increases with length, decreases with cross-sectional area and depends on the material.»?

Answer options

  1. \text{Conductor resistance}
  2. \text{Resistivity}
  3. \text{Rheostat}
  4. \text{Capillary action}
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Given
\text{Description}
\text{The resistance of a homogeneous conductor increases with length, decreases with cross-sectional area and depends on the material.}
Find

\text{name concepts}

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

    A long, thin wire has more resistance than a short, thick wire of the same material.

  2. \text{Let's formulate conclusion}\text{Concept} = \text{Conductor resistance}

Answer\text{Concept} = \text{Conductor resistance}

Position in this set: 5 from 18

Problem #47

Conductor resistance and rheostat

What concept does the description correspond to: “A characteristic of a material that relates the resistance of a conductor to its length and cross-sectional area.»?

Answer options

  1. \text{Resistivity}
  2. \text{Conductor resistance}
  3. \text{Rheostat}
  4. \text{Capillary action}
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Given
\text{Description}
\text{A characteristic of a material that relates the resistance of a conductor to its length and cross-sectional area.}
Find

\text{name concepts}

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

    Copper's low resistivity makes it useful for wiring; alloys with greater resistivity are useful for heaters.

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

Answer\text{Concept} = \text{Resistivity}

Position in this set: 6 from 18

Problem #48

Conductor resistance

A wire 10 m long and with a cross-sectional area of 2 mm² has a resistivity of 0.5 Ohm mm²/m. Find resistance.

Answer options

  1. 2{,}5\,\text{Ом}
  2. 0{,}1\,\text{Ом}
  3. 5\,\text{Ом}
  4. 25\,\text{Ом}
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Given
\rho
0{,}5\,\text{Ом}\cdot\text{мм}^{2}/\text{м}
l
10\,\text{м}
S
2\,\text{мм}^{2}
Find

R

  1. R=\rho\frac{l}{S}R=0{,}5\cdot\frac{10}{2}\,\text{Ом}R = 2{,}5\,\text{Ом}

Answer2{,}5\,\text{Ом}

Position in this set: 7 from 18

Problem #157

Resistance and Ohm's Law

Resistor voltage 12 V, resistance 4 Ohm. Find the current.

Answer options

  1. 3\,\text{А}
  2. 6\,\text{А}
  3. 1{,}5\,\text{А}
  4. 9\,\text{А}
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Given
U
12\,\text{В}
R
4\,\text{Ом}
Find

I

  1. I=\frac{U}{R}I=\frac{12}{4}I=3\,\text{А}

    We apply Ohm's law to a section of the circuit.

Answer3\,\text{А}

Position in this set: 8 from 18

Problem #158

Resistance and Ohm's Law

Resistor voltage 24 V, resistance 6 Ohm. Find the current.

Answer options

  1. 8\,\text{А}
  2. 4\,\text{А}
  3. 2\,\text{А}
  4. 12\,\text{А}
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Given
U
24\,\text{В}
R
6\,\text{Ом}
Find

I

  1. I=\frac{U}{R}I=\frac{24}{6}I=4\,\text{А}

    We apply Ohm's law to a section of the circuit.

Answer4\,\text{А}

Position in this set: 9 from 18

Problem #159

Resistance and Ohm's Law

Under tension 6 The ammeter shows 200 mA. Find the resistance of the circuit section.

Answer options

  1. 60\,\text{Ом}
  2. 30\,\text{Ом}
  3. 15\,\text{Ом}
  4. 90\,\text{Ом}
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Given
U
6\,\text{В}
i
200\,\text{мА}
Find

R

  1. I=\frac{i}{1000}I=\frac{200}{1000}I=0{,}2\,\text{А}

    Before calculating, we convert milliamps to amperes.

  2. R=\frac{U}{I}R=\frac{6}{0{,}2}R=30\,\text{Ом}

    We express resistance from Ohm's law.

Answer30\,\text{Ом}

Position in this set: 10 from 18

Problem #160

Resistance and Ohm's Law

Under tension 12 The ammeter shows 300 mA. Find the resistance of the circuit section.

Answer options

  1. 80\,\text{Ом}
  2. 20\,\text{Ом}
  3. 40\,\text{Ом}
  4. 120\,\text{Ом}
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Given
U
12\,\text{В}
i
300\,\text{мА}
Find

R

  1. I=\frac{i}{1000}I=\frac{300}{1000}I=0{,}3\,\text{А}

    Before calculating, we convert milliamps to amperes.

  2. R=\frac{U}{I}R=\frac{12}{0{,}3}R=40\,\text{Ом}

    We express resistance from Ohm's law.

Answer40\,\text{Ом}

Position in this set: 11 from 18

Problem #161

Resistance and Ohm's Law

Under tension 12 V through a resistor for 10 s charge passed 20 Cl. What current will be at voltage 30 Q, if the resistance does not change?

Answer options

  1. 10\,\text{А}
  2. 2{,}5\,\text{А}
  3. 5\,\text{А}
  4. 15\,\text{А}
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Given
U_1
12\,\text{В}
t
10\,\text{с}
q
20\,\text{Кл}
U_2
30\,\text{В}
Find

I_2

  1. I_1=\frac{q}{t}I_1=\frac{20}{10}I_1=2\,\text{А}

    Based on the transferred charge, we determine the initial current.

  2. R=\frac{U_1}{I_1}R=\frac{12}{2}R=6\,\text{Ом}

    We find constant resistance.

  3. I_2=\frac{U_2}{R}I_2=\frac{30}{6}I_2=5\,\text{А}

    We use new voltage.

Answer5\,\text{А}

Position in this set: 12 from 18

Problem #162

Resistance and Ohm's Law

Under tension 18 V through a resistor for 20 s charge passed 60 Cl. What current will be at voltage 24 Q, if the resistance does not change?

Answer options

  1. 8\,\text{А}
  2. 2\,\text{А}
  3. 12\,\text{А}
  4. 4\,\text{А}
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Given
U_1
18\,\text{В}
t
20\,\text{с}
q
60\,\text{Кл}
U_2
24\,\text{В}
Find

I_2

  1. I_1=\frac{q}{t}I_1=\frac{60}{20}I_1=3\,\text{А}

    Based on the transferred charge, we determine the initial current.

  2. R=\frac{U_1}{I_1}R=\frac{18}{3}R=6\,\text{Ом}

    We find constant resistance.

  3. I_2=\frac{U_2}{R}I_2=\frac{24}{6}I_2=4\,\text{А}

    We use new voltage.

Answer4\,\text{А}

Position in this set: 13 from 18

Problem #163

Conductor resistance and rheostat

Wire length 10 m andmherт section 0{,}5 mm². Resistivity 0{,}1 Ohm mm²/m. Find resistance.

Answer options

  1. 4\,\text{Ом}
  2. 2\,\text{Ом}
  3. 1\,\text{Ом}
  4. 6\,\text{Ом}
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Given
l
10\,\text{м}
S
0{,}5\,\text{мм}^{2}
r
0{,}1\,\text{Ом·мм}^{2}\text{/м}
Find

R

  1. R=\rho\frac{l}{S}R=\frac{0{,}1 \cdot 10}{0{,}5}R=2\,\text{Ом}

    Resistivity units are consistent with length and cross-section.

Answer2\,\text{Ом}

Position in this set: 14 from 18

Problem #164

Conductor resistance and rheostat

Wire length 20 m andmherт section 0{,}4 mm². Resistivity 0{,}02 Ohm mm²/m. Find resistance.

Answer options

  1. 2\,\text{Ом}
  2. 0{,}5\,\text{Ом}
  3. 1\,\text{Ом}
  4. 3\,\text{Ом}
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Given
l
20\,\text{м}
S
0{,}4\,\text{мм}^{2}
r
0{,}02\,\text{Ом·мм}^{2}\text{/м}
Find

R

  1. R=\rho\frac{l}{S}R=\frac{0{,}02 \cdot 20}{0{,}4}R=1\,\text{Ом}

    Resistivity units are consistent with length and cross-section.

Answer1\,\text{Ом}

Position in this set: 15 from 18

Problem #165

Conductor resistance and rheostat

Full wire rheostat length 10 m andmherт resistance 20 Ohm. slider includes B chain 3 m wire. Find force current Under tension 12 B on this parts.

Answer options

  1. 4\,\text{А}
  2. 1\,\text{А}
  3. 2\,\text{А}
  4. 6\,\text{А}
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Given
l
10\,\text{м}
R
20\,\text{Ом}
a
3\,\text{м}
U
12\,\text{В}
Find

I

  1. r=R\frac{a}{l}r=\frac{20 \cdot 3}{10}r=6\,\text{Ом}

    The resistance of a homogeneous wire is proportional to its length.

  2. I=\frac{U}{r}I=\frac{12}{6}I=2\,\text{А}

    Ohm's law applies only to the included part.

Answer2\,\text{А}

Position in this set: 16 from 18

Problem #166

Conductor resistance and rheostat

Full wire rheostat length 12 m andmherт resistance 24 Ohm. slider includes B chain 4 m wire. Find force current Under tension 24 B on this parts.

Answer options

  1. 6\,\text{А}
  2. 1{,}5\,\text{А}
  3. 9\,\text{А}
  4. 3\,\text{А}
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Given
l
12\,\text{м}
R
24\,\text{Ом}
a
4\,\text{м}
U
24\,\text{В}
Find

I

  1. r=R\frac{a}{l}r=\frac{24 \cdot 4}{12}r=8\,\text{Ом}

    The resistance of a homogeneous wire is proportional to its length.

  2. I=\frac{U}{r}I=\frac{24}{8}I=3\,\text{А}

    Ohm's law applies only to the included part.

Answer3\,\text{А}

Position in this set: 17 from 18

Problem #167

Conductor resistance and rheostat

Wire with specific resistance 0{,}5 Ohm mm²/m and cross section 0{,}2 mm² should skip current 2 A Under tension 10 B. Find mass this wire, if her density 8 g/cm³. Please note: 1 m·mm² = 1 cm³.

Answer options

  1. 6{,}4\,\text{г}
  2. 1{,}6\,\text{г}
  3. 9{,}6\,\text{г}
  4. 3{,}2\,\text{г}
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Given
r
0{,}5\,\text{Ом·мм}^{2}\text{/м}
S
0{,}2\,\text{мм}^{2}
I
2\,\text{А}
U
10\,\text{В}
d
8\,\text{г/см}^{3}
Find

m

  1. R=\frac{U}{I}R=\frac{10}{2}R=5\,\text{Ом}

    First we determine the required resistance.

  2. l=\frac{RS}{\rho}l=\frac{5 \cdot 0{,}2}{0{,}5}l=2\,\text{м}

    Express the length of the wire.

  3. V=lSV=2 \cdot 0{,}2V=0{,}4\,\text{см}^{3}

    We use the correspondence of units specified in the condition.

  4. m=dVm=8 \cdot 0{,}4m=3{,}2\,\text{г}

    We find mass by density and volume.

Answer3{,}2\,\text{г}

Position in this set: 18 from 18

Problem #168

Conductor resistance and rheostat

Wire with specific resistance 0{,}4 Ohm mm²/m and cross section 0{,}3 mm² should skip current 3 A Under tension 12 B. Find mass this wire, if her density 8 g/cm³. Please note: 1 m·mm² = 1 cm³.

Answer options

  1. 7{,}2\,\text{г}
  2. 14{,}4\,\text{г}
  3. 3{,}6\,\text{г}
  4. 21{,}6\,\text{г}
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Given
r
0{,}4\,\text{Ом·мм}^{2}\text{/м}
S
0{,}3\,\text{мм}^{2}
I
3\,\text{А}
U
12\,\text{В}
d
8\,\text{г/см}^{3}
Find

m

  1. R=\frac{U}{I}R=\frac{12}{3}R=4\,\text{Ом}

    First we determine the required resistance.

  2. l=\frac{RS}{\rho}l=\frac{4 \cdot 0{,}3}{0{,}4}l=3\,\text{м}

    Express the length of the wire.

  3. V=lSV=3 \cdot 0{,}3V=0{,}9\,\text{см}^{3}

    We use the correspondence of units specified in the condition.

  4. m=dVm=8 \cdot 0{,}9m=7{,}2\,\text{г}

    We find mass by density and volume.

Answer7{,}2\,\text{г}

The theory behind these problems

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

Resistance and Ohm's LawRelationship between current, voltage and resistance of a circuit section.Conductor resistance and rheostatMaterial, length, cross-section and current regulation.

Formulas by topic

Look up symbols and units of measurement in the reference.

Electrical resistanceR · ОмResistivityρ · Ом·м