12.3 Resistance Flashcards

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1
Q

What is the resistance of a component in a circuit a measure of

A

The difficulty of making current pass through the component.

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2
Q

What is resistance caused by

A

The repeated collisions between the charge carriers in the material with each other and with the fixed positive ions of the material

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3
Q

What is the resistance of any component defined as

A

The pd across the component/the current through it

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4
Q

For a component which passes current I when the pd across it is V, its resistance R is given by the equation:

A

R = V/I

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5
Q

What is a resistor

A

A component designed to have a certain resistance, which is the same regardless of the current through it.

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6
Q

How can the resistance of a resistor be measured

A

Ammeter to measure current through, must be in series with resistor
Voltmeter used to measure pd across resistor, must be in parallel, no current should pass through voltmeter otherwise ammeter will not record exact current. Therefore voltmeter must have very high resistance
Variable resistor to adjust current and pd as necessary

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7
Q

What does the graph of an ohmic resistor look like

A

A straight line through the origin

Resistance is same regardless of current

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8
Q

Ohm’s law states that…

A

…the pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change

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9
Q

What statement is Ohm’s law equivalent to

A

The statement that the resistance of a metallic conductor under constant physical conditions (eg. temperature) is constant

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10
Q

For a conductor of length L and uniform cross-sectional area A, its resistance R is…

A

…proportional to L, and inversely proportional to A

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11
Q

Equation for resistivity, ρ

A

ρ = RA/L

Unit of resistivity is the ohm metre Ωm

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12
Q

How to work out the area of a conductor with a circular cross section of diameter d,

A

A = πd^2/4

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13
Q

How to determine the resistivity of a wire

A

Measure diameter of wire using micrometer at several different points along wire to give mean value for d to calculate its cross-sectional area A
Measure resistance R of different lengths L of wire to plot a graph of R against L

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14
Q

Finding resistivity of wire from graph

A

Graph gradient x area

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15
Q

What is a superconductor

A

A wire or device made of material that has zero resistivity at and below a critical temperature that depends on the material
This property of the materia is called superconductivity

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16
Q

The wire or device has zero resistance below the ______ temperature of the material. When a current passes through it, there is no ______ across it because its resistance is ______. So the current has no ______ effect.

A

Critical
pd
Zero
Heating

17
Q

How does a superconductor material lose its superconductivity

A

If its temperature is raised above its critical temperature.

18
Q

What are superconductors used for

A

Making high-power electromagnets that generate very strong magnetic fields in devices such as MRI scanners and particle accelerators