Define electric current .

Electric current is the rate of flow of charge , measured in amperes (A), symbol I .

What is the direction of conventional current , and how does it compare with the direction of electron flow in a metal?

Conventional current flows from the positive to the negative terminal, which is opposite to the direction of electron flow .

An ammeter must always be connected in .......... with the component whose current is being measured.

An ammeter must always be connected in series with the component whose current is being measured.

True or False? The magnitude of charge on a proton is different from the magnitude of charge on an electron .

False. Both carry the same magnitude, the elementary charge , e = 1.60 × 10 -19 C; only the sign differs.

The SI unit of charge ; 1 C = 1 A s , the quantity of charge passing a fixed point per second when a current of 1 A is flowing.

What is meant by saying that charge is quantised ?

Charge can only exist in discrete multiples of the elementary charge , e = 1.60 × 10 -19 C, rather than any arbitrary value.

Define the elementary charge , e .

1.60 × 10 -19 C ; the minimum magnitude of charge, carried (in magnitude) by a single electron (- e ) or proton (+ e ).

Why is a neutral atom uncharged overall?

It contains equal numbers of protons (positive charge) and electrons (negative charge), which cancel to give zero net charge.

An ion that has gained one or more electrons, giving it a negative charge.

An ion that has lost one or more electrons, giving it a positive charge.

What are the charge carriers in a metal, and in an electrolyte?

In a metal : free (delocalised) electrons . In an electrolyte : cations and anions .

Why are metals good conductors of electricity?

Metal ions are arranged in a lattice with many free (delocalised) electrons that can move and carry charge when a potential difference is applied.

In an electrolyte, anions are attracted to the .......... , and cations are attracted to the .......... .

In an electrolyte, anions are attracted to the anode , and cations are attracted to the cathode .

True or False? In a metal, current is due to the movement of the positive metal ions.

False. The metal ions are fixed in a lattice ; current is due to the movement of free (delocalised) electrons .

Give an example of an electrolyte , and state what its charge carriers are.

Copper sulfate dissolved in water; the charge carriers are cations and anions in solution.

Define conventional current .

The current in the direction that positive charge would flow, from the positive terminal of a cell to the negative terminal (opposite to electron flow).

Define Kirchhoff's first law .

The sum of the currents entering a junction equals the sum of the currents leaving the junction.

What physical principle is Kirchhoff's first law a consequence of?

Conservation of charge — charge cannot be created or destroyed.

Define a junction in a circuit.

A point where at least three circuit paths meet.

Define a branch in a circuit.

A path connecting two junctions .

In a series circuit, the current is the .......... at every point.

In a series circuit, the current is the same at every point.

True or False? In a parallel circuit, the current is the same in every branch.

False. Current divides at junctions in a parallel circuit, so different branches can carry different currents — Kirchhoff's first law still applies at each junction.

Define drift velocity .

The average velocity of charge carriers as they travel through a conductor, typically of the order 10 -3 m s -1 .

Although the drift velocity of electrons is relatively .......... , the current appears to flow almost instantaneously because the number density of charge carriers is so large.

Although the drift velocity of electrons is relatively slow , the current appears to flow almost instantaneously because the number density of charge carriers is so large.

For a given current, how does the drift velocity change if the number density , n , of charge carriers is higher?

Drift velocity is inversely proportional to n , so a higher n means a lower (slower) drift velocity is needed to carry the same current.

True or False? If the charge carriers in a conductor are positive, the drift velocity is in the opposite direction to the current.

False. For positive charge carriers, drift velocity is in the same direction as the current; it is only for negative charge carriers that drift velocity is opposite to the current direction.

Define number density , n , of charge carriers.

The number of free charge carriers per unit volume of a conductor, measured in m -3 .

Define a conductor in terms of number density, n .

A material with a very large number density of charge carriers ( n ~ 10 28 m -3 ), giving it many free electrons and making it a very good electrical conductor, e.g. copper , aluminium .

Define an insulator in terms of number density, n .

A material with a number density of charge carriers, n , close to zero, meaning it has very few free electrons and is a very poor electrical conductor, e.g. plastic , rubber , glass .

How does the conductivity of a semiconductor change with increasing temperature?

Its resistivity falls , so its conductivity rises — the opposite behaviour to a metal conductor.

.......... is the process of adding impurities to a semiconductor to increase its number density, n , and improve its conducting properties.

Doping is the process of adding impurities to a semiconductor to increase its number density, n , and improve its conducting properties.

Give two examples of semiconductor materials and one use of them.

Silicon and germanium ; used to make electronic devices such as diodes and transistors .

True or False? As temperature increases, the resistivity of a metal conductor falls, just like a semiconductor.

False. A metal conductor's resistivity rises with increasing temperature (conductivity falls) — the opposite behaviour to a semiconductor.

Define a thermistor .

A component whose resistance depends on its temperature ; as temperature increases, its resistance decreases (and vice versa).

Define a light-dependent resistor (LDR).

A component whose resistance depends on light intensity ; as light intensity increases, its resistance decreases (and vice versa).

What is the function of a diode in a circuit?

It allows current to flow in one direction only ; diodes are used to convert AC to DC current.

How must an ammeter and a voltmeter each be connected in a circuit?

An ammeter is connected in series with the component being measured; a voltmeter is connected in parallel with the component being measured.

For a circuit to work effectively, it needs an energy source, a closed path (complete circuit), and electrical .......... .

For a circuit to work effectively, it needs an energy source, a closed path (complete circuit), and electrical components .

True or False? The resistance of the ammeter and voltmeter must always be included in circuit calculations.

False. In exam questions, the resistance of ammeters and voltmeters is taken as negligible .

State the direction of conventional current flow relative to the terminals of a power supply.

Current flows from the positive terminal to the negative terminal.

Define a light-emitting diode (LED).

A component equivalent to a diode that emits light when a current passes through it; used for aviation lighting and displays .

A LevelPhysicsOCRFlashcards4. Electrons, Waves & PhotonsCharge & Current

Charge & Current (OCR A Level Physics): Flashcards

Exam code: H556

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Define electric current.

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Cards in this collection (43)

Define electric current.
Electric current is the rate of flow of charge, measured in amperes (A), symbol I.
What is the direction of conventional current, and how does it compare with the direction of electron flow in a metal?
Conventional current flows from the positive to the negative terminal, which is opposite to the direction of electron flow.
An ammeter must always be connected in .......... with the component whose current is being measured.
An ammeter must always be connected in series with the component whose current is being measured.
True or False?
The magnitude of charge on a proton is different from the magnitude of charge on an electron.
False.
Both carry the same magnitude, the elementary charge, e = 1.60 × 10^-19 C; only the sign differs.
Define the coulomb.
The SI unit of charge; 1 C = 1 A s, the quantity of charge passing a fixed point per second when a current of 1 A is flowing.
What is meant by saying that charge is quantised?
Charge can only exist in discrete multiples of the elementary charge, e = 1.60 × 10^-19 C, rather than any arbitrary value.
Define the elementary charge, e.
1.60 × 10^-19 C; the minimum magnitude of charge, carried (in magnitude) by a single electron (-e) or proton (+e).
Why is a neutral atom uncharged overall?
It contains equal numbers of protons (positive charge) and electrons (negative charge), which cancel to give zero net charge.
Define an anion.
An ion that has gained one or more electrons, giving it a negative charge.
Define a cation.
An ion that has lost one or more electrons, giving it a positive charge.
What are the charge carriers in a metal, and in an electrolyte?
In a metal: free (delocalised) electrons. In an electrolyte: cations and anions.
Why are metals good conductors of electricity?
Metal ions are arranged in a lattice with many free (delocalised) electrons that can move and carry charge when a potential difference is applied.
In an electrolyte, anions are attracted to the .........., and cations are attracted to the ...........
In an electrolyte, anions are attracted to the anode, and cations are attracted to the cathode.
True or False?
In a metal, current is due to the movement of the positive metal ions.
False.
The metal ions are fixed in a lattice; current is due to the movement of free (delocalised) electrons.
Give an example of an electrolyte, and state what its charge carriers are.
Copper sulfate dissolved in water; the charge carriers are cations and anions in solution.
Define conventional current.
The current in the direction that positive charge would flow, from the positive terminal of a cell to the negative terminal (opposite to electron flow).
Define Kirchhoff's first law.
The sum of the currents entering a junction equals the sum of the currents leaving the junction.
What physical principle is Kirchhoff's first law a consequence of?
Conservation of charge — charge cannot be created or destroyed.
Define a junction in a circuit.
A point where at least three circuit paths meet.
Define a branch in a circuit.
A path connecting two junctions.
In a series circuit, the current is the .......... at every point.
In a series circuit, the current is the same at every point.
True or False?
In a parallel circuit, the current is the same in every branch.
False.
Current divides at junctions in a parallel circuit, so different branches can carry different currents — Kirchhoff's first law still applies at each junction.
Define drift velocity.
The average velocity of charge carriers as they travel through a conductor, typically of the order 10^-3 m s^-1.
Although the drift velocity of electrons is relatively .........., the current appears to flow almost instantaneously because the number density of charge carriers is so large.
Although the drift velocity of electrons is relatively slow, the current appears to flow almost instantaneously because the number density of charge carriers is so large.
Write the equation for current in a conductor in terms of number density (n), cross-sectional area (A), drift velocity (v) and charge (q).
$I = n A v q$
For a given current, how does the drift velocity change if the number density, n, of charge carriers is higher?
Drift velocity is inversely proportional to n, so a higher n means a lower (slower) drift velocity is needed to carry the same current.
True or False?
If the charge carriers in a conductor are positive, the drift velocity is in the opposite direction to the current.
False.
For positive charge carriers, drift velocity is in the same direction as the current; it is only for negative charge carriers that drift velocity is opposite to the current direction.
Define number density, n, of charge carriers.
The number of free charge carriers per unit volume of a conductor, measured in m^-3.
What is the cross-sectional area, A, of a wire, in terms of its radius r?
$A = π r^{2}$
Define a conductor in terms of number density, n.
A material with a very large number density of charge carriers (n ~ 10²⁸ m^-3), giving it many free electrons and making it a very good electrical conductor, e.g. copper, aluminium.
Define an insulator in terms of number density, n.
A material with a number density of charge carriers, n, close to zero, meaning it has very few free electrons and is a very poor electrical conductor, e.g. plastic, rubber, glass.
How does the conductivity of a semiconductor change with increasing temperature?
Its resistivity falls, so its conductivity rises — the opposite behaviour to a metal conductor.
.......... is the process of adding impurities to a semiconductor to increase its number density, n, and improve its conducting properties.
Doping is the process of adding impurities to a semiconductor to increase its number density, n, and improve its conducting properties.
Give two examples of semiconductor materials and one use of them.
Silicon and germanium; used to make electronic devices such as diodes and transistors.
True or False?
As temperature increases, the resistivity of a metal conductor falls, just like a semiconductor.
False.
A metal conductor's resistivity rises with increasing temperature (conductivity falls) — the opposite behaviour to a semiconductor.
Define a thermistor.
A component whose resistance depends on its temperature; as temperature increases, its resistance decreases (and vice versa).
Define a light-dependent resistor (LDR).
A component whose resistance depends on light intensity; as light intensity increases, its resistance decreases (and vice versa).
What is the function of a diode in a circuit?
It allows current to flow in one direction only; diodes are used to convert AC to DC current.
How must an ammeter and a voltmeter each be connected in a circuit?
An ammeter is connected in series with the component being measured; a voltmeter is connected in parallel with the component being measured.
For a circuit to work effectively, it needs an energy source, a closed path (complete circuit), and electrical ...........
For a circuit to work effectively, it needs an energy source, a closed path (complete circuit), and electrical components.
True or False?
The resistance of the ammeter and voltmeter must always be included in circuit calculations.
False.
In exam questions, the resistance of ammeters and voltmeters is taken as negligible.
State the direction of conventional current flow relative to the terminals of a power supply.
Current flows from the positive terminal to the negative terminal.
Define a light-emitting diode (LED).
A component equivalent to a diode that emits light when a current passes through it; used for aviation lighting and displays.

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