Current and Charge

Why does current stop flowing almost instantly throughout a circuit when a switch is opened?

Opening the switch breaks the electric field across the circuit, immediately ending the net directional drift of electrons everywhere.

What causes the temperature of a metal wire to rise when an electric current passes through it?

Collisions between drifting conduction electrons and lattice ions transfer kinetic energy to the lattice, increasing thermal vibration.

What general term describes any movable charged particle whose motion constitutes an electric current?

A charge carrier.

How does the electrical resistance of a pure semiconductor change as its temperature increases?

Its electrical resistance decreases because higher thermal energy releases more charge carriers.

What are the charge carriers in a conducting electrolyte (such as a salt solution)?

Ions (positively and negatively charged atoms or molecules).

What is the SI unit of electric current, and how is it defined in terms of magnetic force?

The SI unit is the ampere (A), defined in terms of the magnetic force between two parallel current-carrying wires.

In a semiconductor, what mechanism frees electrons to become conduction charge carriers?

Thermal excitation (heat energy causes electrons to break free from atomic bonds).

What happens to the motion of conduction electrons in a metal when a potential difference is applied across its ends?

They experience an electric force that causes them to drift towards the positive terminal.

In a test circuit used to test electrical conduction of a material, what component forces charge carriers through the test material?

The battery (or DC power supply), which provides the potential difference.

How do conduction electrons behave inside a metal wire when no potential difference is applied across it?

They move about rapidly and randomly, repeatedly colliding with each other and with the fixed positive ions in the metal lattice.

What equation relates total charge flow ΔQ to the number of electrons n and the elementary charge e?

ΔQ=ne

How is electric current I defined in terms of charge flow ΔQ and time interval Δt?

Electric current is the rate of flow of charge, given by I= ΔQ // Δt

Calculate the total charge transfer ΔQ when a steady current of 5 A flows for 200 s.

ΔQ=IΔt=5 A×200 s=1000 C

How many conduction electrons pass a given point per second to produce a steady electric current of 1 A?

Approximately 6.25×10^18 electrons per second (n=1.6×10^−19 C).

How is potential difference V across a component defined in terms of work done W and charge Q?

Potential difference is the work done (or energy transferred) per unit charge passing through the component (V = W/Q).

How many electrons correspond to a total charge flow of 10 C?

1. n= 10C // 1.6 x 10^-19 C

2. 6.25 x 10^19 electrons

In a circuit carrying a current of 2 A, how much charge passes a fixed point in 5 s?

10C

A component carries a current of 0.5 A under a potential difference of 6 V. What is its electrical resistance?

12 Ω

What is the work done W when a charge Q moves through a potential difference V?

W= QV

What is the agreed convention for the direction of electric current in a circuit?

From the positive terminal (+) to the negative terminal (-).

How is the drift motion of electrons superimposed on their random thermal motion in a current-carrying metal?

The electric field causes a slow net directional drift towards the positive terminal while the rapid, random thermal motion continues in all directions.

What happens to charge carriers in a metal circuit after they pass through a component and enter the positive terminal of a cell?

The cell performs work on them, passing them through to the negative terminal so they can cycle round the circuit again.

What role do fixed positive ions play in the electrical resistance of a metal?

They act as scattering centres, repeatedly colliding with moving conduction electrons and obstructing their drift motion.

Why does the direction of conventional current oppose the actual direction of electron flow in a metal circuit?

The conventional current direction was established before the discovery of electrons, under the assumption that current was a flow of positive charge.

What is the fundamental difference in charge transport between a metallic conductor and a molten ionic salt?

Metals transport charge via delocalised electrons moving through a fixed lattice; molten salts transport charge via mobile positive and negative ions.

In an electrolyte solution, in which direction do positive ions move relative to conventional current?

Positive ions move in the same direction as the conventional current (towards the negative electrode).

Why are ammeters and voltmeters treated as ideal components in standard circuit analysis unless stated otherwise?

Ideal ammeters have zero resistance (no potential drop across them) and ideal voltmeters have infinite resistance (draw no current).

What formula gives average electric current I in terms of electron count n passing a cross-section in time Δt?

I = ne // Δt

What is the net electrical charge on an uncharged copper wire while it carries a steady electric current?

Zero; the rate of negative electron charge entering any region equals the rate leaving it, balancing the positive charge of the ion lattice.

In an electrolyte solution, in which direction do negative ions move relative to conventional current?

Negative ions move in the opposite direction to the conventional current (towards the positive electrode).

How does the electron structure of an insulator differ from that of a metallic conductor?

In an insulator, electrons are localized and bound to atoms; in a metallic conductor, some electrons are delocalised and free to move.

If 1.25×10^19 electrons pass a point in a conductor over 2 s, what is the resulting electric current?

1. ΔQ= (1.25×10^19× 1.6×10^−19)= 2C;

2. I = 2C // 2s

3. 1A

What microscopic property of a material determines whether it is classified as a conductor, semiconductor, or insulator?

The availability and density of mobile charge carriers (delocalised electrons or ions) within its structure.

What are the charge carriers in a metallic conductor?

Free (delocalised) conduction electrons.

What is the definition of electrical resistance R in terms of potential difference V and current I?

Resistance is defined as the ratio of potential difference across a component to the current flowing through it (R=V/I).

How does the charge carrier density of a metallic conductor compare to that of an intrinsic semiconductor at room temperature?

A metallic conductor has a vastly higher charge carrier density than an intrinsic semiconductor.

What necessary conditions must be satisfied for an electric current to continuously pass round a circuit?

The circuit must be complete and there must be a source of potential difference (such as a cell or battery).

Why does the resistance of a metallic conductor increase with temperature, whereas the resistance of a semiconductor decreases?

In metals, carrier density remains constant but lattice vibration collisions increase; in semiconductors, thermal energy significantly increases charge carrier density.

How does an increase in temperature affect the number of charge carriers in a semiconductor material?

The number of charge carriers increases as temperature rises.

Why do the fixed positive ions in a metal lattice not contribute to charge transport?

They are bound in fixed positions within the crystalline lattice structure and cannot move through the metal.

Calculate the total charge flow ΔQ when a current of 10 mA flows for 500 s.

5 C

What does the symbol Δ denote in the formula I= ΔQ //Δt

It represents a change in or a finite quantity of charge flow over a given interval of time.

What is the equivalent unit of 1 Volt in fundamental energy and charge terms?

1 Joule per Coulomb (1 V = 1 J C^−1).

How much work is done when 5 C of charge passes through a potential difference of 12 V?

60J

At which terminal of a battery do electrons enter, and from which terminal do they leave, in a complete metallic circuit?

Electrons enter the battery at its positive terminal and leave from its negative terminal.

If current in a circuit is halved while the time interval is doubled, how does the total charge transferred change?

The total charge transferred remains unchanged, since ΔQ=(I/2)×(2Δt)= IΔt.

What is an intrinsic semiconductor?

A pure semiconducting material where electrical conduction is due solely to electrons thermally broken free from their own atoms.

What is the elementary magnitude of charge e of a single electron in coulombs?

1.6×10^−19 C

How is the coulomb (C), the unit of electric charge, defined in terms of current and time?

One coulomb is the quantity of charge that flows past a point in one second when the current is one ampere (1C =1 A⋅s).

Calculate the charge flow ΔQ when a current of 1 A flows for 10 s.

10C

Why do electrical insulators fail to conduct electricity when a potential difference is applied across them?

All electrons are tightly bound to individual atoms and cannot move freely through the material.

How can work done W in an electrical component be calculated using current I, potential difference V, and time Δt?

W= IVΔt

Define the term: charge carrier

A movable charged particle that constitutes an electric current

Do questions in the textbook relating to this topic please

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