Current and Voltage Relationships

Electrical Quantities

Define electrical resistance.

The opposition to the flow of electric current through a conductor.

What is the symbol for resistance?

R.

What is the unit of resistance?

The ohm (Ω).

What equation is used to calculate resistance?

R = V/I.

What does V represent in R = V/I?

The potential difference across the component, measured in volts (V).

What does I represent in R = V/I?

The current through the component, measured in amperes (A).

How can potential difference be calculated using resistance and current?

V = IR.

How can current be calculated using potential difference and resistance?

I = V/R.

What is Ohm's law?

The current through a component is directly proportional to the potential difference across it, provided its temperature remains constant.

What equation represents Ohm's law?

V = IR.

What is an ohmic conductor?

A conductor that obeys Ohm's law, so current is directly proportional to potential difference at constant temperature.

What happens to the resistance of an ohmic conductor when potential difference changes at constant temperature?

The resistance remains constant.

What shape is the I–V graph of an ohmic conductor?

A straight line through the origin.

Why does an ohmic conductor's I–V graph pass through the origin?

At zero potential difference, there is zero current.

What does a steeper gradient on an I–V graph mean?

A larger current for a given potential difference, so a lower resistance.

What does a shallower gradient on an I–V graph mean?

A smaller current for a given potential difference, so a higher resistance.

How is the gradient of an I–V graph calculated?

Gradient = ΔI/ΔV.

How can resistance be calculated from the gradient of an I–V graph?

R = 1/gradient.

Why is resistance the reciprocal of the gradient of an I–V graph?

Because gradient = I/V, while resistance R = V/I.

When can resistance be calculated from the reciprocal of the gradient of an I–V graph?

When the graph is a straight line, such as for an ohmic conductor.

How can resistance be found from a curved I–V graph?

Use R = V/I at the specific point of interest.

How is an I–V characteristic investigated experimentally?

Change the supply voltage and measure the potential difference across and current through the component for several values.

What instrument measures current in an I–V experiment?

An ammeter.

How is an ammeter connected?

In series with the component.

What instrument measures potential difference in an I–V experiment?

A voltmeter.

How is a voltmeter connected?

In parallel across the component.

Why are the terminals of the power supply reversed during an I–V investigation?

To obtain negative values of potential difference and current.

What is meant by an I–V characteristic?

A graph showing how the current through a component varies with the potential difference across it.

What is a non-ohmic conductor?

A component for which current is not directly proportional to potential difference.

What shape is the I–V graph of a filament lamp?

A curve that becomes less steep as the magnitude of the potential difference increases.

Why is a filament lamp non-ohmic?

Its resistance changes as its temperature changes.

What happens to a filament lamp's temperature as current increases?

Its temperature increases.

What happens to the resistance of a filament lamp as its temperature increases?

Its resistance increases.

Why does the resistance of a filament lamp increase with temperature?

The metal ions vibrate more, causing more collisions between the conduction electrons and the ions.

What happens to the vibrations of metal ions when a metal gets hotter?

The ions vibrate faster and with greater amplitude.

How do increased lattice vibrations affect conduction electrons?

They cause more frequent collisions, making electron flow more difficult.

Why does the filament lamp I–V graph become less steep at higher voltages?

The filament gets hotter, increasing its resistance, so current increases less rapidly.

Is a filament lamp's I–V graph symmetrical about the origin?

Yes, because it behaves similarly when the direction of current is reversed.

What is a diode?

A semiconductor component designed to allow current to flow mainly in one direction.

In which direction does a diode conduct?

The forward direction.

What happens to current through a diode in the reverse direction?

Almost no current flows.

What does the I–V graph of a diode look like for negative potential differences?

The current is approximately zero.

Define threshold voltage for a diode.

The minimum forward potential difference required before a significant current flows.

What is a typical threshold voltage for a silicon diode?

About 0.6 V.

What happens once the threshold voltage of a diode is exceeded?

The current increases very rapidly for a small increase in potential difference.

Why is the I–V graph of a diode not symmetrical?

A diode conducts strongly in the forward direction but blocks current in the reverse direction.

Is a diode an ohmic conductor?

No, because current is not directly proportional to potential difference.

What is a thermistor?

A component whose resistance changes significantly with temperature.

What happens to the resistance of the thermistor shown as its temperature increases?

Its resistance decreases.

What type of thermistor has resistance that decreases as temperature increases?

A negative temperature coefficient (NTC) thermistor.

Why does the resistance of an NTC thermistor decrease as temperature increases?

Higher temperature releases more charge carriers in the semiconductor, increasing conductivity.

What happens to the number of conduction electrons in an NTC thermistor as temperature increases?

It increases.

What happens to the current through an NTC thermistor as it gets hotter for the same potential difference?

The current increases.

How does the I–V graph of an NTC thermistor change as voltage increases?

It becomes steeper because heating reduces its resistance.

How does a thermistor's behaviour differ from a filament lamp?

A thermistor's resistance decreases as temperature increases, whereas a filament lamp's resistance increases.

Is a thermistor an ohmic conductor?

No, because its resistance changes with temperature and current is not directly proportional to potential difference.

What does a straight-line I–V graph through the origin indicate?

The component is ohmic and has constant resistance.

What does a curved I–V graph indicate?

The component's resistance is changing.

How can two ohmic components be compared using their I–V graphs?

The component with the steeper I–V line has the lower resistance.

Why must temperature remain constant for Ohm's law to apply?

Changing temperature can change the resistance of the conductor.

What causes current to flow when a potential difference is applied?

The electric field produces a force on charge carriers, causing them to move.

What happens to current when the potential difference across an ohmic conductor is doubled?

The current doubles, provided temperature remains constant.

What happens to current when the potential difference across an ohmic conductor is tripled?

The current triples, provided temperature remains constant.

What happens to resistance if V and I increase in the same proportion?

The resistance remains constant.

What is the relationship between current and resistance for a fixed potential difference?

Current is inversely proportional to resistance.

For the same potential difference, which component carries more current: one with high resistance or low resistance?

The component with lower resistance.