Electric current

Question 1

What is the definition of current?

Answer 1

The rate of flow of charge in a wire or component.

Question 2

How does current flow in a circuit?

Answer 2

It flows due to the passage of charge carriers.

Question 3

What is the formula linking charge and current?

Answer 3

Q=IT

Question 4

What is potential difference?

Answer 4

Work done per unit charge

Question 5

What is the formula to calculate p.d

Answer 5

V=W/Q - where w = work done.

Question 6

What is the definition of the E.M.F of a source?

Answer 6

The electrical energy produced per unit charge passing through the source.

Question 7

What effect does current passing through a component with resistance have?

Answer 7

Current has a heating effect.

Question 8

What is the formula linking power to current and voltage?

Answer 8

P = IV.

Question 9

What is the definition of resistance?

Answer 9

The measure of difficulty of making a current pass through a component.

Question 10

What is ohm’s law? What is the formula?

Answer 10

P.D across a metallic conductor is proportional to the current through it provided temperature remains constant. V=IR

Question 11

What is the formula for resistivity? What is the unit for resistivity?

Answer 11

p = RA/L. Unit = ohm’s per metre.

Question 12

What is cross sectional area for a conductor with circular cross section of diameter d?

Answer 12

πd (squared) / 4

Question 13

What is the key priority of a diode?

Answer 13

It only allows current passing through in one direction.

Question 14

How does the resistance of a negative temperature coefficient thermistor change with temperature?

Answer 14

Resistance of thermistor decreases with increasing temperature.

Question 15

How does the resistance of an L.D.R change with light intensity?

Answer 15

Resistance of L.D.R decreases with increasing light intensity

Question 16

Why does a filament lamp give a curve with decreasing gradient?

Answer 16

Its resistance increases as it becomes hotter. An increase in current leads to an increase in temperature. This causes an increase in the movement of ions, which causes an increase in the rate of collision, so resistance increases.

Question 17

What defines having a positive temperature coefficient?

Answer 17

If resistance increases with temperature, the component has a positive temperature coefficient.

Question 18

What defines having a negative temperature coefficient?

Answer 18

Resistance decreases with increasing temperature.

Question 19

What is the process to calculate the resistivity of a wire?

Answer 19

Measure the diameter of the wire using a micrometer at several points along the wire, then calculate a mean diameter. Use this to calculate the cross sectional area. Then measure the resistance of the wire of different lengths of wire to plot a graph of R againt L.

Question 20

How do you find the resistivity using a resistance / length graph

Answer 20

Resistivity = graph gradient * Area. R/L * A = RA/L.

Question 21

What is a superconductor?

Answer 21

A wire or device made of material that has zero resistivity at and below a critical temperature, which depends on the material.

Question 22

Why does current not have a heating effect when it passes through a superconductor?

Answer 22

There is zero resistance.

Question 23

What are superconductors used for?

Answer 23

High power electro-magnets and power cables that transfer electrical energy without energy dissipation.

Question 24

Explain why an increase in temperature leads to an increase in resistance?

Answer 24

Positive metal ions vibrate more. Therefore the charge carriers (electrons) can’t pass through the metal as easily and carry a charge.