DC Electricity

Question 1

Current

Answer 1

The rate of flow of charge at a point in a circuit

Question 2

What is used instead of an ammeter for very small currents?

Answer 2

A galvanometer / galvo

Question 3

What is the resistance of an ideal ammeter?

Answer 3

0 ohms

Question 4

Direction of flow of current

Answer 4

Positive to negative

Question 5

Direction of flow of electrons

Answer 5

Negative to positive

Question 6

Potential difference between two points

Answer 6

The energy converted when a unit charge flows between the points

Question 7

What is the resistance of an ideal voltmeter?

Answer 7

Infinite resistance

Question 8

Resistance of a devise

Answer 8

The ratio of the potential difference across its ends to the current flowing through it

Question 9

What is resistance a measure of?

Answer 9

How difficult it is for current to flow

Question 10

Ohm’s law

Answer 10

Provided the temperature and other physical factors remain constant, the current through a conductor is directly proportional to the p.d across it.

Question 11

Power

Answer 11

The rate at which energy is converted

Question 12

A kilowatt-hour

Answer 12

The energy supplied by 1 kilowatt of power in 1 hour

Question 13

Kirchoff’s 1st law

Answer 13

At a junction, the total current entering the junction is equal to the total current leaving the junction

Question 14

Explanation for the shape of a V-I graph

Answer 14

1. Obays Ohm’s law at low PD and I
2. As PD increases, I increases
3. As I increases, more power is dissipated
4. As more power is dissipated, the temp increases
5. As temp increases, resistance increases
6. As resistance increases, there is a smaller increase in I for the same increase in PD

Question 15

Why does resistance increase at high temperatures?

Answer 15

The metal ions vibrate more vigorously, making it more difficult for electrons to flow through a wire

Question 16

Kirchhoff’s seccond law

Answer 16

The sum of the emfs around a circuit loop is equal to the sum of the p.ds around the loop

Question 17

Breakdown voltage of a semiconductor diode?

Answer 17

Around -50V

Question 18

Minimum voltage required for current to flow in a diode

Answer 18

Around 0.7V

Question 19

What is the maximum voltage of a diode?

Answer 19

Around 0.7V regardless of current

Question 20

What is the resistance of an ideal diode when reverse biased?

Answer 20

Infinite resistance

Question 21

What is a protective resistor used for?

Answer 21

Preventing voltage from rising too high and destroying a diode in the circuit

Question 22

What determines resistivity?

Answer 22

The material

Question 23

Units of resistivity

Answer 23

Ωm

Question 24

A low resistivity means that a material is…

Answer 24

A good conductor

Question 25

Effect of increasing temperature on resistivity

Answer 25

Resistivity increases

Question 26

Why does resistivity increase with temperature?

Answer 26

- Metal ions gain KE - More vibrations - More collisions with electrons - Rate of flow of electrons decreases - Current is lower, so resistance increases

Question 27

Superconductors

Answer 27

Materials where the resistivity drops to zero at very low temperatures

Question 28

Critical temperature of supercoductors

Answer 28

The temperature at or below which the resistivity of a material is zero

Question 29

Uses of superconductors

Answer 29

- Mag-lev trains - MRI - Computing - More efficient power cables

Question 30

Electromotive force (emf)

Answer 30

The number of joules of electrical energy transferred to each coulomb that passes through it

Question 31

What causes internal resistance?

Answer 31

The materials inside the power supply

Question 32

Why is the terminal voltage lower than the emf?

Answer 32

Some of the emf goes across the internal resistance

Question 33

Effect of increasing current on terminal voltage

Answer 33

- Current increases internal resistance - A greater proportion of the pd goes across the internal resistance - Terminal pd is lower

Question 34

Why would a low internal resistance be used?

Answer 34

This will cause the current produced by the supply to be high

Question 35

Why would a high internal resistance be used?

Answer 35

In high voltage supplies, this is used to keep current low (prevents electrocution)

Question 36

Short circuit

Answer 36

When something connects the terminals of the supply, causing load resistance to be 0

Question 37

Total emf in series

Answer 37

Sum of the emfs

Question 38

Total internal resistance in series

Answer 38

Sum of the individual resistances

Question 39

Total emf in parallel (identical cells)

Answer 39

Emf of a single cell

Question 40

Total internal resistance in parallel (identical cells)

Answer 40

Combined resistance of all resistances in parrallel

Question 41

Why would cells be used in parrallel?

Answer 41

- Batteries last longer (produce 1/n of the current) - Lower internal resistance reduces heating - Total emf is not effected if a cell brakes

Question 42

Potential divider

Answer 42

A devise that shares the supply voltage between two or more resistors in series

Question 43

Potentiometer

Answer 43

A device that controls voltage across a component

Question 44

What is an ntc thermistor?

Answer 44

A negative temperature coefficient thermistor

Question 45

Effect on resistance of an ntc thermistor when temperature increases

Answer 45

Resistance decreases

Question 46

How can temperature of a thermistor be controlled during an experiment?

Answer 46

Using a water bath (ice to reach colder temperatures)

Question 47

LDR

Answer 47

Light dependant resistor - has a high resistance which decreases greatly as light intensity increases