DC electricity

Question 1

Define current

Answer 1

The rate of transfer of charge

Question 2

Define current quantitively

Answer 2

ΔQ/Δt

Question 3

Define potential difference

Answer 3

The work done per unit between two points in a circuit

Question 4

Define resistance

Answer 4

How difficult it is for charge carriers to pass through a component caused by the ions resisting the flow of charge

Question 5

Define potential difference quantitively

Answer 5

ΔV = ΔE / Q

Question 6

What is the gradient of a charge / time graph?

Answer 6

Current

Question 7

What is the area under a Current / time graph?

Answer 7

The total charge transferred

Question 8

How do you take the resistance from a non linear potential difference / current graph?

Answer 8

Take V at point at I / I

NOT GRADIENT as R is NOT ΔV/ΔI just V/I

Question 9

Define power

Answer 9

The rate of energy transfer

Question 10

What direction is conventional current?

Answer 10

Positive too negative , counter intuitively against the flow of electrons.

Question 11

What end of a cell is the +ive / -ive end?

Answer 11

‘Long bit’ +ive
‘Short bit’ -ive

Question 12

What direction is the potential difference across components?

Answer 12

• ive too + ive across a power supply

Against the flow of current across any other component

Question 13

What is Kirchhoff’s voltage law?

Answer 13

The sum of directed voltages around any loop is zero

Question 14

What is Kirchhoff’s current law?

Answer 14

The sum of currents into a branch = the sum of currents out

Question 15

What is the emf?

Answer 15

The work done in moving 1 Coulomb of charge completely around the circuit.

The voltage across the source ε

Question 16

What equation links the total resistance, emf and current of the cell?

Answer 16

Rt = ε / I cell

Question 17

What is the resistance of an ideal ammeter? Why?

Answer 17

ZERO

So that resistance will not affect the measurement of current in a circuit

Question 18

What is the resistance of an ideal voltmeter? Why?

Answer 18

INFINITE

As there should be no current flowing through the voltmeter

Question 19

Filament lamp power rating assumptions

Answer 19

The resistance is constant
Brightness is proportional too power

Question 20

What is ohms law?

Answer 20

The current through a component is directly proportional too the potential difference across it, provided the physical conditions do not change. (e.g. temperature)

Question 21

Explain the current voltage graph of a filament lamp

Answer 21

As the pd increases, the current increases and so there are more collisions between the electrons every second. The vibration of the metal lattice increases as the temperature increases increasing the resistance.

Question 22

Describe an ohmic graph

Answer 22

Linear graph through the origin

Question 23

What is the relationship between temperature and resistance?

Answer 23

As the temperature increases, the particles vibrate more as their internal energy increases and there are more collisions per second increasing the resistance.

Question 24

Describe an R/ T graph for a metal or conductor

Answer 24

Linear with a positive temperature gradient

Question 25

Describe the R/T graph for semi conductors and thermistors

Answer 25

Negative temperature gradient (decaying exponential)

Question 26

Explain the behaviour of a semi conductor as temperature increases

Answer 26

The electrons gain more energy so the number of charge carriers (electrons that can cross the valence/conduction band gap) increases so there is a large current per given voltage and the resistance decreases.

Question 27

What is a super conductor?

Answer 27

A material who's Resistance, R, drops to 0 ohms below critical temperature

Question 28

Why are super conductors useful?

Answer 28

They do not loose any energy due too I²R heating so efficient anergy transfers (below the critical temperature) Can easily get high currents so good at inducing strong magnetic fields

Question 29

What is the resistivity of a material?

Answer 29

Material property describing how much the material opposes the flow of charge through it

Question 30

What is resistivity measured in?

Answer 30

Ωm

Question 31

What is a potential divider?

Answer 31

A circuit that returns an output voltage a fraction of its input voltage

Question 32

What is the relationship between light intensity of an LDR and the resistance though it?

Answer 32

As light intensity increases, resistance decreases

Question 33

What is the potential divider equation?

Answer 33

V out = (ε . R potential divider) / R total

Question 34

What is internal resistance?

Answer 34

The resistance inside a source of emf which causes the source potential difference to drop when a current flows through the source.

Question 35

Why does the reading across a source with internal resistance change when the switch is closed?

Answer 35

When the switch is open, no current flows through the source, and the voltmeter reads the value of the emf. When the switch is closed, current flows through the source and there is a voltage drop to the voltage of the internal resistor and the voltmeter reads a lower value.

Question 36

What is the emf equation when there is internal resistance?

Answer 36

ε = I . (R+r)

Question 37

What is the maximum power theorem?

Answer 37

The maximum power transferred to the load of a circuit is when the load resistance is equal too the internal resistance

Question 38

What is the external power dissipation of a circuit (with internal resistance)?

Answer 38

I = ε / Rt Rt = (R + r) P = I² R P = ε² .R / (R + r)²