Electricity

Question 1

Current is…

Answer 1

…rate of flow of charge.

Question 2

Current is measured in what?

Answer 2

Amperes.

Question 3

Equation linking current, charge and time:

Answer 3

I=ΔQ/Δt

Question 4

Kirchhoff’s current law:

Answer 4

The total current flowing into a point is equal to the current flowing out of a point.

Question 5

What is Kirchhoff’s current law based on?

Answer 5

Conservation of charge principle. Charge is conserved in a circuit meaning current is also conserved.

Question 6

Potential difference is the energy per…

Answer 6

…coulomb of charge.

Question 7

Potential difference is a measure of what?

Answer 7

The amount of energy per unit of charge transferred between two points in a circuit.

Question 8

Equation linking potential difference, energy and charge:

Answer 8

V = W/Q

Question 9

In series potential difference across the components…

Answer 9

…adds to the potential difference across the circuit.

Question 10

In parallel potential difference…

Answer 10

…remains the same for every branch across the circuit (it is equal throughout).

Question 11

Kirchhoff’s potential difference law:

Answer 11

States that around any closed loop in a circuit the sum of the potential difference across all components equals the potential difference across the supply.

Question 12

What is Kirchhoff’s potential difference law based on?

Answer 12

Conservation of energy principle.

Question 13

Ohm’s Law is stated as:

Answer 13

The voltage drop across an ohmic conductor is proportional to the current flowing through it provided temperature remains constant.

Question 14

Formula for resistance linking potential difference and current:

Answer 14

R = V/I

Question 15

Power is the rate of…

Answer 15

…transferring energy.

Question 16

Equation for power linking energy and time:

Answer 16

P = W/t

Question 17

Equation for power linking current and potential difference:

Answer 17

P = IV

Question 18

Equation for energy linking current, potential difference and time:

Answer 18

W = IVt

Question 19

Equation for power linking current and resistance:

Answer 19

P = I^2R

Question 20

Factors that affect resistivity:

Answer 20

Material
Cross-sectional area
Length
Temperature

Question 21

As the length of material increases, what happens to resistivity?

Answer 21

It also increases.

Question 22

As cross-sectional area increases what happens to resistivity?

Answer 22

Resistivity decreases.

Question 23

Equation for resistance linking length, cross-sectional area and resistivity:

Answer 23

R = ρl/A

Question 24

A potentiometer is a device that does what?

Answer 24

It is a device that provides a pd ranging from 0V up to the voltage of the supply.

Question 25

Equation for potentiometers.

Answer 25

Vout/Vs=x/l

Question 26

Equation for drift velocity.

Answer 26

``` I=nAvq I is in A v in ms^-1 A in m^2 n in m^-3 q in C ```

Question 27

What is resistivity?

Answer 27

The resistance of a 1m^2, 1m long piece of material.

Question 28

EMF stands for...

Answer 28

...electromotive force.

Question 29

A cell has an internal what?

Answer 29

Resistance.

Question 30

Equation for emf?

Answer 30

``` ε=V+Ir Where ε is emf in V. V is terminal pd in V I is current in A. r is internal resistance in Ω. ```

Question 31

If compared to y=mx+c, equation for internal resistance can be used to find r and ε. How?

Answer 31

y is V. x is I. m is -r. c is emf.

Question 32

What is Ir sometimes described as (emf equation)?

Answer 32

The ‘lost volts’.

Question 33

A thermistor is a resistor that varies with what?

Answer 33

Temperature.

Question 34

LDR stands for what?

Answer 34

Light Dependent Resistor.

Question 35

What are the two types of thermistors?

Answer 35

NTC and PTC

Question 36

What does NTC stand for and what does it do?

Answer 36

NTC stands for negative temperature coefficient so as temperature increases resistance decreases.

Question 37

What does PTC stand for and what does it do?

Answer 37

PTC stands for positive temperature coefficient and this means as temperature increases so too does the the resistance.

Question 38

2 factors to consider when talking about resistance in LDRs and thermistors.

Answer 38

1) Increased light/heat energy means increases lattice vibrations which leads to more electron collisions. This reduces the drift velocity (I=nAqv), reduces the current which means increased resistance. (PTC answer and half NTC/LDR answer). 2) Energy absorbed as light/heat results in the release of more conduction electrons. This increases the density of charge carriers (I=nAqv) so increases the current, thus reducing the resistance. In NTC/LDR factor 2 has a bigger effect than factor 1 resulting in reduced resistance.