Electricity

Question 1

What is the definition of current?

Answer 1

The charge flow per unit time

Question 2

What is the definition for potential difference?

Answer 2

The work done per unit charge

Question 3

What is the definition for resistance?

Answer 3

The ratio of potential difference across a component to the current flowing through it

Question 4

What is kirchoff’s current law?

Answer 4

As charge must be conserved, the total current flowing into a junction is equal to the total current flowing out of a junction

Question 5

How does the current law apply to series circuits?

Answer 5

Same current at all points

Question 6

How does the current law apply to parallel circuits?

Answer 6

Current splits between the branches based on resistance

Question 7

Give kirchoff’s voltage law

Answer 7

As energy must be conserved, for a closed loop the total voltage across the cells/batteries is equal to the total voltage across the components

Question 8

How does the voltage law apply to series circuits?

Answer 8

Voltage of the cell is shared based on the resistance of components

Question 9

How does the voltage law apply to parallel circuits?

Answer 9

Each branch gets the full voltage of the battery

Question 10

Give the equation for total resistance in series

Answer 10

Rt = R1 + R2 + R3 + …

Question 11

Give the equation for total resistance in parallel

Answer 11

1/Rt = 1/R1 + 1/R2 + 1/R3 + …

Question 12

What does the IV graph for an ohmic conductor look like?

Answer 12

y = x graph
The steeper the gradient, the lower the resistance

Question 13

What does the VI graph for an ohmic conductor look like?

Answer 13

y = x graph
The steeper the line, the higher the resistance

Question 14

Explain the shape of ohmic conductor graphs

Answer 14

Ohmic conductors obey ohms law
Current directly proportional to potential difference
As resistance is constant theres a straight line through the origin

Question 15

What does the IV graph for a filament lamp look like

Answer 15

Smooth S curve, gradually flattening out at a higher voltage

Question 16

What does the VI graph for a filament lamp look like

Answer 16

Smooth S curve

Question 17

Explain the shape of the filament lamp graphs

Answer 17

As p.d. increases -> current increases -> temperature of filament increases -> metal ions vibrate with greater amplitude -> more frequent collisions between electrons and ions -> resistance increases -> graph curves as V/I increases

Question 18

Explain the shape of the NTC thermistor graphs.

Answer 18

As p.d. increases -> current increases -> temperature increases -> energy of thermistor increases -> more charge carriers released -> resistance decreases -> graph curves as V/I decreases.

Question 19

Explain the shape of the diode graphs.

Answer 19

For negative p.d. -> reverse bias -> very high resistance -> little or no current flows. For positive p.d. -> forwards bias -> no current flows until threshold voltage met -> above this, current increases as p.d increases.

Question 20

Describe a circuit that can be used to investigate I-V characteristics.

Answer 20

Connect test component and a variable resistor in series with a power source. Connect ammeter and voltmeter. Vary resistance of variable resistor to vary V and I

Question 21

Give the relationship between length of wire and resistance

Answer 21

Resistance ∝ Length

Question 22

Give the relationship between area of a wire and resistance.

Answer 22

Resistance ∝ 1 / Area

Question 23

Give the definition for resistivity.

Answer 23

Resistivity is the resistance of a material with a cross-sectional area of 1 m2 and a length of 1 m.

Question 24

What happens to resistivity as temperature increases

Answer 24

As temperature increases, resistivity increases

Question 25

How can you determine resistivity experimentally?

Answer 25

Change length of wire and measure I and V. Calculate R =V/I. Plot a graph of resistance against length of wire. Gradient = resistivity/area.

Question 26

Describe what a superconductor is.

Answer 26

A superconductor is a material that has zero resistivity (and zero resistance) at or below a critical temperature

Question 27

Describe and explain the uses of superconductors.

Answer 27

High speed MagLev trains and MRI scanners -> as superconductors can be used to create very strong magnetic fields. Electrical transmission cables -> zero resistance -> no energy wasted to surroundings -> more efficient.

Question 28

Describe a limitation of using superconductors.

Answer 28

Critical temperature is very low -> cooling with liquid helium/nitrogen is expensive and impractical.

Question 29

Total p.d. for cells in series?

Answer 29

The sum of the p.d. of the individual cells

Question 30

Total p.d. for identical cells in parallel?

Answer 30

Equal to the p.d. of one of the cells.

Question 31

What is the definition for power?

Answer 31

Power is the energy transferred per unit time.

Question 32

How does a potential divider circuit work?

Answer 32

Two or more resistors in series, connected to a power source. P.d. of power source is shared between resistors. V∝R.

Question 33

How are potential divider circuits used in sensory circuits?

Answer 33

LDRs used in automatic lights. Thermistors used in thermostats.

Question 34

What is a potentiometer?

Answer 34

Long coil of wire with a moveable connection. Gives two resistors with varying resistance. Can be used to investigate I-V characteristics – allows V to be varied from 0 V to full voltage of cell/battery.

Question 35

Give the definition of EMF

Answer 35

E.m.f. is the work done moving 1 C of charge through a cell.

Question 36

Define internal resistance.

Answer 36

Resistance of a cell/battery caused by electrons colliding with metal ions and losing energy.

Question 37

Give the equation linking e.m.f., terminal p.d., lost volts

Answer 37

E.m.f = terminal p.d. + lost volts