Electricty

Question 1

Give an equation and definition for current.

Answer 1

I = ΔQ/Δt. Current is the charge flow per unit time.

Question 2

Give the equation and definition for p.d.

Answer 2

V = W/Q. Potential difference is the work done per unit charge.

Question 3

Give the equation and definition for resistance.

Answer 3

R = V/I. Resistance is the ratio of potential difference across acomponent to the current flowing through it.

Question 4

Give 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.

I = V/R gives I ∝ 1/R

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/battery is shared based on resistance of components.

V = IR gives V∝ R

Question 9

How does the voltage law apply to parallel circuits?

Answer 9

Each branch gets the full voltage of the cell/battery.

Question 10

Give the equation for total resistance in series.

Answer 10

R_T = R₁ + R₂ + …

Question 11

Give the equation for total resistance in parallel.

Answer 11

1/R_T = 1/R₁ +1/ R₂ + …

Question 12

Explain the shape of the ohmic conductor graphs.

Answer 12

Ohmic conductors obey Ohm’s law -> current is directly proportional to potential difference -> as the resistance is constant (provided that temperature remains constant) -> straight line through origin.

Question 13

Explain the shape of the filament lamp graphs.

Answer 13

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 14

Explain the shape of the NTC thermistor graphs.

Answer 14

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 15

Explain the shape of the diode graphs.

Answer 15

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 16

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

Answer 16

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. Or use a potentiometer

Question 17

Give the relationship between length of wire and resistance.

Answer 17

Resistance ∝ Length

Question 18

Give the relationship between area of a wire and resistance.

Answer 18

Resistance ∝ 1/ Area

Question 19

Give the equation and units for resistivity.

Answer 19

ρ = RA/ L Units = Ωm

Question 20

Give the definition for resistivity.

Answer 20

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

Question 21

What happens to resistivity as temperature increases?

Answer 21

As temperature increases, resistivity increases.

Question 22

How can you determine resistivity experimentally?

Answer 22

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 23

Describe what a superconductor is.

Answer 23

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

Question 24

Describe and explain the uses of superconductors.

Answer 24

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 25

Describe a limitation of using superconductors.

Answer 25

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

Question 26

Total p.d. for cells in series?

Answer 26

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

Question 27

Total p.d. for identical cells in parallel?

Answer 27

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

Question 28

What is the definition for power?

Answer 28

Power is the work done/energy transferred per unit time.

Or: Power is the rate of work done/energy transfer.

Question 29

Give the power equations from the data sheet.

Answer 29

P = VI = I²R = V²/R

Question 30

How does a potential divider circuit work?

Answer 30

Two or more resistors in series, connected to a power source.

P.d. of power source is shared between resistors. V ∝ R.

Question 31

How are potential divider circuits used in sensory circuits?

Answer 31

LDRs used in automatic lights.

Thermistors used in thermostats.

Question 32

What is a potentiometer?

Answer 32

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 33

Give an equation and definition for e.m.f.

Answer 33

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

Question 34

Define internal resistance.

Answer 34

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

Question 35

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

Answer 35

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

Question 36

Give the equation linking ε, I, R and r.

Answer 36

ε = I (R +r)

Question 37

How can you investigate internal resistence experimentally?

Answer 37

Measure V and I for different values of R.

Plot graph of V against I. V = - Ir + ε

Gradient = -r

y-intercept = ε