Electricity

Question 1

Eq linking power, V and R.

Answer 1

P = V^2 / R

Question 2

What are charge carriers?

Answer 2

Charged particles in current eg electrons.

Question 3

Define resistance.

Answer 3

Ration of voltage across component to current through it.

Question 4

What’s a semiconductor?

Give two examples.

Answer 4

Component where number of charge carriers increases with increase in temperature, therefore resistance decreases as temp increases eg thermistor, LDR.

Effect of more charge carriers > effect of more collisions with lattice.

Question 5

What is p.d ?

Answer 5

The work done to pass through the component and transfer some/all its energy, per UNIT charge.

Question 6

What is emf?

Answer 6

The electrical energy produced per unit charge passing through the source.

OR p.d across terminals of source when no current flowing.

Question 7

What’s Ohms law?

Answer 7

the p.d across a metallic conductor is proportional to the current through it, provided the physical conditions do not change (aka resistance is constant).

Question 8

What’s a superconductor?

What happens when current runs through it?

Answer 8

A wire or device made of material that has zero resistance at and below a critical temperature that depends on the material. When a current passes through it, there’s no p.d across it because its resistance is zero therefore current has no heating effect.

Question 9

Uses of superconductors?

Answer 9

• To make high power electromagnets that generate very strong magnetic fields for eg MRI scanners, accelerators.
• Generators/transmission lines/electrical cables bc low energy-loss conductors so less energy wasted.
• Transformers.
• Computers bc increases speed and less heating problems.
• Amplifiers in radio astronomy bc low noise.

Question 10

Current rule?

Answer 10

At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction - conservation of current.

Question 11

Heating effect is all about…

Answer 11

resistance! See next eq.

(As V increases, current increases. Current heats component therefore R increases. Resistance increases with increasing temp.)

Question 12

Rate of heat transfer to surroundings eq?

Answer 12

Energy per second transferred to the component as thermal energy = I^2 R.
If constant temp then heat transfers at same rate to surroundings therefore the rate of heat transfer, P = I^2R.

If heats up, temp increase depends on power supplied (I^2 R) and rate of heat transfer to surroundings.

Question 13

Define the p.d across terminals of source.

(source = cell/battery/etc)

Answer 13

the electrical energy per unit charge delivered (vs produced for emf) by the source when it’s in a circuit.

Question 14

Define internal resistance of source.

Answer 14

the loss of p.d per unit current in the source when current passes through the source.

Question 15

Power supplied by source eq?

Answer 15

Iε = I^2 R + I^2 r

Question 16

When a source delivers power to a load, the max power delivered to the load when…

Answer 16

when R = r

Question 17

Setup of this circuit?

Purpose of each component?

Answer 17

Source with internal resistance and emf, connected in series with an ammeter and a variable resistor and a light bulb (or fixed resistor). Voltmeter in parallel with source to measure terminal p.d, V.

Current changed by adjusting variable resistor.
Lamp/fixed resistor limits the max current that can pass through the cell. (basically if resistance of variable resistor set to 0 there’s still resistance of lamp therefore it’ll be max current but this will be limited)

Question 18

Why does terminal p.d decrease with increasing current.

Answer 18

When I increases, p.d across r increases therefore terminal p.d decreases. Greater proportion of emf across internal resistance.

Question 19

Given V for two values of I, calculate r and ε :

Answer 19

V1 - V2 = (ε - I1 r) - (ε - I2 r) = I2 r - I1 r = (I2 - I1) r
Therefore r = V1 - V2 / I2 - I1

Sub into eq to find ε .

Question 20

Cell current?

Answer 20

cell current = cell emf / total circuit resistance

Question 21

If cells connected in series in same direction:

Answer 21

ε = sum of individual ε
r = sum of all cells r

Question 22

If cells connected in series in opposite direction:

Answer 22

ε = difference of individual ε
r = sum of all cells r

Question 23

Circuits with n identical cells in parallel:

I in each cell?
v in each cell?
ε ?
r ?

Answer 23

Current through each cell = I / n, where I is total current supplied by cells and n is number of cells.
Therefore lost p.d in each cell = Ir/n.
Cells act a source of emf ε ,
and of internal resistance r/n.

Question 24

Silicon diode

Answer 24

Forward biased diode has forward p.d of 0.6V (silicon diode) when current passes through. If diode was reversed, circuit current would be zero because resistance would be infinite therefore p.d across resistor that’s in series would be zero. P.d across diode would therefore be equal to cell emf.

Question 25

Kirchhoffs laws

Answer 25

1) At any junction in a circuit, the total current entering the junction = the total current leaving the junction (conservation of current).

2) For any complete loop in a circuit, the sum of the emfs around the loop = the sum of the potential drops around the loop (links to conservation of energy).

Question 26

Page 225 worked example clearly demonstrates Kirchhoffs laws.

Answer 26

Look it up :)

Question 27

What’s a potential divider circuit?

Answer 27

Consists of two or more resistors in series with each other and with a source of fixed p.d. P.d is divided between the components in the circuit, as they’re in series with each other.

Question 28

Potential divider circuit used to supply fixed p.d?

(Just resistors in series normally)

Answer 28

V1/V2 = R1/R2. P.d across V1 = R1VT/RT.

Question 29

Potential divider circuit used to supply variable p.d?

(Those resistors with the arrow u can move from side to side)

Answer 29

Enables current to reach zero therefore p.d across it can reach zero.
Larger range of p.d possible??????????????????????????????

Question 30

Sensor circuit - potential divider circuit used to produce output p.d which changes as a result of a change of a physical variable such as temperature of light intensity?

Answer 30

Temperature sensor: when temp constant, source p.d divided between thermistor and variable resistor. Adjusting variable resistance to set thermistor p.d to desired value. Temp increase results in resistance decreases therefore p.d decreases.

Light sensor: pd across LDR changes when light intensity changes. If light intensity increases, resistance of LDR decreases therefore p.d decreases.