Electricity

Question 1

What is meant by alternating current?

Answer 1

the current
changes direction
and value
with time

Question 2

How do you work out the EMF from a graph of V against I?

Answer 2

It’s the voltage where the graph meets the y-axis
(y intercept)

Question 3

What is meant by EMF?

Answer 3

The amount of energy (in J) given to each coulomb of charge as it passes through the battery

Question 4

How do you work out the internal resistance from a graph of V against I

Answer 4

• Work out the gradient using m=y2-y1/x2-x1 (it will be negative)
• Write m = -r
• then write r= (positive of the gradient)

Question 5

What is the purpose of the resistor in a capacitor circuit?

Answer 5

It controls the initial charge/discharge current (using V=IR)

Question 6

What’s the maximum voltage stored on a capacitor charged with a 12V battery?

Answer 6

12 V

the max voltage across the capacitor is the voltage applied across it.

Question 7

How do you work out the T.P.D (terminal potential difference)

Answer 7

It will be the EMF of the battery minus the voltage dropped across the internal resistor (lost volts)

Question 8

What’s the symbol for capacitor on the formula sheet?

Answer 8

C

Question 9

What’s the unit for capacitance?

Answer 9

F

Farads

Question 10

What’s the symbol for charge on the formula sheet?

Answer 10

Q

Question 11

What’s the unit of charge?

Answer 11

C

Coulombs

Question 12

Explain in terms of band theory how an LED emits light?

Answer 12

Voltage applied causes electrons in the conduction band of the n-type semiconductor to

Move towards the conduction band of the p-type semiconductor

The electrons then drop to the valence band

As they drop a photon is emitted

Question 13

Using band theory,

explain how a potential difference is produced

when photons of light are incident on a photodiode

Answer 13

Electrons absorb energy from the photons of light

This moves the electrons from the valence band to the conduction band

The electrons move towards the n-type semiconductor

Creating a potential difference

Question 14

Using band theory, explain why conduction can take place in a semiconductor at room temperature?

Answer 14

In a semiconductor - the energy band between the valence and conduction bands is small

This means some electrons have enough energy at room temperature to move from the valence band to the conduction band

Question 15

Explain what happens to a diode in forward bias?

Answer 15

A negative voltage is applied to the n-type end (and a positive voltage to the p type end)

This reduces the electric field and narrows the depletion layer

Electrons have enough energy to reach the conduction band allowing current to flow

Question 16

Explain what happens to a diode in reverse bias?

Answer 16

A negative voltage is applied to the p-type end (and a positive voltage to the n type end)

This increases the electric field and increases the depletion layer

Very few electrons have enough energy to reach the conduction band so no (or very little) current can flow

Question 18

How do you measure the EMF of a battery?

Answer 18

Measure the voltage across the battery when no current is flowing

this is often by opening a switch so no current can flow