Electricity

Question 1

What is alternative current a.c.?

Answer 1

changes its direction and instantaneous value with time

Question 2

What is direct current d.c.?

Answer 2

The electrons flow in one direction only, and its value is constant

Question 3

What is peak voltage equal to?

Answer 3

number of divisions x volts/divisions

(uses y gain ⬆️⬇️)

Question 4

What is period (T) of the wave equal to?

Answer 4

number of divisions x time per division

(uses time base⬅️➡️)

-> f=1/T

Question 5

What are a.c. voltages stated in terms of?

Answer 5

Their r.m.s (root mean square) values-these are a kind of ‘average’ of the alternating ac supply

-ac readings on meters measure rms

Question 6

What is the rms value of an ac supply equal to?

Answer 6

a dc supply which gives the same power output

Question 7

What is current?

Answer 7

The dow of electric charge per second

Question 8

What is potential difference?

Answer 8

the work energy required to move 1C of charge through an electric field

Question 9

what is supply voltage?

Answer 9

a measure of the energy given to electrons to make them move around the circuit

Question 10

What is power

Answer 10

the energy transformed per second

The energy given to the electrons by the voltage supply is then transformed into other forms of energy by components

Question 11

Current rules

Answer 11

series
same at all points

parallel
add up to total

Question 12

voltage rules

Answer 12

series
add up to supply

parallel
same at all points

Question 13

resistance rules

Answer 13

series
add up to total

parallel
1/Rt = 1/R1 + 1/R2 + 1/R3

Question 14

What is the e.m.f. (electromotive force) of a source?

Answer 14

The electrical energy supplied to each coulomb of charge which passes through the source

Question 15

What is internal resistance? r

Answer 15

The resistance of the battery

Question 16

What is lost volts?

Answer 16

the energy (per coulomb) that will be used up in overcoming the internal resistance of the supply

Question 17

What is the t.p.d. (terminal potential difference)?

Answer 17

The energy (per coulomb) left for the circuit (after energy is used for overcoming r).

The rod is the voltage obtained from a source of electrical energy when a current is being drawn from it.

Question 18

Situation: open circuit

Answer 18

switch is open (oof)/current is 0, voltmeter reading across supply will be emf

no current—>lost volts=0

Question 19

situation: under load (with an external resistance R)

Answer 19

The circuit is working normally->will be lost volts

emf=V+Ir

voltmeter reading across supply is tpd

Question 20

situation: short circuit (no external resistance)

Answer 20

All the energy from the battery is lost because of internal resistance

R=0
E=I(0+r)
E=Ir

The maximum current is the short-circuit current large enough to blow a fuse or circuit breaker

Question 21

Why does Vtpd decrease when current increases?

Answer 21

As current increases, the list colts increases (lost volts=Ir)
This means more energy is lost (per coulomb) as the electrons travel through the supply

If more voltage is lost across the supply, there will be less for the external circuit

Therefore, the rod will decrease (E=V+Ir

Question 22

What is a capacitor?

Answer 22

a component which stores electrical charge

consists of 2 conducting layers separated by an insulator
(simplest:2 metal plates with an air gap between them)

Question 23

What is capacitance measured in?

Answer 23

Farads F

The amount of charge that a capacitor can store depends on its capacitance

Question 24

What is a farad?

Answer 24

One farad of capacitance means that the capacitor can store one coulomb of charge per unit

Question 25

What happens as a capacitor is charging?

Answer 25

-pd across capacitor increases from 0 to the maximum value (supply V)

-the charging current decreases from an initial value of E/R to zero

-at all times the sum of the potential difference across the capacitor and the potential difference across the resistor equals the EMF of the supply

-the potential difference across the resistor decreases from an initial value of E to zero when the capacitor is fully charged

Question 26

What happens as a capacitor is discharging?

Answer 26

-The voltage decreases from the max (Vs) to 0

-the discharging current decreases from an initial value of -E/R to zero

-the potential difference across the capacitor is always equal to the potential difference across the resistor

-the potential difference across the resistor (given by ) decreases from an initial value of to zero when the capacitor is fully discharged

Question 27

Factors affecting the rate of charge and discharge?

Answer 27

-The resistance of the resistor (controls size of current)
-The capacitance of the capacitor (larger capacitances take longer to fill/empty)

SUPPLY VOLTAGE WILL NOT HAVE AN EFFECT

Question 28

How can you get a constant current while charging a capacitor?

Answer 28

Use a variable resistor

Current is constant —> Q=It