Section 2: Electricity

Question 1

Current

Answer 1

Rate of flow of charge (usually electrons) around a complete circuit

Current is measured in amps (A) with an ammeter in series

Question 2

Voltage

potential difference

Answer 2

Energy carried around a circuit by electrons

v = amount of energy transferred per coulomb of charge
1 v = 1 joule per coulomb

Measured in volts (v) with a voltmeter in parallel

Question 3

Current and electrons

Answer 3

Current is always from positive to negative

Electrons in a circuit always flows away from the negative terminal to the positive

Question 4

Battery voltage

Answer 4

Sum of voltage of all other components

Question 5

Draw symbols for:

• cell/battery
• switch
• ammeter
• voltmeter
• bulb
• LDR
• thermistor
• resistor
• variable resistor

Answer 5

check it

Question 6

Equation for current

Answer 6

current = charge / time
I = Q/t

Question 7

Equation for voltage

Answer 7

voltage = energy / charge
V = E/Q

Question 8

Resistance

Answer 8

Resists the flow of the current

Measured in ohms Ω

Question 9

Light dependent resistor

Answer 9

Resistance decreases as light increases, depends on light levels

• In bright light, resistance is low, more current can flow through
• In darkness, resistance is the highest

Useful for various electronic circuits e.g automatic night light and burglar detectors

Question 10

Equation for resistance

Answer 10

resistance = voltage / current

Ω V A

Question 11

Equation for power

Answer 11

power = voltage x current
(watts)
or joules per second

Question 12

Thermistor

Answer 12

Resistance decreases as temp increases, depends on temp

• low temp, resistance high
• high temp, resistance low

Useful for temp detectors e.g fire alarms, thermostats

Question 13

2 things for a circuit to work

Answer 13

1. Complete circuit
   wires runs from battery all the way
2. No short circuits
   no way past a lamp going through other components
   (otherwise lamp will not light)

Question 14

Series Circuit

-current and voltage

Answer 14

1. All components are connected in a line, end to end
2. If one component is removed the circuit is broken and all stops working

-Current is the same everywhere A1 = A2 = A3

-Potential difference is shared between all components
V = V1 + V2
voltage round a series circuit adds up to voltage source

-Total resistance is the sum of all individual resistors
R = R1 + R2 + R3

• Cell voltages add up
  e. g two 1.3 V cells in series would supply 3V in total

Question 15

Parallel Circuit

-current and voltage

Answer 15

1. Each component separately connected to the battery
2. If one component is removed it doesn’t affect all others

-Current is shared between branches
A = A1 + A2
-There are junctions where current split or rejoins, total current into junction = leaving junction
-Total current = total current of separate branches

-Potential difference the same across all components
V1 = V2 = V3
-All components gets the full p.d, so identical bulbs are the same brightness

• Resistance is always less than that of the branch with the smallest resistance
• Resistance lower because current has more than one branch to take
• A circuit with 2 resistors in parallel will have a lower resistance than a circuit with either of the resistors by themselves

Question 16

Current - voltage graph for resistors

V-I graphs

Answer 16

-Current through a resistor (at constant temp) is directly proportional to the voltage (Ohms law)
- Steeper the slope, the lower the resistance
R = V / I

-x-axis : potential difference
-y-axis : current
- straight line through, x = y
(see graph)

Question 17

V - I graph for bulbs

Answer 17

• Bulbs do not follow ohms law
• The resistance increases as the temp increases, so it’s not directly proportional
• x-axis : potential difference
• y-axis : current
• ’s’ shape

(see graph)

Question 18

V - I graph for diodes

Answer 18

• Used to regulate voltage in circuits and make logic gates
• Has very high resistance in one direction, so current can only flow in other direction
• x-axis : potential difference
• y-axis : current
• flat and then a steep rise

(see graph)

Question 19

What can be used to indicate the presence of a current in a circuit?

Answer 19

lamps and LEDs

Question 20

What happens to the current if I increase the resistance ?

What if i decrease it?

Answer 20

increasing resistance will decrease the current, this is achieved by adding more components or components with higher resistance

-increase the current, achieved by removing or replacing with components of a lower resistance

Question 21

Draw a series circuit

draw a parallel circuit

Answer 21

check it

Question 22

What is voltage in terms of energy transfer?

Answer 22

amount of work done per unit of charge

Question 23

What is a series circuit useful for?

Answer 23

everything is connected in a line so the voltage is shared out between components.

• useful for low powered things like fairy lights
• however if one component breaks then whole circuit stops working

Question 24

What is a parallel circuit useful for?

Answer 24

different components connected separately to supply

• practical because if one component breaks the circuit still works
• better for higher powered thing due to p.d being same for all components
• however power supply used quickly

Question 25

What is the risk of water in domestic electricity?

Answer 25

water conducts electricity and energy from circuit can flow through it, creating a fire and electrocution risk

Question 26

What is the risk of frayed cables/damaged plugs in domestic electricity?

Answer 26

the insulation has wore down, exposing live wires, electricity can be conducted

Question 27

What is the risk of over loading in domestic electricity?

Answer 27

can cause fire

Question 28

What are the features of a plug? | Label it

Answer 28

- plastic casing which is an insulator - 3 pins made from brass which is a good conductor - fuse between live terminal and pin. This breaks if the current is too high - cable secured in the plug by cabe grip, which should grip the cable itself, and not individual wires neutral wire : blue earth wire : green/yellow stripes live wire : brown

Question 29

what equation connects energy, current, time and voltage

Answer 29

energy(J) = current(A) x voltage(V) x time(s)

Question 30

What is the use of the earth wire ?

Answer 30

- many electrical appliances such as cookers, fridges have metal casing. - earth wires creates the path of low resistance for current to flow if live wire touches the casing so an electric shock is avoided

Question 31

What is the use of the fuse?

Answer 31

- made from a metal that has a low melting point so if a large current flows through the circuit and the fuse wire gets very hot the metal melts which breaks the circuit - prevents electric shock and fires - needs to be replaced once used

Question 32

What are the use of circuit breakers?

Answer 32

if too large a current flows through a circuit then a switch opens making the circuit incomplete. once fault in circuit corrected, the switch is reset by pressing a reset button

Question 33

What is the use of live wires?

Answer 33

provides the path which electrical energy from power station travels

Question 34

what equation connects power, current and voltage?

Answer 34

``` power(W) = current (A) x voltage(V) P = I x V ```

Question 35

How to choose the correct fuse size to be fitted to a device?

Answer 35

- Fuses come in standard ratings of 3A, 5A and 13A - chose one which allows the correct current to flow but blows if it gets a bit larger - e.g if correct current in circuit is 4A then choose a 5A fuse

Question 36

What is the use of insulation?

Answer 36

- covering live wire with a material that doesnt conduct - Double insulation is when all electrical parts of an appliance is insulated so that the user cannot touch it. e.g when an appliance has a casing made of plastic

Question 37

Prove E = IVt from V = E/Q and Q =It

Answer 37

``` E = Q x V E = (I x t) x V E = I x t x v ```

Question 38

What equation connects charge, current and time?

Answer 38

charge (coulombs) = current(A) x time(s) | Q = I t

Question 39

What is Ohm's law?

Answer 39

``` V = I x R voltage(V) = current(A) x resistance (Ω) ```

Question 40

How does changing resistance affect the current in a circuit?

Answer 40

Increasing the resistance will decrease the current | Decreasing the resistance will increase the current

Question 41

What's an experiment to investigate Current and Voltage characteristics?

Answer 41

Have a circuit with a voltmeter around a lamp and an ammeter. - turn power supply up till p.d across lamp is 12 - record p.d + current - calculate resistance r = V/I - change voltage to see relationship

Question 42

What is the difference between direct current (d.c) and alternating current (a.c)?

Answer 42

d. c: - current flows in only one direction, such as when supplied by batteries and cells - line on graph is flat a. c: - current constantly changes directions, such as when supplied by Mains electricity - It has the frequency of 50 Hz, which means it changes direction, and back again 50 times a second - line on graph like a transverse wave - p.d of the live terminal varies between large + ve and -ve value - p.d of neutral terminal is close to 0 - although the mean voltage is about 230 V, the peak voltage is higher