Chapter 2: Electricity

Question 1

What is voltage/potential difference (V)?

Answer 1

The amount of energy (joules) per coulomb of charge. (or work done by a cell per coulomb of charge transported)

Question 2

What is current (A)?

Answer 2

Amount of charge flowing past any point on a circuit in a given time

Question 3

What is a battery?

Answer 3

2 or more chemical cells (—-+| i—-) in series (—-+| i–| i—-)

Question 4

What is the symbol for a lamp?

Answer 4

Circle with a cross inside it

Question 5

What is a fixed resistor?

Answer 5

A resistor whose resistance does not change (symbol = rectangle)

Question 6

What are the symbols for ammeters and voltmeters?

Answer 6

Circles with A and V inside

Question 7

What is a variable resistor?

Answer 7

A resistor that allows the current to be varied (symbol = rectangle with diagonal arrow)

Question 8

What is a fuse?

Answer 8

A device that melts and breaks the circuit when the current is higher than its rating (symbol = transparent rectangle)

Question 9

What is a diode?

Answer 9

A device that only allows current to flow in one direction (symbol = circle with “play” sign)

Question 10

What is a light-emitting diode (LED)?

Answer 10

A diode that emits light when a current flows through it (symbol = diode symbol with arrows coming off it)

Question 11

What is a light-dependent resistor?

Answer 11

A device with low resistance in bright light and high resistance in dark conditions (symbol = rectangle in circle with arrows going into it)

Question 12

What is a thermistor?

Answer 12

A device with a low resistance at high temperatures and vice versa (symbol = fixed resistor symbol with diagonal line flat at the beginning)

Question 13

What equation links charge flow to current and time?

Answer 13

Q = It
Q = charge (C)
I = current (A)
t = time (s)

Question 14

What happens when voltage is increased but the resistance remains the same?

Answer 14

The current increases (V = IR can be used to answer these types of questions)

Question 15

What is meant by the “voltage” of a battery?

Answer 15

How much electrical potential energy it gives to electrons entering it. The potential difference across a 3V battery is always 3V.

Question 16

What equation links voltage, current and resistance?

Answer 16

V = IR
V = voltage (volts)
I = current (amps)
R = resistance (ohms)

Question 17

What happens to the voltage and the current at different parts of a circuit?

Answer 17

Voltage changes, current stays the same

Question 18

How are ammeters and voltmeters placed?

Answer 18

Ammeters are placed in series, voltmeters are placed in parallel

Question 19

What is an ohmic resistor?

Answer 19

A resistor where, at a constant temperature, the current through the resistor is proportional to the PD across it

Question 20

What does “series” mean?

Answer 20

The current does not split

Question 21

What does “parallel” mean?

Answer 21

The current splits into different paths

Question 22

What are the rules for series circuits?

Answer 22

Same current through each component, voltage is shared between components, the total resistance of two components is the sum of their individual resistances

Question 23

What are the rules for parallel circuits?

Answer 23

Voltage and current are divided evenly among the components, the total resistance of two resistors in parallel is less than the resistance of the smaller individual resistor

Question 24

What is direct current?

Answer 24

Current where the electrons always flow in the same direction

Question 25

What is alternating current?

Answer 25

Current where the electrons change direction multiple times per second

Question 26

How is the alternating current measured?

Answer 26

The frequency is measured in hertz

Question 27

Why is the peak AC potential difference higher than the voltage rating?

Answer 27

To make up for the time where the voltage is close to 0

Question 28

How is mains electricity supplied?

Answer 28

230V alternating current at 50Hz

Question 29

What is the live wire?

Answer 29

The brown wire that carries the AC from the power station (230V)

Question 30

What is the neutral wire?

Answer 30

The blue wire that completes the circuit (<1V)

Question 31

What is the earth wire?

Answer 31

The green and yellow wire whose purpose is to stop an appliance becoming live (0V, only carries a current if there’s a fault)

Question 32

What is earthing?

Answer 32

A method in which an electrical appliance’s metal case is connected to the earth wire, preventing it from becoming live in the case of an accident

Question 33

Why do resistors heat up when charge passes through them?

Answer 33

The electrons collide with atoms in the resistor, converting the electrons’ electrical potential energy to heat energy

Question 34

What is the equation that links energy transferred, voltage and charge?

Answer 34

E = VQ
E = Energy (J)
V = Voltage (V)
Q = Charge (coulombs)

Question 35

What equation links energy, power and time?

Answer 35

E = Pt
E = Energy (J)
P = Power (W)
t = time (s)

Question 36

What equation links power, current and voltage?

Answer 36

P = IV
P = Power (W)
V = Voltage (V)
I = Current (A)

Question 37

What equation links power, current and resistance?

Answer 37

P = RI^2
P = Power (W)
R = Resistance (ohms)
I = Current (A)

Question 38

Why should the current in the overhead wires be reduced as much as possible?

Answer 38

To reduce the power output into the cables (meaning they heat up)

Question 39

What happens to the resistance of a wire as its length increases?

Answer 39

Its resistance increases

Question 40

How is current in transmission cables reduced?

Answer 40

A transformer takes the current from the generator, increases the voltage and decreases the current before it flows through the overhead cables. Near our homes, another transformer step the voltage down to 230V to make it safe.

Question 41

What is static electricity?

Answer 41

Static electrical charge on an insulator

Question 42

How is static electricity created?

Answer 42

When two insulators are rubbed together, electrons are rubbed off one material and onto the other. One material is now positive charged and the other negatively charged.

Question 43

What causes arcs and sparks?

Answer 43

Electrons jumping across the air from one material to another to correct the imbalance in charge

Question 44

What is an electric field?

Answer 44

A field that exerts a force on a charged object

Question 45

What are the properties of electric fields?

Answer 45

A charged object creates an electric field around itself
The electric field is strongest closest to the object
The direction of the field is the direction a positively-charged particle would move
The strength of the field can be increased by adding more charge to the object

Question 46

Why would a human get an electric shock when he touches a live wire?

Answer 46

There would be a potential difference between the wire (230V) and the earthed human (0V), causing a current of charged electrons to flow from the wire to the ground, through the human. This earths the charge.