P4

Question 1

What equation links charge, current and time?

Answer 1

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

Question 2

What equation links potential difference, current and resistance?

Answer 2

V (potential difference, V) = I (current, A) x R (resistance, ohms)

Question 3

What is an ammeter?

Answer 3

Measures the current in amps flowing through the wire. It must always be placed in series with whatever you are measuring.

Question 4

What is a voltmeter?

Answer 4

Measures the potential difference in volts across the wire. It must always be placed in parallel around whatever you’re investigating.

Question 5

Describe the graph and I-V characteristics of ohmic conductors

Answer 5

Ohmic conductors = a wire or a resistor at a constant temperature

The current is directly proportional to the potential difference so you get a linear graph with a straight line. Because it’s at a constant temperature, there is no resistance.

Question 6

Describe the graph and I-V characteristics of a filament lamp (or bulb)

Answer 6

Some energy is transferred to the thermal energy store of the filament which is designed to heat up.
As the current increases, the temperature of the filament increases so the resistance increases.
This means that less current can flow per unit pd so the graph gets shallower, hence the curve.

Question 7

Describe the graph and I-V characteristics of a diode.

Answer 7

They will happily let current flow in one direction so the resistance is very high in the reverse direction.

Question 8

What is an LDR?

Answer 8

It is a light dependent resistor.
It is dependent on the intensity of light so in bright light the resistance falls and in darkness the resistance is very very high.
They are used in automatic night lights, outdoor lighting and burglar detectors.

Question 9

What is a thermistor?

Answer 9

It is a temperature dependent resistor.
When it’s hot, the resistance drops and in cooler conditions, there is the most resistance.
They are used for temperature detectors like a car engine temperature sensors and electronic thermostats.

Question 10

What is a sensing circuit?

Answer 10

turn on or increase the power of components depending on the conditions they are in

Question 11

What is a series circuit?

Answer 11

components are connected in a line between the +ve and -ve of the power supply (except for voltmeters which are always connected in parallel)

• total pd is shared between the various components
• same current flows everywhere
• resistance just adds up
• cell pd add up

Question 12

What is a parallel circuit?

Answer 12

each component is separately connected to the power supply (except for ammeters which are always connected in series)
disconnecting one won’t really affect the others
it is how most things are connected in everyday life
- pd is the same across all components
- current is shared between branches
- adding a resistor in parallel reduces the total resistance

Question 13

What are the results of the experiment which involves adding more and more resistors to series and parallel circuits?

Answer 13

the graph of a series circuit shows a positive straight line - more resistors means more overall resistance of the circuit

the graph of a parallel circuit shows a negative curve - more resistors and more loops increases the total current of the circuit which results in a decrease in total resistance of the circuit

Question 14

What is ac and dc?

Answer 14

alternating current
current is constantly changing direction because the voltage is constantly changing direction
\+ve and -ve ends keep switching
UK mains supply = an ac supply at 230V
frequency of UK mains supply = 50 Hz 
cells and batteries supply dc

Question 15

What is the composition of plugs?

Answer 15

three wires inside, each with a core of copper and a coloured plastic coating: the live, neutral and earth wire.

Question 16

What is the live wire?

Answer 16

bottom right
brown
alternating potential difference
230V

Question 17

What is the neutral wire?

Answer 17

bottom left
blue
0V
completes the circuit
current flows when working normally

Question 18

What is the earth wire?

Answer 18

top
green and yellow
safety
stops the casing of the appliance becoming live
only carries a current when there is a fault
0V

Question 19

What causes an electric shock?

Answer 19

body=0V (like the earth)
the live wire has a current flowing so when there is contact, there is a large pd which causes a shock or a spark

if any link creates a low resistance path from the live wire to the earth, a huge current would flow

Question 20

What are step-up and step-down transformers?

Answer 20

step up= power stations to the national grid - make the size of the ac voltage much bigger

step down= national grid to consumers 230v mains electricity in homes

Question 21

What equation links power, energy and time?

Answer 21

P= E/t

Question 22

What equation links power, pd and current?

Answer 22

P=VI

Question 23

What equation links power, current and resistance?

Answer 23

P=IIR (current is squared)

Question 24

What is a fuse?

Answer 24

between the live pin and the live wire
if too much current passes through, it melts and the live wire is cut off
they come as 3A, 5A or 13A

Question 25

What is static electricity and how does it work?

Answer 25

insulating materials rubbed together
electrons scraped off one and dumped on the other
materials electrically charged (one +ve and one -ve but they are equal)

polythene and acetate rods being rubbed with a cloth duster

Question 26

How does static electricity cause sparks?

Answer 26

electric charge builds up, pd increases between earth and object
when large enough pd, electrons jump the gap (attraction of opposite charge)