P2: electricity

Question 1

What is current?

Answer 1

Current is the flow of charged particles through a circuit.

Question 2

What is potential difference?

Answer 2

Potential difference is the measure of energy, per unit of charge, that is transferred between two points in a circuit

Question 3

What is the relationship between current and resistance?

Answer 3

The greater the resistance, the smaller the current for a given potential difference.

Question 4

Give a method for investigating how the length of a wire affects resistance.

Answer 4

Set up the apparatus so that you have a circuit with a power source, two crocodile clips attached to a wire that is attached to a ruler, an ammeter and voltmeter.
Attatch a crocodile clip to the ruler wire at 0cm, and the second at a fixed increment away from it, e.g. 10cm.
Close the switch and then record the current and potential difference of the wire. Open the switch, move the second crocodile clip e.g. 10cm further away (it would now be at 20cm away from the first), then repeat this step along the whole length of the ruler.
Use these measurements to calculate the resistance for each length of wire.
Plot a graph of resistance against wire length and draw a line of best fit.

Question 5

Why is the i-v graph for an ohmic conductor a straight line?

Answer 5

Ohmic resistors have a constant resistance, that is proportional to potential difference: this produces a straight line graph.

Question 6

Why does the i-v graph for a filament lamp look like that?

Answer 6

As the current increases, so does the temperature of the filament, which in turn increases its resistance. This means that less current can flow per unit potential difference, so the graph gets shallower.

Question 7

Why does the i-v graph for a diode look like that?

Answer 7

Current will only flow in one direction through a diode, as the diode has a very high resistance in the opposite direction.

Question 8

What is the relationship between light level and resistance in an LDR?

Answer 8

As one increases, the other decreases.

Question 9

What is the relationship between temperature and resistance in a thermistor?

Answer 9

As one increases, the other decreases

Question 10

How could an LDR/thermistor be used in a sensing circuit?

Answer 10

• Sensing circuits turn off or on depending on the environment they are in.
• Set up the apparatus so you have a circuit with a thermistor, a fixed resistor, and a fan connected in parallel.
• The potential difference of the power supply is shared between the thermistor and the rest of the circuit (with the fan according to their resistances: the higher the resistance, the larger the share of the p.d. they get.
• As the room gets hotter, the resistance of the thermistor decreases, and therefore takes a smaller share of the pd from the power supply. This means that the pd across the fan increases, so it spins. faster.

Question 11

How is potential difference distributed in a series circuit?

Answer 11

Potential difference is shared between the various components. (adds up like resistance).

Question 12

How is current distributed in a series circuit?

Answer 12

Current is the same everywhere in a series circuit.

Question 13

How is resistance distributed in a series circuit?

Answer 13

Resistance adds up.

Question 14

How is potential difference distributed in a parallel circuit?

Answer 14

Potential difference is the same across all components.

Question 15

How is current distributed in a parallel circuit?

Answer 15

Current is shared between the branches of the circuit.

Question 16

What effect does adding a resistor have in a parallel circuit and why?

Answer 16

Adding a resistor in parallel reduces the total resistance, because the current has more than one direction to go in, increasing the total amount of current that can flow around the circuit: using v = ir, this would decrease the total resistance.

Question 17

Give a method for investigating how adding resistors in series affects the resistance of the whole circuit.

Answer 17

• Gather at least four identical resistors. Build a circuit with a power source, an ammeter, and a resistor.
• Write down the potential difference of the battery, and the current reading on the ammeter. Calculate the resistance of the circuit by rearranging v = ir.
• Add another resistor, in series with the first, and repeat steps 2 and 3 until all the resistors are in the circuit.
• Use your readings to plot a graph: it should show that adding resistors in series increases the total resistance of the circuit.

Question 18

Give a method for investigating how adding resistors in parallel affects the resistance of the whole circuit.

Answer 18

• Gather at least 4 identical resistors. Build a circuit with a power source, an ammeter, and a resistor.
• Write down the potential difference of the battery, and the current reading on the ammeter. Calculate the resistance of the circuit by rearranging v = ir.
• Add another resistor, in parallel with the first, and repeat steps 2 and 3 until all the resistors are in the circuit.
• Use your readings to plot a graph: it should show that adding resistors in parallel decreases the total resistance of the circuit.

Question 19

Which kind of current is supplied by the mains in your home, and what is its frequency and voltage?

Answer 19

AC (alternating current)
230 V
50 Hz.

Question 20

What are the three wires in a plug?

Answer 20

Live, Earth, Neutral.

Question 21

What is the colour, role, and potential difference of the live wire?

Answer 21

Brown. 230V. Provides the alternating potential difference from the mains supply.

Question 22

What is the colour, role, and potential difference of the neutral wire?

Answer 22

Blue. 0V. Completes the circuit.

Question 23

What is the colour, role, and potential difference of the earth wire?

Answer 23

Green and yellow. 0V. Protects the wiring, prevents the appliance from becoming live (only carries current when there’s a fault.)

Question 24

How does the national grid work?

Answer 24

Power stations generate huge amounts of thermal energy, and convert it into electrical energy. How much energy is generated depends on demand, which is generally higher in the morning, the afternoon, and cold and dark months.
This huge amount of power has to be transferred all around the country. To do this, a very high voltage and a relatively low current is used, to prevent energy loss through thermal energy transfer (high current = high resistance). Step-up transformers are used to increase the voltage, and decrease the current. Before reaching the end user, a
step-down transformer reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use.