Physics - electricity

Question 1

What is the purpose of a resistor?

Answer 1

Restricts the flow of electrical current

Question 2

What is the purpose of a variable resistor?

Answer 2

Restricts the flow of electrical current to a varying degree

Question 3

What is the purpose of a thermistor?

Answer 3

Provides a higher resistance at low temperatures, and vice versa

Question 4

What is the purpose of a light-dependent resistor?

Answer 4

Provides a higher resistance at lower light levels, and vice versa

Question 5

What is the purpose of a switch?

Answer 5

To turn the circuit on (closed) and off (open)

Question 5

What is the purpose of a diode?

Answer 5

Current can only flow through it in one direction

Question 6

What is current?

Answer 6

Rate of flow of charge

Question 7

How do you measure voltage in a component?

Answer 7

Place a voltmeter in parallel around it

Question 8

How do you measure current in a circuit?

Answer 8

Place an ammeter in it, in series

Question 9

What is the equation linking voltage, current and resistance?

Answer 9

Potential difference = current x resistance ( V = IR)

Question 10

How does a wire’s length affect its resistance?

Answer 10

The longer the wire, the more resistance

Question 11

Describe a practical to test how wire’s length affects its resistance. Detail steps to visualise the effects.

Answer 11

Create a circuit with a cell, an ammeter in series and a voltmeter connected in parallel around a gap in the circuit. Place 2 crocodile clips on the ends of that gap, 10cm a wire connected in between them.
Take readings of the current and voltage. Move one of the crocodile clips along so they hold 10cm more wire, take the same readings, and repeat that until you have readings up to 60cm.
For each length of wire, multiply current by voltage to get resistance, and plot the results in a table. Graph them.

Question 12

Describe a practical to investigate the relationship between potential difference and current for a bulb, (fixed) resistor and diode.

Answer 12

Connect a circuit with a cell, ammeter, variable resistor and bulb in series, and a voltmeter in parallel around the bulb. Take a reading of the voltage and current. Use the variable resistor to increase potential difference, and take the readings again. Repeat that 3 more times. Reverse the power supply connections, and take the 5 lots of readings at different potential differences again again.
Repeat this but with a resistor instead of a bulb, and then a diode instead.

Question 13

Evaluate the potential difference-current relationship for a resistor

Answer 13

Potential difference is directly proportional to current. In a graph, it’s a straight line with the equation y = x. Ohmic.

Question 14

Evaluate the potential difference-current relationship for a filament bulb

Answer 14

There is a positive correlation between potential difference and current but it’s not linear. In a graph, the line curves up to the origin and curves in the other direction out of it; the closer to the origin, the steeper the curve. Non-ohmic.

Question 15

Evaluate the potential difference-current relationship for a diode

Answer 15

On the graph, the current is at zero as the voltage increases, until at some point after 0 on the x axis, the current shoots up (in a straight line). Non-ohmic.

Question 16

State Ohm’s law

Answer 16

V = IR (potential difference = current x resistance)

Question 17

Why does resistance increase as a filament bulb heats up?

Answer 17

Because metals contain delocalised electrons, which collide with the ions and makes them vibrate, and increasing temperature makes collisions and vibrations more frequent. This increases resistance because it’s more difficult for the electrons to flow through.

Question 18

What are the rules for current and potential difference in series and parallel circuits?

Answer 18

In series, current is the same in all places and potential difference is split between all components. Total resistance = sum of all resistances.
In parallel, total potential difference is the same across every branch and current is split between the branches. Adding more resistors in parallel reduces the total resistance.

Question 19

Explain the difference between AC and DC.

Answer 19

DC (direct current) only flows in one direction. AC (alternating current) constantly switches the direction it flows in.

Question 20

Does mains electricity use AC or DC?

Answer 20

AC

Question 21

What is the standard mains potential difference and current?

Answer 21

230V, 50Hz

Question 22

Describe the colour of the neutral, live and earth wire

Answer 22

The neutral wire is blue; the live wire is brown; the earth wire is yellow and green.

Question 23

Explain two safety measures used in plugs.

Answer 23

Fuse: Connected to the live wire, melts if the current gets too high
Earth wire: In case the other wires fail, current flows through the earth wire instead of to a person who touches it.

Question 24

Describe how and why step-up and step-down transformers are used in the national grid.

Answer 24

Due to the high current produced from the power station, a large amount of energy would be lost as heat, especially since the cables have a high resistance. So, a step-up transformer is used to increase voltage, which decreases current because the two are inversely proportional, reducing energy loss. However, a voltage that high would be dangerous in homes, so a step-down transformer is used to decrease voltage again.

Question 25

Explain why, after being rubbed on a jumper, a balloon sticks to a wall.

Answer 25

When two insulators are rubbed together, electrons are transferred across, leaving an excess of negatively charged electrons on one object (the balloon). Since like charges repel and opposite charges attract, when the balloon is placed against a wall, its negative charge repels the electrons and is attracted to the positively charged protons, sticking to them.