SP10 - Electricity and Circuits

Question 1

What is a series circuit?

Answer 1

A circuit which has one route the current can take (so if a component breaks, the rest of the components all stop working).

Question 2

What is a parallel circuit?

Answer 2

A circuit where there are junctions that allow the current to take different routes (so if a component breaks, the other components on that branch stop working, but components on other branches continue to work).

Question 3

What is current?

Answer 3

The rate of flow of charged particles (how much charge is flowing in a certain amount of time). The unit for current is amperes (often shortened to amps) (A). It is measured using an ammeter.

Question 4

What is potential difference?

Answer 4

Potential difference is the thing that ‘pushes’ current around a circuit. It is the energy transferred to or from a coulomb of charge when it flows between two points. The potential difference of a cell is the amount of potential energy the cell supplies to each coulomb of charge flowing through it. It is also called voltage and is measured in volts (V). It is measured using a voltmeter.

Question 5

What is charge?

Answer 5

Charge is the sum of the positively and negatively charged particles an object contains. Charge is measured in coulombs (C).

Question 6

What is the equation for charge?

Answer 6

Charge (C) = current (A) x time (s)
Q = I x t

Question 7

What is the equation for energy transferred?

Answer 7

Energy transferred (J) = charge moved (C) x potential difference (V)
E = Q x V

Question 8

How is energy transferred in a circuit?

Answer 8

• A cell contains a store of energy
• energy is transferred to the charge
• the charge can now transfer energy to the components in the circuit (it has potential energy)
• energy is transferred from the charge as it moves through the components
• the components transfer energy to the surroundings (often by heating, sometimes by light or sound, depending on which component it is)

Question 9

What are the circuit rules for series circuits?

Answer 9

• current is the same everywhere
• current will reduce as more components are added
• potential difference from the battery is shared between components, so will reduce as more components are added
• if one component breaks, the circuit no longer works

Question 10

What are the circuit rules for parallel circuits?

Answer 10

• current is the same entering and leaving the circuit
• current splits at a junction, so current on each branch adds up to the total entering the circuit
• the more branches, the more current on each branch
• potential difference is the same on each branch but shared between components on the branch
• if one component breaks, the components on other branches will still work

Question 11

What is resistance?

Answer 11

It is a measure of how hard it is for a current to flow. It is measured in ohms. Any component increases the resistance of a circuit. More resistance = less current.

Question 12

What is the equation for resistance?

Answer 12

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

Question 13

What is a resistor?

Answer 13

A resistor increases the resistance of a circuit. The potential difference from a cell is shared between resistors, but it may not be shared equally. There will be greater potential difference across resistors with higher resistances.

Question 14

What is the IV graph like for a resistor at constant temperature?

Answer 14

• linear relationship
• as potential difference increases, current increases
• current and potential difference are directly proportional
• positive correlation
• resistance = potential difference / current
• obeys Ohm’s law

Question 15

What is the IV graph for a filament lamp like?

Answer 15

• current and potential difference are not directly proportional (current stops increasing)
• positive correlation
• non-linear relationship
• does not obey Ohm’s law
• the lamp gets hot as current passes through it, which increases the resistance of the lamp.

Question 16

What is the IV graph of a diode like?

Answer 16

• the diode has a very high resistance in one direction
• this means that the current can only flow in the other direction
• graph is non-linear

Question 17

What is a thermistor?

Answer 17

A resistor that changes its resistance depending on temperature. Most thermistors decrease their resistance as current increases (low temperature = high resistance = low current, high temperature = low resistance = high current). They are often used in fire alarms or freezer buzzers to detect temperature change.

Question 18

What is a light-dependent resistor?

Answer 18

LDRs are resistors which change their resistance based on light levels. Their resistance decreases as light intensity increases (dark = high resistance = low current, bright = low resistance = high current). Often used to detect light levels in automatic security lights and light-up road signs.

Question 19

What is the equation for electrical power?

Answer 19

P = I x V

Electrical power (W) = current (A) x potential difference (V)

Question 20

Why do resistors get hot?

Answer 20

An electric current in a resistor causes an energy transfer by heating, because as electrons move they collide with ions in the metal lattice. The collisions transfer energy by heat to the lattice, so the wires heat up.

Question 21

How can the heating effect be reduced (by reducing resistance)?

Answer 21

• low resistance wires (e.g. copper and aluminium)
• thicker wires (more current can flow, therefore less resistance)
• cooling metals to reduce vibrations in the lattice, therefore reducing collisions

Question 22

What is the equation for energy transferred?

Answer 22

E=IxVxt
Energy transferred (J) = current (A) x potential difference (V) x time (s)

Question 23

What is the equation for electrical power (not involving potential difference)?

Answer 23

P=I^2xR
Power (W) = current^2(A^2) x resistance (ohms)

Question 24

What is a direct current (d.c)?

Answer 24

The direction of the current and the movement of the charge stays the same.

Question 25

What is an alternating current (a.c.)?

Answer 25

Electrons vibrate, passing on energy. The direction of the current changes in alternating currents, as does the voltage, which increases to a peak voltage then decreases to zero repeatedly.

Question 26

What is the frequency and voltage of mains electricity in the UK?

Answer 26

50 Hz
230 V

Question 27

What colours are the wires in a plug and what do each of them do?

Answer 27

Live wire (brown) - connects the appliance to the generators at the power station - voltage 230V.
Neutral wire (blue) - the return path to the power station - voltage 0V
Earth wire (green and yellow) - connect the metal parts of the appliance to a large metal spike or metal tubing that is pushed into the ground (safety feature) - 0V

Question 28

What is a fuse and how does it work?

Answer 28

A fuse is an electrical safety feature connected to the live wire in a plug. It is a thin metal wire in a glass tubing. If the wire gets too hot, the wire melts, braking the circuit. This means that the current of the circuit never exceeds a safe level.

Question 29

What does the earth wire do?

Answer 29

It connects the metal parts of the appliance to the ground so if there is a short circuit (surge in current caused by a very low resistance pathway being created) the current goes into the ground instead of through you.

Question 30

What is a circuit breaker and what does it do?

Answer 30

Circuit breakers are an alternative to fuses. They detect a change in the current and safely switch off the supply. Unlike fuses, they do not have to be replaced every time they are used.

Question 31

Core practical - investigating resistance

Answer 31

• set up a circuit with a power supply, an ammeter, a resistor and a voltmeter
• set the power pack to its lowest voltage and switch it on. Write down the readings on the ammeter and voltmeter, then switch the power pack off
• repeat for five different voltage settings, up to a maximum of 6V
• replace the resistor in the circuit with two filament lamps and repeat the above method
• repeat using a parallel circuit, with two filament lamps, an ammeter and a voltmeter on each branch.