Topic 6 - Electricity

Question 1

Power equations

Answer 1

Power (W) = current (A) × voltage (v)
P = IV

Power (W) = Work done (J) / time (s)
P = W/t

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

Question 2

Voltage equations

Answer 2

Voltage (v) = current (A) × resistance (Ω)
V = IR

Voltage (v) = energy (J) / charge (C)
V = E/Q

Question 3

Charge equation

Answer 3

Charge (C) = current (A) × time (s)
Q = It

Question 4

Energy equations

Answer 4

Energy (J) = charge (C) × voltage (v)
E = QV

Energy (J) = current (A) x voltage (v) x time (s)
E = IVt

Question 5

Current

Answer 5

The rate of flow of charge. Current flows from the positive terminal to the negative terminal of a cell.

Question 6

Voltage

Answer 6

The “electrical pressure” that drives the current around the circuit. It is the energy transferred per unit charge passed.

Question 7

Resistance

Answer 7

Anything in the circuit that slows the flow of current down. The higher the resistance of a circuit, the lower the current.

Question 8

Ohm’s Law

Answer 8

Current is directly proportional to voltage across a fixed resistor and constant temperature

Question 9

Diode

Answer 9

An electronic component that conducts current in primarily in one direction. It has low resistance in one direction and high resistance in the other.

Question 10

Thermistor

Answer 10

A temperature-dependent resistor.

↑ temperature, ↓ resistance, ↑ current

The graph is NON-LINEAR

Question 11

LDR

Answer 11

A light-dependent resistor - changes resistance depending on light intensity.

↑ light intensity, ↓ resistance, ↑ current

Can be used as a sensor in cameras or automatic lights that come on when it gets dark.

Question 12

Ammeter

Answer 12

A device that measures the strength of current flowing through a component. It must be placed in series (anywhere in the main circuit) but never in parallel.

Question 13

Voltmeter

Answer 13

A device that measures the voltage across the component. It must be placed in parallel around the component under test

Question 14

Variable resistor

Answer 14

A type of resistor used to control the value of current and voltage by changing the resistance

Question 15

A.C. supply

Answer 15

A current that continuously changes its direction, going back and forth around a circuit. It is produced by electrical generators (e.g. mains electricity)

Question 16

D.C. supply

Answer 16

A current that is steady, constantly flowing in the same direction in a circuit, either positive or negative. It is produced by cells and batteries

Question 17

LEDs

Answer 17

Light-emitting diodes emit light when a current flows through them in the forward direction. They can be used to indicate the presence of current in a circuit.

Question 18

Series Circuits

Answer 18

• Components are connected in a line (except voltmeter)
• If one component disconnects the whole circuit is broken
• Vtotal = V1 + V2 + V3… = voltage of battery
• Rtotal = R1 + R2 + R3…
• Itotal = I1 = I2 = I3…
• The more components, the lower the current

Question 19

Parallel Circuits

Answer 19

• If one component disconnects the circuit still works
• Vtotal = V1 = V2 = V3
• Itotal = I1 + I2 + I3…
• Rtotal = 1/R1 + 1/R2 + 1/R3…
• The way the current splits depends on the resistance of each individual component
• More components in parallel = lower total resistance

Question 20

Fuse

Answer 20

A safety device designed to cut off the flow of electricity to an appliance if the current becomes too large. The wire inside the glass cylinder heats up and melts.

Question 21

Earthing

Answer 21

Used to protect you from an electric shock. It does this by providing a low resistance path for a fault current to flow to earth

Question 22

Circuit breaker

Answer 22

An electrical safety device used in some circuits to protect it from damage if too much current flows. It breaks the circuit by opening a switch.

Question 23

Electrical power

Answer 23

the rate at which an appliance transfers energy

Question 24

Static electricity

Answer 24

The build up of charge on insulating materials, which can cause sparks when discharged. A common cause of static electricity is friction

Question 25

Spark

Answer 25

Electrons jumping across the gap between the charged object and the earthed object due to large voltage difference.

Question 26

Van de Graaff generator

Answer 26

A machine that causes friction between a rubber belt and plastic rollers in order to build up positive electrostatic charge on a metal dome. 1. When the belt rubs against the lower roller, friction is created, causing electrons to move from the belt to the roller. The belt is left with a positive charge. 2. Electrons from the roller repel electrons away from the tip of the comb, leaving the comb with a positive charge 3. The comb is attracted to the negative roller, but the positive ions cannot reach the roller due to the belt being in the way 4. Therefore, the positive ions get transported up the belt to the dome

Question 27

Insulation

Answer 27

When an insulating material (e.g. rubber) is used to cover metallic wires to cancel the risk of electrocution. Double insulation - insulation around the wires + a non-metallic case for a second layer

Question 28

Movement of electrons in electrostatic experiments

Answer 28

- All objects are initially electrically neutral - When electrons are transferred through friction, one object becomes negative and the other positive - These objects are attracted to other neutral objects by repelling like charges so the opposite charges are in front

Question 29

Use of electrostatic charge in photocopiers

Answer 29

- An image of the document is projected onto a positively charged copying plate - Where light falls onto the plate, the electrical charge leaks away - Negatively charged black toner particles are attracted to the remaining positive areas - Toner is transferred to a piece of paper and heated to make it stick - Paper is taken off the plate and turned over to reveal text

Question 30

Use of electrostatic charge in inkjet printers

Answer 30

- A nozzle makes a fine spray of tiny ink droplets, which are then given an electrostatic charge - The charged droplets can be directed, using pairs of charged plates, with great precision to form letters and images on paper

Question 31

Refuelling an aircraft

Answer 31

- There is buildup of charge when the fuel + pipe (insulators) rub against each other - This can cause an explosion because fuel can get ignited by the spark - You can join a wire between the metal plane and ground, which allows electricity to flow along, preventing the buildup of static charge on the plane - Make nozzles out of metal so charge is conducted away

Question 32

Saw Mill

Answer 32

- Static electricity from electrical equipment can create sparks - Sawing wood creates fine wood dust - If this dust gets dispersed, the spark can land on it and create an explosion in the enclosed space

Question 33

Volt

Answer 33

Joule per coulomb