Physics - electricity

Question 1

Properties of a circuit

Answer 1

Current: the rate of flow of charge
Charge: usually in the form of electrons
Voltage, Potential difference: electrical pressure, what pushes the current from one end of the circuit (+ve) to the other end (-ve)
Resistance: what opposes voltage and potential difference, voltage decreases if there is more resistance

Question 2

The standard test circuit

Answer 2

The standard test circuit
Ammeter: measures the current across the circuit (in amps)
Voltmeter: placed in parallel to the component (measures the voltage of the current)
Ammeter, component and a variable resistor placed in series from one end of the battery to the other
A voltmeter is placed in parallel to the component

Question 3

Types of current

Answer 3

Types of currents
Direct current: the current travels in only one direction (e.g. + to -)
Alternating current: current travels in different directions

Question 4

IV graphs

Answer 4

Current voltage graphs
Current voltage graph of wire: current is directly proportional to voltage
IV graph of resistor: current also directly proportional to voltage, but there can be different slopes as some resistors have more resistance and some have less
IV graph of a diode: current can only travel in one direction and will not travel in the other (0 gradient on negative side)
IV graph of metal filament lamp: slope of graph changes as when temperature increases the resistance also icnreases

Question 5

LEDs

Answer 5

LEDs emit light when a current passes through them. They indicate the presence of a current in a circuit

Question 6

Light dependant resistors

Answer 6

When there is high light intensity, resistant decreases. When light intensity is low, the resistant increases.
Example: burglar detectors

Question 7

Thermistors

Answer 7

Thermistors (temperature dependant resistor)

When temperature is high, resistance decreases, when temperature is low resistance increases

Question 8

Series circuits

Answer 8

Entire circuit is connected along the same line from either side of the battery (+ve and -ve)
Current is the same across the entire circuit
Size of current determined by the total resistance and the total potential difference across the circuit: V=IR
Total PD or voltage of supply is shared between the components of the circuit, pd for each component depends on its resistance
If one part or component of the circuit stops working the entire circuit will shut down
Resistance of each component = total resistance of entire circuit
There is a bigger supply potential difference if you connect more cells in series eg: 2 batteries each with a voltage of 1.5 connected in series will supply a total of 3V)

Question 9

Parallel circuits

Answer 9

Parallel Circuit is in branches, each component is separately connected to the +ve and -ve of the supply, except for the ammeter which is always connected in series
Potential difference or voltage is the same across all the branches
Current is shared between the branches. Total current flowing around circuit is equal to the total of all the currents through the separate components
If one branch, part of the circuit shuts down the other branches are not affected
If resistance is higher in one of the branches, less current passes through that branch. If resistance is same across every branch then current is distributed equally across the branches
In parallel circuit there are junctions where current splits or re-joins, the total current going into a junction equals the total current leaving it.

Question 10

What is charge

Answer 10

Charge is usually carried by negatively charged electrons.
Charge is measured in coulombs
When current travels through a point after a certain amount of time, the charge can be calculated as:
Q = It | Charge = current x time

Question 11

How does a charge transfer energy

Answer 11

When a charge goes through a component in which there is a change in voltage, energy is transferred.
Energy is supplied to the charge at the power source
Components that lead to higher drops in voltage cause more energy to be transferred.
A battery with a higher initial voltage will cause more energy to be transferred as

Voltage (volts) can also be measured as joules per coulomb of charge

Energy transferred = charge x voltage E = QV
Energy transferred = charge x current x resistance E = QIV

Question 12

Wires in a plug

Answer 12

There are 3 wires in a plug:
Live wire: alternates between +ve and -ve at a 230V
Neutral wire: 0V
Earth wire: safety measure, appliances with metal parts need to reduce the danger of electric shock. Earthing means to attach the appliance to an Earth wire. An earthed conductor can never become live.

Question 13

DOuble insulation

Answer 13

when the plug has a plastic casing around it, and no metal parts

Question 14

Fuses

Answer 14

Melts when it detects that there is a big surge in current in the live wire after a fault has occurred (E.g. Live wire touched metal casing.) The Fuse cuts off the live supply, isolating the whole appliance making it impossible to get an electrical shock

Question 15

Circuit breakers

Answer 15

better then fuses as if it detects a surge in current, a switch is opened and the circuit is broken. This requires less time and isiolates the appliance faster then the fuse. It is also reusable and does not have to be replaced, it can be reset by flicking a switch

Question 16

What happens when a na electric current passes through a resistor

Answer 16

Resistors get hot when there is an electric current - transferring energy
This happens because the electrons collide with the ions in the lattice that make up the resistor. This gives the ions more energy causing the ions to heat up.
Heating effect increases the resistor’s resistance so less current will flow, or a greater voltage will be needed to produce the same current

Question 17

Power

Answer 17

Power is the rate at which energy is transferred
Power is measured in volts
Power = current x voltage | P= IV
An appliance with high power rating transfers a lot of energy in a short time
This energy comes from the current flowign through it. Meaning than an appliance with. high power rating will draw a large current from the supply

Question 18

Fuse current ratings

Answer 18

Fuses have current ratings and should be rated as near as possible but just higher than the normal operating current

Question 19

How much Energy is transferred

Answer 19

The energy transferred by an appliance depends on the power of the appliance and how long it is on for. Energy transferred = electrical power x time
Energy transferred = current x voltage x time

Question 20

Conductors and insulators

Answer 20

Materials that are electrical conductors conduct charge easily - a current can flow through the,
Electrical insulators don’t conduct charge well so a current can’t flow

Question 21

Static electricity

Answer 21

Static electricity or charge is Caused by the rubbing of two insulators creating friction, when two insulators rub onto each other, electrons are dumped from one insulator to the other, causing one of the insulators to become negatively charged and the other to become equally positively charged.

As charge builds up voltage also builds up. As charge builds up on an isolated object, the voltage between this object and the ground (at zero volts) increases, if the voltage gets large enough electrons can jump across the gap between the charged object and the earth - this is the spark. They can also jump to any earthed conductor that is nearby which si why you experience static shocks.

Question 22

How is static electricity investigated

Answer 22

Usually investigated with polythene rod and acetate rods being rubbed with a cloth duster.
When the polythene rod is rubbed with the duster, electrons move from the duster to the polythene rod making it negatively charged. When the acetate rods are being rubbed with a cloth duster, the electrons move from the acetate rods to the duster, making the acetate rod positively charged.

Question 23

Gold leaf electroscope

Answer 23

Attach a disk to a metal rod, attach two thin pieces of gold leaf to the metal rod.
When the metal disk is contacted by the rods, electrons will either be attracted or repelled from the metal disk depending on the charge of the rod.
This induces a charge in the metal disk which induces a charge in the gold leaves, the gold leaves will both be induced the same charge and will repel each other causing them to rise.
However when u detatch the gold leaves from the metal rod they will both fall back down and discharge
If the foil does not rise when the rod is brought near the disc the rod is not charged

Question 24

Suspending a charged rod

Answer 24

Suspending a known charged rod:
Suspend a rod of known charge, and bring test rod near it. If the two rods attract then they are of opposite charge, if they repel they are of like charge

Question 25

Van de graff gen

Answer 25

Inside: rubber belt constantly rubbing over plastic rollers underneath a metal dome, creating friction and generating static electricity, this charge is built up in on the dome as the belt goes around. if you stand on an insulated chair and put your hands on the dome, electrons will move between your body and the dome, giving oyur body a charge. Thhe human body conducts charge, and like charges repel so the charges will spread out as much as possible throughout your body

Question 26

Inkjet printers

Answer 26

Inkjet printer:
Droplets of Ink forced out of a fine nozzle giving them a charge
Droplets of ink pass through 2 charged plates, one positive and one negative, as droplets of ink passes through the 2 charged plates they are deflected from one and attracted to one. size and direction of the voltage across each plate changes so each droplet is deflected to hit a different place on the paper.