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# Elect Know @ AJHS NZ

### knowledge questions for Yacapaca too

A current will flow through an electrical component only if there is . . . a VOLTAGE or POTENTIAL DIFFERENCE (p.d.) across its ends. Potential difference makes charges flow
The bigger the potential difference across a component . . . The bigger the CURRENT that flows through it.
Components resist a current flowing through them. The bigger their resistance . . . The smaller the current produced by a particular voltage
The p.d. across a component in a circuit is measured in . . . VOLTS. 1 volt is equal to 1 joule of electrical energy for every 1 coulomb of charge
Voltage (p.d.) is measured using . . . a VOLTMETER connected IN PARALLEL with the component. Remember: you measure the potential difference between two points in a circuit
The CURRENT flowing through a component in a circuit is measured using . . . an AMMETER connected IN SERIES with the component. You need to put the ammeter in line with the current so it flows through the meter
The unit of current is . . . the AMPERE (AMP). It is a rate of flow of charge
A current of 1 A is equal to a charge flowing of . . . 1 COULOMB per SECOND
In metals, a current is a flow of charged particles called . . . ELECTRONS. They are the negatively charged parts of atoms
In a circuit, we say that CURRENT always flows from . . . The POSITIVE terminal of a cell, towards the NEGATIVE terminal. Red is positive - black is negative
In metals, electricity is actually carried by NEGATIVELY charged electrons, which flow from . . . the NEGATIVE [electron] terminal towards the POSITIVE terminal of a cell.
The behaviour of a component in a circuit can best be studied by plotting . . . a current-voltage graph. Put voltage on the x-axis and current on the y-axis
A resistor at constant temperature has a constant ratio of . . . current to voltage. Its graph is a straight line.
The current-voltage graph for a filament lamp is NOT a straight line. The reason is . . . Resistance increases as temperature increases. The graph is a curve - current stops increasing at quickly because it gets harder for it to flow through the higher resistance
A diode is a device which . . . only allows a current to flow one way through it. The graph shows a sudden rapid rise in current when the p.d. exceeds 0.7 volt ( but in reverse the current is just zero )
When components are connected in series . . . the SAME CURRENT flows through each component.
The total resistance of components in series is . . . equal to the SUM of their separate resistances. If you keep adding components in series the current will just keep getting less and less
The total potential difference in a series circuit is . . . shared between the individual compoents.
If a p.d. of 12 volts is shared between two equal resistances, each one will get . . . 6 volts. If the same current flows in each, the voltage is proportional to the resistance
If a p.d. of 12 volts is shared between two resistors of 6 ohms and 12 ohms, the bigger resistor will get . . . 8 volts. ( The smaller one gets 4 volts because it has half the resistance of the other one.) Voltage is directly proportional to resistance according to Ohm's Law
When components are connected IN PARALLEL . . . There is the SAME p.d. across each component.
The total current in a parallel circuit is equal to . . . the SUM of the currents through each of the separate components.
If one lamp is connected to a cell it gets a current of 1 amp. If two identical lamps are connected in parallel (to the same cell) they will get . . . a current of 1 amp each - that makes the total current 2 amps.
If one lamp gets a current of 1A from one cell, two lamps joined in series will get . . . approximately half as much current ( 0.5A) because they have twice as much resistance. Three lamps get 1/3 amp, etc.
How many different p.ds. can you get from 4 cells, each of 1.5volts, connected in series? THREE ( 6v, 3v, 0v)
The resistance of a component is measured in OHMS. 1 ohm is 1 volt per amp
Potential difference, current and resistance are related by Ohm's Law which states P.d. = current x resistance
The resistance of a component is calculated by applying Ohm's law as follows: R = p.d. / current
If a current of 2A flows in a resistor of resistance 10 ohms, the p.d. across the resistor is . . . V= IR = 2x10 = 20
20 volts
What is the current when a p.d. of 12v is applied across a 10 ohm resistor? I = V/R = 12/10
1.2 A
The current through a resistor at constant temperature is . . . directly proportional to the p.d. across the resistor.
The resistance of a light dependent resistor . . . decreases as the light intensity increases. They are used to switch on street lights at dusk
The resistance of a thermistor with a negative temperature coefficient will . . . decrease as the temperature increases. Can be used as a thermostat to control central heating
As an electric current flows through a circuit . . . energy is transferred from the battery or power supply to the components in the circuit. A battery is a source of electrical energy which can easily be converted into more useful forms such as light, heat and sound
A lamp converts electrical energy into . . . light and heat.
When electric charge flows through a resistor, electrical energy is transferred as . . . HEAT
The rate of energy transfer is called . . . POWER
Power is measured in watts. 1W is equal to . . . 1 joule per second. 1 watt means that 1 joule of energy is transferred EVERY second !
In an electric circuit, POWER = current x potential difference
Energy transferred = Power x Time
CHARGE = Current x Time
The higher the p.d. the greater the energy transferred for a given amount of charge which flows. Energy transferred = charge x potential difference
The letters D.C. stand for . . . Direct current
The letters A.C. stand for . . . Alternating current. It goes back and forth 50 times a second
A direct current can be obtained from . . . a cell or battery ( or dc power supply)
A direct current is one in which . . . charge carriers flow in one direction continuously.
An alternating current is one which flows . . . first in one direction then in the opposite direction alternately. It is constantly changing. It is produced using a dynamo or a generator which has a coil spinning in a magnetic field
The frequency of alternating current in the UK is . . . 50 Hertz ( or cycles per second). It is 60Hz in the USA - which makes it difficult to watch imported DVDs or videos
The voltage of the UK mains supply is . . . About 230 volts (AC)
In Europe, the colour of the LIVE cable in a 13 amp plug is . . . BROWN
In Europe, the earth cable is coloured . . . GREEN & YELLOW
In Europe,the neutral wire in a 13A plug is coloured . . . BLUE
If a fault occurs in an electrical circuit the current is interrupted by a . . . FUSE (or circuit breaker). Circuit breakers can be reset - fuses need to be replaced
The fuse in a plug is designed to MELT when . . . the current exceeds the value of the fuse e.g. 5amp.
Fuses and circuit breakers should always be fitted in the . . . LIVE wire. So that the current is stopped before it enters the appliance
An appliance with a metal casing should always be EARTHED because . . . the current can flow to earth preventing the user from receiving a shock.
It is safe to use an appliance fitted with a 2-pin plug because . . . there are no metal parts which can become LIVE
A loudspeaker converts electrical energy into . . . sound energy.
A microphone converts sound energy into . . . electrical energy.
A motor converts electrical energy into . . . kinetic energy.
A dynamo converts kinetic energy into . . . electrical energy.
The amount of electrical energy transferred from the mains is measured in units called . . . kilowatt hours. That's kilowatts x hours
1 kWh = 1kW x 1hour
1000 W x 3600 seconds = 3 600 000 joules
Cost of electricity used = Number of units (kWh) x cost per unit
If 1 unit of electricity costs 8cents, what would it cost to run a 500W lamp for 6 hours? Cost = 0.5 x 6 x 8 = 24c
Metals are good conductors of electricity because . . . some of their electrons can move freely throughout the metal. They are 'delocalised' or mobile and not firmly attached to any one atom
When a current flows through a solution the process is called . . . electrolysis
In an electrolyte, current is carried by charged particles called . . . IONS. An atom which has either gained or lost electrons is called an ion
Positve ions move towards the . . . CATHODE
Negatively charged ions move towards the . . . ANODE
During electrolysis the mass or volume of the substance deposited or released at the cathode depends on . . . the current and the time for which it flows.
A capacitor is a device which is used to store . . . electric charge (energy)
A capacitor takes time to charge up or to discharge. This can be used in . . . a timing circuit
When two different materials are rubbed together . . electrons are transferred from one to the other
Certain materials can be used as electrical insulators. This is because . . . they do not conduct electricity
When a substance loses electrons, it becomes . . . POSITIVELY charged.
Opposite electric charges will . . . ATTRACT each other
Like charges . . . REPEL each other
A charged object can be discharged by . . . connecting it to EARTH with a conductor
In a photocopier, electric charge is used to create an image on . . . a copying plate which conducts when light shines on it
Smoke particles can be extracted from the emissions from power stations by means of . . . an electrostatic precipitator
When walking along a nylon carpet with plastic-soled shoes, you may pick up a static charge. This happens because . . . electrons are transferred by friction from the shoes to the carpet
When you touch a metal door handle you sometimes receive an electric shock because . . . a charge passes from your body to earth through the door handle
When filling a car with petrol, the nozzle should make good electrical contact with the neck of the tank. This is to prevent . . . a spark which could ignite the fuel.
Created by: johncl