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Elect Know @ AJHS NZ
knowledge questions for Yacapaca too
| Question | Answer |
|---|---|
| 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
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