Voltage and Resistance

The amount of current (in amps) flowing through a circuit will partly depend on electromotive force (EMF) of the electrical supply. Electromotive force is measured in volts (V), and is generally called voltage. The voltage depends on the ‘strength’ of the electrical supply.

The electric current flows only when there is a potential difference between the two points in the circuit. The opposition to the current flow is the resistance. Thus we can say that the current is directly proportional to the voltage and inversely proportional to the resistance (Ohm’s law) : I=U/R.

Thus, the amount of current will also depend on electrical resistance (or resistance). Resistance is a measure of how easily current can flow through the conductors and components in a circuit. For example, a lamp creates resistance because the filament (the metal wire inside it) is very thin. This limits the amount of current that can flow. Resistance also depends on the materials used as conductors (a material or device that conducts or transmits electricity ). For example, copper has a low resistance and so is a good conductor.

Materials with very high resistance, such as plastics, are called electrical insulators (or insulators). Only very high voltages can cause current to flow through them. Materials that are good insulators are used to insulate conductors. An example is plastic insulation around electric wires. This stops people from touching the conductor and – if it is live (carrying current) – from getting a dangerous electric shock.

Electrical Power

The amount of current, in amps, required by an electrical appliance – such as a TV or an electric kettle – depends on the power of the appliance. This number, expressed in watts (W), will be marked somewhere on the appliance. To calculate the required current, simply take the wattage and divide it by the voltage of the electrical supply in your home (from ‘Professional English in Use. Engineering’).

Exercise 19. Solve the clues and complete the puzzle with the terms from the text above.

1. another term for amperage

2. provided by a battery, for example

3. measured as a wattage

4. allows current to flow through it

5. has very high electrical resistance

6. carried by moving electrons

7. another term for an electrical ‘device’

8. the consequence of a person touching a live conductor

		1						E
		2						L
				3				E
	4							C
5								T
				6				R
		7						I
				8				C

ELECTRIC(AL)

Exercise 20. Match the terms in the box with the appropriate definition.

step-down transformer transformer current Watt direct current resistance generator Ampere alternating current conductor inverter Ohm step-up transformer insulator battery wire voltage

1. ­­__________ – the flow of charge between two points caused by voltage or electric field.

2. __________ – substance that allows heat or electricity to pass through it.

3. __________ – the ability of something to stop the flow of electricity.

4. __________ – the unit of electric current, a flow of one coulomb per second.

5. __________ – the unit for power (the rate of transfer the energy), the name given to a rate of one joule (one Newton meter) per second.

6. __________ – a unit used for measuring electrical resistance.

7. __________ – the amount of power in an electric current, measured in volts.

8. _________ – direct current motor that drives an alternator, or any other system that converts input DC to AC.

9. _________ – a continuous electric current that flows in one direction only, without substantial variation in magnitude.

10. _________ – a continuous electric current that periodically reverses direction, usually sinusoidally.

11. __________– substance with low conductivity for the energy transfer.

12. _________ – metal sample drawn into a long uniform thread.

13. _________ – voltage electromagnetic machine used in alternating current contexts whose design output tends to be at a different voltage from the input.

14. _________ – machine with a mechanical input and an electric output.

15. _________ – a linked set of one or more electric (electrochemical) cells.

16. _________ – a transformer that increases the voltage.

17. _________ – a transformer that decreases the voltage.

Exercise 21. Complete the table below with appropriate information.