Chapter 5: Electric currents

Question 1

electric potential difference

Answer 1

the amount of work needed to move a positive charge a certain distance (W/q=voltage)

Question 2

Electronvolt

Answer 2

the amount of work done or energy needed to move an electron across an potential difference of 1 volt

Question 3

Electric current

Answer 3

the force per unit length between parallel current-carrying conductors

Question 4

Resistance

Answer 4

a measure of the opposition of the flow of electrical charge throught a component (R=V/I)

Question 5

Ohm´s law

Answer 5

The current in a system is directly proportional to the voltage and inversly proportion to the resistance, provided the temperature remains constant(V=IR)

Question 6

Compare ohmic and non-ohmic behaviour

Answer 6

If current and p.d. are proportional (like the metal at constant temperature) the device is said to be ohmic. Devices where current and p.d. are not proportional (like the filament lamp or the diode) are said to be non-ohmic

Question 7

Electromotive force (emf)

Answer 7

The maximum p.d. available from a voltage source represented as energy per unit charge (volt)

Question 8

Internal resistance

Answer 8

A small resistance produced within a voltage souce (battery or generator) (this can also be found in an emf)

Question 9

The use of ideal ammeters

Answer 9

An ammeter is a device that measures the current in a circuit. It must be placed in series with the circuit element so the same current can pass throught both. An ammetter must have low resistance so it does not reduce the current it is trying to measure.

Question 10

The use of ideal voltmeter

Answer 10

a voltmeter measures the voltage between two points of a circuit. It must be placed in parallel with the circuit element being measured and ideally should have a very high resistance - so current is not diverted from the current to the meter

Question 11

Describe a potential divider

Answer 11

As current go through a line of in series resistors the voltage dropsacross each one - this effect can be used to lower the voltage down to obtain fraction of the original voltage

Question 12

Explain the used of sensors in potential divider circuit

Answer 12

Many sensors work by changing there resistance when sensing their environment - if the sensor put in series with another resistor, the resistance change in the sensor can strongly effect the voltage across the other resistor and thus be easily measured. LDR´s, Light dependent resistors lower their R with increased light, thermisors measure temperature by increasing their R with increasing temperature etc.