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AS-Level Physics > Electricity > Flashcards

Flashcards in Electricity Deck (73)
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1

What does EMF stand for?

Electromotive force

2

What is a ideal cell?

A source of EMF

3

What is a real cell?

A source of EMF with an internal resistance.

4

Explain why a real cell has internal resistance.

-The current produced has to flow through the cell itself.
-The chemicals in the cell have an electrical resistance.
-The resistance inside the cell forms part of the circuit.

5

Calculate the current that would flow from a 1.5V AA cell when short circuited and describe the heating effect. (R=0, r=1))

-1.5A
-The cell will get warm.

6

Calculate the current that would flow from a 1.2V AA NiMH cell when short circuited and describe the heating effect. (R=0, r=0.1))

-12A
-Substantial heating effect.

7

Calculate the current that would flow from a 12V car battery when short circuited and describe the heating effect. (R=0, r=0.01))

-1200A
-Explosive heating effect.

8

Which is higher v(emf) or V (terminal pd)?

v

9

Define emf

The potential difference across the cell terminals when no current is drawn from the cell.

10

What property must a voltmeter have to measure emf?

Infinitely high resistance

11

What two instruments can be used to measure emf?

-Voltmeter
-Oscilloscope

12

What does the term rated mean?

The max p.d. that can be applied to a component without damaging it.

13

Give five equations that relate to emf.

- ε = V + v
- V = IR
- v = Ir
- ε = I( R+r )
- ε = V + Ir

14

What is a conductor?

A component with a very small resistance and allow current to pass through with ease.

15

What is an insulator?

A component with a high resistance making it hard for current to pass through.

16

What is a semiconductor?

Materials that can change resistance by electrical means or by external factors.

17

Define resistance

R = V/I
where: R is the components RESISTANCE measured in OHMS
V is the p.d. across the component measured in VOLTS.
I is the current through the component measured in AMPS.

18

What is Ohm's Law?

For a metallic conductor, the current flowing through the conductor is directly proportional to the potential difference across the conductor provided the temperature remains constant.

19

What is a diode?

A component that only allows current to flow in one direction.

20

What is forward bias?

When a diode allows current through with little or no resistance.

21

What is reverse bias?

When a diode doesn't allow current though due to almost infinite resistance.

22

What is an LED?

-Light emitting diode.
-Emits light in forward bias.

23

What does LDR stand for?

Light dependent resistor

24

Describe how the resistance of an LDR changes.

The resistance of the LDR falls as the light intensity increases.

25

Explain how an LDR works.

-Made from CdS.
-CdS forms a covalent structure with almost all available electrons in covalent bonds.
-When a photon hits, the energy is transferred to an electron giving it enough energy to break the bond.
-The electron is then free for conduction.
-More electrons means a lower resistance.

26

Describe how the resistance of a thermistor changes.

The resistance falls as the temperature increases. (ntc)

27

What does ntc stand for?

Negative temperature coefficient

28

Explain how an ntc thermistor works.

-Made from silicon.
-Silicon forms a giant covalent structure.
-Heat energy causes bonds to break releasing electrons.
-More electrons means a lower resistance.

29

Explain how the reading of the voltmeter will change as the temperature of the thermistor falls.

-As the temperature increases, the resistance of the thermistor falls.
-As the resistance of the thermistor falls the total resistance in the circuit falls.
-The current in the circuit will increase.
-The pd across the fixed resistor increase so the pd across the thermistor decreases.

30

What is the relationship between pd and current of an Ohmic conductor?

They are directly proportional as long as the temperature remains constant.