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Physics > Unit 1: Topic 3: Internal Resistance and Wheatstone Bridges > Flashcards

Unit 1: Topic 3: Internal Resistance and Wheatstone Bridges Flashcards

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1
Q

What equation is used to calculate power?

A

P = IV.

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2
Q

What is power measured in?

A

Watts.

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3
Q

What 3 equations can be used to calculate power?

A

P = IV, P = I^2R, P = V^2/R.

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4
Q

What does resistance measure?

A

The opposition of a circuit component to the flow of charge or current through the component.

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5
Q

If the resistance through a component is greater, what would the current be described as?

A

Smaller.

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6
Q

What 4 things does the resistance depend on?

A

Type, length, thickness, temperature.

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7
Q

Resistance depends on type, describe this.

A

Better conductor, lower resistance.

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8
Q

Resistance depends on length, describe this.

A

Longer material, higher resistance.

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9
Q

Resistance depends on thickness, describe this.

A

Thinner material, higher resistance.

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10
Q

Resistance depends on temperature, describe this.

A

Higher temperature, higher resistance.

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11
Q

What is the relationship/equation used to calculate electrical resistance?

A

R = V/I.

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12
Q

What is the name given to resistors which obey this equation, ‘R=V/I’?

A

Ohms Law.

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13
Q

Resistors which obey the electrical resistance equation are Ohmic conductors, what does this mean?

A

Most metals.

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14
Q

What does an electrical source supply to an electrical circuit?

A

Supplies energy to the charges.

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15
Q

Give examples of electrical sources.

A

Cell, battery, mains supply, thermocouple.

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16
Q

What is the meaning of an electromotive force?

A

The energy supplied to charges in an electrical circuit from an electrical source.

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17
Q

What is another name for ‘electromotive force’?

A

E.m.f.

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18
Q

What is the electromotive force/E.m.f measured in?

A

Volts.

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19
Q

What does a voltmeter measure?

A

The open circuit p.d aross the terminals of the source (terminal potential difference - t.d.p).

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20
Q

What is an open circuit t.p.d equal to?

A

E.m.f of source.

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21
Q

What is the meaning of a load resistor?

A

The electrical charges which flow around the external circuit.

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22
Q

What is the meaning of internal resistance?

A

Resistance from within a cell or battery (only when a current is flowing).

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23
Q

Internal resistance obeys Ohm’s law so ‘r’ is ____ and is independent of the _____ through it.

A

Constant, current.

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24
Q

What hint can be given during an assessment that an internal resistance question is being asked?

A

A dashed line around the source and resistor of the circuit can be used to represent an internal resistor question.

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25
Q

What does the e.m.f of an internal resistance source equal in a circuit?

A

Sum of all p.d across all resistors in the circuit. Including p.d across Ir and IR.

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26
Q

What does V represent in an internal resistance question?

A

T.p.d.

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27
Q

What does Ir represent?

A

The p.d ‘lost’ across the Ir of the source and never appears in external circuit.

28
Q

If no current flows then E=V, as there are no ‘___ ____’.

A

‘Lost volts’.

29
Q

During internal resistance, if the current increases then the lost volts increase, what happens to the t.p.d?

A

Decreases.

30
Q

What does the energy gained per coulomb from the battery equal in this equation ‘E=IR+Ir’?

A

Energy ‘lost’ per coulomb (heat) in resistors.

31
Q

What does the lost volts represent?

A

Energy per coulomb used in the cell.

32
Q

What is termed an open circuit?

A

When the ‘E’ in the energy equation equals the terminal voltage when no current is drawn.

33
Q

What is the meaning of a short circuit?

A

When the cell delivers it’s maximum current and there is zero external resistance.

34
Q

When does the maximum power transferred to a load occur?

A

When the external resistor equals the internal resistance.

35
Q

What is the equation used to calculate the current in a short circuit?

A

Imax = E/r.

36
Q

Apart from measuring the terminal voltage drawn from the cell, what is another way internal resistance can be found?

A

Connect known resistors across the supply and measure the current.

37
Q

What would a graphs line look like in an internal resistance experiment if R is against 1/I?

A

Straight line.

38
Q

Can charge be created or destroyed?

A

No, it can only be transformed.

39
Q

For resistors in series, the energy per charge supplied by the cell is equal to the total ____ dissipated by the _____ (heat) in the circuit.

A

Energy, resistors.

40
Q

For resistors in series, what is the ‘E’ equal to?

A

Sum of all p.d across all resistors in the circuit.

41
Q

What is the relationship/equation used for calculating resistors in series to equal the sum of all individual resistances?

A

Rt = r + R1 + R2 + R3.

42
Q

The charge passing any point in the circuit must be 1 _____ per second.

A

Coulomb.

43
Q

One coulomb of charge moving round the circuit will gain _____ potential energy in the ____.

A

Electric, battery.

44
Q

What does the sum of e.m.f’s equal?

A

The sum of p.d around a closed circuit.

45
Q

What is the Wheatstone bridge used for?

A

Finding resistance.

46
Q

How is the Wheatstone bridge more accurate than using the voltmeter and ammeter method?

A

The meter shows a true value of voltage or current with only a very small error.

47
Q

Explain how the Wheatstone bridge is sensitive.

A

The meter shows a large deflection when a small current flows through it. It doesn’t need to have a scale but a mark will show where the pointer is when no current flows.

48
Q

What will the voltmeter reading be if it is connected between 2 pairs of resistors in a Wheatstone bridge?

A

Zero.

49
Q

In a Wheatstone bridge, if the voltmeter has a zero reading and there is a Null of Deflection, what does this mean for the bridge?

A

It is balanced.

50
Q

In a Wheatstone bridge, usually opposing resistors (T->B) are fixed resistors, what happens to the resistor (L->R)?

A

It is adjusted until a balance is found, so the reading on the voltmeter is zero.

51
Q

In a Wheatstone bridge, what would occur if the voltmeter is replaced with a sensitive ammeter?

A

The current would flow if the bridge wasn’t balanced.

52
Q

Sometimes in a Wheatstone bridge, the voltmeter can be replaced with a sensitive ammeter, what is this often called?

A

Galvanometer.

53
Q

If the voltmeter in a Wheatstone bridge was replaced with a sensitive ammeter, what would happen on the way to balance a large current in the circuit?

A

The large current could flow through the galvanometer and damage it.

54
Q

In a Wheatstone bridge, sometimes the galvanometer can become damaged, suggest a way to prevent this.

A

Using a protection resistor.

55
Q

To prevent the galvanometer becoming damaged in a Wheatstone bridge, what are the 2 resistors which can be used to protect it?

A

Variable resistor, resistor R.

56
Q

How can a variable resistor be used to protect a galvanometer?

A

It’s reduced to zero near balance point.

57
Q

How can an R resistor be used to protect a galvanometer?

A

It is by-passed (short circuited) by closing the switch near the balance point.

58
Q

How are Wheatstone bridges useful when they are out of balance?

A

They can be used as ‘sensing’ devices to measure small changes.

59
Q

With an ‘out of balance’ Wheatstone bridge they are initially balanced, the value of the variable resistor is altered by small amounts, what does this do to the balance of this bridge?

A

The bridge will no longer be balanced and the reading will be obtained on the voltmeter.

60
Q

A graph of change in resistance against a change in voltage, describe what the graph would look like?

A

A straight line through the origin. With small changes in resistance, the change in voltage is proportional to change in resistance.

61
Q

In a Wheatstone bridge, when measuring temperature, what happens when the resistance box is replaced by a thermistor?

A

The out of balance current can be used to indicate temperature change.

62
Q

In a Wheatstone bridge, when measuring strain, what do strain gauges rely on?

A

The fact that the resistance of a wire increases as the thickness decreases?

63
Q

In Wheatstone bridges, when measuring strain, what is a typical gauge made from?

A

Film or thin wire of conducting material on an insulating base with gold contacts at each end.

64
Q

What is the resistance of a Newtonmeter?

A

About 120 ohms.

65
Q

If a Wheatstone bridge is set up with 2 strain gauges, what happens to the bending?

A

Very small amounts of bending is detected.

66
Q

Where can strain gauges be fixed to?

A

Buildings, bridges.

Physics (15 decks)

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