Define electrical power .

Electrical power is the rate of doing work (energy transferred per second), measured in watts (W) .

For a fixed resistor , if the current through it doubles, what happens to the power dissipated?

The power increases by four times (since P ∝ I ²).

When calculating electrical power in watts, the time used in any equation must always be in .......... .

When calculating electrical power in watts, the time used in any equation must always be in seconds .

Define the kilowatt-hour (kW h) .

A unit of energy equal to 1 kW of power sustained for 1 hour .

A cooker transfers 1.2 × 10 9 J of electrical energy to heat. How much will this cost if 1 kW h costs 14.2 p?

Convert to kW h: (1.2 × 10 9 ) ÷ (3.6 × 10 6 ) = 333.3 kW h Cost: 333.3 × 14.2 p = 4733 p = £47.33

To convert an energy value from kilowatt-hours to joules, multiply by .......... .

To convert an energy value from kilowatt-hours to joules, multiply by 3.6 × 10 6 .

Why is the kilowatt-hour , rather than the joule, used to calculate household electricity bills?

The kilowatt-hour is a much larger unit of energy , making it more convenient for the amounts of energy used in homes.

True or False? The number of kilowatt-hours used can be found by multiplying the power in watts by the time in seconds.

False. Energy in kW h is found using power in kilowatts multiplied by time in hours , not watts and seconds (which would give an answer in joules).

Define Kirchhoff's second law .

The sum of the e.m.f.s in a closed circuit equals the sum of the potential differences .

In a parallel circuit made up of multiple closed loops, what must the sum of the voltages in each individual loop equal?

The total e.m.f. of the power supply.

Why are parallel circuits useful for home wiring systems?

Each appliance receives the same voltage as the supply, and if one component breaks, current and voltage can still flow to the rest.

In a series circuit, the sum of the voltages across the components is equal to the .......... of the power supply.

In a series circuit, the sum of the voltages across the components is equal to the e.m.f. of the power supply.

True or False? In a parallel circuit, the sum of the voltages across all components in every branch combined must equal the e.m.f.

False. It is the sum of the voltages within each individual closed loop that equals the e.m.f, not the sum across all branches combined.

Define Kirchhoff's first law .

The sum of the currents entering a junction always equals the sum of the currents leaving the junction.

Define Kirchhoff's second law .

The sum of the e.m.f.s in a closed circuit equals the sum of the potential differences .

Name three common mistakes made when applying Kirchhoff's laws to circuit calculations.

Any three from: thinking the current is the same through every branch of a parallel circuit; thinking the voltage is the same across every component in a series circuit; not accounting for multiple resistors; not using the correct series or parallel resistance equation; the sum of series voltages not adding up to the e.m.f.; current into a junction not equalling current out.

A common mistake is assuming the current is the same in every branch of a .......... circuit.

A common mistake is assuming the current is the same in every branch of a parallel circuit.

What technique helps avoid errors when solving circuit problems with several components?

Annotate the circuit diagram with all known values, making it easier to calculate the values that are missing.

True or False? In a series circuit, the voltage across each component is always the same.

False. The voltage is shared among the components depending on their resistance , so it is only equal across components of equal resistance.

How does the combined resistance of resistors in parallel compare with any individual resistor's resistance?

The combined resistance is always less than the resistance of the smallest individual resistor.

Two resistors of equal resistance are connected in parallel. What happens to the combined resistance compared with one resistor alone?

The combined resistance halves .

To find the value of R from 1/ R , you must calculate 1 .......... your answer.

To find the value of R from 1/ R , you must calculate 1 divided by your answer.

True or False? Connecting two resistors in parallel always increases the total resistance.

False. Combining resistors in parallel always decreases the total resistance; it never increases it.

How does the current compare across all components in a series circuit?

It is the same through all components.

What is true of the total current entering and leaving a junction in a parallel circuit?

The total current into a junction must equal the total current out of the junction.

What determines the amount of current in each branch of a parallel circuit?

The total resistance of the components within that branch.

How is the e.m.f. of the power supply shared in a series circuit?

It is shared among all the components in different amounts , depending on their resistance .

In a .......... circuit, the voltage across each branch is equal to the e.m.f. of the power supply.

In a parallel circuit, the voltage across each branch is equal to the e.m.f. of the power supply.

True or False? The current in each branch of a parallel circuit is always equal.

False. The current splits according to the resistance of each branch, so it is only equal if the branch resistances are equal.

What is the total potential difference of identical cells connected in parallel?

The total potential difference is the same as the potential difference of one cell.

In circuits with multiple sources of e.m.f, current flowing from the positive terminal to the negative terminal is taken as .......... , while current in the opposite direction is taken as negative.

In circuits with multiple sources of e.m.f, current flowing from the positive terminal to the negative terminal is taken as positive , while current in the opposite direction is taken as negative.

True or False? Two cells connected in parallel to supply a circuit can have different voltages.

False. If cells of different voltages are connected in parallel, the higher-voltage cell discharges into the lower-voltage cell until they are equal, which can cause sparking, overheating and failure of both cells.

In the experiment investigating circuits with multiple e.m.f sources, what are the independent and dependent variables?

Independent variable: the total potential difference of the cells, V total Dependent variable: the potential difference of the resistor, V

Why should the voltmeter be checked before starting the experiment?

To make sure it starts from zero , avoiding a zero error (a systematic error).

Why might the measured potential difference across the resistor differ from the calculated combined p.d of the cells?

The internal resistance of the cells affects the voltmeter reading, introducing a random error .

A LevelPhysicsOCRFlashcards4. Electrons, Waves & PhotonsSeries & Parallel Circuits

Series & Parallel Circuits (OCR A Level Physics): Flashcards

Exam code: H556

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Define electrical power.

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Cards in this collection (43)

Define electrical power.
Electrical power is the rate of doing work (energy transferred per second), measured in watts (W).
State the equation linking electrical power, current and potential difference.
$P = I V$
Using Ohm's law, write the electrical power equation in terms of resistance and current, and in terms of resistance and voltage.
$P = I^{2} R$
$P = \frac{V^{2}}{R}$
For a fixed resistor, if the current through it doubles, what happens to the power dissipated?
The power increases by four times (since P ∝ I²).
When calculating electrical power in watts, the time used in any equation must always be in ...........
When calculating electrical power in watts, the time used in any equation must always be in seconds.
True or False?
If the potential difference across a fixed resistor doubles, the power dissipated also doubles.
False.
Since $P = \frac{V^{2}}{R}$ , the power quadruples (increases four times) when the voltage doubles.
Define the kilowatt-hour (kW h).
A unit of energy equal to 1 kW of power sustained for 1 hour.
Write the equation for electrical energy transferred in terms of power and time, and in terms of current, voltage and time.
$W = P t = I V t$
A cooker transfers 1.2 × 10⁹ J of electrical energy to heat. How much will this cost if 1 kW h costs 14.2 p?
Convert to kW h: (1.2 × 10⁹) ÷ (3.6 × 10⁶) = 333.3 kW h
Cost: 333.3 × 14.2 p = 4733 p = £47.33
To convert an energy value from kilowatt-hours to joules, multiply by ...........
To convert an energy value from kilowatt-hours to joules, multiply by 3.6 × 10⁶.
Why is the kilowatt-hour, rather than the joule, used to calculate household electricity bills?
The kilowatt-hour is a much larger unit of energy, making it more convenient for the amounts of energy used in homes.
True or False?
The number of kilowatt-hours used can be found by multiplying the power in watts by the time in seconds.
False.
Energy in kW h is found using power in kilowatts multiplied by time in hours, not watts and seconds (which would give an answer in joules).
Define Kirchhoff's second law.
The sum of the e.m.f.s in a closed circuit equals the sum of the potential differences.
Kirchhoff's second law is a consequence of which conservation law?
The conservation of energy.
In a parallel circuit made up of multiple closed loops, what must the sum of the voltages in each individual loop equal?
The total e.m.f. of the power supply.
Why are parallel circuits useful for home wiring systems?
Each appliance receives the same voltage as the supply, and if one component breaks, current and voltage can still flow to the rest.
In a series circuit, the sum of the voltages across the components is equal to the .......... of the power supply.
In a series circuit, the sum of the voltages across the components is equal to the e.m.f. of the power supply.
True or False?
In a parallel circuit, the sum of the voltages across all components in every branch combined must equal the e.m.f.
False.
It is the sum of the voltages within each individual closed loop that equals the e.m.f, not the sum across all branches combined.
Define Kirchhoff's first law.
The sum of the currents entering a junction always equals the sum of the currents leaving the junction.
Define Kirchhoff's second law.
The sum of the e.m.f.s in a closed circuit equals the sum of the potential differences.
Name three common mistakes made when applying Kirchhoff's laws to circuit calculations.
Any three from: thinking the current is the same through every branch of a parallel circuit; thinking the voltage is the same across every component in a series circuit; not accounting for multiple resistors; not using the correct series or parallel resistance equation; the sum of series voltages not adding up to the e.m.f.; current into a junction not equalling current out.
A common mistake is assuming the current is the same in every branch of a .......... circuit.
A common mistake is assuming the current is the same in every branch of a parallel circuit.
What technique helps avoid errors when solving circuit problems with several components?
Annotate the circuit diagram with all known values, making it easier to calculate the values that are missing.
True or False?
In a series circuit, the voltage across each component is always the same.
False.
The voltage is shared among the components depending on their resistance, so it is only equal across components of equal resistance.
Define the combined resistance of resistors connected in series.
It is equal to the sum of the individual resistances: $R = R_{1} + R_{2} + R_{3} + . . .$
Define the combined resistance of resistors connected in parallel.
The reciprocal of the combined resistance equals the sum of the reciprocals of the individual resistances: $\frac{1}{R} = \frac{1}{R_{1}} + \frac{1}{R_{2}} + . . .$
How does the combined resistance of resistors in parallel compare with any individual resistor's resistance?
The combined resistance is always less than the resistance of the smallest individual resistor.
Two resistors of equal resistance are connected in parallel. What happens to the combined resistance compared with one resistor alone?
The combined resistance halves.
To find the value of R from 1/R, you must calculate 1 .......... your answer.
To find the value of R from 1/R, you must calculate 1 divided by your answer.
True or False?
Connecting two resistors in parallel always increases the total resistance.
False.
Combining resistors in parallel always decreases the total resistance; it never increases it.
How does the current compare across all components in a series circuit?
It is the same through all components.
What is true of the total current entering and leaving a junction in a parallel circuit?
The total current into a junction must equal the total current out of the junction.
What determines the amount of current in each branch of a parallel circuit?
The total resistance of the components within that branch.
How is the e.m.f. of the power supply shared in a series circuit?
It is shared among all the components in different amounts, depending on their resistance.
In a .......... circuit, the voltage across each branch is equal to the e.m.f. of the power supply.
In a parallel circuit, the voltage across each branch is equal to the e.m.f. of the power supply.
True or False?
The current in each branch of a parallel circuit is always equal.
False.
The current splits according to the resistance of each branch, so it is only equal if the branch resistances are equal.
What is the total potential difference of cells connected in series?
The total potential difference is the sum of the potential difference across each cell:
$V_{t o t a l} = V_{1} + V_{2} + . . . + V_{n}$
What is the total potential difference of identical cells connected in parallel?
The total potential difference is the same as the potential difference of one cell.
In circuits with multiple sources of e.m.f, current flowing from the positive terminal to the negative terminal is taken as .........., while current in the opposite direction is taken as negative.
In circuits with multiple sources of e.m.f, current flowing from the positive terminal to the negative terminal is taken as positive, while current in the opposite direction is taken as negative.
True or False?
Two cells connected in parallel to supply a circuit can have different voltages.
False.
If cells of different voltages are connected in parallel, the higher-voltage cell discharges into the lower-voltage cell until they are equal, which can cause sparking, overheating and failure of both cells.
In the experiment investigating circuits with multiple e.m.f sources, what are the independent and dependent variables?
Independent variable: the total potential difference of the cells, V_total
Dependent variable: the potential difference of the resistor, V
Why should the voltmeter be checked before starting the experiment?
To make sure it starts from zero, avoiding a zero error (a systematic error).
Why might the measured potential difference across the resistor differ from the calculated combined p.d of the cells?
The internal resistance of the cells affects the voltmeter reading, introducing a random error.

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