Energy Transfers in Circuits Flashcards

All 39 cards in this deck

What happens inside a resistor when there is an electric current in it?

There is an energy transfer that heats the resistor.

What causes a resistor to become warm, in terms of particles?

Collisions between the moving electrons and the ions in the lattice.

What happens to the lattice of a resistor as electrons collide with it?

The lattice vibrates more, so its kinetic energy increases (and the electrons' kinetic energy decreases).

What happens to the electrical energy when a current does work against electrical resistance?

It is dissipated as thermal energy in the surroundings.

True or false? Connecting wires stay at exactly the same temperature because only resistors get warm.

False. All wires have some resistance, so they also warm up slightly and dissipate thermal energy.

True or false? The energy dissipated by a resistor is stored inside the resistor for ever.

False. It is transferred to the thermal energy store of the surroundings.

What property must connecting wires have to reduce unwanted energy transfer?

Low resistance.

Give two ways of making the resistance of a connecting wire lower.

Make it thicker (larger cross-sectional area) and make it shorter.

Which metal is usually chosen for connecting wires and mains cables, and why?

Copper, because it is a very good conductor (low resistance), so less energy is dissipated as thermal energy.

True or false? Using a thicker wire increases the unwanted energy transfer to the surroundings.

False. A thicker wire has lower resistance, so less energy is dissipated as thermal energy.

True or false? For the same current, halving a wire's resistance halves the power dissipated in it.

True. $P = I^2R$, so power is proportional to resistance at constant current.

Give three appliances that make use of the heating effect of an electric current.

Kettle, toaster, electric heater (also hair dryer, iron, filament lamp).

Give two disadvantages of the heating effect of an electric current.

Energy is wasted, lowering efficiency; overheating can melt insulation or start a fire.

Why does the heating effect reduce the efficiency of a device such as an electric motor?

Some electrical energy is dissipated as thermal energy instead of being usefully transferred.

True or false? The heating effect of an electric current is always a disadvantage.

False. It is useful in devices such as kettles, toasters and electric heaters.

True or false? Too large a current in a cable can make the insulation melt and cause a fire.

True. The greater the current, the greater the heating effect in the cable.

What is the equation linking energy transferred, charge moved and potential difference?

energy transferred (J) = charge moved (C) × potential difference (V), $E = Q \times V$

What is the equation linking energy transferred, current, potential difference and time?

energy transferred (J) = current (A) × potential difference (V) × time (s), $E = I \times V \times t$

What are the steps to find the energy transferred by a supply from the charge moved?

e.g. a charge of $5\,\mathrm{C}$ moves through a $6\,\mathrm{V}$ supply

1. Write $E = Q \times V$
2. Substitute: $E = 5 \times 6$
3. Energy transferred $= 30\,\mathrm{J}$

What are the steps to find the energy transferred by a component from current, p.d. and time?

e.g. $2\,\mathrm{A}$ at $12\,\mathrm{V}$ for $10\,\mathrm{s}$

1. Write $E = I \times V \times t$
2. Substitute with time in seconds: $E = 2 \times 12 \times 10$
3. Energy transferred $= 240\,\mathrm{J}$

True or false? For a given charge, a higher potential difference means more energy is transferred.

True. $E = Q \times V$, so energy is proportional to p.d.

What is power?

The energy transferred per second.

What is the unit of power, and what is 1 watt equal to?

The watt (W); $1\,\mathrm{W} = 1\,\mathrm{J/s}$ (one joule per second).

What is the equation linking power, energy transferred and time?

power (W) = energy transferred (J) ÷ time taken (s), $P = E / t$

What are the steps to calculate the power of a device from the energy it transfers?

e.g. $600\,\mathrm{J}$ transferred in $20\,\mathrm{s}$

1. Write $P = E / t$
2. Substitute with time in seconds: $P = 600 / 20$
3. Power $= 30\,\mathrm{W}$

How is $P = E/t$ rearranged to find the energy transferred?

$E = P \times t$ (energy = power × time in seconds).

True or false? A device with a power of $2000\,\mathrm{W}$ transfers $2000\,\mathrm{J}$ of energy each second.

True. A watt is a joule per second.

What is the equation linking electrical power, current and potential difference?

power (W) = current (A) × potential difference (V), $P = I \times V$

What is the equation linking electrical power, current and resistance?

power (W) = (current)$^2$ (A$^2$) × resistance (Ω), $P = I^2 \times R$

How does the power transfer in a circuit device depend on the current and the potential difference?

Power is the current multiplied by the p.d., so increasing either increases the power transferred.

What are the steps to find the current in a device from its power and the p.d. across it?

e.g. a $60\,\mathrm{W}$ lamp on a $12\,\mathrm{V}$ supply

1. Write $P = I \times V$
2. Rearrange: $I = P / V$
3. Substitute: $I = 60 / 12 = 5\,\mathrm{A}$

What are the steps to find the resistance of a component from its power and the current in it?

e.g. $20\,\mathrm{W}$ dissipated with a current of $2\,\mathrm{A}$

1. Write $P = I^2 \times R$
2. Rearrange: $R = P / I^2$
3. Substitute: $R = 20 / 4 = 5\,\Omega$

True or false? Doubling the current in a fixed resistor doubles the power dissipated.

False. $P = I^2R$, so doubling the current makes the power four times greater.

What does the power rating of a domestic appliance tell you?

The energy it transfers each second when in use, in joules per second (watts).

What are the steps to find the energy transferred by an appliance from its power rating?

e.g. a $1500\,\mathrm{W}$ heater used for $10\,\mathrm{s}$

1. Write $E = P \times t$
2. Substitute with time in seconds: $E = 1500 \times 10$
3. Energy transferred $= 15\,000\,\mathrm{J}$

Describe the energy transfer in an electric kettle connected to the a.c. mains.

Electrical energy from the mains is transferred to the thermal energy store of the heating element and water.

Describe the energy transfer in a battery-powered fan.

The chemical energy store of the battery is transferred electrically to the kinetic energy of the motor and fan blades (some is dissipated as thermal energy).

True or false? For the same time of use, a $2\,\mathrm{kW}$ heater transfers twice as much energy as a $1\,\mathrm{kW}$ heater.

True. $E = P \times t$, so energy is proportional to power rating for a fixed time.

True or false? A low power appliance can never transfer more energy than a high power one.

False. If it is used for long enough, a low power appliance can transfer more energy in total.