2.04-2.06 Alternating & Direct Current

Power, Current, and Voltage relationship (for power formula)

Power = Current x Voltage

P = I x V

Power, Current, and Voltage relationship (for current formula)

Current = Power / Voltage

I = P / V

Power, Current, and Voltage relationship (for voltage formula)

Voltage = Power / Current

V = P / I

Power is … , current is … , voltage is … Therefore power can also be considered as …

rate of doing work (transferring energy), rate of flow of charge, energy transferred per unit charge passed, rate of energy transferred for charge

Energy transferred, power, and time relationship (for energy formula)

Energy transferred = Power x Time

E = P x t

Energy transferred, power, and time relationship (for power formula)

Power = Energy transferred / Time

P = E / t

Energy transferred, power, and time relationship (for time formula)

Time = Energy transferred / Power

t = E / P

Energy transferred = … , and power = …, therefore we can also say that energy transferred = …

power x time, current x voltage, current x voltage x time (E = I x V x t)

Power

Rate of energy transferred per second; rate of work done.

It’s equal to energy transferred/time or work done/time

Rate

Something/time

Electrical power of a component/appliance depends on the … and … flowing through it, and is measured in … (…)

current, voltage, Watts, W

1 Watt =

1 Joule/second

1W = 1J/s

Energy transfers involved in a battery

Chemical energy of battery → transferred to electrical energy to the component and surroundings

… is done when … flows through the circuit and is … to the … being transferred

Work, charge, equal, energy

What happens when charge flows through a resistor?

Charge flows through resistor → thermal energy transferred (making it hot) because electrons flowing collide with the lattice of atoms in the metal conductor → provides resistance. Collisions → atoms vibrate more → conductor heats up (used by electrical heaters/ovens/stoves/toasters/ kettlers, etc)

Relationship between energy supplied by battery and energy transferred to all circuit components and why?

Conservation of energy → Energy supplied by battery = Energy transferred to all circuit components

Thermistor

Temperature-dependent resistor.

↑ Temperature → ↓ Resistance

↓ Temperature → ↑ Resistance

Temperature ∝ 1/Resistance (inversely proportional)

LDR

Light-dependent resistor

↑ Light intensity → ↓ Resistance

↓ Light intensity → ↑ Resistance

Light intensity ∝ 1/Resistance (inversely proportional)

Diodes

Semiconductor device or electric component that only allows current to flow in 1 direction.

LED

Light-emitting diode, diodes that only light up when current flows through them when placed in the correct direction.

What can lamps and LEDs do in a circuit?

Acts as visual indicator of current flowing in a circuit.

Lighted LED/lamp → Current is present → Circuit working

LED/lamp not lighted → Current is absent → Circuit may be broken

Light-dependent circuit components have … pointing … from their …

arrows, towards, circuit symbol

Light-emitting circuit components have … pointing … from their …

arrows, away, circuit symbol

Fuse

Safety device that cuts off electricity flow if current is too high (can cause electrical fires) due to a fault or surge.

How do fuses work?

Fuses are glass cylinders containing a thin metal wire which heats up and melts if the current is too high → gap in circuit created and current cannot flow anymore → protects circuit/device from damage or preventing a fire. Fuses have different current ratings (e.g. 3A, 5A, 13A, etc.), the rating indicates the current at which or over which the fuse will melt. They should only be slightly higher than the current used by the device in the circuit. To be safe always select a fuse with a current rating that is the next size up (greater than) the circuit current but is as close to the circuit current as possible (e.g. 4A circuit → use 5A fuse, using 13A fuse is useless as fire or damage may occur before 13A is reached to melt the fuse).

Circuit breakers

Electrical safety device designed to protect an electrical circuit from damage due to a current that is too high. They have an automatic electromagnet switch that breaks the circuit if the current exceeds a specific threshold.

Are circuit breakers or fuses better? Why?

Circuit breakers are better than fuses because they can be reset and reused. They’re also faster than a fuse. However, fuses are cheaper than circuit breakers.

Direct Current (D.C.)

Steady current constantly flowing in the same direction in a circuit from positive to negative, and which has the same value.

D.C. power supplies (e.g. batteries and cells) have fixed positive and negative terminals → voltage (potential difference) across a cell in a D.C. circuit travels in 1 direction only → current is either positive OR negative, only 1 NOT both.

Alternating Current (A.C.)

Current that continuously changes its direction, going back and forth around a circuit, and which can have varying values.

A.C. power supplies (e.g. electrical generators and mains electricity) have 2 identical terminals that switch between positive and negative → current can hence be positive OR negative (can be 1 of both) depending on which direction it’s flowing in at that time.

Frequency of alternating current is the …

number of times the current changes direction back and forth per second.

What is the approximate frequency and voltage of mains electricity in the UK?

50Hz frequency

230V voltage

Fill in this D.C. vs A.C. comparison table

D.C. graph

A.C. graph