Physics, Paper 1 - Section 2 (Electricity)

What are 5 safety precautions you should take when using plugs?

1) Check the plug isn't damaged - live parts could be exposed? Shock? 2) Check cable isn't frayed e.g. wires exposed - live parts could be uncovered? 3) Check the cables aren't too long - trip hazard? 4) Check water isn't nearby - water is a very good conductor 5) Never push a metal object into a plug socket - metal is a conductor (shock)

How does a fuse work?

1) If a fault develops (live wire touches the metal case) because the case is earthed - a big current flows in through the live wire, through the case and out down the earth wire. 2) The surge in current 'blows'/melts the fuse (or trips the circuit breaker. This cuts off the live supply. 3) The whole appliance is isolated, making it impossible to get an electric shock, also prevents fire risk caused by the heat of a large current.

What is the UK mains approximate electricity supply?

230 volts - it is an a.c. (alternating current) supply (current is constantly changing direction)

What are some common UK fuse ratings?

3A, 5A and 13A fuses

What voltage is the live wire at?

Alternates between a high +VE amnd -VE Voltage of about 230V

What voltage is the neutral wire at?

Always at 0 Volts

What are circuit breakers?

An electrical safety device used in circuits. Like fuse, it protects the circuit from damage if too much current is flowing Can be reset easily by flicking a switch on the device - more convenient than fuses which have to be replaced once they melt

What is resistance?

Anything in the circuit that slows the flow down. The more components you add to a circuit, the higher the overall resistance. Measured in ohms, Ω

What should be fuses be rated at?

As near as possible but just higher than the normal operating current

Voltage-Current Graph for a Metal Filament Lamp

As the temperature of the metal filament increases, the resistance increases

Disadvantage of the heating effect of an electric current

Can cause circuit components to melt - which means the circuit will stop working, or not work properly/fully. Fuses use this heating effect to melt/break the circuit if the current gets too high.

What is charge?

Charge can be positive or negative, and when charge flows it is called current. In solid metal conductors (e.g. copper wire) charge is carried by negatively charged electrons. More charge passes around a circuit when a bigger current flows.

How are components connected in series circuits?

Components are connected in a line. Current flows through all the components or none of them. If you remove one component, the circuit is broken and they all stop. Isn't very handy, only a few things are connected in series e.g. fairy lights.

How are components connected in parallel circuits?

Components are separately connected to the +ve and -ve of the supply. Removing a component will barely affect the others. In a circuit diagram, each component is in its own loop in the circuit. To add a component, you just add another loop. This is how most things are connected as you can switch everything off/on separately. Household electrics (each light switch part of a loop - it just turns one light/set of lights on and off).

Describe how electrical appliance transfer electrical energy.

Current flows through a component and energy is being transferred. (e.g. when current goes into a light bulb, electrical is transferred to light/heat energy).

Voltage-Current Graph for different Resistors

Current through a resistor (at constant temperature) is proportional to voltage. Different resistors have different resistances, hence the different slopes

Voltage-Current Graph for a Diode

Current will only flow through a diode in one direction

What type of current do batteries and cells supply?

Direct current (d.c.) (the current keeps flowing in the same direction.

How can a plug be wired correctly?

Each coloured wire is connected to each pin, and firmly screwed in No bare wires should be showing inside the plug The cable grip is tightly fastened over the cable outer layer

Advantage of the heating effect of an electric current

Good if you want to heat something - toasters contain a coil of wire with a really high resistance. When current passes through, its temp. increases and it glows, giving off infrared (heat) radiation which cooks the bread.

Describe the balance in a circuit.

If you increase voltage - more current will flow. If you increase the resistance - less current will flow (or more voltage will be required to keep the same current flowing)

Where must a voltmeter be placed?

In parallel around the component under test.

What are LEDs? How do they work?

Light-Emitting Diode They emit light when current flows through them in the forward direction. They don't have a filament that can burn out like light bulbs.

What is an LDR? How does it work? What is it useful for?

Light-dependent resistor - A special type of resistor that changes its resistance depending on how much light falls on it. In bright light - resistance falls In darkness- - resistance is highest Useful device for various electronic circuits e.g. burglar detectors

What do conductors do that insulators don't?

Materials that are electrical conductors conduct easily - a current can flow through them. Usually metals e.g. copper, silver. Electrical insulators don't charge very well, current can't flow e.g. plastic, rubber.

What is an ammeter?

Measures the current in amps flowing through the component. Must be placed in series.

What is a voltmeter?

Measures the voltage in volts across the component. Must be placed in parallel - not around the variable resistor or the battery.

What stuff is a plug made of in order to make it safe?

Metal parts made of copper/brass - very good conductors Case, cable grip and cable insulation are made of rubber/plastic (which is flexible) - really good insulators

3 Advantages of Circuit Breakers over Fuses

Normally the same current flows through the live and neutral wires. If somebody touches the live wire, a small and deadly current will flow through them to the earth. This means the neutral wire carries less current than the live wire. The RCCB notices this difference in current and quickly cuts off the power by opening a switch. Much faster - they break the circuit as soon as there is a current surge - not time is wasted waiting for the current to melt a fuse (safer). RCCBs work for small current changes which may not be large enough to melt a fuse. Even small changes could be fatal - RCCBS are more effective at protecting against electrocution.

Uses for LEDs

Numbers on digital clocks Traffic lights Remote controls Can also be used to show appliances are switched on as like lamps, LEDs indicate the presence of current in a circuit.

Name a type of circuit breaker.

Residual Current Circuit Breaker (RCCB)

What is the function of the earth wire?

Stops you getting hurt, for safety Works together with fuse/circuit breaker All appliances with metal parts must be "earthed" - reduces the danger of electric shock. An earthed conductor can never become live.

What is a thermistor? How does it work? What is it useful for?

Temperature-dependent resistor In hot conditions - resistance drops In cool conditions - resistance goes up Useful temperature detectors e.g. car engine temperature sensors

What are basic circuits used for?

Testing components and getting V-I graphs for them.

How can the equation (v = I x R) be used to create a Voltage-Current Graph?

The Gradient of an I-V graph is equal to "1 / resistance". The steeper the graph the lower the resistance. A straight line graph has a constant gradient and shows a constant resistance. If the graph curves the resistance is changing.

What happens when you vary the variable resistor?

The current flowing through the circuit is altered.

Voltage-Current Graph for a Wire

The current through a wire (at constant temperature) is proportional to voltage

What is voltage?

The driving force that pushes the current afround. Kind of like "electrical pressure". Measured in volts, V

How a resistor works.

The electrical current in the resistor is an energy transfer (which heats the resistor). This heating effect increases the resistance of the resistor - so less current will flow, or a greater voltage will be required to provide the same current.

What is current?

The flow of charge around the circuit. Current will only flow through a component if there is a voltage across that component. Measured in ampere/amps, A

What is electrical power?

The rate at which an appliance transfers energy An appliance with a high power rating - transfers a lot of energy in a short time (this energy comes from the current flowing through it). Therefore, an appliance with a high power rating will draw a large current from the supply.

How does current work in series circuits?

The same current flows through all parts of the circuit. The current size is determined by the total voltage of the cells and the total resistance of the circuit (I = V/R). The total resistance depends on: - the number of components - the types of components used

How does electricity flow in a plug?

Through the live wire and out through the neutral wire

How are voltmeters and ammeters connected?

Voltmeters - In parallel with a component even in a series circuit Ammeters - In series even in a parallel circuit

When would an appliance be double insulated?

When the appliance has a plastic casing and not metal parts showing Plastic (insulator) stops current flowing - you can't get a shock. If an appliance is double insulated it doesn't need an earth wire - just a live and neutral wire.

What is the equation for charge?

charge = current × time

What is the equation for electrical power?

electrical power (Watts) = current × voltage

What is the equation for energy transferred ?

energy transferred = current × voltage × time

What does +ve/-ve mean?

positive/negative

What is the equation for voltage?

voltage = current × resistance