P5: Electric Circuits

Definition: an electric charge that has built up on an object

Electric charge:

A substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes positively charged. Charges that are the same (eg positive and positive) repel, while unlike charges (eg positive and negative) attract.

When the materials are rubbed against each other:

• Negatively charged particles called electrons move from one material to the other.
• The material that loses electrons becomes positively charged.
• The material that gains electrons becomes negatively charged.
• Both materials gain an equal amount of charge, but the charges are opposite.

Metal atoms release some of their electrons, and these electrons are free to move through the structure of the metal. This is why metals are good conductors of electricity.

When a circuit is made, the battery causes these free charges to move. They move in a continuous loop around the circuit.

The strength of an electric current is measured in amperes, usually shortened to amps. Amps are a measure of how much electric charge is flowing round the circuit.

The moving electrons can collide with the atoms of the conductor. This is called resistance and it makes it harder for current to flow.

Resistance is measured in ohms, Ω. The greater the number of ohms, the greater the resistance.

The equation below shows the relationship between resistance, voltage, current:

resistance = voltage / current

ohms (Ω) = volts (V) / amperes (A)

Thermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases:

• At low temperatures, the resistance of a thermistor is high and little current can flow through them.
• At high temperatures, the resistance of a thermistor is low and more current can flow through them.

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:

• In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it.
• In bright light, the resistance of an LDR is low and more current can flow through it.

The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not be lit.

The diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still be lit. Because a parallel circuit has more paths for charges to flow along, the current is bigger, and the resistance of the whole circuit is smaller.

Potential difference

A 'potential difference' across an electrical component is needed to make a current flow in it. Cells or batteries often provide the potential difference needed.

'Potential difference' is often called 'voltage'.

Measuring potential difference:

• Potential difference is measured in volts, V.
• Potential difference across a component in a circuit is measured using a voltmeter.
• The voltmeter must be connected in parallel with the component.

When two or more components are connected in series, the total potential difference of the supply is shared between them.

Current

When two or more components are connected in series, the same current flows in each component.

Potential difference

When two or more components are connected in series, the total potential difference of the supply is shared between them.

The potential differences across resistors in series must add up to the battery voltage. This is because the total energy transferred by the battery must equal the amount of energy transferred to the other components in the circuit. Energy is always conserved.

Current

When two or more components are connected in parallel, the total current flowing in the circuit is shared between the components.

Potential difference

When two or more components are connected in parallel, the potential difference across them is the same. This means that if a voltage across a lamp is 12V, the voltage across another lamp connected in parallel is also 12V.

Voltage

In parallel circuits, the voltage across each component is the same as the voltage of the battery. Each component in parallel has the same current as it would have if it were connected to the battery without the other components present.

When you move a megnet into a coil of wire a voltage is Induced. This is called Electromagentic induction.

You can induce a voltage in the opposite direction by:

- Moving the magnet out of the coil

- Moving the other pole of the magnet into the coil

Increasing the induced voltage

To increase the induced voltage:

• move the magnet faster
• use a stronger magnet
• increase the number of turns on the coil
• increase the area of the coil

One side of a coil in a generator moves up during one half turn, and then down during the next half turn.

This means that as a coil is rotated in a magnetic field, the induced current reverses direction every half turn. This is called alternating current (AC).

It is different from the direct current (DC) produced by a battery, which is always in the same direction.

A transformer needs an AC that will create a changing magnetic field. A changing magnetic field also induces a changing voltage in a coil. How a transformer works:

• The primary coil is connected to an AC supply.
• An alternating current passes through a primary coil wrapped around a soft iron core.
• The changing current produces a changing magnetic field.
• This induces an alternating voltage in the secondary coil.
• This induces an (AC) in the circuit connected to the secondary coil.

The ratio between the voltages in the coils is the same as the ratio of the number of turns in the coils.

Primary voltage / secondary voltage = turns on primary / turns on secondary

Step-up transformers have more turns on the secondary coil than they do on the primary coil. Step-down transformers have fewer turns on the secondary coil than they do on the primary coil.

The energy supplied in household electricity is measured in kilowatt hours (kWh). Energy is transferred from the power source to components in an electric circuit. 

When an electric current flows in a circuit, energy is transferred from the power supply to the components in the circuit.

• Energy is measured in joules, J.
• The rate of energy transfer is called the power.
• Power is measured in watts, W.

The equation

The relationship between power (watt, W), PD (volt, V) and current (amp, A).

power = potential difference x current

If the potential difference is 12V and the current is 5A, the power is 12 x 5 = 60W.

The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1kWh.

The equation below shows the relationship between energy transferred, power and time:

energy transfered (kilowatt-hour, kWh) = power (kilowatt, kW) x time (hour, h)

Power is measured in kilowatts, instead of the more usual watts. To convert from W to kW you must divide by 1000. For example, 2000W = 2000 ÷ 1000 = 2kW.

Time is measured in hours here, instead of seconds. To convert from seconds to hours you must divide by 3600. For example, 1800s = 1800 ÷ 3600 = 0.5 hours.

'Wasted' energy

Energy cannot be created or destroyed. It can only be transferred from one form to another or moved. Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings.

Electric lamps: Most of the electrical energy is transferred as heat energy instead of light energy. This is the Sankey diagram for a typical filament lamp:

Modern energy-saving lamps transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.

Effiency Calculation: Effiency = (Useful energy transferred / energy supplied) x 100

The efficiency of the filament lamp is (10 ÷ 100) ×100 = 10%. The efficiency of the energy-saving lamp is (75 ÷ 100) × 100 = 75%. The efficiency of a device will always be less than 100%