Circuits and Resistance
The atom consists of a central nucleus containing protons (relative mass 1, relative charge +1) and neutrons (relative mass 1, relative charge 0), surrounded by shells of electrons (relative mass 1/1835, relative charge +1). Metals have delocalised electrons which can flow and carry a current.
Circuit diagrams are drawn using symbols such as the ones to the right. Conventional current direction goes from the positive to the negative terminal, while electrons flow towards the positive terminal.
In a series circuit, there is only one route for current to take, but in parallel circuits there are junctions which allow current to flow along different routes. Energy is transferred in a circuit. A cell contains a store of energy which is transferred to the charge. The charge has potential energy as it can transfer energy to the components, which can transfer energy through light, heating and movement.
Voltmeters are connected in parallel with a component to measure the potential difference across it while ammeters are connected in series to measure the current in the component.
Potential difference, or voltage, is the energy transferred per unit of charge passed and therefore one volt in a joule per coulomb.
where E = Energy Transferred (J) , Q = Charge (C) and V = Potential Difference (V)
In a closed loop in a circuit, the total potential difference across the battery is equal to the total p.d. across the components in that loop
Electric current is the rate of flow of charge, measured in coulombs.
where Q = Charge (C), I = Current (A) and t = Time (s)
When a closed circuit includes a source of potential difference then there will be a current flowing through it. Current is the same throughout a series circuit. For parallel circuits, current is conserved at a junction in a circuit, and the sum of currents into the junction is equal to the sum of current out of the junction, as the current splits as it flows down the different branches which may have different resistance.
Resistance is measured in ohms, and changing the resistance changes the current in the circuit. A variable resistor (circuit symbol to the left) can be used to measure resistance of a resistor by taking measurement of potential difference as current also changes.
where V = Voltage (V), I = Current (A) and R = Resistance (Ω)
If two resistors are in series the total resistance of the circuit increases as the circuit becomes more difficult for current to flow through. There will be a greater p.d. across the resistor with higher resistance
When resistors are connected in parallel the total resistance of the circuit decreases as there are more paths for the current to flow through.
| Component | Graph | Explanation |
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Increasing current causing the filament to become hotter and to glow brighter. As the temperature increases, the resistance increases, meaning current does not increase proportionally with p.d. |
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Allows current flow in only one direction, for which resistance is very high and increases exponentially. |
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High resistance in the dark but as light intensity increases, this resistance decreases. |
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Thermistors have high resistance at low temperatures but as temperature increases this resistance increases | |
| The resistance change can be explored using the component in series with a variable resistor, battery and ammeter while a voltmeter is connected in parallel | ||
CORE PRACTICAL: Investigating Resistance
A – Set up a series circuit with resistor, battery to provide potential difference, ammeter and voltmeter
B – Measure voltage and current using voltmeter and ammeter and slowly increase voltage, measuring how current changes. Overall, resistance should be constant by using V = IR, and current should be directly proportional to voltage. Direction of current has no effect on resistance
C – Replace the resistor with two filament lamps. Repeat process, slowly increasing voltage. Overall, the resistance of the lamps should be the same and resistance should increase with potential difference in a non-linear way as the filament gets hotter
D – Set up a parallel circuit with lamps in different branches and repeat with same voltages. Overall, the resistance will be lower in the circuit as there are more branches for current to flow through
Energy and Electricity
When there is an electric current in a resistor, there is an energy transfer which heats the resistor as a result of collisions between ions in the lattice structure and the electrons. Electrical energy is dissipated as thermal energy in the surroundings when the electrical current does work against electrical resistance. This thermal energy is useful in appliances like heaters and kettles, but in other uses like laptops and wires, this transfer is not useful as energy is lost. Resistance can be reduced by using low resistance wires like copper. Thicker wires have lower resistance. Resistance can also be decreased by cooling metals so the ions in the lattice do not vibrate as much.
Use the equation
where E = Energy transferred (J), V = Voltage (V) and t = Time (s)
Power is the energy transferred per second, measured in watts.
where P = Power (W), E = Energy (J) and t = Time (s)
Power transfer in any circuit device is related to the potential difference across it and the current in it
where P = Power (W), I = Current (A) and V = Voltage (V)
where P = Power (W), I = Current (A) and R = Resistance (Ω)
In different domestic devices, energy is transferred from batteries and the a.c. mains to the energy of motors and heating devices. Direct Current (d.c.) is where cells have a positive and negative terminal and the movement of charge stays the same. Alternating Current (a.c.) is where direction is always changing due to rotating generators. The voltage also changes, increasing to a peak voltage then decreasing to zero. In the UK, the domestic supply is a.c. at a frequency of 50Hz and a voltage of 230V.
Electrical Safety
In the UK, appliances are connected to the mains with a 3-pin plug. Each of these pins are connected to a wire
- Live Wire: connects appliance to generators at the power station. Voltage of 230V is carried
- Neutral Wire: return path to power station. Voltage is 0V if the circuit is connected properly
- Earth Wire: connects the metal parts of appliance to a metal that is pushed into the ground. Used for safety and is 0V if the circuit is connected correctly
Switches are connected in the live wire, so that if they are off, no current goes through the appliance
A fuse is a tube with a thin wire inside. The current passes through and the wire gets hotter. If the current exceeds a certain voltage, the wire melts, breaking the circuit and stopping the current flow
If a faulty appliance draws too much current, it can cause overheating, which could lead to fires. The metal parts may be at a high voltage. If this was touched, a current would flow through the person to the ground, and as a result the earth wire goes straight into the ground. If the live wire touched the metal part, this makes a low resistance circuit between 230V and the earth, causing the current to flow to the earth, heating the wire and maybe causing a fire. The current blows a fuse and cuts off the mains supply. Circuit breakers, which detect a change in current and switch off supply, can be alternatives to a fuse as they do not need to be replaced as fuses do, and they work very quickly, potentially saving lives.
The power rating of an appliance is measured in watts, and an appliance which has a Z kW power rating transfers Z × 1000 W of energy each second from the mains supply to a store of energy in the appliance