Electricity

3E: Tuesday 30th November

ResistanceLearning Objectives:• Know what resistance is and what units

we measure it in

Starter

Write down as many meanings for the word resistance as you can in 1 minute!

Experimental setup

1. Set up the circuit as shown2. Turn on the power supply and adjust the supply voltage to 6V3. Take readings of current, I (A), through, and the voltage, V (V) across the

resistor and record in a table in your workbook – use a ruler!4. Repeat this for several different values (6V down to 0V) of the voltage

and record both voltage and current through the resistor in your table of results

5. Plot a graph of your results (voltage, V (V) on y-axis; current, I (A) on x-axis) using a best fit line – use a ruler!

Battery Pack

A

VResistor

Analysing results:

I (A) V (V)

Voltage, V (V)

Current, I (A)

0V0A

Gradients

• How do we find a gradient of a graph in maths?

• Instead of y we have Voltage, V, instead of x we have Current, I

• If we have a straight line, the gradient is constant• Gradient, m = V/I = constant: RESISTANCE

Definition of Resistance• Electrical Resistance of an object is a measure of its opposition to the passage

of an electric current• Resistance is measured in Ohms (Ω)

Resistance = voltage / current

R=V/I V=IR I=V/R

George Simon Ohm

German Physicist

V

I R

Ohm’s Law questions

• V = IR

• Complete the questions on the handout

Resistors

• Resistors are simple components used to control the current and voltage in a circuit

Resistive heating

• For a given component, power P = IV where I is the current through that component and V is the voltage across that component

• Substituting from V=IR we get:P = I2R = V2/R = IV

Power is measured in Watts

Resistors in series

• If we join resistors in series we increase the resistance of the circuit. The current will decrease

• The total resistance in series is equal to the sum of the individual resistances:

Rs = R1 + R2 + R3

where Rs = equivalent series resistance

Resistors in parallel

• If we join components in parallel we decrease the resistance of the circuit. The current will increase.

• The combined resistance in parallel is calculated using the formula:

1/RP = 1/R1 + 1/R2 + 1/R3

where RP = equivalent parallel resistance